Honeywell
Variable Frequency Drive (VFD)
APPLICATION GUIDE
CONTENTS
S A F E T Y ........................................................................................................................... 3
W h e n In s ta llin g th is P r o d u c t............................................................................ 3
In s p e c tio n P r o c e d u r e .......................................................................................... 3
M o n ito r th e L E D .....................................................................................................3
IN T R O D U C T IO N ........................................................................................................ 4
V F D T y p e s ............................................................................................................... 4
W h y B u y a V F D .....................................................................................................4
W h e re a re V F D A p p lic a t io n s .......................................................................... 4
B e n e fits S u m m a r y .................................................................................................5
V F D M A R K E T ............................................................................................................... 5
E n e rg y C o s t s .......................................................................................................... 5
V F D C o s ts .................................................................................................................5
W h a t's N e w .............................................................................................................. 5
U s e r B e n e f it s .......................................................................................................... 6
M O T O R F U N D A M E N T A L S ................................................................................... 7
A d v a n ta g e s .............................................................................................................. 7
D is a d v a n ta g e s ....................................................................................................... 7
V o lta g e a n d C u rre n t W a v e fo rm E x a m p le s .............................................. 8
D iffe re n c e s B e tw e e n S ta r D e lta a n d D e lta S t a r ....................................... 12
L o a d T y p e s ................................................................................................................ 14
P o w e r F a c to r ............................................................................................................. 15
U s e fu l F o r m u la e ..................................................................................................... 16
V F D F U N D A M E N T A L S ............................................................................................. 16
C o n s tru c tio n ...............................................................................................................16
B ra k in g R e s is t o r ..................................................................................................... 17
V F D V o lta g e to F re q u e n c y R a tio ..................................................................... 18
C h a rg in g R e s is to r ...................................................................................................19
A C L in e C h o k e ......................................................................................................... 19
M o to r a n d V F D T e s t s ............................................................................................2 0
IN S T A L L A T IO N G U ID E L IN E S ................................................................................2 4
G e n e ra l G u id e lin e s ................................................................................................2 4
S u r v e y in g ................................................................................................................... 2 4
V F D L o c a tio n : E n c lo s u re s a n d V e n t ila t io n ................................................2 4
E M C W ir in g a n d R F I F ilt e r s .............................................................................. 2 5
L in e R e a c to rs ............................................................................................................2 6
R F I ....................................................................................................................................... 2 6
S o u rc e s O f E m is s io n s ......................................................................................... 2 6
R o u te s fo r E m is s io n s ............................................................................................2 7
E q u ip m e n t C a t e g o r ie s ......................................................................................... 2 8
G e n e ra l W ir in g S t a n d a r d s ................................................................................. 2 8
A P P L IC A T IO N S .............................................................................................................29
V F D O V E R V IE W .......................................................................................................... 30
A p p lic a tio n D e s c r ip tio n ........................................................................................30
B e n e fits ........................................................................................................................31
V F D F e a tu re s U s e d .............................................................................................. 31
O p e ra tio n (S e e F ig. 3 6 ) ...................................................................................... 31
C o n s ta n t A ir V o lu m e w ith A ir Q u a lity C o m p e n s a tio n (S e e Fig. 3 7 )3 2
O p e r a tio n ....................................................................................................................33
V a ria b le A ir V o lu m e (V A V ) P rim a ry P la n t C o n tro l (S e e Fig. 4 0 ).... 34
C o o lin g T o w e r F a n s (S e e F ig. 4 1 ) ................................................................. 35
A p p lic a tio n D e s c rip tio n ...................................................................................... 35
B e n e fits ........................................................................................................................35
V F D F e a tu re s U s e d .............................................................................................. 36
O p e ra tio n (S e e F ig. 4 2 ) ..................................................................................... 36
P rim a ry C h ille d W a te r O p tio n 1 (S e e Fig. 4 3 a n d 4 4 ) ......................... 36
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P rim a ry C h ille d W a te r P u m p w ith D D C C o n tro l o f P la n t O p tio n 2 (S e e Fig. 4 5
a n d 4 6 ) ........................................................................................................................ 37
S e c o n d a ry C h ille d W a te r P u m p O p tio n 1 (S e e Fig. 4 7 ) ..................... 39
S e c o n d a ry C h ille d W a te r P u m p w ith D D C C o n tro l O p tio n 2 (S e e Fig. 4 8 )
........................................................................................................................................ 4 0
H e a tin g S e c o n d a ry W a te r C irc u it (S e e Fig. 4 9 ) ......................................41
S te a m B o ile r M a k e -U p P u m p (S e e Fig. 5 0 a n d 5 1 ) ............................. 4 2
B o ile r F lu e G a s -In d u c e d D ra u g h t (ID ) F a n (S e e F ig. 5 2 ) .................. 4 3
B o ile rs a n d F o rc e d D ra u g h t F a n (S e e Fig. 5 3 ) ....................................... 4 4
A ir c r a ft P a s s e n g e r J e tty L o a d in g (S e e F ig. 5 4 ) ....................................... 4 5
S c re w P re s s (S e e Fig. 5 5 ) ..................................................................................4 6
E le v a to rs (S e e Fig. 5 6 ) ........................................................................................ 4 7
L is t o f A b b r e v ia tio n s ..............................................................................................4 8
P R E P O W E R U P C H E C K S .................................................................................... 4 8
P O S T P O W E R U P C H E C K S ................................................................................. 4 9
T R O U B L E S H O O T IN G O N S I T E ...........................................................................4 9
U n s ta b le S p e e d C o n t r o l......................................................................................4 9
F a u lts a n d T r ip s ....................................................................................................... 50
T e s tin g B rid g e R e c tifie r a n d O u tp u t P o w e r T r a n s is t o r s ......................53
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2
SAFETY
When Installing this Product...
1. R e a d th e s e in s tru c tio n s c a re fu lly . F a ilu re to fo llo w th e m c o u ld d a m a g e th e p ro d u c t o r c a u s e a h a z a rd o u s c o n d itio n .
2. C h e c k th e ra tin g s g iv e n in th e in s tru c tio n s a n d on th e p ro d u c t to m a k e s u re th e p ro d u c t is s u ita b le fo r y o u r a p p lic a tio n .
3. In s ta lle r m u s t be a tra in e d , e x p e rie n c e d s e rv ic e te c h n ic ia n .
4. A fte r in s ta lla tio n is c o m p le te , c h e c k o u t p ro d u c t o p e ra tio n a s p ro v id e d in th e s e in s tru c tio n s .
WARNING
E le c t r ic a l S h o c k T h a t C a n C a u s e D e a th .
C a p a c it o r d is c h a r g e c a n p r o d u c e le t h a l s h o c k .
E n s u re th a t a ll c irc u its a re c o m p le te ly s a fe b e fo re c o m m e n c in g w o rk .
E a c h V F D h a s a b a n k o f c a p a c ito rs th a t, u n d e r n o rm a l c irc u m s ta n c e s , d is c h a rg e s s h o rtly a fte r d is c o n n e c tin g m a in p o w e r (fiv e
o r 10 m in u te s ). D u rin g n o rm a l o p e ra tio n , th e s e c a p a c ito rs c h a rg e to a t le a s t 5 0 0 V d c a n d a s h ig h a s 8 0 0 V d c.
Inspection Procedure
IMPORTANT
I n s p e c t i o n a n d R e p a i r a r e o n l y to b e u n d e r t a k e n b y a c o m p e t e n t p e r s o n .
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M o s t V F D m a n u fa c tu re rs in c lu d e an L E D th a t illu m in a te s w h e n e v e r th e c a p a c ito r v o lta g e is a b o v e 3 0 V d c.
D o n o t re ly u p o n th e s e re s is to rs a n d L E D c irc u its to in d ic a te a s a fe c o n d itio n . S o fa r, w e h a v e n o t e x p e rie n c e d a fa ilu re o f
th e L E D c irc u it o r d is c h a rg e re s is to rs b u t th is c o u ld h a p p e n .
T h e fo llo w in g is th e c o rre c t p ro c e d u re to fo llo w :
T e s t In s tru m e n ts R e q u ire d : S ta n d a rd M u ltim e te r w ith V a c /V d c ra n g e in e x c e s s o f 6 0 0 V .
IMPORTANT
C h e c k t e s t i n s t r u m e n t s i m m e d ia t e l y p rio r to c o m m e n c i n g w o rk b y c o n n e c t i n g to v o l t a g e s u p p l y .
C a rry o u t n e c e s s a ry is o la tio n a n d s a fe ty p ro c e d u re s fo r w o rk in g on e le c tric a l e q u ip m e n t. C h e c k fo r c o m p lia n c e w ith lo c a l
re q u ire m e n ts . A p e rm it-to -w o rk m a y be re q u ire d . W h e re p o s s ib le , re m o v e fu s e s a n d lo c k o ff is o la to rs .
IMPORTANT
I f t h e L E D is illu m i n a t e d , d o n o t t o u c h a n y o f t h e i n te r n a l c o m p o n e n t s o f t h e V F D o r a s s o c i a t e d w i rin g .
1. D is c o n n e c t p o w e r s u p p ly fro m V F D . A ll in d ic a to rs , d is p la y s , a n d L E D s h o u ld e x tin g u is h a fte r a fe w s e c o n d s .
2 . W a it 5 m in u te s b e fo re ta k in g fu rth e r a c tio n .
3 . C a re fu lly re m o v e a n y p ro te c tiv e co v e rs .
4 . D o n o t to u c h a n y c o n d u c to rs w ith in th e V F D .
5 . C o n firm th a t th e L E D c h a r g e in d ic a to r is illu m in a te d . T h is L E D is b rig h t a n d c a n n o t be m is ta k e n fo r a n o th e r in d ic a to r.
6 . If L E D in d ic a to r is e x tin g u is h e d , id e n tify D C b u s c irc u it a n d c h e c k a ll b u s b a rs a n d te rm in a ls fo r v o lta g e . P a y p a rtic u la r
a tte n tio n to te rm in a ls m a rk e d P a n d N.
7 . E n s u re th a t no v o lta g e s a re p re s e n t th e n u s e th e v o lta g e te s te r to c h e c k b e tw e e n c o n d u c to rs a n d e a rth.
N O T E : T h e v o lta g e te s te r ca n d is c h a rg e th e c a p a c ito rs b y c o n n e c tin g it b e tw e e n b u s b a rs /c o n n e c to rs a n d e a r th .
Monitor the LED
It s h o u ld be e a s y to se e th e le v e l o f illu m in a tio n d e c a y a n d w ith in 1 o r 2 m in u te s m o re , th e L E D s h o u ld be e x tin g u is h e d . A n
L E D m a in ta in in g illu m in a tio n le v e l c a n in d ic a te d a m a g e d d is c h a rg e re s is to rs th a t c re a te an o p e n c irc u it. A ls o , an a d d itio n a l
p o w e r s u p p ly m a y h a v e b e e n in s ta lle d a n d is s till p o w e rin g th e V F D . C a re fu lly c h e c k fo r V a c s u p p lie s u s in g th e p ro c e d u re
o u tlin e d in th e In s ta lla tio n P ro c e d u re s e c tio n .
IMPORTANT
R e c h e c k w ith m u l t i m e t e r b e f o r e c o m m e n c i n g w o r k .
NOTE:
If no V a c s u p p lie s a re fo u n d , it is p ro b a b le th a t d is c h a rg e re s is to rs a re fa u lty .
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IN T R O D U C T IO N
VFD Types
T h e fo llo w in g lis t g iv e s m e c h a n ic a l, e le c tric a l a n d h y d ra u lic V F D e x a m p le s (m a n y h a v e a c o n s id e ra b le m e c h a n ic a l c o n te n t
n e e d in g re g u la r m a in te n a n c e ):
1.
2.
3.
M e c h a n ic a l D riv e s.
a.
A d ju s ta b le s h e a v e b e lt d riv e .
b.
C lu tc h .
c.
T ra c tio n d riv e .
E le c tric a l D riv e s .
a.
E d d y — c u rre n t c lu tc h .
b.
D C (ro ta tin g a n d s o lid s ta te ).
c.
S o lid s ta te V a c.
d.
M u lti-s p e e d m o to rs .
F lu id d riv e s .
M a n y o f th e s e c a n be re p la c e d w ith a s ta n d a rd in d u c tio n m o to r a n d a g e n e ra l p u rp o s e P W M V F D to p ro v id e a m o re re lia b le
a n d c o s t e ffe c tiv e s o lu tio n to th e V F D re q u ire m e n t. U s in g a V F D o fte n p ro v id e s e n e rg y c o n s e rv a tio n b e n e fits a n d
im p ro v e m e n ts in th e a c c u ra c y o f c o n tro l a s a b o n u s . M a n y o p p o rtu n itie s fo r o b ta in in g s u b s ta n tia l s a v in g s a re m is s e d e v e n
th o u g h a d v ic e is a v a ila b le fro m n u m e ro u s s o u rc e s , in c lu d in g G o v e rn m e n t, p ro fe s s io n a l b o d ie s , c o n s u lta n ts a n d e q u ip m e n t
s u p p lie rs . T h e re a s o n s fo r th is in c lu d e :
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L a c k o f a w a re n e s s a n d /o r s k e p tic is m re g a rd in g la te s t te c h n o lo g ie s .
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O v e ra ll fin a n c ia l a n d o p e ra tio n a l b e n e fits a re o fte n n o t fu lly a p p re c ia te d .
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E n e rg y s a v in g p ro je c ts to o o fte n ta k e s e c o n d p la c e to o th e r p ro d u c tio n re la te d e x p e n d itu re .
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L o w e s t firs t c o s tta k e s p re c e d e n c e o v e r life cycle c o s t (th a t is, in itia l c o s t p lu s ru n n in g a n d m a in te n a n c e c o s ts ).
Why Buy a VFD
M e c h a n ic a l, e le c tric a l, a n d flu id p o w e r a d ju s ta b le s p e e d d riv e s a re a v a ila b le th a t o ffe r s o m e o f th e a fo re m e n tio n e d b e n e fits .
N o n e c o m b in e a ll o f th e a d v a n ta g e s o f a V F D :
1. V a ria b le s p e e d a n d flo w c a p a b ility w ith s ta n d a rd in d u c tio n m o to r.
a.
Im p ro v e d p ro c e s s c o n tro l.
b.
E n e rg y s a v in g s .
2. R e d u c e d v o lta g e s ta rtin g c h a ra c te ris tic s .
a.
S o ft s ta rt/s m o o th a c c e le ra tio n .
b.
R e d u c e s p o w e r s u p p ly p ro b le m s in th e fa c ility .
c.
R e d u c e s m o to r h e a tin g a n d s tre s s .
3. U s e d w ith s ta n d a rd A C in d u c tio n m o to r.
Where are VFD Applications
In d u s try s e g m e n ts a re im p o rta n t, b e c a u s e m a n y a p p lic a tio n s a re in d u s try s p e c ific . S o m e cla s s ic V F D a p p lic a tio n s fo r v a rio u s
in d u s trie s a re p ro v id e d b e lo w :
1. H V A C , fa n s a n d p u m p s .
2. F o o d P ro c e s s in g : a g ita to rs , m ix e rs , c o n v e y o rs fo r fo o d tra n s p o rt, p a c k a g in g a n d b o ttlin g , p re p a ra tio n m a c h in e s (s lic e rs ,
d ic e rs , c h o p p e rs ), e x tru d e rs , fa n s a n d p u m p s .
3. P e tro c h e m ic a ls : d e e p w e ll p u m p s , oil fie ld re c o v e ry , lo c a l d is trib u tio n p u m p s , fa n s a n d p u m p s .
4. M in in g a n d M e ta ls : re h e a t fu rn a c e s , c o o lin g b e d s, run in /o u t ta b le s , fa n s a n d p u m p s .
5. P u lp a n d P a p e r/F o re s t P ro d u c e rs : w a s h e rs , k iln s, s litte rs , d e c k e rs , c h ip p e rs , s a w s , s a n d e rs , p e e le rs , d e -b a rk e rs , fa n s
a n d p u m p s , v a c u u m re m o v a l s y s te m s .
6. M a c h in e T o o l: re p la c e s p in d le d riv e s , g rin d e rs , s a w s , la th e s , to o l p o s itio n in g d riv e s , b a la n c in g m a c h in e s , fa n s a n d
pum ps.
7. T ra n s p o rta tio n : m a te ria l h a n d lin g c o n v e y o rs , c ra n e s a n d h o is ts , s m a ll v e h ic le d riv e s , fa n s a n d p u m p s .
8. A n y m a c h in e o r p ro c e s s th a t c a n be im p ro v e d b y v a ry in g s p e e d o r flo w is a c a n d id a te fo r a V F D .
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Benefits Summary
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—
—
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Im p ro v e d c o n tro l.
R e d u c e d p la n t w e a r.
Q u ie te r o p e ra tio n a t lo w lo a d .
R e d u c e d c o m p la in ts .
L o w e r o p e ra tin g c o s ts .
T y p ic a l p a y b a c k p e rio d is le s s th a n 2 y e a rs .
VFD M ARKET
Energy Costs
T h e fo rc e s fo r in d u s tria l a u to m a tio n , re q u ire m e n ts fo r e v e r in c re a s in g e ffic ie n c ie s fro m p la n t a n d m a c h in e ry , to g e th e r w ith
d e m a n d s fo r h ig h e r p e rfo rm a n c e a t a lo w e r c o s t, c o n tin u e to fu e l ra p id V F D m a rk e t g ro w th .
VFD Costs
M o d e rn m a n u fa c tu rin g te c h n iq u e s , u s in g n e w te c h n o lo g y M ic ro -C o n tro lle rs , D ig ita l S ig n a l P ro c e s s o rs , A p p lic a tio n S p e c ific
In te g ra te d C irc u its (A S IC ) a n d o th e r h ig h ly in te g ra te d d e v ic e s , h a v e s u b s ta n tia lly re d u c e d th e V F D c o m p o n e n t c o u n t. T y p ic a lly ,
V F D c o m p o n e n t c o u n ts h a v e d ro p p e d fro m s e v e ra l th o u s a n d fo r e a rly d e s ig n s , to a ro u n d 5 0 0 fo r m o d e rn m a c h in e s . T h e re s u lt
o f th is is n o t o n ly s m a lle r p h y s ic a l size , b u t a ls o s u b s ta n tia l re d u c tio n s in o v e ra ll c o s t a n d in c re a s e in re lia b ility (s e e F ig. 1).
O th e r fa c to rs w h ic h h a v e a lo w e rin g e ffe c t on m a rk e t p ric e s
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W id e V F D a c c e p ta n c e w ith in in d u s try a n d c o m m e rc e .
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L a rg e r v o lu m e s , p ro d u c e b e n e fits o f s c a le : g re a te r p u rc h a s in g p o w e r b y m a n u fa c tu re rs a n d s u p p lie rs , la rg e r in v e s tm e n t in
a u to m a te d m a n u fa c tu rin g p ro c e s s e s .
Size
C ost
Functions
R eliability
F ig . 1 . T r e n d s in t h e V F D M a r k e t p la c e .
What's New
A n o th e r b e n e fit o f u s in g a d v a n c e d te c h n o lo g ie s in m a n u fa c tu rin g a n d h ig h p e rfo rm a n c e c h ip se ts, h a s b e e n th e a v a ila b ility to
p ro d u c e c o n tro lle rs w ith m a n y m o re fe a tu re s .
In th e e a rly V F D d a ys, m o re th a n 2 0 y e a rs a g o , th e m a c h in e h a d s im p le fu n c tio n a lity , p ro d u c e d b y th e u s e o f b a s ic a n a lo g u e
te c h n o lo g y . T h e s e m a c h in e s h a d lim ite d fu n c tio n s , p ro v id in g th e b a s ic s o f s p e e d c o n tro l a n d s o ft s ta rt a n d sto p .
M o d e rn c o n tro l o f a g e n e ra l p u rp o s e V F D is d ig ita l, w ith w h a t s e e m s to be an e v e r g ro w in g lis t o f fu n c tio n s :
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M u ltip le p ro g ra m m a b le d ig ita l in p u ts .
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M u ltip le p ro g ra m m a b le d ig ita l o u tp u ts .
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M u ltip le p ro g ra m m a b le a n a lo g u e in p u ts .
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M u ltip le p ro g ra m m a b le a n a lo g u e o u tp u ts .
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C lo s e d lo o p h ig h a c c u ra c y s p e e d c o n tro l.
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A n a rra y o f m o to r p ro te c tio n fu n c tio n s .
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S im p le plc. ty p e fu n c tio n s .
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S im p le b u t e ffe c tiv e to rq u e c o n tro l.
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C o m m u n ic a tio n bus.
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F ig . 2 . V F D F u n c t io n a lit y .
For non specialized use, there have always been four clearly defined areas of motor speed control, which require four different
types of machines:
General Purpose AC Machine
Used on fans, pumps and machinery of all types, where a v e r a g e p e r f o r m a n c e is required using either single or three-phase
Vac supply driving a standard induction motor.
Flux Vector
High performance machine providing very high accuracy in speed and torque control, over wide operating range. Newer
designs challenging the DC drive for High Torque at very low speeds. Special motors often required.
Spindle Drive
Very high performance machine used in mechanical handling and machine tool applications where high dynamic response is
critical. These machines have many features of the Vector Flux VFD but often have much more input and output capability, to
provide the necessary interfacing with the o u t s i d e w o rld . They almost always require a special motor.
DC Drive
DC drives are still prevalent in the drive industry, making up some 40% of the market. VFD performance has improved so much
in the recent past, that DC drives are often replaced with AC machines that provide added benefits. There are applications, in
particular very large, high-power, low-speed drives, where DC will be used for many years to come.
Advancements in design and expertise now allow the combination of functionality and performance of General Purpose, Flux
Vector, Spindle Drive and to some extent the DC drive in the o n e package. However, the motor will have to be an A c type.
User Benefits
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One machine covers all applications.
Lower spares cost.
Fewer different variants for technicians to work with.
Possible general reduction is costs: higher volume production.
Interchangeability.
This u n i v e r s a l approach is adopted by most high volume manufacturers as it is seen as the ultimate goal. (Fig. 3)
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6
DC Drive
Itfl-
F ig . 3 . U n iv e r s a l D r iv e .
M O TO R FUNDAM ENTALS
T h re e -p h a s e e le c tric m o to rs a re v e ry ru g g e d m a c h in e s th a t h a v e to w ith s ta n d a lm o s t e n d le s s a b u s e fro m e n d u s e rs a n d still
c o n tin u e to p e rfo rm to s p e c ific a tio n . T h e re a re s e v e ra l v a ria b le s o f w h ic h th e e n d u s e r is o fte n u n a w a re . M a n y h a v e s ig n ific a n t
e ffe c t on th e g e n e ra l p e rfo rm a n c e o f th e m o to r. T h e m o to r te rm in a l v o lta g e is a m a jo r v a ria b le th a t ca n h a v e s ig n ific a n t e ffe c t
on th e p e rfo rm a n c e o f th e m o to r. T a b le 1 s h o w s th e e ffe c ts o f ra is in g a n d lo w e rin g th e m o to r te rm in a l v o lta g e , a b o u t its
n a m e p la te v a lu e . T h e p o w e r s u p p ly fe e d in g th e m a jo rity o f d o m e s tic fa c ilitie s is n o m in a lly 2 0 0 V a c 1 P h a s e a n d n o m in a lly 4 0 0
V a c th re e - p h a s e fo r c o m m e rc ia l a n d in d u s tria l a p p lic a tio n s . T h is p o w e r is tra n s m itte d a t a fr e q u e n c y o f 6 0 Hz. T h e v a s t
m a jo rity o f m o to rs in in d u s try a n d c o m m e rc e a re th re e -p h a s e in d u c tio n m o to rs o f th e S q u irre l C a g e D e s ig n .
T a b l e 1. A C M o t o r s : V o l t a g e V a r i a t i o n E f f e c t s ( C o n s t a n t F r e q u e n c y ) .
9 0 % R a te d V o lta g e
T o rq u e (V a rie s d ire c tly a s S q u a re o f V o lta g e )
-1 9 %
1 1 0 % R a te d V o lta g e
+21%
S p e e d , S y n c h ro n o u s
No C hange
No C hange
S p e e d , F u ll L o a d (In d u c tio n M o to rs )
-1 .5 %
+ 1%
C u rre n t, S ta rtin g
-1 0 to -1 2 %
+ 10 to + 1 2 %
C u rre n t, F u ll L o a d
+ 11%
-7 %
S lip
+23%
-1 7 %
E ffic ie n c y
-2 %
+ / to + 1 %
P o w e r F a c to r
+ 1%
-3 %
Advantages
—
—
—
—
—
—
—
—
L o n g life.
H ig h p ro te c tio n le v e ls a v a ila b le .
H ig h e ffic ie n c y : 8 0 % + .
R e a d ily a v a ila b le r e p la c e m e n ts w o rld -w id e .
M in im a l m o v in g p a rts th e re fo re lo w m a in te n a n c e co st.
H ig h s ta rtin g to rq u e , s u ita b le fo r w id e v a ria tio n s o f a p p lic a tio n s .
S im p le to re v e rs e .
L o w co st.
Disadvantages
—
—
D e s ig n e d fo r fix e d s p e e d o p e ra tio n .
D e s ig n e d to be run fro m s in e w a v e p o w e r s u p p ly .
7
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Voltage and Current Waveform Examples
No Load
Fig. 4 s h o w s th e c u rre n t a n d v o lta g e w a v e fo rm s o f o n e p h a s e o f a s ta n d a rd th re e - p h a s e 10 H p in d u c tio n m o to r u n d e r no lo a d
c o n d itio n s . F u ll lo a d c u rre n t o f th e m o to r is 1 3 .5 a m p s fro m a 4 1 5 s u p p ly . It is c le a r fro m th e g ra p h th a t e v e n u n d e r no lo a d
c o n d itio n s th e m o to r d ra w s s u b s ta n tia l c u rre n t.
A
m
p
s
Time( ms)
F ig . 4 . S t a n d a r d t h r e e - p h a s e 1 0 H p in d u c t io n m o t o r w a v e f o r m s ( n o lo a d c o n d i t i o n s ) .
N o te th e s c a le on th e rig h t o f th e g ra p h . T h e p e a k to p e a k c u rr e n t is 15 a m p s . C u rre n t la g s s u b s ta n tia lly b e h in d v o lta g e . T h is
d e m o n s tra te s th a t th e m o to r p o w e r fa c to r is m u c h le s s th a n u n ity. T h e m a jo rity o f th e c u rre n t d ra w n is fo r th e p u rp o s e o f
m a g n e tiz in g th e m o to r a n d is c a lle d m a g n e tizin g c u rre n t N o te th e s h a p e o f th e v o lta g e a n d c u rre n t w a v e fo rm s , b o th c u rv e s
s h o u ld be s in e w a v e s . It is lik e ly th a t th e re a s o n fo r th e d is to rte d w a v e fo rm s is lo a d s h a rin g o f th e s a m e p o w e r s u p p ly th a t is
n o n -s in u s o id a l a n d d is to rts th e v o lta g e w a v e fo rm . T h is re fle c ts in th e d is to rtio n o f th e c u rre n t w a v e fo rm .
Full Load
Fig. 5 s h o w s th e re s u lts o f in c re a s in g th e lo a d on th e m o to r in th e te s t a b o v e , to fu ll lo a d c o n d itio n s . C u rre n t d ra w ro s e b y a
fa c to r o f th re e a n d th e d is to rtio n fa c to r h a s n o w in c re a s e d s u b s ta n tia lly . N o te th e s c a le c h a n g e on th e rig h t h a n d s id e o f th e
g ra p h a n d th e p e a k to p e a k c u rre n t is 5 0 a m p s . T h e c u rre n t w a v e fo rm m o v e d to w a rd s th e v o lta g e , in d ic a tin g an in c re a s e in
p o w e r fa c to r. H o w e v e r, th e c u rre n t s till la g s b e h in d th e v o lta g e .
Phase L1
25
18.75
12.5
V
0
1
t
s
6.25
0
6.25
A
m
p
s
- 12.5
18.75
- 25
F ig . 5 . S t a n d a r d t h r e e - p h a s e 1 0 H p i n d u c t io n m o t o r w a v e f o r m s ( f u ll lo a d c o n d i t i o n s ) .
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8
Sine Wave Supply Voltage to Frequency Relationship
T h e m o to r n a m e p la te c a rrie s s e v e ra l im p o rta n t p a ra m e te rs : k W o r H P, v o lta g e , fu ll-lo a d c u rre n t, fu ll-lo a d m o to r s p e e d , a n d
p o w e r s u p p ly fre q u e n c y . T w o o f th e s e , te rm in a l v o lta g e a n d p o w e r s u p p ly fr e q u e n c y a re v e ry im p o rta n t a s th e y d e fin e th e
v o lta g e to fr e q u e n c y ra tio fo r th e m o to r. T h is c re a te s a c o n s ta n t flu x (m a g n e tic fie ld ) c o n d itio n th ro u g h o u t th e fu ll s p e e d ra n g e
o f th e m o to r. T h e v o lta g e to fr e q u e n c y r e la tio n s h ip is a s im p le lin e a r re la tio n s h ip . F o r e x a m p le : If fr e q u e n c y c h a n g e s fro m 50
H z to 2 5 Hz, th e v o lta g e s h o u ld c h a n g e fro m 4 1 5 V (th re e p h a s e ) to 2 0 7 v o lts . if th e m o to r w e re c o n n e c te d to a n y s in e w a v e
s u p p ly , w ith a fr e q u e n c y o th e r th a n th a t d e fin e d on th e n a m e p la te , th e c o rre c t v o lta g e fo r th is fr e q u e n c y ca n be d e te rm in e d .
F ig . 6 . T y p ic a l m o t o r s p e e d t o r q u e c u r v e .
Motor Torque Characteristics
Fig. 7 s h o w s th e s ta rtin g to rq u e c h a ra c te ris tic s o f a ty p ic a l A /C in d u c tio n m o to r. T h is is a to rq u e c u rv e o f a g e n e ra l p u rp o s e
m o to r. T h e re a re m a n y o th e r ty p e s , d e p e n d in g on lo a d ty p e a n d p e rfo rm a n c e re q u ire d . P o in t 1 is th e lo c k e d ro to r to rq u e ,
ty p ic a lly 2 0 0 % o f n o rm a l fu ll lo a d to rq u e . A s th e m o to r b e g in s to a c c e le ra te th e lo a d , th e to rq u e p ro d u c tio n a b ility o f th e m o to r
d e c re a s e s . T h e n , a t p o in t 2, ty p ic a lly 2 0 % o f fu ll s p e e d , th e to rq u e h a s fa lle n to an a p p ro x im a te m in im u m o f 1 7 5 % . T h is p o in t is
c a lle d th e run up p o in t. T h e to rq u e p ro d u c tio n a b ility o f th e m o to r in c re a s e s fro m th e run up p o in t u n til a b o u t 9 0 % s p e e d , p o in t
th re e , m a x im u m to rq u e , th e p u ll o u t p o in t, is a c h ie v e d .
F ro m h e re , th e m o to r ra p id ly re a c h e s fu ll lo a d s p e e d , p o in t 4, w h ic h c o rre s p o n d s to th e n a m e p la te s p e e d on th e m o to r. If th e
lo a d is v e ry lig h t, th e m o to r w ill c o n tin u e to a c c e le ra te to c lo s e to th e s y n c h ro n o u s s p e e d , w h ic h in th e c a s e o f a 4 p o le
m a c h in e on a 5 0 H z s u p p ly w o u ld be 1 5 0 0 rpm .
NOTE:
T h e d iffe re n c e b e tw e e n th e s y n c h ro n o u s s p e e d a n d th e fu ll lo a d s p e e d is c a lle d slip. S lip is g e n e ra lly a fe w p e rc e n t o f
th e s y n c h ro n o u s s p e e d , e q u iv a le n t to 4 0 -5 0 rp m
W h e n a s ta n d a rd in d u c tio n m o to r is s w itc h e d d ire c tly o n to a 6 0 H z p o w e r s u p p ly , th e re is a la rg e in ru s h o f c u rre n t. T h is c a n
e x c e e d s ix tim e s th e m o to r n a m e p la te fu ll-lo a d c u rre n t. A s th e m o to r a c c e le ra te s th e lo a d , th is la rg e c u rre n t flo w fa lls o ff ra p id ly
u n til a p o in t a t w h ic h m o to r s p e e d a n d lo a d b a la n c e . T h is p o in t is s o m e w h e re b e tw e e n no lo a d s p e e d a n d fu ll lo a d s p e e d . S e e
Fig. 7. M o to rs d riv e n b y a V F D h a v e th e la rg e in ru s h o f c u rre n t c o n tro lle d b y th e V F D . O n ly u n d e r w o r s t c a s e c o n d itio n s w ill th e
in ru s h re a c h 1 5 0 % o f m o to r n a m e p la te c u rre n t. H o w e v e r, th is s e ttin g is a d ju s ta b le w ith in th e V F D a n d ca n be lim ite d to a lo w e r
v a lu e .
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Factors Affecting Motor Temperature and Insulation Life
As indicated earlier, three-phase electric motors are extremely rugged machines. With few moving parts sealed from the local
environmental effects, except temperature, the motor can be expected to provide reliable service for many years. When
discussing this subject with end users, often the comments made are significantly different. Where the motor is considered
relatively unreliable and regularly b u r n i n g o u t, random motor inspections are made on various sites. The majority are found to
work at only 80% of full load. Fig. 8 indicates the life expectancy of Grades A, B, F and H types of insulation.
NOTE:
Most modern motors are manufactured using Class F insulation.
H
F
B
A
Temperature (C°)
F ig . 8 . In s u la t io n life e x p e c t a n c i e s .
NOTE:
The T e n D e g r e e R u l e indicates that a ten degree temperature increase cuts insulation life span approximately in half
and vice-versa.
Clearly, with a life expectancy of in excess of 1,000,000 operational hours (over 100 years) at 284°F, motor burn-out should not
be the problem it is often claimed to be. When investigations are made into a motor b u r n o u t , the failure can often be attributed
to one of the variables indicated in Fig. 9.
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10
F ig . 9 . C a u s e s f o r in s u la t io n f a ilu r e .
T h re e o f th e e le m e n ts in Fig. 9 ca n be o r a re a ffe c te d s u b s ta n tia lly b y a p p lic a tio n o f a V F D . T h e re fo re , th e y h a v e a b e a rin g on
th e te m p e ra tu re o f th e m o to r a n d th u s its life e x p e c ta n c y :
V ib r a t io n
A lth o u g h a m o d e rn V F D p ro d u c e s g o o d q u a lity c u rre n t w a v e fo rm s , th e re is a s m a ll a m o u n t o f a d d itio n a l v ib ra tio n p ro d u c e d a t
th e m o to r. T h u s , th e re is p o te n tia l fo r s m a ll re d u c tio n in th e m o to r life e x p e c ta n c y . H o w e v e r, m o to rs a re o fte n in s ta lle d on
in a d e q u a te fra m e s o r m a c h in e ry th a t h a s a te n d e n c y to v ib ra te . C o n s e q u e n tly , th e life e x p e c ta n c y o f th e w in d in g is a ffe c te d fa r
m o re b y th e in s ta lla tio n th a n th e V F D .
V e n t ila t io n
T h e in s ta lla tio n o f a V F D s h o u ld h a v e no a ffe c t on m o to r v e n tila tio n , a s th is is a p u re ly m e c h a n ic a l fu n c tio n . H o w e v e r, a V F D
te n d s to c a u s e m o to rs to run s lig h tly w a r m e r th a n th e y w o u ld if d riv e n fro m a c o m m e rc ia l p o w e r s u p p ly : ty p ic a lly 5 d e g re e s
F a h re n h e it. N o rm a lly th is is w e ll w ith in th e m o to r d e s ig n lim its a n d th e re a re no a d v e rs e e ffe c ts fro m th e s m a ll in c re a s e in
te m p e ra tu re .
R a t e d T e m p e r a t u r e R is e
T y p ic a lly , m o to rs a re d e s ig n e d to p ro d u c e an 1 7 6 °F te m p e ra tu re ris e a t fu ll lo a d w ith n a m e p la te c o n d itio n s , in an a m b ie n t
te m p e ra tu re o f 1 0 4 °F . A s a d d itio n a l te m p e ra tu re ris e c a u s e d b y V F D a p p lic a tio n is o n ly 5 °F a n d e x p e rie n c e s h o w s th a t m o s t
m o to rs a re s e le c te d w ith 8 0 % o f d e s ig n c a p a c ity a n d ra re ly run w ith an a m b ie n t te m p e r a tu r e o f 1 0 4 °F . A g a in th e re is little
e ffe c t on th e life e x p e c ta n c y o f th e m o to r, w h e n d riv e n b y a V F D .
L o a d ( D u t y C y c le )
G e n e ra lly s m a ll m o to rs fro m fra c tio n a l H p up to m a y b e 5 o r 10 H p ca n w ith s ta n d m a n y s ta rts p e r h o u r, ty p ic a lly 1 0 0 -2 0 0 ,
w ith o u t o v e rh e a tin g . T h e re is an in c re a s e in th e m o to r te m p e ra tu re o v e r th a t re a c h e d in a m o to r le ft ru n n in g c o n s ta n tly . T h is
in c re a s e in o p e ra tin g te m p e r a tu r e is d u e m a in ly to:
—
T h e s u rg e c u rre n t a t s ta rt up, w h ic h m a y be a s h ig h as 5 o r 7 tim e s fu ll lo a d c u rre n t.
—
P e rio d s w h e n th e m o to r is s to p p e d , w h e re th e m o to r c o o lin g fa n is n o t ru n n in g , th e re fo re th e b o d y te m p e ra tu re w ill
in c re a s e to c o m p e n s a te .
A s in c re a s e in te m p e ra tu re re d u c e s m o to r life, fr e q u e n tly s ta rtin g e v e n s m a ll m o to rs c a n s u b s ta n tia lly re d u c e th e life
e x p e c ta n c y . A s th e m o to rs in c re a s e in c a p a c ity a b o v e 10 H p, th e fr e q u e n c y o f s ta rts p e r h o u r s h o u ld be re d u c e d to 3 o r 4
s ta rts p e r h o u r fo r m o to rs o f 1 2 5 H p to 2 5 0 Hp.
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63-7062
D u rin g s ta rtin g , m e c h a n ic a l s tre s s e s in c re a s e ra p id ly w ith in c re a s e s in m o to r s ize s. M e c h a n ic a l fa ilu re th ro u g h m e ta l fa tig u e is
c o m m o n on la rg e m a c h in e s th a t a re s ta rte d to o fre q u e n tly . W h e n a V F D is a p p lie d to a m o to r, s u rg e c u rre n t is a lm o s t
e lim in a te d d e p e n d in g on th e in e rtia o f th e lo a d a n d th e tim e re q u ire d fo r th e m o to r to re a c h fu ll o r re q u ire d s p e e d . T h e re s u lt is
th a t th e re is v ir tu a lly no lim it to n u m b e r o f m o to r s ta rts p e r h o u r, re g a rd le s s o f size .
A m b ie n t T e m p e r a tu r e
A s in d ic a te d e a rlie r, g e n e ra l p u rp o s e m o to rs a re n o rm a lly d e s ig n e d to run a t fu ll lo a d w ith n a m e p la te c o n d itio n s a n d an
a m b ie n t te m p e r a tu r e o f 1 0 4 °F .
M o to rs a re o fte n in s ta lle d in a re a s w h e re th e re is little o r no v e n tila tio n . T h e h e a t lo s s e s fro m th e m o to r m a y c a u s e th e
s u rro u n d in g a ir te m p e r a tu r e to e x c e e d th e o rig in a l d e s ig n c o n d itio n s o f th e m o to r. T h e a p p lic a tio n o f a V F D in th is c a s e h a s
little e ffe c t on life e x p e c ta n c y (s e e a b o v e ).
A ltitu d e
T h e a ltitu d e , h e ig h t a b o v e s e a le ve l, h a s a b e a rin g on m o to r te m p e r a tu r e s in c e th e a ir d e n s ity , a n d th u s its a b ility to a b s o rb
h e a t, re d u c e s w ith a ltitu d e . G e n e ra lly 3 2 8 0 fe e t is th e h e ig h t a t w h ic h d e ra tin g s h o u ld c o m m e n c e .
T e r m in a l V o l t a g e
V F D c o n tro l o f th e m o to r te rm in a l v o lta g e is d ire c t. T h e V F D o v e ra ll c o n tro l s tra te g y in c lu d e s d e s ig n to p ro v id e th is fu n c tio n . A s
m o to r te rm in a l v o lta g e h a s a m a jo r im p a c t on m o to r p e rfo rm a n c e , g re a t e ffo rt h a s b e e n m a d e b y m a n u fa c tu re rs to o p tim iz e
v o lta g e c o n tro l u n d e r all c o n d itio n s .
P o w e r S u p p ly F r e q u e n c y
T h e m o to r d e s ig n fr e q u e n c y is u s u a lly 5 0 o r 6 0 c y c le s . W h e n a V F D d riv e s a m o to r, it is u s u a lly to c o n tro l th e m o to r s p e e d . It
th e re fo re fo llo w s th a t th e V F D h a s d e s ig n fu n c tio n s e m b e d d e d w ith in its c o n tro l s tra te g y to e n s u re th a t th e m o to r is n o t
a d v e rs e ly a ffe c te d b y c h a n g e s in th e fr e q u e n c y s u p p lie d b y th e V F D .
C u rre n t W a v e fo rm
M a n u fa c tu re rs h a v e ta k e n m a jo r s te p s o v e r th e la s t fe w y e a rs to im p ro v e th e q u a lity o f th e m o d e rn V F D c u rre n t w a v e fo rm .
M o to rs a re m a c h in e s d e s ig n e d to o p e ra te w ith a s in e w a v e in p u t. A fe w y e a rs a g o , th e V F D p ro d u c e d c u rre n t w a v e fo rm s th a t
w e re c o a rs e a n d a n y th in g b u t s in u s o id a l. T h e re s u lt o f th is w a s ro u g h ru n n in g a n d on s o m e o c c a s io n s o v e rh e a tin g o f th e
m o to rs . A m o d e rn V F D p ro d u c e s c u rre n t w a v e fo rm s th a t s h o w little d is to rtio n fro m th e s in e w a v e a n d th u s m o to r a n d d riv e
p e rfo rm a n c e h a s d ra m a tic a lly im p ro v e d . M o to r lo s s e s a n d te m p e ra tu re s a re b o th re d u c e d .
Further Protection
U n d e r all o p e ra tin g c o n d itio n s th e V F D m o n ito rs th e s p e e d a n d lo a d im p o s e d on th e m o to r. A m o d e l o f th e s e c o n d itio n s is
c o n tin u o u s ly u p d a te d a n d c h e c k e d a g a in s t s ta n d a rd a c c e p ta b le lim its. In th e e v e n t th a t th e s e lim its a re e x c e e d e d , th e m o to r is
in an o v e rlo a d e d c o n d itio n . U n le s s th is o v e rlo a d is re m o v e d , th e V F D ta k e s th e d e c is io n to trip th e d riv e to s a fe g u a rd th e
m o to r. If th is m o to r m o d e l d o e s n o t to p ro v id e th e d e s ire d p ro te c tio n le ve l, th e V F D ca n ta k e d ire c t m e a s u re m e n t o f m o to r
te m p e ra tu re v ia in te rn a l th e rm is to rs .
Differences Between Star Delta and Delta Star
L o w p o w e r th re e - p h a s e m o to rs , 5 H P a n d b e lo w , a re g e n e ra lly w o u n d so th e m o to r ca n o p e ra te on 2 0 0 o r 4 0 0 V a c . T o o p e ra te
on 2 0 0 V a c , c o n n e c t th e m o to r in d e lta . T o o p e ra te on 4 0 0 V a c , c o n n e c t th e m o to r in sta r. S e le c t th e c o rre c t V F D b a s e d on
p o w e r s u p p ly v o lta g e a n d p h a s e n u m b e r. C o n n e c t th e m o to r to m a tc h th e p o w e r s u p p ly v o lta g e .
M o to rs a b o v e th is c a p a c ity g e n e ra lly a re w o u n d to be o p e ra te d in s ta r m o d e fo r s ta rtin g , to re d u c e s u rg e c u rre n t, a n d d e lta
m o d e w h e n th e lo a d is up to s p e e d , to p ro v id e n a m e p la te p o w e r o u tp u t. M u ltip le c o n ta c to rs a n d v a rio u s o th e r d e v ic e s a re u se d
to p e rfo rm th is c h a n g e o v e r fu n c tio n .
U n de r norm al circu m sta n ce s w hen a V F D is applied to a m otor, c o n n e ct the m o to r in D elta m ode. The V F D w ill start th e m otor
sm o o th ly and w ith o u t cu rre n t surge. No ch a n g e o ve r co n ta cto rs are required.
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Star Delta
S ta r D e lta is a m e th o d to re d u c e a p p lie d v o lta g e to th e s ta to r w in d in g s ta rtin g la rg e m o to rs (s e e Fig. 10). T h is re d u c e s s ta rtin g
c u rre n t. T h is a ls o re d u c e s lo c k e d ro to r to rq u e . B o th p o w e r a n d c u rre n t in S ta r a re less.
F ig . 1 0 . 4 1 5 V p h a s e w i n d i n g s in S t a r .
Delta
F ig . 1 1 . 4 1 5 V p h a s e w i n d i n g s in D e lta .
F u ll n a m e p la te P o w e r a v a ila b le in D e lta (s e e F ig. 11).
Delta Star
A m e th o d to p ro d u c e a D u a l V o lta g e m o to r. S e e F ig. 12.
F ig . 1 2 . 2 4 0 V t h r e e - p h a s e w i n d i n g s in C o n n e c t D e lta .
F u ll n a m e p la te P o w e r a v a ila b le in C o n n e c t D e lta .
F ig . 1 3 . 2 4 0 V t h r e e - p h a s e w i n d i n g s in C o n n e c t S t a r .
F u ll n a m e p la te P o w e r a v a ila b le in C o n n e c t S ta r (s e e F ig. 13).
H o rs e p o w e r is th e s a m e in b o th c a s e s , b u t c u rre n t c h a n g e in a ra tio o f 1 .7 3 2 (s q u a re ro o t o f th re e ). D e lta is 1 .7 3 2 tim e s
g re a te r th a n S ta r.
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Load Types
There are four basic load types:
Constant Torque
Care is required. Examples are: cranes, elevators, agitators, mixers, and conveyors. The torque remains constant throughout
the range of speed of operation. This application is probably the most difficult for a general purpose VFD. It requires the motor
to produce full load torque at zero speed. Rarely can a general purpose VFD provide this capability. Usually a Vector Type VFD
would be applied, with some form of feedback of motor speed, such as an encoder. If accurate speed holding at very low
speeds is not required a general purpose VFD could be used.
Square Law Torque Load
Usually a simple application. Examples are: fluid movers and reciprocating pumps. The torque demand increases with the
speed of operation, but the power increases as the square of the increase in speed: hence the name Square Law. See Fig. 14.
This is not usually a problem for a general purpose v F d . At the most critical speed, zero and just above, torque demand is low.
Beware if the customer wishes to over speed the driven machine as the power requirement increases rapidly as the speed
increases.
% Speed
F ig . 1 4 . S q u a r e la w t o r q u e lo a d t o r q u e /p o w e r c u r v e .
Cube Law Torque Load
General HVAC and usually simple. Examples are: centrifugal fans and pumps. In the HVAC industry, it is generally easy to
identify the type of load being driven by the motor. Most loads are of the fan or centrifugal pump types and therefore can be
described as c u b e l a w l o a d s . The torque and power requirement at zero speed is zero. See Fig. 15. Power requirement
increases with the cube of the increase in speed. Therefore, there is a large increase in power requirement for a small increase
in speed. For example: To double the machine speed would require eight times the power to drive it. Conversely, if the speed
of the machine is halved, the power requirement is reduced to 1/8 of the original power.
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Centrifugal pumps and fans
operate like this.
Power changes sharply
with small speed changes.
% T o rq u e /
Pow er
Torque
Power
% Speed
F ig . 1 5 . C u b e la w t o r q u e lo a d t o r q u e /p o w e r c u r v e .
Constant Power Type Loads
N o t u s u a lly e n c o u n te re d in H V A C in d u s try . E x a m p le s a re m a c h in e to o ls . C a re m u s t be ta k e n a t a ll tim e s . If a m is ta k e is m a d e
in ty p e o f lo a d id e n tific a tio n th e n th e a p p lic a tio n w ill be a t b e s t tr o u b le s o m e a n d a t w o r s t a fa ilu re . S e e F ig. 16.
Torque is higher at
lower speeds, but power
demand holds steady.
150
125
Most often found
in machine tools.
% T o rq u e/
P ow er
100
75
50
Torque
Power
25
“I---------1-------- 1-------- ----------1--------- !"
0
25
50
75
100
125
150
% Speed
F ig . 1 6 . C o n s t a n t P o w e r lo a d t o r q u e /p o w e r c u r v e .
Power Factor
P o w e r fa c to r is a te rm c o m m o n ly e n c o u n te re d w ith in th e V F D o r d riv e s in d u s try . M a n y V F D m a n u fa c tu re rs c la im th a t th e p o w e r
fa c to r o f th e ir p ro d u c ts is a lm o s t U n ity a n d re m a in s so a c ro s s m o s t o f th e lo a d im p o s e d u p o n th e V F D . M o s t g e n e ra l p u rp o s e
V F D m a n u fa c tu re rs u s e th e s a m e ty p e o f in p u t c irc u itry a n d th e re fo re a s e le c tric m a c h in e s , h a v e s im ila r p o w e r fa c to rs . S o m e
fa c to rs a ffe c tin g th e p o w e r fa c to r o f th e V F D :
—
—
—
—
T h e p o w e r s u p p ly c a p a c ity o f th e tra n s fo rm e r.
T h e le n g th o f c a b le fe e d in g th e V F D .
L o a d on th e s u p p ly tra n s fo rm e r.
S iz e o f A C lin e re a c to r a n d /o r D C b u s re a c to r in th e V F D .
T h e p o w e r fa c to r o f th e m o to r d riv e n b y a V F D v a rie s a c c o rd in g to th e lo a d im p o s e d on th e m o to r. T h is ca n ra n g e fro m 0 .3 fo r
a s m a ll m o to r on lig h t lo a d to 0 .9 fo r a la rg e m o to r on h e a v y lo a d . T h e V F D w ill e ffe c tiv e ly is o la te th e m o to r fro m th e p o w e r
s u p p ly a n d a p o w e r fa c to r o f o n e w ill a lw a y s be s e e n b y th e p o w e r s u p p ly . T h e p o w e r fa c to r o f an e le c tric m a c h in e c a n be
c o n s id e re d a s th e r e la tio n s h ip b e tw e e n th e to ta l c u rre n t d ra w n b y th e m o to r a n d th e c u rre n t d ra w n to p ro d u c e u s e fu l p o w e r.
S e e T a b le 2.
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Table 2. Power Factor of Some Devices.
D e v ic e
P o w e r F a c to r
D e v ic e
P o w e r F a c to r
In c a n d e s c e n t L a m p
100%
S in g le P h a s e In d u c tio n M o to r
6 0 % -8 0 %
Neon Lam p
4 0 % -5 0 %
T h re e P h a s e In d u c tio n M o to r
7 0 % -9 0 %
F lu o re s c e n t L a m p (w /S ta b iliz e r)
90%
T a b le T o p F an
6 0 % -8 0 %
R a d io
100%
C e ilin g F an
5 0 % -6 0 %
Useful Formulae
M o to r s p e e d = (S u p p ly fre q u e n c y * 1 2 0 ) / N u m b e r o f m o to r p o le s
T h e a b o v e fo rm u la p ro v id e s th e s y n c h ro n o u s (n o lo a d ) s p e e d o f th e m o to r. H o w e v e r, a s lo a d is a p p lie d to th e m o to r, it b e g in s
to d e v e lo p s lip . T h e a c tu a l s p e e d d e v ia te s fro m s y n c h ro n o u s s p e e d . T h is ca n be p o s itiv e s lip o r n e g a tiv e s lip in o rd e r to
d e v e lo p to rq u e .
S h a ft P o w e r in k W = (N m * R P M ) / 9 5 4 9
T o rq u e in N m = (9 5 4 9 * k W ) / R P M
C O S $ (P F ) ty p ic a lly 0 .8 a n d e ffic ie n c y 0 .8 fo r s m a ll m o to rs
E le c tric a l P o w e r in k W = (V * I * P F * 1 .7 3 ) / 1 0 0 0
VFDFUNDAM ENTALS
Construction
T h e m a jo rity o f g e n e ra l p u rp o s e V F D s p ro d u c e d to d a y h a v e fo u r fu n d a m e n ta l s e c tio n s (s e e F ig. 17). T h e s e are :
1. T h e in p u t re c tifie r o r c o n v e rte r.
2. T h e D C bus.
3. T h e o u tp u t s ta c k o r V F D .
4. T h e c o n tro lle r.
T h e in p u t r e c tifie r o r c o n v e rte r c a n be e ith e r th re e - p h a s e or, in s m a ll m a c h in e s , s in g le p h a s e . T h is in p u t r e c tifie r c o n v e rts th e
V a c in p u t in to V d c a n d c h a rg e s th e c a p a c ito rs in th is p a rt o f th e c irc u it.
T h e D C b u s a c ts a s a s m a ll re s e rv o ir fo r p o w e r on w h ic h th e o u tp u t V F D d ra w s . If a n y re g e n e ra te d e n e rg y fro m th e lo a d
re m a in s , it is s to re d on th e D C b u s in th e c a p a c ito rs .
T h e o u tp u t s ta c k o r V F D d ra w s p o w e r fro m th e D C b u s a n d c re a te s a s y n th e s iz e d V a c p o w e r s u p p ly , th e fr e q u e n c y o f w h ic h
c a n be v a rie d by th e c o n tro lle r. T h e o u tp u t o f th e c o n v e rte r is u s e d to d riv e th e e le c tric m o to r.
S u p e rv is in g th e w h o le m a c h in e is a c o m p u te riz e d c o n tro lle r, w h ic h is c a p a b le o f m a k in g d e c is io n s b a s e d on th e d e m a n d s a n d
on s ta te o f m o to r a n d lo a d . It is d riv in g a n d ta k in g p ro te c tiv e m e a s u re s to e n s u re th a t no d a m a g e o c c u rs to th e m a c h in e ry it is
c o n tro llin g o r th e V F D itse lf.
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F ig . 1 7 . V F D C o n s t r u c t io n .
Braking Resistor
U n d e r n o rm a l c irc u m s ta n c e s , w ith th e m o to r u n d e r lo a d , th e flo w o f e n e rg y th ro u g h th e s y s te m is fro m th e c o m m e rc ia l p o w e r
s u p p ly to th e V F D a n d fro m th e V F D to th e m o to r a n d fin a lly fro m th e m o to r to th e lo a d . T h e re a re o p e ra tin g c o n d itio n s w h e re
th e lo a d trie s to o v e r run th e m o to r. A n e x a m p le o f th is w o u ld be a h ig h in e rtia lo a d . F o r e x a m p le : a la rg e d ia m e te r fa n , ru n n in g
a t h ig h s p e e d , w h e re th e c o n tro l s y s te m c a lls fo r th e fa n to run a t lo w sp e e d .
T h e V F D b e g in s to lo w e r its o u tp u t fr e q u e n c y a n d th e m o to r fo llo w s . H o w e v e r, d u e to th e in e rtia in th e fa n , th e fa n re s is ts th e
c h a n g e in s p e e d , c a u s in g th e m o to r to run a b o v e th e fr e q u e n c y o u tp u t fro m th e V F D . T h is s itu a tio n w ill c a u s e e n e rg y to flo w
fro m th e lo a d b a c k th ro u g h th e m o to r a n d in to th e V F D . A g e n e ra l p u rp o s e V F D d o e s n o t n o rm a lly h a v e th e a b ility to p a s s th is
e n e rg y b a c k in to th e c o m m e rc ia l p o w e r s u p p ly . H o w e v e r, if re q u ire d , a d d itio n a l e q u ip m e n t ca n d o th is .
T h e re s u lt o f th is re g e n e ra tio n is a b u ild up o f e n e rg y in th e D C b u s c a p a c ito rs , w h ic h m a n ife s ts its e lf a s an in c re a s in g v o lta g e .
If th is w e re a llo w e d to o c c u r u n c h e c k e d , d a m a g e w o u ld o c c u r to th e c o m p o n e n ts in th e V F D d u e to e x c e e d in g th e o p e ra tin g
v o lta g e lim its . T o e n s u re p ro b le m s d o n o t o c c u r, th e V F D h a s a b u s v o lta g e m o n ito rin g c irc u it. T h is c irc u it a tte m p ts to re d u c e
re g e n e ra tio n u n til b u s v o lta g e fa lls to an a c c e p ta b le le ve l. If h o w e v e r it is im p o rta n t th a t th e m o to r a n d lo a d fo llo w th e c o n tro l
s ig n a l e x a c tly , it m a y be n e c e s s a ry to a d d a b ra k in g r e s is to r s y s te m to th e V F D . (S e e Fig. 18.)
T h is w o u ld ta k e th e fo rm o f a p o w e r tra n s is to r, a p o w e r r e s is to r a n d a c o n tro l c irc u it, th e la y o u t o f w h ic h is s h o w n a b o v e . In th e
e v e n t th a t th e D C b u s v o lta g e e x c e e d s th e th re s h o ld o f th e c o n tro l c irc u it, th e p o w e r tr a n s is to r s w itc h e s on. It a ls o c o n n e c ts th e
p o s itiv e a n d n e g a tiv e s id e s o f th e D C b u s to g e th e r, v ia a la rg e p o w e r re s is to r. T h is a c tio n d is s ip a te s th e e x c e s s e n e rg y a s h e a t
fro m th e re s is to r. T h is r e s is to r is s u b je c te d to h ig h v o lta g e s a n d c u rre n ts , so it is a h ig h ly s tre s s e d c o m p o n e n t a n d h a s to be
c a re fu lly s e le c te d to e n s u re re lia b ility .
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F ig . 1 8 . V F D b r a k in g r e s is t o r .
Pulse Width Modulation (PWM) Control Principles
V a rio u s m e th o d s p ro d u c e a s y n th e s iz e d th re e - p h a s e p o w e r s u p p ly s u ita b le fo r d riv in g a s ta n d a rd th re e - p h a s e e le c tric m o to r.
H o w e v e r, th e in d u s try h a s s ta n d a rd iz e d on th e P W M m e th o d o f c o n tro l. S e e Fig. 19.
F ig . 1 9 . P u ls e W id t h M o d u la t io n .
VFD Voltage to Frequency Ratio.
W h e n c o n n e c te d to a V F D , m o to r s p e e d is no lo n g e r fix e d b y s u p p ly fre q u e n c y , s in c e th e V F D ca n v a ry its o u tp u t fre q u e n c y .
U n d e r p e rfe c t c o n d itio n s , a t z e ro s p e e d th e te rm in a l v o lta g e w o u ld a ls o be z e ro . O b v io u s ly if th is w a s th e c a s e th e n th e m o to r
w o u ld p ro d u c e z e ro to rq u e a n d in m a n y c a s e s th is w o u ld be u n a c c e p ta b le . A ls o a t v e ry lo w s p e e d s th e m o to r w in d in g a p p e a rs
m o re lik e a re s is tiv e lo a d th a n an in d u c tiv e lo a d . T o o v e rc o m e th is p ro b le m w ith a g e n e ra l p u rp o s e V F D , a d e g re e o f fix e d
v o lta g e b o o s t is a p p lie d a t z e ro s p e e d . A s th e m o to r a c c e le ra te s , a p ro p o rtio n o f fix e d b o o s t is re p la c e d b y n o rm a l V /F ra tio
u n til, a t s o m e s p e e d a b o v e z e ro , g o v e rn e d b y th e a m o u n t o f fix e d b o o s t a p p lie d , a ll b o o s t is re p la c e d b y th e n o rm a l V /F ra tio . If
an e x c e s s iv e a m o u n t o f fix e d b o o s t is a p p lie d , th e m o to r ca n b e c o m e o v e rh e a te d d u e to o v e r flu x in g . (S e e F ig. 2 0 .)
It is a ls o p o s s ib le to p ro g ra m th e d riv e to a d ju s t th e V /F ra tio a u to m a tic a lly a c c o rd in g to th e lo a d a p p lie d to th e m o to r. W ith in
lim its , a s th e c u rr e n t d ra w n in c re a s e s th e d riv e re s p o n d s to th is a s an in c re a s e in lo a d a n d to m a in ta in to rq u e a n d s p e e d , th e
d riv e in c re a s e s th e te rm in a l v o lta g e , w ith in p re d e fin e d lim its.
If th e d riv e h a s b e e n p ro g ra m m e d c o rre c tly , th e m a x im u m te rm in a l v o lta g e w ill be re a c h e d a t m a x im u m s p e e d . H o w e v e r, th e
a p p lic a tio n c a n re q u ire th a t th e m o to r run o v e r s p e e d . N o rm a lly , 2 0 % o v e r s p e e d is a c c e p ta b le , p ro v id in g th e lo a d ca n
w ith s ta n d th e s tre s s e s c a u s e d b y th is a d d itio n a l s p e e d . Fig. 2 0 illu s tra te s th a t a t m a x im u m s p e e d te rm in a l v o lta g e is a t
m a x im u m . T h e re fo re , th e re ca n be no in c re a s e in v o lta g e d u e to s p e e d in c re a s e s . T h is p o in t is c a lle d th e fie ld w e a k e n in g
p o in t, a s a c o n s ta n t m o to r flu x c a n no lo n g e r be m a in ta in e d . A c c o rd in g to th e V /F ra tio , th e re fo re th e m o to r to rq u e c a p a b ility
b e g in s to re d u c e . If th e m o to r c o n tin u e s to in c re a s e in s p e e d th e n it p a s s e s in to an a re a o f o p e ra tio n c a lle d th e c o n s ta n t p o w e r
a re a .
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Voltage
F ig . 2 0 . V F D v o l t a g e t o f r e q u e n c y r a t io .
Charging Resistor
The charging resistor is included in the DC bus to provide current limiting during the initial power up stages of the VFD. When a
fully discharged VFD is switched on to the power supply, the capacitors on the DC bus are seen by the power supply as a very
low impedance load. If the design does not include a charging resistor, the current surge magnitude would be so high that the
input bridge can be damaged, or require up-rating far beyond that required for normal running.
The power supply capacity, cable lengths have a bearing on the magnitude of this surge current. With the charging resistor in
the circuit, the current surge is limited until the Vdc rises above about 380 volts. At this point, a contactor or solid state switch
shorts out the resistor. The charging circuit is short time rated, repeated power down/up cycles will ultimately cause failure.
Typically 10 power up/down cycles per hour are acceptable.
AC Line Choke
The AC Line choke design provides some smoothing on the DC bus and reduces the amount of ripple current that must be
tolerated by the main capacitors. This has an effect of extending the life of these components. The choke provides a limiting
function to the magnitude of the DC bus current during normal operation. This results in an improved overall power factor of the
VFD and reduced harmonic currents flowing in the power distribution network.
NOTES:
—
—
—
—
Be aware that if multiple VFDs or one large VFD installed on a distribution network that supports equipment that
also produces harmonic currents, the effects are cumulative.
If power factor correction equipment is also installed on the network, damage can occur to the correction
equipment capacitors.
Limiting the amount of harmonic current flowing in the network can require additional power line reactors, or
under extreme conditions, a filter network.
Only an Harmonic Survey can guarantee the quality of the power supply.
To illustrate this point, a test was carried out with a standard 4 pole three-phase motor connected first to no load and then full
load, driven by a VFD. Fig. 21 and 22 show the voltage and current waveforms under these two conditions.
19
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7.5
5
A
m
0 p
-2.5 s
2.5
5
7.5
0
2
4
6
8 10 12 14
Time ( ms)
7.5kWMotor NoLoad
16
18
20
F ig . 2 1 . M o t o r v o l t a g e a n d c u r r e n t u n d e r n o lo a d c o n d it io n s .
Note the shape of the current waveform in Fig. 21 with respect to the voltage. It is the non sinusoidal format of the current
waveform that causes the harmonics on the network and can ultimately cause voltage waveform distortion.
7.5 M o to r F u ll Load
F ig . 2 2 . M o t o r v o l t a g e a n d c u r r e n t u n d e r f u ll lo a d c o n d it io n s .
Note that the peak current in Fig. 22 has risen from 7.5 amps to almost 40 amps. Motor full load current is 13.5 amps.
Motor and VFD Tests
In the past, the VFD was seriously criticized for causing motor problems- in particular causing the motor to overheat to the
extent where motor winding insulation was damaged, resulting in m o t o r b u r n o u t. As a result, it was often recommended by
suppliers/manufacturers/consultants to down rate a VFD driven motors by 10 percent.
Many advancements have been made in the control design of the modern VFD to an extent now that for centrifugal fan and
pump type loads, no derating is required. The following sections contain results of a sequence of tests indicating the heating
effects of running a standard 7.5 kW squirrel cage motor connected to the commercial power supply and to a general purpose
VFD. The loading was provided by a calibrated dynamometer. Thermal imaging equipment was used to identify the h o t s p o t s
on the outside of the motor body. Type K thermocouples were secured to the cleaned surface of the motor body at these points
using epoxy resin and then thermal insulation applied.
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Test 1. Motor Run with Varying Loads on Commercial Power Supply
Fig. 23 indicates performance of a standard squirrel cage motor driven directly by a commercial power supply at 100%, 6 6 %
and 33% load. On 100% load, motor body temperature rises quickly to 64°C, while ambient temperature remains stable around
20°C. When the load is reduced to 6 6 %, body temperature quickly falls to 45°C, as expected under reduced load conditions.
Reducing load to 33% produced a further reduction in motor body temperature, to 40°C. However, the test was curtailed due to
lack of time. It is evident from the slope of the graph at this load, that the motor temperature would have continued to fall
several degrees before stabilizing at an estimated 38°C.
7.5 kw 4 pole m o tor
F ig . 2 3 . M o t o r r u n a t 1 0 0 % , 6 6 % , a n d 3 3 % lo a d o n c o m m e r c ia l p o w e r s u p p ly .
Test 2. Motor Driven by VFD with Varying Loads.
Fig. 24 indicates the results of a similar test to test 1. However, the motor was driven by a VFD running at 50Hz, rather than a
commercial power supply. Early in the test, the VFD tripped. The diagnosed cause was VFD thermal overload. Caused by
setting the VFD to a level too low for the load. This overload setting was adjusted and the test restarted. At 100% load, the
motor body temperature reached 65°C when the load was reduced (see Fig. 24). However, had time allowed, the temperature
would have probably reached 70°C. As the load was reduced to 6 6 percent, the motor body temperature fell to 46°C. Reducing
the load to 33 percent caused the motor body temperature to fall to 38°C. During the test, ambient temperature was recorded
and was similar to that of test 1 .
A comparison between the two tests indicates that when the motor under test was run on a VFD, under maximum load
conditions, at rated speed i.e. 50 Hz, the motor body temperature was increased by 5°C over that reached when run on the
commercial power supply. This small increase in temperature is still within the operating range of the machine and clearly
indicates that there is no need to derate a motor driven by a VFD, when the motor is run at full rated speed.
7 .5 k w 4 p o le
F ig . 2 4 . M o t o r r u n a t 1 0 0 % , 6 6 % , a n d 3 3 % lo a d o n V F D .
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Test 3. Motor Run on VFD with Fan Load Simulation.
The purpose of this test is to prove that the motor will not suffer harmful effects of overheating, when driving a fan or pump type
centrifugal load at varying speeds. The dynamometer was configured to produce similar characteristics to those of a centrifugal
fan. The motor was then run up to 50 Hz an the load adjusted to 1 0 0 %. After a short time, it can be seen from Fig. 25, that the
motor body temperature rose to a similar level as that reached in test 1 and 2 in the same time frame. The speed of the motor
was then reduced in steps from 50 Hz to 40, 30 and 20 Hz. In each case the temperature of the motor body fell as the speed of
the motor reduced. No auxiliary cooling was provided for the motor i.e. the only cooling air flow was provided by the shaftmounted fan on the motor. Clearly, there are no harmful heating effects caused by driving a motor with a VFD, on centrifugal
type loads and there is no need to derate the motor.
7 .5 k w 4 p o le
F ig . 2 5 . F a n lo a d w it h m o t o r r u n n in g a t v a r i o u s f r e q u e n c i e s .
Test 4 Motor Run on VFD with Constant Torque Load.
This test was by far the most arduous for the motor and VFD since the dynamometer was configured to hold the torque stable
across a speed range from 50 Hz down to 15 Hz. This was not a test to prove the capabilities of the VFD to produce full load
torque at reduced speeds, rather it was to show the heating effects under these operating conditions. Most modern VFDs can
cause the motor to produce full load torque down to 1.5 Hz or less.
Time constraints limited the amount of test time at the higher speeds, thus not allowing the motor body temperature to stabilize
at each step in the speed range, however a full load 50 Hz test had already been undertaken in test 2. It can be seen from Fig.
26 that under constant torque conditions, the motor body temperature will rise with each step reduction in speed.
30 Hz is probably the minimum speed at which a standard motor can be run continuously under full load torque conditions,
however for short periods of time, much lower speeds at full load torque can be tolerated without adverse effects. If regular
operation at speeds lower than 30 Hz and at full load torque are anticipated, then an auxiliary fan should be installed, in order
to produce the desired cooling for the motor. Brooke Hansen and other motor manufacturers can provide standard motors with
auxiliary cooling fan fitted as standard.
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22
7 .5 k w 4 p o le
Test 5 Motor Run on Old and Modern-Design VFDs at 50 Hz on Full Load torque.
As a final test, Fig. 27 shows a comparison in the heating effects of running a motor first on a 10 year old design VFD and a
modern VFD. The graph shows that under similar load and ambient temperatures, the old design VFD caused the motor
temperature to rise to 75°C whereas on the modern design, the motor temperature reached 6 8 °C. The reason for this reduction
in temperature is the improvement in the synthesis of the output waveforms, to the motor.
7 .5 k w 4 p o le m o t o r
Early Generation
80 70 °C
60 50 40 30 20 10 F ig . 2 7 . C o m p a r i s o n o f e a r l y g e n e r a t io n a n d m o d e r n V F D s .
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IN S T A L L A T IO N G U ID E L IN E S
General Guidelines
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Do not install a VFD in a hazardous or flameproof area.
Do not install a VFD to drive motors installed in hazardous areas. This requires special testing and approvals.
Every Honeywell VFD has a three-phase output.
Do not assume that all three-phase AC motors can be driven by a general purpose VFD. Some types require a special
VFD.
Do get full motor name plate details - do not assume anything.
Do not select VFD on Hp alone - some motors have high current to Hp ratio and the VFD could be too small if selected on
Hp alone.
On lightly loaded motors, do not select a VFD with capacity more than one frame size less than motor. If you do, expect
problems.
On very lightly loaded motors, do not select VFD on motor current drawn alone, i.e. 60 Hp VFD drawing 40 amps at 60 Hz
may draw 60 amps at 10 Hz. If this situation arises, an output line reactor will be needed. Make allowance in proposal.
Do not install a VFD in a hot, dirty or humid environment without special precautions.
Do use shielded cable between the VFD and motor.
Do not install control cables in same trunking, conduit as VFD power cables.
Do include RFI filters on input to VFD.
Do use shielded control cables as far as possible - and segregate from power cables at every opportunity.
Do add additional input reactors, if VFD is installed within 92 feet of a power supply of 1 MVA or more.
Do allow for output reactors if motor cable length is more than 320 feet or always if mineral insulated type cable is used.
Do allow for correct sized earth cable as described later.
Do allow for individual motor protection, if the VFD is driving more than 1 motor.
Do interface any switches, isolators, contactors on output of the VFD, with the VFD control inputs.
Do not feed a Star Delta starter with a VFD unless the VFD is held off until Delta sequence is complete and use la te
m a k e / e a r l y b r e a k contacts.
Do ensure when using Excel Classic type controllers, that transformers are connected correctly - otherwise 24 Vac can be
output from DC analogue control terminals.
On first time power up always disconnect VFD output cables. Transposed input and output cables can damage VFD.
Beware: Installing a VFD on a fan supplying air to direct expansion refrigeration coils. Reduction in air flow could cause coil
icing or liquid floodback to the compressor, which can cause mechanical failure.
Do not mount sensors close to an electric motor driven by a VFD. The motor emits higher than normal electrical noise
which may interfere with the accuracy of sensors.
Surveying
As with any system, it is crucial that the system is surveyed thoroughly to ensure that the application of the VFD is to be
successful and provide trouble free operation. The survey should encompass six areas:
1. Location for the VFD.
2. The motor, control gear and power supply.
3. Mode of control.
4. Type of load.
5. The effect of varying the motor speed on the performance of the machine or service provided by the machine to be speed
controlled.
6 . Discussion with user.
VFD Location: Enclosures and Ventilation
The VFD is, by comparison with the motor, a relatively expensive machine. It should therefore be installed in a suitable
environment. The VFD although very efficient still has some losses which manifest themselves as heat. The VFD should be
installed in a well ventilated area, and obviously if installed within a cubicle, the cubicle must have sufficient through flow of air
to cool the VFD or its surface area must be sufficiently large to provide natural cooling.
The ambient temperature (surrounding the VFD) must be below 122°F in order to provide correct reliable operation of the VFD.
If the ambient temperature is liable to rise above 122°F then either derating of the VFD capacity will be required or additional
cooling fans or possibly even a refrigeration unit will be required.
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24
Where the environment is dusty and in particular if the dust is of a conductive nature, then the cooling air should be filtered
prior to injection into the VFD enclosure. Again, if the environment is particularly dirty, the enclosure may be sized to provide
sufficient natural cooling or a refrigeration unit may be installed. Refer to the technical manual for the individual machine for
specific details on mounting and spacing between machines.
Under no circumstances must the VFD be installed within a hazardous area, an area where there is potential for presence of
an explosive mixture surrounding the VFD. It may be that the VFD can be installed outside the hazardous area driving a motor
within the hazardous area. In this particular circumstance, approvals and special testing are required.
Motor, Control Gear and Power Supply
Take the motor full name plate details, do not assume anything. If the name plate is missing then take full load current readings
and measure the supply voltage.
Where there is a local isolator or contactors on the output of the VFD, try to ensure that these devices are interfaced with the
VFD via l a te m a k e / e a r l y b r e a k auxiliary contacts. This ensures that the VFD is aware that the switch is about to be opened
before the main contacts open. If programmed correctly, it ensures trouble-free operation. During the opening of a switch
controlling an inductive load, such as a motor, very high voltages can be created as the magnetic flux decays. Values as high
as 4000 volts have been measured on a standard 400 volt circuit. If these high voltages are allowed to reach the VFD,
unpredictable results may occur at the very least or possibly at worst, damage may occur to the power devices within the VFD.
What type of load is the motor driving - if its a fan or a centrifugal pump, then normally there will be no problems, however with
process machines, the type of load characteristics can be very important e.g. grinding and mixing machines have very high
starting torque requirements (and therefore current demand) whilst once up to speed, the current demand may only be 50% of
motor nameplate current.
Beware of old motors. Many motors still in operation have not been rewound in 30 or 40 years of operation. The quality of the
insulation materials used in old motors is not often able to cope with the stresses imposed by a VFD. If an old motor is being
considered for speed control it would be wise to check if the machine has been rewound in that last 10-15 years. If it has not,
then there is potential for a motor failure within a very short period after the VFD is installed.
Effects of Speed Control
Be aware that the system being served by the motor, which is being considered for speed control, may not be able to accept a
varying volumes or special consideration may be required. Typical examples would be chilled water pumps feeding chillers and
heating water pumps feeding boilers. Centrifugal or screw-type chillers usually have temperature control on the leaving water
side of the machine. This will provide the feedback to the chiller so that it can reduce capacity without adverse effects. There is
however a minimum flow rate that the machine will accept before problems occur. Laminar flow through the tube bundle is a
typical example. Refer to manufacturers details carefully before considering this type of application - but it can be done
successfully.
On chillers with reciprocating compressors, the temperature controls often placed in the flow on to the machine. In this case the
temperature control system would not see the reduced flow and a trip on low leaving water temperature will occur. Special
consideration is required.
Boilers are another example where if the flow rate through the machine is reduced too much, problems can occur. The boiler
requires a minimum flow rate to produce turbulence within the heat exchanger in order to control h o t s p o t s . If the flow rate is
reduced below this value then there is a possibility of h o t s p o t s occurring resulting in damage to the heat exchanger
IMPORTANT
E n s u r e t h a t t h e u s e r is fu lly a w a r e o f a n d a g r e e s w ith t h e i m p l ic a t io n s o f a p p l y i n g s p e e d c o n tr o l b e f o r e p r o c e e d i n g .
EMC Wiring and RFI Filters
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Mount the RFI Filter as close as possible to the VFD input terminals, with cables as short as possible. Install at least one
ferrite ring on the VFD output. Pass all conductors through the same ring. If possible, put one full turn of each conductor
around the ring. If this is not possible and there are problems, add additional ferrite rings.
Connect the filter to ground via its own earth strap, at least 1/2 in. (10 mm) wide and as short as possible.
Use armored or shielded cable from the VFD to the motor. Connect the armor gland to the motor frame and the VFD main
earth terminal with a heavy cable at least as large as the power conductors and as short as possible.
Do not connect any signal 0V common to ground at the VFD. Ground at the controller end.
25
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Line Reactors
Line reactors can be used on both VFD inputs and outputs.
Input Line Reactors
The installation of a large number of VFDs or other similar equipment can distort the current and voltage waveforms of the
power supply, to the point which exceeds the power supply company guidelines or to the a level that other equipment served by
this power supply is prevented from operating correctly. In such a case, including input line reactors provides additional
smoothing to that provided by the DC choke. This reduces the overall VFD disturbance. Input line reactors can reduce the
amount of radio interference produced by the VFD on the power line back into the distribution system. Tests have shown that
typically 10 dB reductions in noise levels can be achieved, however the optimum situation is to install both input line reactors
and RF filters.
Output Line Reactors
Reactors may need to be fitted on the VFD output for several reasons:
— If the VFD to motor cables are long, 320 feet or more, then capacitive leakage from the cables to earth can be so high that
the VFD sees this as a fault and can trip on earth fault or over-current. It is difficult to be specific on this point as the
capacitance varies according to: type of cable, proximity of individual conductors relative to each other, and the conductor
shielding.
— With one large VFD driving multiple small motors, it is often specified that individual motors can be switched on and off the
VFD without stopping the whole system. When the motor is disconnected from the VFD, a line reactor in the supply to
each motor: acts as a current limiter, and helps absorb voltage spikes.
— Lightly loaded motors, when driven from a commercial power supply, can often mislead the surveying engineer into
assuming that a VFD several frame sizes smaller than the motor can be selected for the application. Problems can occur
when the VFD starts the motor. Even though the load is small, the motor current draw at low speed can be much higher
than at full speed. This is due to the fact that the motor winding at low speed is a more resistive than inductive load.
Typically, the VFD is unable to accelerate the motor due to over-current and it finally trips. Adding a line reactor to the VFD
output reduces current magnitude and can r e s c u e the situation.
— When sudden shock loads, contactors, or switches on the VFD output are encountered, an output line reactor installed can
reduce the impact of these occurrences that cause trips or damage to the VFD.
— Mineral insulated cables from the VFD to motor have often been a problem. The conductors in these cables are so close to
each other that higher than normal leakage losses occur. Also, there have been occurrences of gland failure at cable ends
for no apparent reason. On installations where output line reactors have been installed together with mineral insulated
cables, no problems have been reported. This may be a coincidence, as there is insufficient data to form a positive
conclusion. It appears that the s o f t e n i n g of the output pulses by the line reactor has a beneficial effect where mineral
insulated cables are used.
RFI
Sources Of Emissions
VFD emissions fall into 2 categories: conducted and radiated (see Fig. 28). It is important to recognize that the VFD itself does
not radiate much RF energy. Within 8 inches of the VFD, the field strength can be high and can interfere with the correct
operation of sensitive equipment. Beyond the region shown in Fig. 28, the field strength quickly diminishes by the inverse cube
law. Beyond 300 mm the field strength is generally insignificant.
C o n d u c te d
R a d ia t e d
\ i s
m
m
o
m
/
m
\ \
E m is s io n
F
150 kHz
W4 500m
30MHz
1GHz
2.5m
75cm
F ig . 2 8 . C o n d u c t e d a n d r a d ia t e d e m is s io n s .
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26
Routes for Emissions
The main route for the RF energy is out through the VFD output terminals. See Fig. 29.
Su p p ly
Radiation
M oto r
Winding
Capacitance
Drive
Machinery and Building
C able
C a p a cita n c e
F ig . 2 9 . E m is s io n r o u t e s .
Without Input Filter
The VFD can be considered as a source of radio frequency (RF) current which leaves the output terminals. The capacitance of
the output cable (motor cables) and the motor windings represents a relatively low impedance path to this n o i s e current which
flows back to the input terminals of the VFD via the main safety earth and then the input conductors. Any equipment coupled to
the distribution network will effectively s e e this RF at its power supply input. If this path is not well defined and low impedance
then the route back to the VFD input may be via unexpected path which may disturb nearby sensitive equipment.
To ensure the minimum possibility of disturbance to sensitive equipment, ensure that a low impedance path is provided for this
current via a conductor at least the same cross section as the power conductors.
With Input Filter
When a correctly designed input filter is used with a VFD, then the path back to the VFD, for the RF current in the safety earth,
is greatly reduced. The capacitors in the filter effectively provide a short circuit for the high frequency current.
D rive
Winding
Supply
Filter
Capacitance Motor
JLflJL,
v
w
r Q00
JULR
RF Current
ble
Capacitance
F ig . 3 0 . E f f e c t o f a n in p u t f ilt e r .
Shielding Motor Cables
Where further reductions in RF emissions are required, shielded motor cables can be employed. Standard armored cable has
proven to be adequate for this purpose, resulting in 1/30 of the original emissions. The motor chassis should be connected to
the safety earth, in the normal manner, with the both ends of the armored cable also connected to the safety earth. The safety
earth cable should then be taken directly to the VFD earth terminal, before progressing to the main distribution earth. See Fig.
31.
H
H2
N o is e C u r r e n t
*1
N o is e C u r r e n t
F ig . 3 1 . S h i e l d i n g m o t o r c a b le .
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Equipment Categories
Non Sensitive
Equipment falling into this category are simple electromechanical device:
— Relays.
— Contactors.
— Electric motors.
— Solenoid devices.
Care Required
Many electronic devices are insensitive to VFD emissions due to the spectrum at which this interference is generated and the
ability for the target equipment to locally filter out the noise. Computers, and digital equipment generally working above 1 Vdc
should not normally suffer from these emissions, unless they have circuits within them or externally connected, which fall into
the following category. Many control systems have this combined categorization.
Sensitive
Any analog measuring circuit using low level signals:
— Thermocouples.
— Resistive temperature sensor.
— Humidity sensors.
— Strain gauges.
— pH sensors.
— Audio circuits.
— Proximity sensors.
Computer digital circuits, Ethernet and RS 232 have good immunity to VFD RFI, provided the cabling is correctly installed with
high quality screening.
Very Sensitive
Systems that are specifically designed to be sensitive to electromagnetic radiation the band 100 kHz to 5 MHz:
—
Radios designed to work in the long wave and medium wave bands.
—
Inductive-loop pagers and communication systems.
—
Power line carrier systems.
Radio receivers with input filters and shielded cables have little chance of problems outside a 3 to 9 foot radius around the
system. Televisions, v H f radios, and mobile telephones using high frequencies are generally unaffected by VFD RFI.
General Wiring Standards
Sensitive equipment should not be installed within 1 foot of the VFD and its associated input and output cables. Parallel runs of
any control signal cables and the input and output cables should be avoided. Where this is not possible then the control cables
should be correctly shielded but not installed with 1 foot of the input and output cables of the v F d . See Fig. 32.
In p u t
F ilte r
F ig . 3 2 . D i s t a n c e s t o k e e p a s s h o r t a s p o s s ib le .
63-7062
28
Where control signal cables need to cross VFD input and output cables this should be done at right angles with no parallel
runs. See Fig. 33.
Incorrect
Correct
Inverter
Inverter J
Inverter
Inverter
Filter
1
F ig . 3 3 . L o c a t in g f i lt e r a n d V F D c o n t r o l s ig n a l c a b le s .
Filter Location
Fig. 33 adjacent shows various VFD and filter installations. Where possible the filter should be installed as close as possible to
the VFD and most importantly, the earth conductor from the filter to ground, should be at least the same cross section as the
power conductors. The optimum configuration is 1 filter per VFD. VFD output cables should not be run close to the VFD input
power cables as there will be some pickup from the output cable, even though it may armored or screened in some other way.
VFDs are a major source of interference. However, with good engineering practices, large numbers can be installed in
proximity with sensitive equipment, with no adverse effect. As design technologies advance, more sophisticated control of the
VFD output devices will make significant reductions in the area of RFI emissions. VFDs are here to stay for the foreseeable
future, reducing plant running costs and enhancing our environment.
A P P L IC A T IO N S
Fig. 34 indicates the typical duty expected from a fan or a pump used in Heating, Ventilating and Air Conditioning (HVAC)
applications (data supplied by the DTI) It can be seen from the graph that for the majority of the running life of this fan or pump,
it has o v e r c a p a c i t y . The reason for this excess capacity is to accommodate fluctuations in loads due to weather conditions,
changes in occupancy. The excess in flow is usually controlled either by throttling, using dampers or valves, or redirecting the
excess capacity via by-pass ductwork or pipelines. The result of this is type of control is increased running costs and in some
cases increased noise and vibration from the plant. This situation also often occurs in industrial applications, as the through-put
of the process is inevitably variable. In many cases a VFD can be installed to control the speed of a motor and thus the
capacity of the plant, providing reduced running costs and enhanced environmental conditions, as a result of reduced noise
and drafts.
T ypical System Load Requirements
% Flow o r Volum e
F ig . 3 4 . T y p ic a l s y s t e m lo a d r e q u ir e m e n t s .
29
63-7062
EXCEL VRL
APPLICATION GUIDE
A P P L IC A T IO N S
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Fans.
Pumps.
Mixers.
Boilers.
Compressors.
Conveyors.
Elevators.
Machine Tools.
Textile machinery.
Paper and film manufacturing.
V F D O V E R V IE W
Honeywell has long been aware of the full benefits of VFDs, and has installed them in a wide range of applications from fans,
pumps, and compressors to machine tools, elevators and textile machines. The following applications are a sample of some of
the installations and are intended to give an understanding of how a VFD can be utilized in the heating, ventilating, air
conditioning (HVAC) and other industries. See Fig. 35.
Application Description
A Constant Air Volume (CAV) System controls space temperature by altering the supply air temperature while maintaining
constant airflow. The system design provides sufficient capacity to delivery supply air to the space for design load conditions.
Only at maximum load conditions - full heating or full cooling - is maximum air flow required, at other periods it is perfectly
feasible to vary the airflow while maintaining design conditions. Installing a VFD and linking into the existing temperature
controls, allows the air flows to be varied relative to load conditions (see Fig. 36). The motor will only run at full speed when full
heating or full cooling is called for.
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Benefits
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Reduced running costs - typical savings 40% and 14 months payback, dependent on power consumption.
Maintenance savings arise from running the motor at reduced speed which lowers the load torque; lessens mechanical
stress on belts, bearings.
Environmental comfort increased as a result of reduced air velocity - less draughts - and lower background noise levels.
VFD Features Used
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Soft Start - Reduced starting current and avoids pressure surges.
Motor Speed Low Limit - Minimum air movement levels programmable.
Automatic Continuous Operation - VFD will still run if control/speed reference signal is lost.
IMPORTANT
1.
2.
E n s u r e m i n i m u m a ir flo w is m a i n t a in e d .
B e w a r e o f l o w a ir q u a lity : S e e C A V + A i r Q u a lity
Operation (See Fig. 36)
C o n tro l
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Constant Air Volume with Air Quality Compensation (See Fig. 37)
Application Description
The application is as the previous standard CAV application, but with the addition of an air quality (AQ) or CO 2 detector fitted in
either the extract duct or within the space. The air quality detector will control the dampers and the VFD in sequence.
As an example the temperature control could be satisfied with minimum heat output and supply/extract fan running at slow
speed. With a large density of people in the space the air quality could be low (cigarette smoke, stale air and CO2 ), while
maintaining temperature control conditions.
The AQ detector could compensate for this by opening the fresh air dampers and increasing the fan speed in sequence. The
control circuit will be modified to include two highest of two signal selector relays on the inputs to the VFD and the fresh air
dampers.
Possible Usage
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Airport arrival and departure lounges.
Theaters.
Cinema.
Lecture halls.
Exhibition halls.
Benefits
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63-7062
Reduced running costs.
Maintenance savings due to reduced mechanical stress.
Improved indoor air quality.
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VFD Features Used
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Power failure recover.
Speed search.
Possible DC injection on start.
Overspeed capability for increased performance.
IMPORTANT
E n s u r e m i n i m u m n u m b e r o f a ir c h a r g e s p e r h o u r is s till m e t , b y u s i n g V F D l o w lim it.
Operation
C o n tro l
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Variable Air Volume (VAV) Primary Plant Control (See Fig. 40)
Application Description
A Variable Air Volume (VAV) system controls the space temperature by varying the volume of supply air, rather than the supply
air temperature. The interior zones of most large buildings normally require cooling only, because of occupancy and lighting
loads. Air terminal units serve these zones and operate under thermostatic control to vary the airflow into the space to maintain
the required temperature. The perimeter zones often have a variable load, dependent on the season and the losses through
the building fabric. Heating in these areas may be supplied via reheat coils while the air terminal units maintain minimum
airflow.
Airflow in the supply duct varies as the sum of the airflow through each unit varies. In light load conditions the air terminal units
reduce airflow; as more cooling is required, the units increase airflow. When the VAV terminal unit dampers open, the static
pressure drops in the supply duct; the sensor detects the pressure drop and the controller increases the speed of the supply
fan. The opposite occurs when the VAV terminal unit dampers close.
A feature of VAV systems is that the minimum outside air delivered by the system is determined by the difference in air flow
between the supply and return fans (and not the position of the outdoor air damper). To increase the volume of outside air, the
return air damper is regulated. This airflow control provides a slightly positive building static pressure with respect to outdoor
air, in a properly designed system. As supply air volume is reduced, so is return air volume. The return air fan is normally sized
smaller than the supply air fan. Air velocity sensors are located in both the supply and return ducts, so the control of the return
air fan mimics the supply air fan; the airflow in both ducts are controlled with a constant differential.
Benefits
-
Reduced electrical running costs; especially when compared with the traditional guide vane control—with no energy
savings—where the motor is running at full speed all the time.
Maintenance savings arise from running the motor at reduced speed which lowers the load torque; lessens mechanical
stress on belts, bearings.
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34
VFD Features
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Soft Start: Reduced starting current and avoids pressure surges.
Motor Speed Low Limit: Minimum air movements levels programmable.
Cooling Tower Fans (See Fig. 41)
F ig . 4 1 . A p p l i c a t i o n d ia g r a m .
Application Description
A chilled water system is made of three main elements: refrigeration unit (water chiller), chilled water distribution network, and a
means of dissipating heat collected by the system. The towers dissipate this collected heat by cooling the condenser water.
The cooling effect in evaporative type cooling towers is dependent on the ratio of water to air contact and wet bulb temperature.
Traditionally when the condenser water is flowing fully over the tower the fan or fans are cycled on and off to control the water
temperature. Partial load operating characteristics have a strong influence on operating costs. As most operation is at less than
design load, fitting a VFD and varying the speed of the fan is a far more efficient means of controlling the condenser water
temperature. For maximum chiller efficiency the condenser water temperature should be as low as can be used by the
refrigeration system dependent on outdoor air conditions (i.e. wet bulb temperature).
When outside air wet bulb temperature is lower than design, the tower can cool water to a lower temperature, (but can never
reach the actual wet bulb temperature). Therefore, the controller setpoint should be at the lowest setpoint attainable (by the
tower) to save chiller energy and not waste fan energy trying to reach an unobtainable value.
Benefits
Reduced wear and tear.
Improved control (straight line)— more stable chiller operation.
Fewer start/stops.
Energy saving.
35
63-7062
VFD Features Used
—
—
—
—
Power failure recovery.
Speed search.
Possible DC injection on start.
Overspeed capability for increased performance.
IMPORTANT
In m u lti- fa n a p p l i c a t i o n s u n d e r V F D s p e e d c o n tr o l, a ll f a n s s h o u l d o p e r a t e in u n i s o n .
Operation (See Fig. 42)
Full
C o n d e n s e r T e m p e r a tu r e R e s e t b y W e t-B u lb
Primary Chilled Water Option 1 (See Fig. 43 and 44)
E vap o rato r 1
E vap o rato r 2
F ig . 4 3 . A p p l i c a t i o n w i t h o u t V F D .
63-7062
36
Inverter
E vap o rato r 1
CC1
V1
4
V2
E vap o rato r 2
F ig . 4 4 A p p l i c a t i o n w it h V F D .
Application Description
Chillers with evaporators piped in parallel (see Fig. 43) and controlling on either flow or return from the machine. Often, under
light load conditions, chiller two will close down (to prevent short-cycling). This results in a rise in CHW supply temperature to
the building, as 50% of water circulates through the o f f chiller (supply temperature is average of flow from the two machines).
A more energy efficient means of controlling the plant would be to install valves V1 and V2 and a VFD on the circulation pump
(as in Fig. 44). Under light load conditions, one machine running and the other isolated, by closing valve V2 the VFD speed can
be reduced to give the design flow rate through the “on” machine. As the demand increases the VFD speed can be increased
to give full design flow for the two machines, the valve for number two chiller opened and the second machine started.
Benefits
—
—
—
—
Energy savings: Chilled water pump and second chiller off for longer periods.
Reduced maintenance: Fewer starts and stops and machine not having to run under light loads.
Supply temperature to the building will be more stable.
Reduced noise.
VFD Features Used
—
—
Digital speed control.
Auto-restart.
IMPORTANT
T h e V F D s p e e d m u s t b e i n c r e a s e d a n d fu ll f lo w a c h i e v e d , b e f o r e t h e s e c o n d v a l v e is o p e n e d . O t h e r w i s e a f lo w fa ilu r e
trip m a y o c c u r o n t h e n u m b e r 1 m a c h i n e . It is a l s o a d v i s a b l e to p u l s e th is v a l v e o p e n a s s l o w l y a s p o s s i b l e .
Primary Chilled Water Pump with DDC Control of Plant Option 2 (See Fig. 45 and 46)
E vap o rato r 1
E vap o rato r 2
F ig . 4 5 . A p p l i c a t i o n w i t h o u t V F D .
37
63-7062
In v e rte r
CC1
CC1
E
o rr a
a tt o
o rr 1
1
Ev
va
ap
po
C o n t r o lle r
M
V
1
4
V
2
F ig . 4 6 A p p l i c a t i o n w i t h V F D a n d E x c e l P lu s
Application Description
If the chillers in the previous example (option 1) are controlled on the flow from the machine and also have Direct Digital
Control or a building management system installed on the air handling and chiller plants, an even more efficient and effective
means of control is available.
The
—
—
—
control software is made up of three elements :
Safety factors, ensuring the minimum design flow rates for each chiller.
Flow control for supplying the volume of water to suit the needs of the AHU. with the highest demand.
Load control for sequencing the two machines.
The first element is the low limit control to set the minimum speed of the VFD (minimum design flow) while one machine runs
and then increase the pump speed when both machines run to satisfy the minimum design flow rate (plus a 15% safety factor).
This would act as a 2 position low limit.
The flow rates (pump speed) can then be varied to suit demand. This is achieved by inputting the feedback position of each
AHU. bypass valve into a control loop. Using a predetermined setpoint, e.g. 85% open and P+I control, the output from each
loop is fed into a load selector relay (highest signal). The output from the load selector is the control signal to the VFD. If this
signal is higher than the low limit, the pump motor speed is increased and hence increase the flow rate. Conversely, as the
AHU. with the highest demand begins to close its valve the flow rate would be reduced accordingly.
The third element in the control function is the load control or sequencing of the two machines. With one machine running its
total load can be measured, as the chiller approaches full load e.g. 95% it can signal for the second machine to start. The total
load of the machine can be measured by a current transducer measuring the current drawn by the chiller motor. Another
possibility is by using pressure transducers to measure the differential pressure and by sensing the differential temperature
across the evaporator and then calculating the instantaneous load. This later method is probably the safer as the differential
pressure signal can be used as an input into the low limit flow control. The former is more accurate for starting the second
machine at set load conditions. A variation of this system could be to have more than one chilled water pump and to sequence
them on and off according to load.
Benefits
—
—
—
—
—
Energy savings: Lag chiller will be off for longer periods.
Reduced maintenance: Few starts and it is unwise to cycle chillers on and off under light load.
Supply temperature to the building will be more stable.
Reduced maximum demand: Fewer starts especially during winter.
Reduced noise.
VFD Features
—
—
—
—
—
—
Auto restart.
Speed search.
Overspeed.
Overtorque protection.
Stall prevention.
Analog or digital control.
63-7062
38
IMPORTANT
V e r y c a r e f u l c a lib r a tio n o f c o n tr o lle r s is r e q u i r e d to e n s u r e t h a t t h e c h ille r s a r e s a f e g u a r d e d u n d e r a ll c o n d it i o n s . T h e
c h ille r s b u ilt-in c o n tr o ll e r s w ill s till c o n tr o l a t t h e i r o r ig in a l v a lu e . T h e s u p p l y t e m p e r a t u r e to b u ild in g w ill n o t c h a n g e .
W h e n s t a r t i n g t h e s e c o n d c h iller, t h e is o l a ti n g v a l v e m u s t b e o p e n e d a s s l o w l y a s p o s s i b l e , to p r e v e n t t h e o n m a c h i n e
tr ip p in g u n d e r l o w f lo w c o n d itio n .
Secondary Chilled Water Pump Option 1 (See Fig. 47)
i
Secondary Chilled Water
T
2
M
V1
EMC
In v e r te r
C o n tr o lle r C1
V1 = Mixing Valve
■Primary Chilled Water
F ig . 4 7 . A p p l i c a t i o n d ia g r a m
Application
Conventional Control Valve V1 is modulated by controller C1 with sensor T 1 located in the secondary chilled water flow. The
objective is to give a constant supply temperature at T1. The return temperature at T2 will vary relative to the load. Only at full
load (hottest period of the year) will the return temperature be at design.
At all times a fixed speed circulating pump supplies the design water volume required to satisfy maximum load conditions.
When full load capacity is not required (for most of the year) energy is continuously being wasted - a more economic system
would be to install a v F d to control the pump circulating rate against the return temperature measured at T2. The controller
setpoint would be the (design full load) return temperature.
As the flow rate is now being varied, the need for the three port valve is reduced, the bypass balancing values can be set down
to a minimum.
Benefits
—
—
—
—
—
—
Reduced noise from water pipework.
Reduced maximum demand.
Reduced wear and tear on machinery.
Ability to overspeed the pump to increase capacity.
Energy-saving on chillers.
Energy-saving on heat loss through lagging. Return water temperature will be higher.
39
63-7062
Secondary Chilled Water Pump with DDC Control Option 2 (See Fig. 48)
¿L______
V1
n
EMC
In v e rte r
C o n tro lle r C1
V1, V2 = Mixing Valve
.P rim ary Chilled W ater
Fig. 48. A p p lic a tio n d ia g ram .
Application
If the existing plant is controlled by Direct Digital Control or connected into a Building Management System, an even more
effective method of controlling the rate of flow of the secondary chilled water pump is available. The bypass port balancing
valves can be set down to a minimum position and the VFD can be set for a low limit speed than will satisfy minimum
requirements. The feedback position from each bypass valve can be used as an input to a control loop. Using a predetermined
setpoint, e.g. 85% open and P+I control, the output from each loop is fed into a load selector (highest signal). The output from
the load selector (equivalent to the AHU with the highest demand) is the input to the VFD. Any increase in valve position above
85% would cause the VFD to increase speed and hence increase the flow rate. Conversely as the AHU with the highest
demand begins to close its valve it would cause the flow to decrease. Hence optimizing the pump flow volumes at all times.
Benefits
—
—
—
—
—
—
Reduced noise from water pipework.
Reduced maximum demand.
Reduced wear and tear on machinery.
Ability to overspeed the pump to increase capacity.
Energy-saving on chillers.
Energy-saving on heat loss through lagging as return water temperature will be higher.
VFD Features Used
—
—
—
—
—
—
Auto restart.
Speed search.
Overspeed.
Overtorque protection.
Stall prevention.
Analog or digital control.
IMPORTANT
T a k e c a r e w ith c h o o s i n g t h e s e t p o i n t f o r e a c h c o n tr o l lo o p .
63-7062
40
Heating Secondary Water Circuit (See Fig. 49)
S e c o n d a ry H o t W a te r
t
2
Honeywell
I : :
V1
EMC
In v e r te r
C o n tr o lle r C1
V1 = Mixing Valve
• P r im a r y H o t W a t e r
F ig . 4 9 . H e a t in g s e c o n d a r y w a t e r c i r c u i t a p p lic a t io n .
Application Description
Conventional Control Valve V1 is modulated by controller C1 with sensor T 1 located in the secondary hot water flow. The
objective is to give a constant supply temperature at T1. The return temperature at T2 will vary relative to the load. Only at full
load (coldest period of the year) will the return temperature be at design. At all times a fixed speed circulating pump supplies
the design water volume required to satisfy maximum load conditions. When full load capability is not required (for most of the
year) energy is continuously being wasted - a more economic system would be to install a VFD to control the pump circulating
rate against the return temperature measured at T2. The controller setpoint would be the (design load) return temperature. As
the flow rate is varied, the need for three port valve bypass circuit reduces, and balancing values can be set down to a
minimum. The circulated water volume is always at an optimum as setpoint is equivalent to full load conditions, thus energy
consumption is at a minimum.
Benefits
—
—
—
—
—
—
Reduced noise from water pipework.
Reduced maximum demand.
Reduced wear and tear on machinery.
Ability to overspeed the pump to increase capacity.
Reduced demand on the boilers.
Energy-saving on heat loss from lagging: Return water temperature will be higher.
VFD Features Used
—
—
—
—
—
—
Auto restart.
Speed search.
Overspeed.
Overtorque protection.
Stall prevention.
Analog or digital control.
IMPORTANT
T h e s e t p o i n t o f T 2 m u s t b e c a r e fu l l y a d j u s t e d a s u n d e r c e r ta in c i r c u m s t a n c e s t h is m a y n o t s a t i s f y t h e r e q u i r e m e n t s o f
a ll o f t h e a ir h a n d l in g u n i ts s e r v e d b y t h e p u m p . In th is c a s e , t h e s e t p o i n t s h o u l d b e i n c r e a s e d .
41
63-7062
Steam Boiler Make-Up Pump (See Fig. 50 and 51)
to System
I
Steam
L e v e l T r a n s m it t e r
Water
|
L e v e l C o n tro l
X
M ) - l
M
INVERTER
Micronik 100
from System
*
P
R e c e iv e r
F ig . 5 0 . S i n g l e b o ile r , s i n g l e p u m p a p p lic a t io n .
to System
Steam
Steam
Bo,.r 1
J,
LT
Water
Steam
Bo.r 2
^
LT
Water
M
^
LC
X
LT
Water
LC
Z
Bo,.r3
M
LC
X
M
s ...............
EMC
1^
Inv ;rter
—
P
---------------------
R e c e iv e r
F ig . 5 1 . M u lt i- b o ile r , s i n g l e m a k e - u p p u m p a p p lic a t io n .
63-7062
from System
42
Boiler Flue Gas-Induced Draught (ID) Fan (See Fig. 52)
C h im n ey/S tack
rsi
C om m on Flue
M ic r o n ik 1 0 0
Inverter
F ig . 5 2 . B o i le r f lu e g a s ID f a n a p p lic a t io n .
Application Description
Traditionally a damper in the common flue from all boilers is controlled by a static pressure sensor located before the ID fan.
Individual dampers for each boiler are located in their own flue, these dampers close when the boiler is off line to prevent
airflow through the boiler which can cause condensation and corrosion.
The common flue gas static pressure sensor detects the change in static pressure when the individual dampers close and
modulates the common damper to prevent the induction of too much air into the on-line boilers. Only when all the boilers are on
full load will the dampers be fully open.
Fitting a VFD to control the speed of the main ID fan will enable more precise control of the static pressure within the flue, thus
reducing losses and preventing excess air flow through the on-line boilers. Thereby, matching the fan speed to the exact static
pressure requirement, overcoming the need for the dampers, increasing the efficiency of the boiler and reducing electrical
energy used by the ID fan.
Other Applications
—
—
—
Process furnaces (metal refining).
Hospitals.
Incinerators (waste burning).
Benefits
—
—
Reduced electrical energy.
Prevents too much air from passing through o n - lin e boilers, thus improving combustion efficiency.
VFD Features Used
—
—
Power fail recovery.
Auto restart on trip.
IMPORTANT
C a r e f u l c o m m i s s i o n i n g is r e q u i r e d to e n s u r e e f f i c i e n t c o m b u s t i o n is m a i n t a i n e d a n d t h a t e m i s s i o n s a r e k e p t u n d e r
c o n tr o l.
43
63-7062
Boilers and Forced Draught Fan (See Fig. 53)
P re s s u re
T r a n s m itt e r
Pressure
R eg u lato r
M ain Gas
M anual
Valve
Gas
C ontrol
Valve
T a k e c a r e to e n s u r e a ll s a fe ty d e v ic e s on
th e b o ile r s till fu n c tio n a f t e r c o n v e rs io n .
F ig . 5 3 . B o i le r w i t h f o r c e d d r a u g h t f a n a p p lic a t io n .
Application Description
On large boilers modulating control is used to regulate the boiler output to match the building load. The steam pressure or hot
water temperature is controlled by varying the volume of fuel (gas or oil) fed to the burner.
To maintain efficient combustion the volume of air supplied to the burner is regulated, relative to the amount of fuel. A simple
combustion control system adjusts the air flow by means of a damper and linkage connected to the same modulating motor
that adjusts the fuel supply. Fitting a VFD to the motor of the forced draught fan allows the fan speed to be varied relative to the
volume of air required, giving more precise control and ensuring efficient combustion is achieved. Only when the boiler is at
maximum load will the forced draught fan be running at full speed.
Other Applications
—
—
Process furnaces.
Incinerators.
VFD Features Used
—
—
—
Power failure recovery.
Auto restart on trip.
Over speed.
Benefits
—
—
Reduced electrical energy.
Improved combustion control.
IMPORTANT
C a r e f u l c o m m i s s i o n i n g is r e q u i r e d to e n s u r e e ff i c i e n t c o m b u s t i o n a n d t h a t e m i s s i o n s a r e k e p t u n d e r c o n tr o l.
63-7062
44
Aircraft Passenger Jetty Loading (See Fig. 54)
U ltraso nic D e te c to r
M o to r/G e arb o x
Inverter
F ig . 5 4 . A i r c r a f t p a s s e n g e r j e t t y lo a d in g a p p lic a t io n .
Application Description
With the existing control system the jetty is started by hand and the operator drives the jetty out towards the aircraft. As the
flexible skirt on the extremity of the jetty approaches the side of the fuselage, the operator stops the jetty and then slowly
inches the jetty into its final docking position, where limit switches disable the control circuit and the motor powers down.
Occasionally, the operator makes a mistake and the jetty skirt is driven into the fuselage at full speed. There is sufficient inertia
in the equipment to damage the fuselage of the aircraft.
Fitting the VFD to the existing jetty motor allows smooth and progressive acceleration/deceleration with precise control even at
low speeds. In addition an ultrasonic sensor is required to detect the proximity of the aircraft. Several control modifications are
designed to prevent these accidents without loss of functionality and speed.
On start-up from a full retracted position, the VFD accelerates the jetty drive motor up to full speed within 4-6 seconds and the
jetty extends as normal. As the jetty approaches the aircraft the ultrasonic sensor detects the proximity of the fuselage and
signals the VFD to reduce speed (within 2-3 seconds) by 75 to 80% for the final docking phase.
The existing limit switches still operate and stop the jetty. This slower docking speed reduces the likelihood of damage to the
fuselage. The reduction in human control also reduces the chance of accidents. The jetty control circuit is not disabled when
retracting from the aircraft, as under some circumstances it is paramount to remove the jetty from the aircraft as quickly as
possible.
Fit the VFD into the electrical circuits before the existing contractors. The VFD is then interfaced with these contactors to
provide the normal start/stop functions, whilst retaining the existing limits and emergency control. Install the ultrasonic detector
at a suitable angle to be able to detect all types of aircraft. It may be necessary to install a second detector to guarantee
reliability. To enhance reliability the sensors should be mounted in a heated weatherproof box to prevent rain and frost affecting
performance.
Other Applications
—
—
—
—
—
—
Benefits.
Reduction in potential damage to fuselage.
Smooth acceleration/deceleration.
Precise positioning.
Shock free starting and stopping.
Semi-automatic docking.
45
63-7062
VFD Features Used
—
—
—
—
—
Digital or analog speed control.
Stall prevention.
Forward - reverse.
Variable torque.
Overtorque trip.
IMPORTANT
E x i s t i n g s a f e t y lim its m u s t n o t b e a l te r e d .
Screw Press (See Fig. 55)
W e t Product
-a ^ " - < r .
M anual
■r
I
A dj u »tm en t
M echanism
To Process
F ig . 5 5 . S c r e w p r e s s a p p lic a t io n .
Application Description
A screw press is used to reduce the moisture content of a wet product. This application was used on a whisky distillery in
Scotland. The product was the left over malted barley, after having the moisture removed by the screw press, it is then dried
and used in cattle feed. Conventional control uses a set of mechanically variable pitch pulleys. The operator fixes the speed of
the screw in order to meet the process requirements. However, differences in the wetness of the mixture means that the end
product suffers from inconsistency. The mechanical speed variation is also unreliable and wear can take place thus changing
the speed with respect to the adjuster position.
Modification
Replace the variable pitch pulleys with fixed pulleys (selected for the correct ratios), install VFD and vary speed electronically.
Benefits
—
—
—
No mechanical speed variation to wear or brake-increased reliability and consistent control.
Speed control can be automated since the VFD can be easily connected to a product sensor measuring wetness, thus
providing closed loop control.
Reduced possibility of damage to the screw press since the VFD can be programmed to maintain a fixed torque or to trip if
overtorque conditions occur.
63-7062
46
VFD Features Used
—
—
—
—
—
—
—
A u to re s ta rt.
O ve r speed.
O v e r to rq u e .
S ta ll p re v e n tio n .
A n a lo g o r d ig ita l c o n tro l.
F o rw a rd /re v e rs e fo r c le a rin g b lo c k a g e s .
V a ria b le to rq u e c o n tro l.
IMPORTANT
E n s u r e t h a t t h e V F D c a n h a n d l e t h e b r e a k a w a y t o q u e o f t h e lo a d . It m a y b e n e c e s s a r y to s e l e c t o n e s i z e l a r g e r V F D
t h a n t h e m o t o r to a c h i e v e th is .
Elevators (See Fig. 56)
C
E le v a t o r M o t o r
0 0
;-
E le v a t o r G e a r b o x
E le v a t o r B r a k e
In v e rte r
F lo o " 8
Landi g
S w itC
12 3 4
F lo o
6 78
7
F lo o
6 78
6
S
1/
12 3 4
IS
s
F ig . 5 6 . E l e v a t o r a p p lic a t io n .
Application Description
A s th e re a re g o v e rn in g re g u la tio n s c o v e rin g e q u ip m e n t u s e d in th e tra n s p o rta tio n o f p e o p le , th is a p p lic a tio n re q u ire s th e
in v o lv e m e n t o f an o rg a n iz a tio n a lre a d y in v o lv e d in e le v a to r m a in te n a n c e a n d in s ta lla tio n . S m a ll e le v a to rs te n d to u s e s in g le o r
tw o s p e e d m o to rs d ire c tly c o n n e c te d to th e e le v a to r g e a rb o x . W h e n th e c a r a p p ro a c h e s th e la n d in g a s w itc h is m a d e a n d th e
e le v a to r m o to r is d is e n g a g e d fro m th e p o w e r s u p p ly a n d th e c a r is b ro u g h t to a h a lt b y u s e o f a b ra k e . If th e m o to r is 2 s p e e d
th e n a s e c o n d s p e e d p h a s e is s e le c te d b e fo re th e m o to r is d is e n g a g e d fro m th e p o w e r. T h is fo rm o f c o n tro l is te m p e ra m e n ta l
a n d th e s to p p in g a c c u ra c y (u n p re d ic ta b le ) d u e to th e te m p o f th e b ra k e , lo a d a n d m o is tu re , all a ffe c tin g th e a c c u ra c y o f
s to p p in g .
47
63-7062
Starting is also coarse as the motor is generally switched directly on to the power supply causing harsh acceleration. By
interfacing a VFD to the existing control, smoother starting is achieved and stopping is more precise. These both to accuracy of
approximately 1.5 mm. This is a fully controlled stop, so it is a repeatable process. It is not easy to give a design for a control
interface as there are so many different possibilities, but the VFD application engineer should be conversant with the
capabilities of the VFD to ensure that the correct mode of operation is employed.
Benefits
—
—
—
—
—
Smooth start/stop.
Accurate positioning.
Easy retrofit.
More reliable.
Reduced wear on brakes.
VFD Features Used
—
—
—
Digital control.
Multi-fixed speed control.
Auto restart.
IMPORTANT
1.
2.
3.
T h e d e c e l e r a t i o n p h a s e g e n e r a l l y is m o r e a r d u o u s f o r t h e V F D a s it h a s to a b s o r b a l a r g e a m o u n t o f e n e r g y f r o m t h e
car.
U n d e r n o r m a l c i r c u m s t a n c e s t h e V F D is N O T c a p a b l e o f h a n d l in g t h is e n e r g y w i th o u t tr ip p in g . A r e g e n e r a t i v e b r a k i n g
u n i t s h o u l d b e c o n n e c t e d to d i s s i p a t e t h is e n e r g y s a f e ly .
T h is is o n e o f t h e m o s t d iffic u lt a p p l i c a t i o n s a s t h e p e r f o r m a n c e h a s to b e r e p e a t e d m a n y t i m e s t h r o u g h o u t th e
w o r k in g d a y . A s m a n y a s 2 0 0 s t a r t s p e r h o u r a r e n o t u n c o m m o n .
List of Abbreviations
AHU
AQ
CAV
CC
CHW
DDC
DP
HVAC
ID
M
PS
P+ I
T
TS
TT
V
VAV
Air Heating Unit
Air Quality
Constant Air Volume
Chiller Controller
Chilled Water
Direct Digital Control
Differential Pressure Sensor
Heating, Ventilation, Air Conditioning
Induced Draught
Motor
Pressure Sensor
Proportional and Integral Control
Temperature Sensor
Temperature Switch
Temperature Transmitter
Valve
Variable Air Volume
PRE P O W E R UP C H E C K S
1. Inspect internal condition of VFD for physical damage.
2. If the output (motor) cables from the VFD have been connected to the VFD terminals, disconnect them before powering
up for the first time.
NOTE It is very easy to confuse the input and output cabling resulting in one or more of the commercial electrical
supply phases to be connected to the VFD output. If this occurs when power is applied, it is likely to damage to
the VFD.
3. Make a general inspection of the internal condition of the VFD and its cubicle. Check for debris that can cause short
circuits. Be aware that debris can fall down through the heat sink and become lodged in internal cooling fan.
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P O S T P O W E R UP C H E C K S
1.
2.
3.
4.
5.
6
.
7.
.
9.
8
10.
11.
12.
13.
Power the VFD (with output disconnected).
Ensure that all input phases are present and the display is correct. Refer to the device technical manuals for details.
If all is well, power down and wait until the DC bus voltage has decreased to a safe level.
Reconnect the output cables.
Power the VFD again, ensuring that the s t o p command is present in the VFD terminal.
NOTE: If the s t a r t command is present on the VFD terminal, the VFD will likely start the load using default parameters.
Program the following parameters before giving the s t a r t command:
a. Motor name plate voltage.
b. Motor name plate full load current.
c. Acceleration ramp.
d. Deceleration ramp.
e. Any minimum or maximum speed limits.
NOTE: Typical values for acceleration and deceleration times for first start:
Up to 10 Hp: 30 seconds acceleration and deceleration.
15 Hp to 50 Hp: 45 seconds acceleration and deceleration.
60 Hp to 1 0 0 Hp: 74 seconds acceleration and deceleration.
125 Hp to 160 Hp: 1 2 0 seconds acceleration and deceleration.
Use either the terminal s t a r t command or keypad command fo r w a r d j o g , and observe the direction of rotation. If the load
rotates in the wrong direction, reverse the connection of the two cables on the VFD output. Reversing the cabling on the
input of the VFD has no effect on direction of rotation at the output.
Start the machine with minimum speed reference and monitor the current drawn.
If the system will safely allow this, while monitoring current drawn, gradually increase the speed until maximum speed is
achieved. Note any areas where overload or overcurrent occurs.
Reduce the speed reference to minimum and monitor the DC bus voltage as the VFD decelerates the load.
a. If the deceleration rate is much too fast, an over voltage trip may occur immediately.
b. If the deceleration ramp is only marginally too fast the over voltage trip may occur at some speed approaching
zero.
NOTE: In either case, the deceleration ramp should be extended. If there is a specific need to decelerate the load at a
defined rate, this can require the addition of a braking circuit in order to dissipate regenerated energy.
Make several starts and stops to simulate what happens during normal operation. Include power down and power up
during motoring. If the VFD trips, there is a problem with programming, cabling, or interfacing. A modern VFD should not
trip.
Tune the primary control loop ensuring that the operation is stable and producing the desired results.
Check the operation of any bypass circuits or downstream switching to ensure that these functions work correctly without
causing the the VFD to stress or trip.
T R O U B L E S H O O T IN G O N S ITE
NOTE:
Refer to the Safety section for guidance on safety.
With correct wiring, set-up and application, a modern VFD should rarely trip. Usually, if a VFD trips, the problem lies with the
application or programming. However, situations can exist that cause operational problems, but do not cause the VFD to trip.
These conditions can be difficult to diagnose.
Unstable Speed Control
If the speed reference value is available for display, this should be monitored when the VFD is not driving the motor and then
when the motor is running. The displayed speed reference value will normally be available to a high degree of accuracy and
therefore any deviation in this value will be immediately evident on the display. The VFD will attempt to follow this value. It is
also be possible to monitor the input terminals for this speed reference, with an appropriate instrument - again any instability
will be evident.
The reference value is normally Vdc or mA, so it is worthwhile to check reference terminals for any voltage level. Problems can
occur with control instruments and with poor wiring, because Vac can be superimposed on the DC reference signal. Any level
of Vac, particularly if it is at low frequencies, typically 60Hz power supply frequency produces modulation of the real reference
signal causing instability.
In some cases, the level of induced Vac can be so high that physical damage can occur to the components on the control
board of the v F d .
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A simple check for stable control from the VFD is to install a temporary control potentiometer, (maybe 15 - 20K ohms in value)
using the VFD power supply, usually 10 or 15 Vdc and 0V (ground). Connect the ends of the potentiometer to the DC power
supply and to 0V ground and connect the wiper of the potentiometer to the voltage speed reference input. Vary the setting of
the potentiometer and monitor the reference value on the VFD display for stability. Also monitor the display whilst the VFD is
driving the motor - again it should be stable.
Where Vac is present on the speed reference lines, several courses of action can be taken:
1. Check for correct shielding and segregation of the control cables.
2. Check that correct input RFI filter has been applied.
3. Check for correct earth grounding on control and power cable shields.
4. Check for malfunction of control instruments.
5. Install filters in the reference lines to reduce the amount of Vac present.
6. Some VFDs have software filter options which can be enabled. However, these do cause delay in response and may not
be acceptable.
Faults and Trips
Most VFDs have some form of error indication and an error log. The error indication is usually either on the VFD display, or an
indicator LED (or relay output), or both.
Where an error log exists, it usually has the capacity for logging multiple errors and may also store other details that may have
caused the trip. However, in most cases this information is only available on the last trip that has occurred and then only if the
power has been maintained to the VFD.
Faults causing trips can be in 1 of 2 categories i.e. m i n o r or m a jo r.
Normally resetting the VFD after a minor fault will not cause damage to the VFD or immediate damage to the motor.
Casual resetting of the VFD after a major fault, could cause serious damage to the VFD, and may cause damage to the motor
and or cables between the motor and the VFD.
Examples of Minor faults would be:
— VFD overheat.
— Motor thermal trip.
— Loss of control loop.
— DC bus overvoltage.
— Loss of input phase.
Examples of Major faults would be:
— Instantaneous over current.
— Earth fault.
— Open circuit output phase.
— DC bus fuse open circuit.
Further inspection of the VFD technical manual will show any additional error messages.
Action if the VFD is Tripped
If the display indicates that the DC bus fuse has become open circuit, it is likely that one or more of the output power transistors
have become short circuited or damaged in some way. Casual replacement of the DC bus fuse should not be made without full
internal inspection and testing of power transistors (covered later in document.)
If the VFD is still powered, attempt to call up the motor voltage, motor current, speed, DC bus voltage and any other parameter
available from the VFD. If this monitoring is available, log these parameters before powering down the VFD, as this important
information may be lost. Also, if the VFD is fitted with a cooling fan, check if it is operating correctly.
If the error can be categorized as m in o r , the VFD can be reset. If the error can be categorized as m a jo r , it should be powered
down at this stage.
Once the DC bus voltage has decayed to a safe level further work can continue.
The VFD should be inspected for internal damage, overheating. If no damage is apparent then the power can be reinstated.
Care should be taken when restoring the power supply, as this will normally reset the trip and if the command to run is still
present on the VFD, it will begin to accelerate the motor.
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Minor Faults
VFD Overheat
This fault is usually caused by problems with cooling air circulation over the VFD heat sink or within the VFD chassis. It may
also be caused through a combination of high motor load and high ambient temperature. The outcome of the above is that the
VFD has sensed that its internal components have become overheated and has closed down, in order to prevent thermal
damage.
1. Check for correct rotation of fans, both mounted on the VFD and any external cooling fans within the VFD enclosure.
2. Clean any air filters and heat sinks.
3. Check for any loose internal connections associated with remote mounted temperature sensors.
Motor thermal trip
The VFD has the capacity to monitor the motor load via the motor current and make an estimate of the degree of heating of the
motor caused by this load. There is usually at least one parameter that allows the user to match the overload to the motor load
conditions:
— Adjust the motor overload parameter to match the motor load condition, or
— Reduce the actual load on the motor until the overload condition is removed.
IMPORTANT
There may be an intermittent overload condition not present during inspection. Adjusting the motor overload
parameter can result in a damaged motor.
The VFD may also have the ability to actually measure the motor temperature via sensors mounted within the motor body. If
this option is available, there is usually no adjustment available. In this situation the only option is to reduce the motor load
condition. It s also possible that the motor temperature sensors have become defective.
Loss of Control Loop
If the VFD is using a 4-20 mA source for speed reference, the VFD will normally monitor for a reference value below 4 mA. If
this occurs, the VFD may respond by tripping, as it understands that the control loop has been lost and is therefore out of
control. This will usually be caused by broken external cabling or a control instrument failure
DC bus over voltage
This problem can have several causes:
— Regeneration from high inertia loads.
— Excessively fast deceleration speeds.
— Open circuiting contactors or other types of switches between the motor and VFD.
— Regeneration from high inertia loads.
Under normal stable operation, the flow of energy is from the power supply through the VFD, to the motor and then out to the
load. There can be situations where energy flow is reversed and the motor becomes a generator. This would occur if the load
tries to over-run the motor, or the VFD tries to decrease the motor speed rapidly, and the load inertia is particularly high. In
either case the regenerated energy flows back into the VFD. As it cannot flow past the bridge rectifier, it will be stored within the
DC bus capacitors, causing a voltage rise. If this voltage rises to 740 volts the VFD will begin to ignore the speed reference and
attempt to limit the DC bus voltage rise by reducing or stopping the rate of deceleration.
If this control loop is inadequate for the load, the DC bus voltage will continue to rise and at 800V (on a 400V VFD) the VFD will
trip, to prevent over voltage damage occurring.
Extending the deceleration ramp time and or installing a braking unit would be options to prevent tripping. Also, it might be
possible to use coast to stop, if the problem only occurred when the VFD was trying to bring the load to a halt.
Excessively fast deceleration speeds
If the deceleration rate is sufficiently short, it is almost guaranteed that an over voltage trip will occur, irrespective of the
magnitude of the load. A motor without any connected load can regenerate a substantial amount of energy if attempts are
made to bring it to a halt in one or two seconds. Simply extending the deceleration ramp times usually accommodates most
problems in this area
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S w itc h e s o n o u tp u t o f V F D
Isolating switches or contactors will often be located in the output cables from the VFD. As when switching an inductive load,
an arc develops at the point where the contact first breaks. This arc can cause high voltage spikes, as high as 3-4 kV, on the
cables back to the VFD. The high voltage spike can completely disrupt the DC bus voltage monitoring circuit, causing a false
high DC bus voltage trip.
To ensure optimum reliability, the switch on the output of the VFD must be interfaced with the VFD controls circuit, by a late
make early break auxiliary contact. When programmed correctly, the VFD will see the opening of this switch and turn off - into a
safe controlled state, thus preventing the trip.
L o s s o f in p u t p h a s e
A VFD does not usually have the ability to monitor the input power supply integrity. If the VFD is single phase input, loss of a
phase will cause the VFD to close down safely and possibly log a power supply loss. If one phase of a three-phase input VFD
is lost, there is usually sufficient capacity within the input bridge rectifier to cope with it. Heavy loading of the motor causes
overheating of the input bridge rectifier and ultimately cause a trip on V F D o v e r h e a t .
Some VFDs monitor the phases feeding the input bridge rectifier. If one phase voltage deviates from the acceptable limit, the
VFD trips and record p h a s e lo s s .
Major Faults
In s t a n t a n e o u s o v e r c u r r e n t
This fault may be caused for various reasons:
—
Motor phase to phase short circuit.
—
Short circuit between two or more motor cables.
—
Closing switches between VFD and motor while VFD still running.
—
Acceleration rate too fast.
With short circuits, the VFD detects a low resistance between two or more phases that allows high current to flow at a level
beyond the normal absolute rating of the VFD. High speed protection circuits have taken effect to protect the output power
transistors of the VFD.
E a r t h f a u lt
This fault will be reported if either the motor or motor cables have become shorted to earth. Most VFDs monitor the current
balance between all three-phases and at maybe 40% imbalance on one phase, the VFD will trip and report an e a r t h fa u lt.
NOTE:
With phase-to-phase and phase-to-earth faults, the speed of VFD response usually prevents any major damage from
occurring in the fault area.
In some cases, there can be little or no apparent damage and even with a simple high voltage m e g g e r t e s t the fault may not
show itself. There can be situations where a high voltage f la s h t e s t may have to be undertaken on the motor to identify the
problem.
NOTE:
The VFD should be disconnected from the motor when a megger test or high voltage test is to be undertaken. Failure
to do so may cause damage to output transistors.
O p e n c ir c u it o u tp u t p h a s e .
Many VFDs do not allow the motor to run with an open circuit on one of the phases to the motor. If the currents flowing in each
phase do not balance to a reasonable level, the VFD will trip.
D C b u s fu s e o p e n c ir c u it
Most VFDs have a fuse located in the DC bus circuit. This fuse is usually a high speed semi-conductor protection fuse and is
designed to protect the input bridge rectifier in the event of a major failure on the VFD output stage. If the fuse has become
open circuit it should not be replaced without first checking the VFD power devices. It is unlikely that the fuse will have blown
without some major component having failed elsewhere. See Fig. 57 for bridge rectifier and output power transistor testing
information.
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Closing a Switch on a Running VFD Output
There are no electrical defects in the circuit. It is caused by incorrect programming and application. If the VFD is running and a
switch is closed in the output cables from the VFD, the motor and VFD will try to effect a direct on-line (DOL) start. Typically,
surge currents in excess of 6 times full-load motor current flow during a DOL start. The VFD has a normal rating of 150% of its
nameplate current for one minute and 200% current for 0.5 seconds. Under these conditions the VFD will trip.
Correcting the control wiring and reprogramming the VFD resolves this problem.
Testing Bridge Rectifier and Output Power Transistors
Diode and Power Transistor Test
IMPORTANT
Before undertaking these tests, ensure that main power to the VFD is isolated, additional power supplies are made
safe, and the DC bus voltage has decreased to 0V. Remove any components from the connections shown to ensure
readings are not misleading.
Table 3 and Fig. 57 indicate the various points for testing the power devices on the VFD. For this test, use a standard
multimeter set to read resistance.
U
V
W
T a b l e 3 . P r o p e r P o w e r D e v ic e T e s t R e a d in g s .
D1
Converter
Module
D2
D3
TR1
VFD
Module
TR3
TR5
T e s te r
P o la r it y
+
-
L1
P
L2
P
L3
P
U
P
V
P
W
P
P
L1
P
L2
P
L3
P
U
P
V
P
W
M e a s u re m e n t
Open
Closed
Open
Closed
Open
Closed
Open
Closed
Open
Closed
Open
Closed
C ir c u it
T e s te r
Sym bol
+
D4
D5
D6
TR4
TR6
TR2
53
L1
N
L2
N
L3
N
U
N
V
N
W
N
P o la r it y
N
L1
N
L2
N
L3
N
U
N
V
N
W
M e a s u re m e n t
Closed
Open
Closed
Open
Closed
Open
Closed
Open
Closed
Open
Closed
Open
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Miscellaneous
C o n t r o l P o w e r T e r m in a ls
The VFD will generally have an on board power supply for use with external equipment e.g. transmitters. A simple voltage
check of this power supply together with an other terminals that output voltages will give a good indication of the state of the
control board.
U s e o f M u lt ip le A u t o R e s e t s
Most VFDs have a function which provides auto reset in the event of a trip. This function is designed to enhance the reliability
of the VFD service provided.
IMPORTANT
Use the auto reset function with caution.
Example: When an over current trip occurs, the over current condition causes abnormal heating within the input bridge rectifier
and output power transistors. If the over current condition occurs repeatedly within a short space of time and the VFD is
programmed to reset itself automatically. The reset can be programmed with insufficient time between resets to allow for
cooling. This results in the power devices becoming thermally stressed, shortening their life expectancy.
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