EE   2145230   Chapter   8   Electric   Motor
8.1
Motor   Theory
•
An   electric   motor   is   a   device   that   changes   electrical   energy   to   mechanical   energy.
Electrical
Energy
Motor
Generator
Mechanical
Energy
•
Motors   can   be   classified   as   DC   and   AC   motors.
•
Electromagnetism :   Whenever   an   electric   current   flows   in   a   conductor   a   magnetic   field   is   set   up   in   the   space   surrounding   the   conductor.
The   field   spreads   out   around   the   conductor   in   concentric   circles   with   the   greatest   density   of   magnetic   flux   nearest   to   the   conductor.
•
If   we   place   a   current ‐ carrying   conductor   in   a   magnetic   field,   the   conductor   has   a   force   exerted   on   it.
If   the   conductor   is   free   to   move   this   force   will   produce   motion.
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8.2
DC   Motor
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DC   motors   are   described   in   part   by   the   type   of   field   winding   and   armature   winding   connection.
There   are   four   types:   separately   excited,   series,   shunt   and   compound   DC   motor.
•
The   principle   of   DC   motor:
JK
F
=
(
×
B ) →
F
=
Bli → T
= φ a
•
Characteristics   of   DC   motors
Repulsion   of   like   magnetic   poles   and   attraction   of   unlike   magnetic   poles.
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There   are   two   main   windings:
1.
Field   winding   (Stator):   This   winding   provides   magnetic   flux   and   is   static.
2.
Armature   winding   (Rotor):   Current ‐ carrying   wires   in   B   (from   Stator)
⇒
Force
⇒
Torque.
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•
The   action   of   the   commutator   continually   switches   the   input   current   to   new   sections   of   the   armature   winding   so   that   the   top   of   the   armature   is   always   a   north   pole;   hence   the   armature   continues   to   rotate   in   an   effort   to   align   itself   with   the   field   poles.
•
Counter   or   Back   emf :   As   an   armature   rotates   and   cuts   through   the   magnetic   flux   from   the   field   circuit,   an   induced   voltage   occurs.
This   is   because   an   electromotive   force   (emf)   will   be   induced   across   the   ends   of   a   conductor   when   there   is   relative   motion   between   it   and   a   magnetic   field.
K K JK e
(
) →
E b
=
KVL   on   armature   circuit:   E b
V t
I R a
t
Separately   excited   DC   motor:   Field   circuit   and   armature   circuit   are   separated.
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Shunt   DC   motor:   I
L
I I f
‐ Low   starting   torque
‐ The   speed   is   almost   constant   for   all   reasonable   loads.
‐ Loads:   electric   fans   and   pumps
Series   DC   motor:   V
T
=
E
A
+
R I
+
R I
‐ High   starting   torque
‐ If   a   series   motor   is   not   connected   mechanically   to   a   load,   the   speed   of   the   motor   will   continue   to   increase   for   as   long   as   the   counter   emf   is   substantially   below   the   applied   emf.
The   speed   may   increase   far   above   the   normal   operating   speed   of   the   motor,   and   this   may   result   in   the   armature   flying   apart   because   of   the   centrifugal   force   developed   by   the   rapid   rotation.
A   series   motor   should   always   be   connected   mechanically   to   a   load   to   prevent   it   from   “ running   away ”.
‐ Loads:   engine   starter,   landing   gear
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•
DC   motor   efficiency :
η =
P out
×
100%
=
P in
P out
+
P out
Losses
×
100%
=
P in
−
Losses
×
100%
P in
Four   major   losses   are
1.
Copper   losses   (Cu   losses):   losses   from   field   and   armature   windings
2.
Core   loss:   Hysteresis   and   Eddy   current   losses
3.
Mechanical   losses:   Friction   and   windage   losses
4.
Stray   load   losses:   1%   of   output   power
Note   that   core   and   mechanical   losses   are   called   rotational   losses.
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Speed   control
1.
For   separately   excited   motors,   speed   can   be   controlled   by   varying   the   field   current,   varying   the   armature   source   voltage   or   by   inserting   additional   resistance   in   series   with   the   armature.
2.
For   shunt ‐ connected   motors,   speed   can   be   controlled   by   varying   the   field   current   or   by   inserting   additional   resistance   in   series   with   the   armature.
3.
For   series ‐ connected   motors,   its   torque   is   almost   inversely   proportional   to   the   square   of   its   speed.
This   type   of   motor   is   suitable   for   starting   heavy   loads.
It   can   reach   dangerous   speeds   if   the   load   is   totally   removed
8.3
AC   Motor
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Because   the   speed   of   an   AC   motor   is   determined   by   the   frequency   of   the   AC   supply   that   is   applied   it,   AC   motors   are   well   suited   to   constant   speed   applications.
The   principle   of   all   AC   motors   is   based   on   the   generation   of   a   rotating   magnetic   field.
It   is   this   rotating   field   that   causes   the   motor’s   rotor   to   turn.
•
AC   motors   can   be   grouped   into   1)   Synchronous   motors   and   2)   Induction   motors
•
If   three   windings   are   placed   round   a   stator   frame,   and   three ‐ phase   AC   is   applied   to   the   windings,   the   magnetic   fields   generated   in   each   of   the   three   windings   will   combine   into   a   magnetic   field   that   rotates.
At   any   given   instance,   these   fields   combine   together   in   order   to   produce   a   resultant   field   that   which   acts   on   the   rotor.
The   rotor   turns   because   the   magnetic   field   rotates.
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•
Synchronous   motor :   AC   is   applied   to   the   stator   and   DC   is   applied   to   the   rotor.
•
If   the   rotor   winding   is   energized   with   DC ,   it   will   act   like   a   bar   magnet   and   it   will   rotate   in   sympathy   with   the   rotating   field.
The   speed   of   rotation   of   the   magnetic   field   depends   on   the   frequency   of   the   three ‐ phase   AC   supply   and,   provided   that   the   supply   frequency   remains   constant,   the   rotor   will   turn   at   a   constant   speed.
Furthermore,   the   speed   of   rotation   will   remain   constant   regardless   of   the   load   applied.
•
The   synchronous   motor   is   so   called   because   its   rotor   is   synchronized   with   the   rotating   field   set   up   by   the   stator.
Its   construction   is   essentially   the   same   as   that   of   a   simple   AC   generator   (alternator).
•
Synchronous   motors   are   not   self ‐ starting   and   must   be   brought   up   to   near   synchronous   speed   before   they   can   continue   rotating   by   themselves.
When   the   motor   speed   reaches   approximately   97%   of   nameplate
RPM,   the   DC   field   current   is   applied   to   the   rotor   producing   Pull ‐ in   Torque   and   the   rotor   will   pull ‐ in  ‐ step   and   “synchronize”   with   the   rotating   flux   field   in   the   stator.
The   motor   will   run   at   synchronous   speed   and   produce   Synchronous   Torque.
•
The   requirement   to   have   an   external   DC   voltage   source   as   well   as   the   AC   field   excitation   makes   this   type   of   motor   somewhat   unattractive!
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•
Advantages   of   synchronous   motors
1)   Precise   speed   regulation   makes   the   synchronous   motor   an   ideal   choice   for   certain   industrial   processes   and   as   a   prime   mover   for   generators.
2)   Synchronous   motors   have   speed   /   torque   characteristics   which   are   ideally   suited   for   direct   drive   of   large   horsepower,   low ‐ rpm   loads   such   as   reciprocating   compressors.
3)   Synchronous   motors   operate   at   an   improved   power   factor,   thereby   improving   overall   system   power   factor   and   eliminating   or   reducing   utility   power   factor   penalties.
An   improved   power   factor   also   reduces   the   system   voltage   drop   and   the   voltage   drop   at   the   motor   terminals.
•
Induction   motor   (IM) :   most   commonly   used   AC   motors.
•
The   induction   motor   derives   its   name   from   the   fact   that   AC   currents   are   induced   in   the   rotor   circuit   by   the   rotating   magnetic   field   in   the   stator.
The   stator   construction   of   the   induction   motor   and   of   the   synchronous   motor   is   almost   identical,   but   their   rotors   are   completely   different.
•
Two   types   of   rotor   winding   are   1)   Squirrel   cage   and   2)   Wound   rotor
•
The   essential   parts   of   an   induction   motor   are   the   rotor   and   the   stator.
The   stator   is   in   the   form   of   a   shell   with   longitudinal   slots   on   the   inner   surface.
The   rotor   in   an   induction   motor   consists   of   a   laminated   iron   core   in   which   are   placed   longitudinal   conductors.
Additional   resistance   can   be   added   to   wound   rotor   type   through   slip   rings,   while   no   resistance   can   be   added   for   squirrel   cage   type.
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End ring
3 supply
Stator Winding Squirrel cage rotor
3 supply
Slip ring
Brushes
Stator Winding
(Stator Winding may be Y or connection.)
Wound rotor winding
External variable resistor
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The   induction   motor   is   the   most   commonly   used   AC   motor   because   of   its   simplicity,   its   robust   construction   and   its   relatively   low   cost.
These   advantages   arise   from   the   fact   that   the   rotor   of   an   induction   motor   is   a   self ‐ contained   component   that   is   not   actually   electrically   connected   to   an   external   source   of   voltage.
•
The   induction   motor   has   the   same   stator   as   the   synchronous   motor.
The   rotor   is   different   in   that   it   does   not   require   an   external   source   of   power.
Current   is   induced   in   the   rotor   by   the   action   of   the   rotating   field   cutting   through   the   rotor   conductors.
This   rotor   current   generates   a   magnetic   field   which   interacts   with   the   stator   field,   resulting   in   a   torque   being   exerted   on   the   rotor   and   causing   it   to   rotate.
•
The   rotating   magnetic   field   generated   in   the   stator   induces   an   e.m.f.
in   the   rotor.
The   current   in   the   rotor   circuit   caused   by   this   induced   e.m.f.
sets   up   a   magnetic   field.
The   two   fields   interact,   and   cause   the   rotor   to   turn.
Note   that   the   rotor   circuit   is   a   closed ‐ loop   circuit.
•
The   rotor   of   an   induction   motor   rotates   at   less   than   synchronous   speed,   in   order   that   the   rotating   field   can   cut   through   the   rotor   conductors   and   induce   a   current   flow   in   them.
This   percentage   difference   between   the   synchronous   speed   and   the   rotor   speed   is   known   as   slip .
Slip   varies   very   little   with   normal   load   changes,   and   the   induction   motor   is   therefore   considered   to   be   a   constant ‐ speed   motor.
If   the   speeds   were   exactly   the   same,   no   relative   motion   would   exist   between   the   two,   and   so   no   e.m.f.
would   be   induced   in   the   rotor.
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Three ‐ phase   IM
•
Synchronous   speed   (Ns)   can   be   calculated   from   N s
2 f 60
=
P
120 f
P
Slip   (s)   can   be   calculated   from   s
=
N s
−
N r
N s where   N r
is   the   rotor   speed
•
Developed   torque   T
=
2 ksE R
2 2
R
2
2
+
( sX
2
)
2
•
When   the   load   on   an   induction   motor   becomes   so   great   that   the   torque   of   the   rotor   cannot   carry   it,   the   motor   will   stop.
This   is   called   the   pull ‐ out   point .
•
Induction   motor   efficiency
3 V I
L L cos
θ
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Four   major   losses   are
1.
Stator   Copper   losses   (P
SCL
):   losses   from   the   stator   winding
2.
Core   loss:   Hysteresis   and   Eddy   current   losses
3.
Rotor   Copper   losses   (P
RCL
):   losses   from   the   rotor   winding
4.
Mechanical   losses:   Friction   and   windage   losses
5.
Stray   load   losses:   1%   of   output   power
Two ‐ phase   IM
•
In   the   case   of   a   two ‐ phase   induction   motor,   two   windings   are   placed   at   right   angles   to   each   other.
By   exciting   these   windings   with   current   which   is   90 degrees   out   of   phase,   a   rotating   magnetic   field   can   be   created.
Single ‐ phase   IM
•
A   single ‐ phase   induction   motor,   on   the   other   hand,   has   only   one   phase.
This   type   of   motor   is   extensively   used   in   applications   which   require   small   low ‐ output   motors.
The   advantage   gained   by   using   single ‐ phase   motors   is   that   in   small   sizes   they   are   less   expensive   to   manufacture   than   other   types.
Also   they   eliminate   the   need   for   a   three ‐ phase   supply.
•
A   single ‐ phase   induction   motor   has   only   one   stator   winding;   therefore   the   magnetic   field   generated   does   not   rotate.
A   single ‐ phase   induction   motor   with   only   one   winding   cannot   start   rotating   by   itself.
Some   means   of   starting   is   required   for   all   single ‐ phase   induction   motors.
Once   the   rotor   is   started   rotating,   however,   it   will   continue   to   rotate   and   come   up   to   speed.
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•
Three   types   of   commonly   used   single ‐ phase   IM
1)   Split ‐ phase   IM:   This   IM   should   not   start   frequently.
Rating   between   60W   –   250   W   (1/12   hp ‐  1/3   hp).
Split   stator   winding   into   main   winding   and   auxiliary   winding   with   different   wire   sizes.
2)   Capacitor ‐ type   IM:   This   IM   can   start   often   and   gives   high   starting   torque.
Rating   between   120   W   –   7.5
kW
(1/6   hp ‐ 10   hp).
The   performance   is   better   due   to   the   capacitor.
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3)   Shaded ‐ pole   IM:   This   IM   gives   low   starting   torque.
Rating   lower   than   0.05
hp   (40   W).
In   the   shaded   pole   induction   motor,   a   section   of   each   pole   face   in   the   stator   is   shorted   out   by   a   metal   strap.
This   has   the   effect   of   moving   the   magnetic   field   back   and   forth   across   the   pole   face.
The   moving   magnetic   field   has   the   same   effect   as   a   rotating   field,   and   the   motor   is   self ‐ starting   when   switched   on.
References:
Textbooks   #   1   and   #   2
EE   2145230   Aircraft   Electricity   and   Electronics
Asst.
Prof.
Thavatchai   Tayjasanant,   Ph.D.
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