€x - 3- szg
EXPERTUENT
TRANSFORMF:R VOT,TAGE,
CITRRENT ANn
I{O.
2
TMpr':nANcF: RATTOS
P URP O S F ::
To study the rerationship
transformer.
between vort,age and current
in a
I ' T T S CUS S T O N :
when a road is connected to the secondary of a transformer,
the
current
which flows sets up an MMF which opposes the flux
produced by the.appried
prirnary voltage.
As a result,
the primary
flux and the prirnary counter
EMF (primary
induced voltage)
are
reduced.
The prirnary current
increases
because the appried
prinary
voltage
has less opposition
from the induced prinary
voltage.
This increase
in primary current
supplies
the energy
required
by the load. Note that the prirnary ampere-turns
increase
the flux while the secondary ampere-turns
decrease the flux.
The
ne t effe ct
re rn a in s p ra e t ic a ' l ly u n c h a n g e d f o r
is that tfte fluv ar ior r s
c o n d i ti o n s
of
'load-
Neglecting
the smarl exciting
current
and other transformer
losses,
the primary and secondary ampere-turns
are egual:
flil"
w he r e :
Thus,
tu r n s
Io
I,
No
N,
the relationship
i s as follows:
:
=
=
:
= I"Ng
primary
amperes
Secondary amperes
primary
turns
secondary turns
between voltage,
E^
iV.
r"
E,
/V,
-=a
I^
J =J =
currents,
and number of
Where a is calIed the transformation
ratio.
The voltaqes
E^ and
E" are the primary and secondary vortage values r"=pec-ti.r"fy.
The impedance,
or total
opposition
oo p -T-
F:
-p
to
z"=+
P
Conbining the last
current
two equations gives
flow
under
load
is:
tfti'2"
zo=
t7
4^
And solving
for
v
= d' 2,
Z" gives
o
," = L.
a'
zp
t\ll 2-t2
/
tts
APPARA.rUS
1.
2.
3.
4.
5.
6.
RRQUTRF:N:
on e Hampden Transforme r
(E T -1 0 0 , T -1 o o A ,
One Hampden RLC-100 Resistance-Reactance
Three 0-1 anp AC ammeters
One O-15O volt AC voltmeter
One O-75 volt AC voltmeter
One O-L?O volt AC power source
o r T -i-0 0 -3 A )
Load
PROCF:NIIRF:
l-.
Make the connections
shown in figure
7. Apply approximately
O.4 amperes resistive
1oad to the secondary of-thL
transformer.
Record the prirnary and secondary voltages
and
amperes in table 2.
2-
Ca lculate
Enter the
3.
Make the connections
shown in figure
B. Apply approximately
0 .5 amperes, 0.8 ragg in g p o we r f a c t o r
ro a d t o t h e s e c o n da r y
g for a simple method of
of the transformer.
Refer to figure
applying
a 0.8 power factor
load. Record the pr-imary and
secondary volts
and amps in table 2.
4.
calcul-ate
th9 voltage,
for the lagging power
2.
5.
Determine the power dissipation,
a n d 3. E nter the data in t a b le
the voltage,
c u rre n t ,
data in table 2.
t u rn s
a n d imp e d a n c e ra t io s .
current,
turns and impedance ratios
factor
1oad. Enter the data in table
p, of
the
load
in
steps
2.
Q UF S T T O N. q :
l-.
2.
3.
What load conditions
mu s t b e me t s o t h a t Z p rs e e s r a n
irnpedance match in the secondary?
What commercial use is made of impedance transformation?
what apparent effect
did power factor
have on turns,
voltage,
current,
and impedance ratios?
2
1
Prepare
a report
containing:
1-.
Diagrams of each circuit.
2.
A Il calculatio n s
a n d re q u ire d
3.
A n error an a ly s is
4.
Ansrarers to questions.
data.
n_1 n
H1
HJ
0-150v
RESI ST IVE
L!AD
H4
H5
I'i g,rrre
7
H1
H3
H4
F " ' igr r r e
c ^r i
na
1 OOO
R TS IS TIV f
LOA D
IN D UCTI VE
LIA D
I
Iab2-1.w p5
Ta b le
2z
Load
Re s is t iv e
0.8
P.F.
Lagging
Eo
I
E,
I
E./8"
T"/I
N../N
z
z"
z./z
PI
qnr i
nd
1 O qq
4
1ab2 -l .w p5
A lagging or leading
.8 power factor
load is easily
obtained
graph below. This graph is a 3-4-5 right
using the trianglar
triangle.
it provides
the correct
relative
magnitudes of
resistive,
reactive
and total
current
vectors
for a 0.8 power
factor
load situation.
The power factor
may be either
lagging
Ieading.
The inductance
load bank would provide
the reactj-ve
power factor
current
for a lagging
load. The capacitance
load
power
bank would provide
the reactive
current
for a leading
factor
load.
or
To obtain a 0.8 lagging power factor
load, first
apply the
desired nurnber of resistive
load steps, next, travel
vertically
(from an ammeter in
from the value of resistive
current
obtained
series with the resistance
load bank) to the hypotenuse of the
trj-angle.
The value indicated
on the scale is the value of total
(as indicated
current
by an ammeter in series with the line)
to produce 0.8 power factor.
required
Adjust the inductance
load
bank to yield
this value of total
current.
Recheck the resistive
load current
to determine
if voltage
variations
have changed the
va lu e s1ightly.
If so read ju s t
the total
c u rre n t
a c c o rd in g ly .
L!,
(_)
Z
O
{
L!
u.( .rQ
\2
a\
(h"
I RIS IS TA NCE
zG
a..$?\2
i RES = 0,8 i TDTA L
4 - 0 ,8(5)
0,5
V)
LJ
u
LJ
L,-J
u
nr
..r
{
U
u
0.?
0,3
-.ll n
0 .5 -
RI S ] S T ] V T A MP E RI S
Sp ring 19 99
l -ab2-1.w p5
ET332b
l'
2'
DiscussionPointsfor Lab 3
Define in words and mathematical equationsthe turns
ratio
of an ideal transformer.
Definein wordsandmathematical
equationstherelationshipbetweenthe transformerswindings
andwindingcurrents.(Curent ratio)
3. How areimpedances
affectedby the turnsratioof the ideal
4'
transformer?
How doesthepowerfactorof the load affectthecurrentand
the voltageratiosin actual
transformeroperation
?
5. what is therelationship
for primaryandsecondary
powerin an
ideartransformer?
6. List or discussusesof tmnsformers
in industrv.
Spring2002
dpJab3.doc