MA’DIN POLYTECHNIC
COLLEGE
MALAPPURAM
DEPARTMENT OF ELECTRICAL
ENGINEERING
AC MACHINE LAB(437)
SEMESTER 6
CONTENTS
1. Load test on single phase transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Load test on 3−φ squirrel cage induction motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. O.C.C of dc shunt generator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
4. Load test on dc shunt generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20
5. Load test on dc series motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6. Measurement of coupling coefficient of transformer coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7. O.C and S.C tests on single phase transformer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
8. Three phase power measurement by two wattmeter method. . . . . . . . . . . . . . . . . . . . . . . . . . .42
9. Calibration of single phase energy meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
10. Resistance measurement using Wheatstones bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
11. Resistance measurement using Kelvins double bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
12. V-I Characteristics of incandescent lamp and linear resistance . . . . . . . . . . . . . . . . . . . . . . . . 58
13. Open circuit and short circuit test on three phase alternator . . . . . . . . . . . . . . . . . . . . . . . . . . 60
14. No load and blocked rotor tests on 3−φ slip ring induction motor. . . . . . . . . . . . . . . . . . . . .
CONNECTION DIAGRAM
0-10A MI
R
L
C
V
V
R
0-600V
MI
Y
400V φ3-
50Hz AC
M
A
10A
600V,10A,upf
IM
Y
10A
B
C
B
10A
V
M
L
600V,10A,upf
D.O.L
STARTER
MACHINE DETAILS
Voltage
Current
Synchronous speed =
120f
P
V
I
-
Power
-
Connection
-
Speed(rpm)
Phase
-
S1
S2
Experiment 1
LOAD TEST ON 3−φ SQUIRREL CAGE
INDUCTION MOTOR
AIM
To conduct load test on the given 3-φ squirrel cage induction motor and plot the performance
characteristics.
APPARATUS REQUIRED
1. Voltmeter 0-600V
MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
2. Ammeter 0-10A
MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
3. Wattmeter 600V,10A, upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 nos.
4. Tachometer To measure speed
THEORY
A squirrel cage induction motor essentially consists of a stator and a rotor. The stator is a hollow
cylindrical structure with slots on the inner periphery and carries a three phase winding. The
winding can be connected in star or delta and is connected across a 3-φ supply.
The rotor is also a cylindrical structure with slots on the outer periphery. The slots carry
thick Al or Cu bars. These bars are short circuited at both ends by means of end rings.
◦
When a 3-φ supply is given to a 3-φ winding displaced by 120 in space, a magnetic field
of constant magnitude but rotating at synchronous speed is produced. This flux links with the
stationary rotor, thus inducing an emf in it. As the rotor circuit is closed, a current flows
through it. The direction of the induced current is such as to oppose the cause producing it.
The cause is the relative motion between the stator magnetic field and the rotor. So the rotor
starts rotating in the same direction as the stator magnetic field and tries to catch up with it.
But practically it is never able to do so. Because if it does so, there would be no relative
motion, no emf and hence no torque.
9
OBSERVATIONS
Sl.NO
V(VOLTS
)
%slip =
I(AMPS)
Ns − N
W1(WAT
TS)
W2
(WATTS
)
S1(KG)
S2(KG)
N(rpm)
T
(Nm)
Output
(watts)
Input
(watts)
slip
(%)
pf
× 100
Ns
Where,
Ns - Synchronous speed =
120 × f
P
N - rotor speed
f - frequency
P - No. of poles of the machine
An induction motor can never operate at s=0. It always operates between s=0 and s=1(starting). The
performance characteristics are plots of efficiency, torque, speed, slip, pf and line cur-rent versus
output.
Current and torque increases with increase in output. The induction motor is essentially a
constant speed motor. However speed reduces gradually with increase in output and slip increases
gradually with increase in output. The pf is low at low loads and increases with increase in output.
The efficiency increases with increase in output, reaches a peak value and then gradually drops with
further increase in output.
PROCEDURE
The load on the motor is completely removed by loosening the brake drum. The motor is to be always
started and stopped at no load, The supply is switched on and the motor is started using a Direct On
Line Starter (DOL Starter).
The readings of the voltmeter, ammeter, wattmeters and spring balance are noted down. The speed
is measured using a tachometer. The load is then increased in steps, each time noting down all the
above readings. The experiment is repeated for different values of load currents till the rated current of
the machine is reached.
During the experiment, the machine may get heated up. It is cooled by pouring some water into
the brake drum.
At low loads,(when pf < 0.5) one of the wattmeters read negative, in such cases, the supply is
switched off and the connections to the M and L terminals of the wattmeter are interchanged. The
meter now reads positive, but it is to be recorded as negative.
The load on the machine is removed completely and the supply is switched off. The
readings are tabulated and the performance characteristics are plotted.
Sample Calculation (set no )
Voltage (V) = . . . . . . . . .
.
Effic
ienc
y
..
Current (I) = . . . . .
..
...
Wattmeter Reading 1 (W1)
=
. .....
...
Wattmeter Reading 2 (W2)
=
. .....
...
Spring balance Readings S1 =
. . . . . . S2 = . . . . . . . . .
..
Speed (N) = . . . . . .
.
Torque (T) = (S1 − S2)Rg =
.. . . . .
Where,
R = Radius of brake drum = . . . . . . . . .
g = 9.8 m/s2
Synchronous speed =
slip(%) =
P
(Ns − N )
×
Ns
=
120 × f
100 =
. ..
... ...
Input power = (W1 + W2) = . . . . . . . . .
(W1 + W2) power
factor(cos φ) =
√3V I
Output power= 2πNT
60
Efficiency =
Output
Input = . . . . . . . . .
Performance Characteristics
η
Efficiency
(%)
(rpm)
T N
(Nm)
(%)
Slip pf
Speed
pf
Torque
Slip
Output (watts)
RESULT
Conducted load test on the given 3-φ squirrel cage induction motor and plotted the performance
characteristics.
VIVA QUESTIONS
1. How are the meter ratings selected for this experiment?
2. Why does one of the wattmeters read -ve at starting?
3. What is ‘slip’ in an induction motor?
4. What are the two types of 3-φ induction motors and what is the difference between
the two?
5.
6.
7.
8.
9.
10.
What is the value of slip at starting?
What are the advantages and disadvantages of squirrel cage induction motor?
What is the condition for maximum torque in an induction motor?
What are the different losses in an induction motor?
Give some applications of 3-φ squirrel cage induction motor?
Explain a typical Torque-slip characteristic.
11. What is the effect of increased rotor resistance on the performance of an in-duction
machine?
CIRCUIT DIAGRAM - OCC & SCC
+
0-10A MI
LFA
15A
Rh1
300Ω
1.7A
220V
DC
A
A
1
M
F
1
15A
-
A
V
R
0-300V
MI
R
GS
B
2
Y
N
F
F
1
S3
Y
B
2
F2
S1
+
D.C motor
Rh2
2A
VIH.P rpm-
1000Ω 1.2A
-
+
S
2
2A
A
-
alternator
VV
IKV ArpmConn.-Star
0-2A MC
MEASUREMENT OF ARMATURE RESISTANCE
+
0-5A MC
5A
+
A
-
45Ω 5A
R
+
20V
DC
0-10V
MC
V
-
5A
N
OBSERVATIONS AND CALCULATIONS
O.C
TEST
I
V
S.C TEST
I
f
a
OC
If
Measurement of Ra
V
I
R
a
OPEN CIRCUIT AND SHORT CIRCUIT
TEST ON A THREE PHASE
ALTERNATOR(EMF METHOD)
AIM
To conduct open circuit and short circuit tests on a three phase alternator and predetermine the regulation curve
by emf method at half load and full load.
APPARATUS REQUIRED
Voltmeter
Ammeter
Rheostat
- 0-300V, MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 0-10V, PMMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
....
- 0-10A, MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 0-2A, PMMC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
- 0-5A, PMMC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
- 300Ω, 1.7A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 1000Ω, 1.2A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
- 45Ω, 5A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 no.
1 no.
1 no.
1 no.
1 no.
1 no.
1 no.
1 no.
PRINCIPLE
As the load on the alternator is varied the terminal voltage also varies. This is due to
1. Voltage drop due to armature resistance IR.
2. Voltage drop due to armature reactance IXL.
3. Voltage due to armature reaction effect.
The voltage regulation of a synchronous generator is defined as the rise in voltage at the terminals
when the load is reduced from full load rated value to zero, speed and field current remaining constant
%Reg =
E-V
× 100
V
Where E - Generated emf
V - Terminal voltage
For small machines the regulation may be found by direct loading. For large machines the voltage
regulation is predetermined by using indirect methods like emf method, mmf method. These methods
require open circuit characteristics and short circuit characteristics.
The open circuit characteristics is a plot of no load terminal voltage versus field excitation with
the machine running at rated speed. Under these conditions the induced voltage is directly
proportional to the flux. The shape of curve is therefore a typical B-H curve or magnetization curve.
The short circuit characteristics is a plot between armature current and field excitation with a
symmetrical short circuit applied across the terminals. Under these conditions current in the armature
winding
61
O.C.C and S.C.C of
Regulation curve
3φ alternator
%Reg
(half load)
Isc
I Eg(V
a
(full load)
S.C test
O.C test
(upf)
0
PF
(lead)
0.2
0.4
0.6 0.8
0.8
0.6 0.4
0.2
0
PF
(lag)
If (A)
%Reg
Line voltage
VL
=.
V. L
V
Effective value
From graph
PH
= √ 3
=.... .....
Ra(dc) = . . . . . . . . .
R
a
= 1.6 × Ra(dc) = . . . . . . . . .
Zs
= VOC /ISC = . . . . . . . . .
=
∴ X s2
Zs2 − Ra2 . . . . . . . . .=
XS
=
Sample Calculation
2
Eo = √V cos φ + IRa) + (V sin φ ± IXS )
2
‘-ve’ for leading
‘+ve’ for lagging
% regulation =
Eo − V
× 100
V
LOAD FACTOR
POWER
FACTOR
E0 VOLTS
LAGGING
REGULATION%
E0 VOLTS
LEADING
REGULATION%
wholly depends on the internal impedance consisting of synchronous reactance X sand the
wind-ing resistance Ra.
Now Ra being small compared to Xs the pf under short circuit condition is zero power
factor lagging and therefore the armature reaction mmf is almost wholly demagnetizing.
The short circuit characteristics is a straight line. From O.C.C & S.C.C the synchronous
impedance is evaluated as follows.
For any value of excitation or field current If , if VOC is the open circuit voltage & ISC is
the short circuit current, then synchronous impedance Zs=VOC /ISC . The value of Zs is
calculated for the unsaturated region. For the computation of regulation, it is convenient to
take Zs at such a value of excitation which give rise to Vph[normal voltage per phase]on open
circuit. The armature resistance is measured using ammeter-voltmeter method. Under
working conditions the effective value of Ra is increased due to skin effect and temperature
effect. The effective value of Ra is generally taken as 1.6 times the d.c value.
Synchronous reactance per phase Xs = √ ( Za2 − Ra2) Ω per phase.
Eo = √ (V cos φ + IRa)2 − (V sin φ ± IXs)2
where +ve sign for lagging power factor and -ve for leading. Now percentage regulation
for each case is computed as
% Regulation = Eo − V × 100
V
PROCEDURE
O.C test
Connections are made as shown in the connection diagram. Switches S3 and S2 are kept in the
open position. The motor field rheostat Rh1 is kept in minimum position and the alternator
field rheostat Rh2 in the maximum position. Supply is switched on by closing switch S1. The
dc motor is started using the 3-point starter. The motor field rheostat Rh1 is varied till the
speed becomes equal to the rated speed. Switch S2 is closed. Rh2 is varied in steps and the
field current and voltmeter reading are noted down. The experiment is repeated for different
values of field current till the voltmeter reading shows 120% of the rated voltage of the
alternator. Rheostat Rh2 is brought back to the maximum resistance position.
S.C test
Switch S3 is closed and rheostat Rh2 is varied till the ammeter reading in the alternator (A2)
reads the rated current of the machine. The corresponding value of field current is noted
down.
Armature resistance is found by voltmeter-ammeter method.
The regulation is then determined at various power factors for half and full loads and the
regulation curve is plotted.
RESULT
The open circuit and short circuit test was conducted on the given 3-φ alternator by emf
methode and the regulation curves for half load & full load are plotted.
CIRCUIT DIAGRAM - OCC & SCC
+
Rh1
300Ω
1.7A
220V
0-10A MI
LFA
15A
DC
A
A
1
M
F
1
15A
-
A
V
R
0-300V
MI
GS
B
2
Y
N
F
F
1
S3
R
Y
B
2
F2
S1
D.C motor
Rh2
2A
+
VIIH.P rpm-
1000Ω 1.2A
-
+
S
2
2A
A
-
VV
IKV ArpmConn.-Star
0-2A MC
MEASUREMENT OF ARMATURE RESISTANCE
+
0-5A MC
5A
+
A
-
45Ω 5A
R
+
20V
DC
0-10V
MC
5A
-
alternator
V
N
OBSERVATIONS AND CALCULATIONS
O.C
TEST
I
V
S.C TEST
I
f
a
OC
If
Measurement of Ra
V
I
R
a
OPEN CIRCUIT AND SHORT CIRCUIT
TEST ON A THREE PHASE
ALTERNATOR(MMF METHOD)
AIM
To conduct open circuit and short circuit tests on a three phase alternator and predetermine the regulation curve
by mmf method at half load and full load.
APPARATUS REQUIRED
Voltmeter
Ammeter
Rheostat
- 0-300V, MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 0-10V, PMMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
....
- 0-10A, MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 0-2A, PMMC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
- 0-5A, PMMC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
- 300Ω, 1.7A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- 1000Ω, 1.2A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
- 45Ω, 5A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 no.
1 no.
1 no.
1 no.
1 no.
1 no.
1 no.
1 no.
PRINCIPLE
Field ampere turns required to produce a voltage V on full load is the vector sum of follows
1. Field ampere turns(If1) required to produce the V or IaRa Is taken in account then V+
IaRa COSф on no load this can be founds from OCC
2. Field ampere turns(If2)requird to over come the demagnetizing effect of armature
reaction on full load. This values found from SCC
The impedence cn be neglected because Ra is very small and Xs is also small under short
circuit conditions. Hence power factor on SC is almost zero lagging and the ampere turns is
used entirely to over come to the armature reaction. Which is wholly demagnetizing. The
resultant If1 and If2. Curresponding to the unity powerfactor. Lagging PF and Leading PF can
be obtained by Drawing phasor as shown in figure.
PROCEDURE
O.C test
Connections are made as shown in the connection diagram. Switches S3 and S2 are kept in the
open position. The motor field rheostat Rh1 is kept in minimum position and the alternator
field rheostat Rh2 in the maximum position. Supply is switched on by closing switch S1. The
dc motor is started using the 3-point starter. The motor field rheostat Rh1 is varied till the
speed becomes equal to the rated speed. Switch S2 is closed. Rh2 is varied in steps and the
field current and voltmeter reading are noted down. The experiment is repeated for different
values of field current till the voltmeter reading shows 120% of the rated voltage of the
alternator. Rheostat Rh2 is brought back to the maximum resistance position.
S.C test
Switch S3 is closed and rheostat Rh2 is varied till the ammeter reading in the alternator (A2)
reads the rated current of the machine. The corresponding value of field current is noted
down.
Armature resistance is found by voltmeter-ammeter method.
The regulation is then determined at various power factors for half and full loads and the
regulation curve is plotted.
RESULT
The open circuit and short circuit test was conducted on the given 3-φ alternator
by mmf methode and the regulation curves for half load & full load are plotted.
CONNECTION DIAGRAM - NO LOAD TEST
600V,5A,lpf
M
L
0-5A
R
A
400V 3 - φ
50Hz AC
10A
B1
C1
Y
10A
C
E1
V
R
R1
V
0-500V
B2
Y
B
R3
STATOR
C2
B
10A
R2
ROTOR
E2
B3
C
C3
E3
V
M
L
600V,5A,lpf
BLOCKED ROTOR TEST
250V,10A,upf
M
L
0-10A
R
A
400V 3 - φ
50Hz AC
10A
C1
Y
10A
C
B1
E1
V
R
V
0-250V
B2
Y
BR
3
STATOR
C2
B
10A
S1
R1
ROTOR
R
2
E
2
B3
C
C3
E3
V
M
L
250V,10A,upf
Machine Details
Voltage
Current
speed
Phase
H.P
- 415V
- A
- 1440rpm
3-φ
-
BLOCKED
ROTOR
S2
NO LOAD AND BLOCKED ROTOR
TESTS ON 3 PHASE SLIP RING
INDUCTION MOTOR
AIM
To perform no load and blocked rotor test on a three phase slip ring induction motor and determine
the equivalent circuit.
APPARATUS REQUIRED
Voltmeter
Ammeter
Wattmeter
Rheostat
Autotransformer
- (0-500V) MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- (0-250V) MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(0-30V) PMMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- ...
- (0-5A) MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- (0-10A) MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(0-10A)PMMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- ...
- 500V, 5A, lpf. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
250V, 10A, upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- ...
- 9Ω, 8.5A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 no.
1 no.
1 no.
1 no.
1 no.
1 no.
2 nos.
2 nos.
1 no.
PRINCIPLE
Slip ring motors are always started with full line voltage applied across the stator terminals. The value
of starting current is adjusted by introducing a variable resistance in the rotor circuit.The controlling
resistance is in the form of resistances connected in star. The resistance is gradually cut out of the
rotor circuit as the motor gathers speed.
OBSERVATIONS AND CALCULATIONS
No load test:
V0(V )
I0(A)
W1
W2
W 0=W 1 + W 2
Blocked rotor test:
VSC (V )
ISC (A)
W1(w) W 2(w) W SC =W1 + W 2
MEASUREMENT OF STATOR RESISTANCE
0-5A
5A
+
+
50Ω 5A
-
A
R
+
220V
DC
V
0-20V
-
5A
-
Y
B
For finding stator resistance, Rs :
No.
V (V )
I(A)
Rs
R
R
R
(meas)
R
R
(meas)
=
R × 2R
3R
2
=3R
R /ph(dc) =2 R
s
3 (meas)
2
Rs/ph(ac) = 1.6 × 3 × R(meas) = . . . . . . . .
By introducing the rotor resistance, the rotor current is reduced at starting and the starting torque is
increased the latter due to improvement in power factor.
No load test:If the motor is run at rated voltage and frequency without any mechanical load, it will draw power necessary to
supply the no load losses. The no load current will have two components. The active component and the
magnetizing component, the former being very small as the no load losses are small. The power factor at no
load is therefore very low. The no load power factor is always less than 0.5 and hence at no load one of the
wattmeter at input side reads negative.
The no load input W0 to the stator consists of
1. Small stator copper loss
2. Core losses
3. The loss due to friction and windage.
The rotor copper loss can be neglected, since slip is small at no load.
Blocked rotor test :The stator is supplied with a low voltage of rated frequency just sufficient to circulate rated current through the
stator with the rotor blocked and short circuited. The power input, current and the voltage applied are noted
down.
The power input during the blocked rotor test is wholly consumed in the stator and rotor copper losses. The
core loss is low because the applied voltage is only a small percentage of the normal voltage. Again since the
rotor is at stand still the mechanical losses are absent. Hence the blocked rotor input can be taken as
approximately equal to the copper losses.
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From no load test:V0 = . . . . . . . . .
I0 = . . .
V0/ph = V0 = . .
.......
=.........
I
0
Line current(IL) =
=.........
IL
phase current(I0/ph) =
Power consumed =
W0 = . . . . . . . . .
. . .. . .
√3
=.........
W0
W0
√3V0I0
cos φ0
=
∴ φ0
= . ..
=. ..
sin φ0
=.........
....
....
=.........
.
.
V0/ph
R0/ph
=
I0 /ph cos φ0
V0/ph
X0/ph
=
=.........
I0 /ph sin φ0
From blocked rotor test:VSC = . . .
..... .
ISC = . . . . . . . . .
VSC /ph
= VSC
=.........
W
WSC /ph
=
SC
ISC
ISC /ph
.
.
=
3
√3
WSC = . . . . . . . . .
=.........
=.........
(Total winding resistance as
referred to the stator side)
(Total leakage reactance as
re-ferred to the stator side)
(Rotor resistance as referred
to the stater side)
(Electrical equivalent of the
mechanical load)
R
01
WSC /ph
=
2
/ph
I
Z
X
VSC /ph
01
=
01
= q Z0 1 − R0
=R −R
R2
′
2
01
R0
2
1
S(EFF )
=.........
=.........
1−s
=R
R
=.........
ISC /ph
2′
L
R01=
V/ph=
=.........
SC
s
X01 =
RL
=
=
X0
PROCEDURE
No load test:Connections are made as shown in the diagram for no load test. Brake drum is made free to
rotate by loosening the belt. The autotransformer is placed in zero position. Then the supply
is switched on and the auto transformer is adjusted to supply the rated voltage to the machine.
The handle of the starter is rotated to cut out the rotor resistance. Readings of the wattmeters,
voltmeter and ammeter are noted and tabulated.
Blocked rotor test:Connections are made as shown. The rotor is blocked by tightening the belt on the brake
drum. The auto transformer is set to the zero voltage position. Then the three phase supply is
switched on. By adjusting the autotransformer, the ammeter reading is made equal to rated
current of the machine. Readings of the two wattmeters, voltmeter and the ammeter are noted
and tabulated.
Measurement of stator resistance:Connections are done for the stator resistance measurements.
It is measured using the
voltmeter-ammeter method. The measured value is 23 Rph as the machine is connected. Thus
Rph = 1.5Rmeas. Rs(eff ) is taken as 1.6 times Rph to account for skin effect and heating effect.
RESULT
No load and blocked rotor tests were conducted on the given three phase slip ring induction
motor and the equivalent circuit parameters were determined.
69