Electric Machinery
SPRING 2007
Experiment No. 4
Parameters of a Squirrel Cage Induction Machine
Experiment No. 4
Parameters of a Squirrel Cage Induction Machine
Objective
The objective of this experiment is to determine the parameters for the per phase circuit
model of an induction machine shown in Fig. 1.
Apparatus
1. Squirrel-cage induction motor-dc generator set
2. DC motor starting box
3. 2 test tables
4. 1 0-150 V DC voltmeter
5. 1 0-50 A DC ammeter
6. 1 0-5 A AC ammeter
7. 1 0-300 V AC voltmeter
8. 1 current transformer
9. 1 wattmeter
10. 1 3-φ transformer bank
11. 1 3-φ resistive load bank
12. 1 3-φ variac
13. 1 Strobe
14. 2 5 A Kenwood DC power supply
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Electric Machinery
SPRING 2008
Experiment No. 4
Parameters of a Squirrel Cage Induction Machine
Procedure
1. Blocked-Rotor Test
a) Set up the circuit shown in Figure 2.
b) Starting with low current, adjust the variac until the ac line current equals approximately rated current. Measure and record I1, I2, I3, V1−2, V2−3, V3−1, and
PBR.
From the blocked-rotor test data the resistance RBRand the reactance XBRare determined.
PBR
RBR=3IBR2=r1+ r2
VBR
XBR= √3IBRsinθBR= x1+ x2
P
BR
θ = cos−1 sqrt3VBRIBR
where
1 I
IBR=3(1+ I2+ I3)
1 V
and VBR=3(1−2+ V2−3+ V3−1)
Let x1= 0.4XBRand x2= 0.6XBR.
2. Approximate No-Load Test
Normally the magnetizing reactance (Xm) is determined from the no-load test; however, to conduct this test it is necessary for the induction machine to be uncoupled
from the dc machine. In this experiment we will only approximate the no-load test.
a) Set up the circuit shown in Figure 3.
b) Do not turn on dc power. Start the induction motor. Note the direction of
rotation and speed (n) of the shaft. Shut off ac power.
c) Using the dc starting box, start up the dc machine. If the shaft is not turning in
the same direction as in step 2, shut off the dc machine and reverse the two leads
connected to the shunt field terminals. Restart the dc machine. Adjust the field
rheostat until the speed of the machine set is the same as in step 2.
d) Turn on ac power. Check for balanced 3-φ voltages and currents. Adjust IFuntil
I2is at a minimum. Measure and record I1, I2, I3, V1−2, V2−3, V3−1, PN L, and n.
From the no-load test data the reactance Xmis determined.
VN L
Xm=IN L
− x1
where
1
IN L= (I1+ I2+ I3)
3
1
and VN L= (V1−2+V2−3+V3−1)
3
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Experiment No. 4
Parameters of a Squirrel Cage Induction Machine
Electric Machinery
SPRING 2008
3. Direct-Current Test
a) Set up the circuit shown in Figure 4.
b) Starting at zero current, adjust the resistive bank until Idcequals approximately
rated line current. Measure and record Vdcand Idc.
From the direct-current test data the resistances r1and r2are determined.
r1=
Vdc
2Idc
r2= RBR− r1
4. Full Load Operation
a) Set up the circuit shown in Figure 5.
IFand the resistive load bank until the
b) Start the induction machine. Adjust
induction machine is drawing rated line current. Measure and record I1, I2,
V1−2,V2−3,V3−1,P3−φ, n,Ia, and Va.
Report
1. A drawing of the induction machine equivalent circuit showing the calculated values
of all parameters.
2. In the approximate no-load test what condition existed between the two machines for
minimum ac line current?
3. For the full load operating condition of part 4 calculate the efficiency (η)
4. Using the data gathered in part 4, calculate the slip (s).
5. Using the data gathered in part 4 and your equivalent circuit, calculate the mechanical
torque per phase (τem).
The electromechanical torque per phase is given by:
2
τem= |I2| r2
sωe
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I3,
Electric Machinery
SPRING 2008
I
r1
Experiment No. 4
Parameters of a Squirrel Cage Induction Machine
x
1
I
1
x2
r2
2
r2
Xm
V 1m
1-s
s
FIGURE 1. CIRCUIT MODEL FOR ONE PHASE OF AN INDUCTION MACHINE
TEST TABLE
I1
V
120 V
3-PHASE
VARIAC
AC POWER
SUPPLY
I2
BR
INDUCTION
MACHINE
I3
DC
MACHINE
FIGURE 2. CIRCUIT SCHEMATIC FOR BLOCKEDROTOR TEST
I1
L1
115 V DC
L2
V
A
230 V
NL
F
I
F
I2
3-PHASE
POWER
SUPPLY
INDUCTION
MACHINE
I3
DC MACHINE
FIGURE 3. CIRCUIT SCHEMATIC FOR APPROXIMATE NO-LOAD TEST
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Electric Machinery
SPRING 2008
Experiment No. 4
Parameters of a Squirrel Cage Induction Machine
TEST TABLE
I
120 V
DC
POWER
SUPPLY
V
DC
DC
INDUCTION
MACHINE
FIGURE 4. CIRCUIT SCHEMATIC FOR DIRECT-CURRENT TEST
I1
RESISTIVE
I
230 V
AC POWER
SUPPLY
V
1-2
I2
I3
DC
INDUCTION
MACHINE
Va
MACHINE
IF
120 V
DC POWER SUPPLY
FIGURE 5. CIRCUIT SCHEMATIC FOR FULL LOAD OPERATION
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a
LOAD
BANK