Kertas Ujikaji Makmal Mesin Asas
SEE 3742
http://fke.utm.my/makmal/mesin
INDUCTION MOTOR
Fakulti Kejuruteraan Elektrik
Universiti Teknologi Malaysia
2006
INDUCTION MOTOR
Objectives
To obtain the equivalent circuit of an induction motor.
To study the characteristic of a three phase induction motor.
Introduction
The induction motor is one of ac machine, in which there are alternating currents in both stator and
rotor windings. The alternating current is supplied directly to the stator winding and by induction, i.e.
transformer action, to the rotor winding. The rotor winding may be either of two types, the squirrelcage type or the wound-rotor type. In the three-phase squirrel–cage induction motor, the three phase
stator winding is a distributed winding (in Fig. 1 is represented by three concentrated coils). The ends
of these phase windings can be connected in a wye or a delta
Stator
a
a'
b
c
Rotor
a'
c'
c'
b'
a
b'
c
b
(b)
(a)
a
Figure 1. Three-phase squirrel-cage
induction motor. (a) Cross-sectional view.
(b) Y-connected stator winding. (c) ∆connected stator winding.
c'
a
a'
c
b'
b
b
c
(c)
When excited from a balanced three phase source, it will produced magnetic field in the air gap
rotating at synchronous speed as determine the number of poles and the applied stator frequency.
The winding of a squirrel-cage rotor consists of conducting bars embedded in slots in the rotor iron
and short-circuited at each end by conducting end rings. The extreme simplicity and ruggedness of
the squirrel-cage construction are outstanding advantages of the induction motor.
The stator rotating magnetic field cuts the conductor of the rotor to induced voltage which caused
current to flow through the short circuited rotor winding. This rotor current interacts with stator
rotating field to produce a torque on the rotor. The rotor will thus accelerate to a speed determined
by the load torque.
The equivalent circuit is very useful in testing and analysis of performance of induction motor. The
parameter of equivalent circuit of induction motor can be determined from stator resistance
measurement, no load test and blocked rotor test.
Based on the no load test, the magnetizing reactance can be determined by
Xm =
V NL
I NL sin φ o
where
φ o = cos −1
PNL
V NL I NL
PNL = no load power supply per phase
VNL = no load voltage supply per phase
INL = no load current supply per phase
In the blocked-rotor test, the rotor is blocked so that the motor cannot rotate, and balanced voltages
are applied to the stator terminals. The parameter of equivalent circuit are thus found from
PBL
2
I BL
R1 + R2' =
R2' =
PBL
− R1
2
I BL
X 1 + X 2' =
VBL sin φ s
I BL
where
φ s = cos −1
PBL
VBL I BL
R1 = average dc resistance per stator phase.
PBL = power supply on blocked-rotor per phase
VBL = voltage supply on blocked-rotor per phase
IBL = current supply on blocked-rotor per phase
Reference:
1. P. C. Sen, Principles of Electric Machines and Power Electronics, Second Edition.
2. A. E. Fitzgerald, Charles Kingsley, Jr, Stephen D. Umans, Electric Machinery, Sixth Edition.
Procedure
In this laboratory experiment four procedures are performed.
A. Stator resistance measurement
Switch off all supplies. Measure the resistance between any two stator terminals of induction motor
by using a digital multimeter. Obtain the average dc resistance per stator phase.
B. No load test
voltmeter
ammeter
B1
A1
Adaptor
C1
V
A
W
Figure 2. Stator circuit for no-load and blockedrotor tests.
W
3-phase watt
meter
Figure 3. Power measurement circuit
Switch off all supplies. Connect the stator windings to form a delta-connected induction motor.
Connect the voltmeter, ammeter and wattmeter to measure the voltage, current and power. Make
sure that the induction motor is connected to a 415 V, 50 Hz, 3-phase supply through the circuit
breaker, push button starter and variac transformer.
ASK THE LAB SUPERVISOR TO VERIFY YOUR CIRCUIT CONNECTION BEFORE YOU PROCEED
Start the motor using the supply from the variac transformer by gradually increasing its voltage to
415 V. Record the readings for the voltage VNL, current INL and power PNL.
C. Blocked-rotor test
Switch off all supplies. Blocked-the rotor of induction motor. Set voltage range of wattmeter to 100 V
and current appropriately. Increase the current in steps of 1 A by increasing the voltage to the rated
current (make sure that the current does not exceed the rated value). Record the readings for the
voltage VBL, current IBL and power PBL. Tabulate your results.
D. Load test
Switch off all supplies. Use the stator winding ∆-connected as shown in Figure 2. Connect the dc
generator as a compounded generator. Connect the resistance load from the resistor bank to the
output of the dc generator.
ASK THE LAB SUPERVISOR TO VERIFY YOUR CIRCUIT CONNECTION BEFORE YOU PROCEED
Run the induction motor by increasing the supply voltage gradually until it reaches 415 V. By
increasing the armature (load) current from 0 until the full load current (refer the dc generator spec.)
in steps of around 1 A and at the same time maintaining the 220 V load voltage. Record the readings
for stator current and speed of induction motor.
R load
Figure 4. DC generator and resistance load circuit for load test.
Discussion/Report
1. Determine the parameters of the IEEE-recommended equivalent circuit of induction motor based
on the results obtained from no-load test (415 V) and blocked-rotor test (full load current).
2. Using the parameters obtained (no. 1) and from measurement of the dc resistance of the stator
winding, draw the per-phase IEEE-recommended equivalent circuit of the induction motor.
3. Based on the results of the load test, plot the stator current as a function of speed. Make
discussion on the results.