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EE 448 Laboratory Experiment 5
3-Phase Induction (Squirrel-Cage) Machines
EE 448
Experiment No. 5
04/12/2007
3-PHASE INDUCTION
(SQUIRREL-CAGE)
MACHINES
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EE 448 Laboratory Experiment 5
3-Phase Induction (Squirrel-Cage) Machines
I. INTRODUCTION
The objectives of the experiment are: To examine the construction of the threephase squirrel cage motor; To determine its starting, no load and full load
characteristics. INSTRUMENTS AND COMPONENTS:
Squirrel Cage Induction Motor Module
Electro-dynamometer Module
Three-Phase Wattmeter Module
Power Supply Module (0-120/208V 3)
AC Metering Module (250V)
AC Metering Module (2.5/2.5/2.5/8A)
Timing Belt
Strobotac
EMS 8221
EMS 8911
EMS 8441
EMS 8821
EMS 8426
EMS 8425
EMS 8942
General Radio 1531-AB
Squirrel Cage Induction Motor and Electro-dynamometer
Timing belt
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EE 448 Laboratory Experiment 5
3-Phase Induction (Squirrel-Cage) Machines
Strobotac
II. BACKGROUND
The simplest and most widely-used rotor for induction motors is the so-called
squirrel cage rotor, from which the squirrel cage induction motor gets its name.
The squirrel cage rotor consists of a laminated iron core which is slotted lengthwise around its periphery. Solid bars of copper or aluminum are tightly pressed
or embedded into the rotor slots. At both ends of the rotor, short-circuiting rings
are welded or brazed to the bars to make a solid structure. The short-circuited
bars, because their resistance is much less than the core, do not have to be
specially insulated from the core. In some rotors the bars and end rings are cast
as a single integral structure for placement on the core. The short-circuiting
elements actually form shorted turns that have high currents induced in them by
the stator field flux.
Compared to the intricately wound and arranged wound rotor or the armature of
the DC motor, the squirrel cage rotor is relatively simple. It is easy to
manufacture and is essentially trouble-free in actual service.
In an assembled squirrel cage induction motor, the periphery of the rotor is
separated from the stator by a very small air gap. The width of this air gap, in
fact, is as small as mechanical clearance needs will permit. This insures that the
strongest possible electromagnetic induction action will take place.
When power is applied to the stator of a practical induction motor, a rotating
magnetic field is created by any one of the means you learned about. As the field
begins to revolve, its flux lines cut the shorted turns embedded around the
surface of the squirrel cage rotor and generate voltages in them by
electromagnetic induction. Because the turns are short-circuits with very low
resistance, the induced voltages cause high currents to circulate in the rotor bars.
The circulating rotor currents then produce their own strong magnetic fields.
These local rotor flux fields produce their own magnetic poles, which are
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EE 448 Laboratory Experiment 5
3-Phase Induction (Squirrel-Cage) Machines
attracted to the rotating field. Thus, the rotor revolves with the main field.
If torque from an external source is applied to the shaft of the induction machine
such that it rotates at a speed greater than its synchronous speed, then electrical
power may be generated in the stator windings. The generation of electrical
power relies on the existence of the stator rotating magnetic field. The
magnetizing reactive power must be supplied to the stator windings and may
come from a capacitor or from synchronous machines connected to the stator.
Courtesy of: http://www.knoware-online.com/motors.html
Induction generators are not commonly used as sources of electrical power
because of the need for a separate source of magnetizing reactive power.
Synchronous alternators are the common means of generating electricity.
However, small generators used to supply a farm or residence from wind energy
can be induction generators. These generators draw their magnetizing reactive
power from the utility system and generate electrical power for the local load
and/or supply power to the utility system.
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EE 448 Laboratory Experiment 5
3-Phase Induction (Squirrel-Cage) Machines
III. PRELAB EXERCISES
Fill in the blanks.
A. The relative difference in speed between the actual rotor speed and the
synchronous speed of the stator field is called the _______________.
B. The direction of rotation of an induction motor may be changed by
_____________.
C. In order to operate an induction machine as a generator the rotor has to
be driven at a speed _______________ than the synchronous speed.
Answer the following questions.
A. The general formula relating power, torque and speed is P = T with P in
watts, T in Newton meters,  in radians/sec. Derive the formula with
appropriate constants for P in hp, T in lbf-in,  in r.p.m.
B. The 3 induction machine in your lab has the following rated values.
current = 1.2 A
voltages (L-L) = 208 V
speed = 1670 r.p.m.
power = ¼HP
From these values, calculate the rated torque in lbf-in.
Trated = _______ lbf-in.
C. A 3, y connected induction motor is operating at rated load conditions.
At this rated load, the following data is obtained.
TLOAD = 9 lbf.in
I = 1.0A
W1 + W2 = Total Power Input = 250W
VLL = 200V
N = 1680 r.p.m.
Calculate the following quantities.
a. Apparent power S _______
b. Reactive power, Q _______
c. Input power factor, pf _______
d. Horse power, HP _______
e. Efficiency  _______
D. Draw a typical torque versus speed curve for an induction motor.
Carefully show the location of the following points on the axes of the
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EE 448 Laboratory Experiment 5
3-Phase Induction (Squirrel-Cage) Machines
graph.
a.
b.
c.
d.
IV.
Synchronous speed, ns,
Starting torque, Ts
Maximum torque, Tmax
Identify the region of the curve in part 8a where the motor cannot
be operated.
LABORATORY EXPERIMENT
CAUTION: HIGH VOLTAGES ARE PRESENT IN THIS LABORATORY
EXPERIMENT! DO NOT MAKE ANY CONNECTIONS WITH THE
POWER ON! THE POWER SHOULD BE TURNED OFF AFTER
COMPLETING EACH INDIVIDUAL MEASUREMENT!
A. Examine the construction of the Squirrel Cage Induction Motor Module
EMS 8221, paying particular attention to the motor, connection
terminals and the wiring.
a. Identify the stator windings. Note that they consist of many turns
of small diameter wire evenly spaced around the stator. (The stator
windings are identical to that of the wound rotor induction motor).
b. Identify the end rings of the squirrel cage rotor.
c. Note the thickness of the air gap between the stator and the rotor.
d. Is there any electrical connection between the rotor and any other
part of the motor?
______________________________________
B. Viewing the front face of the module:
a. The three separate stator windings are connected to terminals
___________ and ___________,
__________ and
__________, __________ and ___________.
b. What is the rated current of the stator windings? ___________
c. What is the rated voltage of the stator windings? ___________
d. What is the rated speed and horsepower of the motor?
r/min = __________
hp = __________
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EE 448 Laboratory Experiment 5
3-Phase Induction (Squirrel-Cage) Machines
Ammeter
C. Using your EMS Squirrel Cage Motor, Electrodynamometer, ThreePhase Watt-meter, Power Supply and AC Metering Modules, connect the
circuit shown in Fig. 1. DO NOT COUPLE THE MOTOR TO THE
DYNAMOMETER AT THIS TIME! Note that the stator windings are wye
connected through the wattmeter to the variable 3 output of the power
supply, terminals 4, 5 and 6. To measure the speed of the motor two
methods can be used. One method is to connect the tachometer to the
shaft of the motor. The other method is to aim the strobotac at the
pulley of the motor and adjust the strobotac till a constant line is visible
on the pulley while the motor is running.
+
+
-
I1
+
Power Meter
Ammeter
W2
W1
+
+
5
6
6
4
3
5
6
5
4
3
2
1
-
-
I2
4
1
4
2
5
3
6
5
6
Ammeter
4
Voltmeter
-
200 Volt
Line-Line
3-Phase
Source
8821
2
1
+
V1
Squirrel Cage
Induction Motor
+
+
N
-
I3
1
1
Electro-Dynamometer
120 V
AC Source
8821
-
N
-
Figure 1: Motor Connection Diagram
D. a. Turn on the power supply and adjust V1 to 200Vac. The motor should
be running.
b. Measure and record in Table 1, the three line currents, the two
wattmeter indications, and the motor speed.
c. Return the voltage to zero and turn off the power supply.
E. a. Couple the motor to the electrodynamometer by using the timing belt.
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EE 448 Laboratory Experiment 5
3-Phase Induction (Squirrel-Cage) Machines
Make sure the belt rides in-between the belt tensioners as show in fig 2.
Figure 2: A Properly Installed Timing Belt.
b. Set the dynamometer control knob at its full ccw position.
c. Repeat Procedure 2 for each of the torques listed in Table 1,
maintaining the input voltage at 200Vac. Torque is measured on
the scale attached to dynamometer frame, not on the control knob.
To adjust the torque make sure the voltage V1 is 200Vac, and slowly
turn the dynamometer control knob cw until the desired torque can
be read on the frame of the dynamometer.
d. When finished return the voltage to zero and turn off the power
supply.
TORQUE
(Ibf.in)
I1
(amps)
Table 1
I2
I3
(amps) (amps)
W1
(watts)
W2
(watts)
SPEED
(rpm)
0
3
6
9
_____
12
F.
a. Connect the circuit shown in Fig. 3. Note that the variable 3
output of the power supply, terminals 4, 5, and 6 are being used. Make
sure the dynamometer is still properly connected to the induction motor.
b. Set the dynamometer control knob at its full cw position (to provide
a maximum starting load for the motor).
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EE 448 Laboratory Experiment 5
3-Phase Induction (Squirrel-Cage) Machines
+
+
Line-Line
3-Phase
Source
8821
4
5
6
4
-
I1
V1 Voltmeter
5
6
1
4
2
5
3
6
-
200 Volt
+
+
Ammeter
8 Amp
Squirrel Cage
Induction Motor
-
N
1
1
Electro-Dynamometer
120 V
AC Source
8821
-
N
-
Figure 3: Starting Torque Connection Diagram
G. a. Turn on the power supply and quickly increase the supply voltage to
maximum and measure V1, I1 and the developed starting torque. Turn off
the power supply immediately after the measurements are made.
NOTE: Measurements can be taken very easily if each member of
the group is assigned one measurement to record. This helps
reduce the amount of times the test must be preformed and thereby
reduces the stress on the motor. The torque measurement should be
the highest reading achieved during the quick startup.
V1 =___________ Vac,
I1 = ___________ Aac
starting torque = ____________ lbf-in
b. Calculate the apparent power to the motor at starting conditions.
apparent power = ___________ VA
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