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
I. INTRODUCTION
The objectives of the experiment are: To examine the construction of the three-phase squirrel cage motor; To determine its starting, no load and full load characteristics. INSTRUMENTS AND COMPONENTS:
Squirrel Cage Induction Motor Module EMS 8221
Electro-dynamometer Module EMS 8911
Three-Phase Wattmeter Module EMS 8441
Power Supply Module (0-120/208V 3f) EMS 8821
AC Metering Module (250V) EMS 8426
AC Metering Module (2.5/2.5/2.5/8A) EMS 8425
Timing Belt EMS 8942
Strobotac General Radio 1531-AB
Squirrel Cage Induction Motor and Electro-dynamometer
Timing belt
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 length-wise 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 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.
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 = Tw with P in watts, T in Newton meters, w in radians/sec. Derive the formula with appropriate constants for P in hp, T in lbf-in, w in r.p.m.
B. The 3f 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 3f, 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 VLL = 200V
W1 + W2 = Total Power Input = 250W 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 h ______
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 graph.
a. Synchronous speed, ns,
b. Starting torque, Ts
c. Maximum torque, Tmax
d. Identify the region of the curve in part 8a where the motor cannot be operated.
IV. 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 = ______
C. Using your EMS Squirrel Cage Motor, Electrodynamometer, Three-Phase 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 3f 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.
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. 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.
Table 1
TORQUE(Ibf.in) / I1
(amps) / I2
(amps) / I3
(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 3f 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).
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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