Lab 3 Squirrel Motor

Lab 3: AC Induction Motors Part A: The Three Phase Squirrel Cage Induction Motor Introduction: The following lab report details the operation and results of an AC induction motor using a four pole squirrel cage induction module. An induction motor is an AC electric motor in which the electric current in the rotor needed to produce torque is induced by electromagnetic induction from the magnetic field of the stator winding. The rotor used in this experiment is known as a squirrel cage. This kind of rotor r otor winding is a rugged, reliable r eliable and economical and is commonly used in the industry today. A squirrel cage rotor is composed of usually bare copper bars, slightly longer than the rotor, which are pushed into slots. ither ends of the bars are welded together so that they short circuit and in turn this makes the component look like a squirrel cage, as seen in the figures below.

Figure 1: Squirrel Cage Induction Motor

Figure 2: Schematic o Squirrel Cage

Aim: The aim of this experiment is to demonstrate and analysis the operating characteristics of a three!phase induction motor using the four pole squirrel cage induction motor module. "rocedure: A nominal line #oltage will be applied to the squirrel cage induction motor, and the characteristics such as the motor rotation direction and no load speed will be measured. $lowly the mechanical load will be increased at steady increments% for each augmentation the #arious electrical and mechanical results will be tabulated

and graphed. Then using the data recorded, #arious graphs will be plotted to help determine the characterises of the squirrel cage induction motor. &astly we will interchange the two leads of the power supply and record how this affects the direction of rotation of the squirrel cage induction motor. Apparatus: '. Install the power supply, prime mo#er( dynamometer, four pole squirrel cage induction motor, and data acquisition interface )*AI+ m odules in the $  workstation. -. echanically couple the prime mo#er( dynamometer to the four!pole squirrel cage induction motor. . Turn on the power supply, making sure the #oltage control knob is fully counter clockwise. /. Connect the 0$1 port cable from the computer to the *AI module. Connect the low power inputs of the *AI and prime mo#er( dynamometer modules to the -/ 2 3 AC output of the power supply. 4. $tart the metering application 5. 6inally connect the following circuit in such configuration as shown below: aking sure the prime mo#er( dynamometer controls are as follows: •

7* switch 3 *89

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&7A* C79T7& 7* switch 3 A9

•

&7A* C79T7& knob 3 I9 )CC;+

•

*I$"&A8 switch 3 Torque )T+

Figure 3: !"#erimental Circuit $iagram

esults: )The following results are numbered in direct relation to the question numbers given in the lab manual) <. The direction of rotation of the squirrel cage induction motor is cloc%&ise and the motor speed indicated by the meter is, n ' 1()* r+min and ,es the no load speed is almost equal to the speed of the rotating magnetic field. =. nnom ' 13-* r+min

Tnom ' 1.21 /.m Inom ' 0.A >. Ad?ust load control so that the torque indicated on the prime mo#er( dynamometer reads @ 9.m. $lowly increase the load control knob so that the torque increases by increments of @. till '.= 9.m is reached, then continue to increase but only at increments of @.' 9.m until the motor speed starts to decrease rapidly. ecord all data in a table:

Figure (: $T(11 Table o esults4

'@. 5es the motor line current indicated in column I' increases as the mechanical load applied to the squirrel cage induction motor increases.

''. Plot a gra#h of induction motor torque against induction motor s#eed: As the torque increases the speed decreases, an in#erse relationship.

Figure *: SCI Motor Torque 7s SCI Motor S#eed

'-.6rom the graph )figure 4+: The brea%do&n torque  of the squirrel cage induction motor is 3.1(/.m The loc%ed6rotor torque  of the squirrel cage induction motor is approximately 2./.m ;hen comparing the breakdown torque and locked! torque with the nominal torque of the squirrel cage induction motor, the  nominal torque is less than those torques, as breakdown region is not yet reached.

'. Plot a gra#h of squirrel cage induction motor s#eed against squirrel cage induction motor acti7e and reacti7e #o&ers.

Figure 9: SCI Motor S#eed 7s SCI Motor Acti7e and eacti7e Po&ers

The graph does conirm that the squirrel cage induction motor always draws reacti#e power from the ac power source. The graph does conirm that the squirrel cage induction motor draws more electrical power from the ac power source as it dri#es a hea#ier load )as speed and torque are in#ersed+. ;hen the squirrel cage induction motor rotates without load the reacti#e power exceeds the acti#e power re#ealing that the motor &ill still dra& #o&er &ith 8ero load as the rotating magnetic ield is still #resent &ithin the induction motor.

'/. Plot a gra#h of squirrel cage induction motor s#eed against squirrel cage induction motor line current.

Figure ): SCI Motor S#eed 7s SCI Motor Line Current

As the motor s#eed increases the line current decreases, this can be seen by the negati#e slopping line shown abo#e. The relationship is in#ersely proportional. '4.The starting line current is roughly * times greater than the nominal line current. '5.The direction of rotation of the squirrel cage induction motor is counter cloc%&ise when we interchanged the leads connected to the stator windings. 5es the squirrel cage induction motor rotates in the o##osite direction than pre#iously noted in this exercise. e#iew uestions: '. The speed of the rotating magnetic field created by three phase power is called s,nchronous s#eed. -. The difference between the synchronous speed and rotation speed of a squirrel cage induction motor is known as sli#. . eacti#e power is consumed by a squirrel cage induction motor because it requires reacti7e #o&er to create the rotating magnetic ield.

/. *oes the speed of a squirrel cage induction motor increase or decrease when the motor load increasesB It decreases. 4. ;hat happens when two of the three leads supplying power to a squirrel cage induction motor are re#ersedB The motor re7erses its direction o rotation. Conclusion: 7#erall se#eral key characteristics of a squirrel cage induction motor can be stated from this experiment: !

;hen the nominal line #oltage is applied to the stator windings of a squirrel! cage induction motor without mechanical load, the rotor turns at approximately the same speed as the rotating magnetic field )synchronous speed.+

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Interchanging any of the two leads supplying power to the stator windings re#erses the phase sequence and thereby causes the motor to rotate in the opposite direction.

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Additionally as the mechanical load increases so does the motor line current, thus pro#ing that the squirrel cage induction motor requires more electrical power to dri#e hea#ier loads.

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The squirrel cage induction speed in in#ersely related to the squirrel cage induction torque.

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The squirrel cage induction motor draws reacti#e power from the ac source to create its magnetic field

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The starting current for the squirrel cage induction motor is roughly 4 times greater than the nominal line current.