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UMD ENEE 473 - Experiment #6: Three Phase Induction Motor

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1ELECTRICAL ENGINEERING DEPARTMENT UNIVERSITY OF MARYLAND COLLEGE PARK, MARYLAND 20742 Prof. Fawzi P. Emad e-mail: [email protected] Office: AVW-1429 FAX: 301-314-9281 Tel: 301-405-3640 Experiment #6: Three Phase Induction Motor The three phase induction motor (IM) is very similar to the three phase transformer in ex-periment #3. Besides transformer action, the IM has motion. If the IM were restrained so it would not rotate, then it would be similar to a transformer. If there are no connections to the rotor, then the rotor is shorted on itself and the IM (blocked rotor) is similar to a short circuited transformer, the short being on the secondary side. As discussed in the lecture, the model of the IM (for each phase) is similar to that of the transformer except that motion is now included via the slip parameter s. Since there are three phases, the torque of the motor is three times the torque for each phase, similarly for the shaft power, it will be three times that found per phase. The slip parameter is defined as s n n ns s=−( )/ , where ns is the synchronous speed of the shaft in rpm (revolutions per minute) and n is the actual ma-chine speed in rpm (usually, s is a positive number, i.e. n ns<). The single phase IM is one of the most common types of electrical motors in the world to-day. If time permits, we shall study it in a future experiment. The three phase IM is also one of the most commonly used three phase motors. The reason is the rugged construction of the motor and the absence of brushes. The figure above illustrates the per phase equivalent circuit of the IM. On the rotor side, the total resistance is R RssRs2 221+−= . This Rs2 is broken into two parts (as shown above) so the equivalent has a symmetry similar to the T-model of a transformer. If the ro-tor speed is ns, (n ns=), then the slip s is equal to zero, thus Rs becomes infinite (open circuit secondary) and the IM is similar to the transformer T-model. Like in the analysis of the transformer, we will move the rotor parameters to the stator side thus we get the sim-plified T-model as shown below. It is noted that we do not know the turns ratio, thus in-2xfrcI1I2ImV1V2++--Simplified approximate model.reqxeqR2(1-s)/s'''stead of a R22 on the stator side, we use ′R2. It is also noted that the shunt branch was moved to the input for added simplicity. Instead of using I a2/ , we use ′I2. Similarly for ′X2 and ′V2. In the figure below, x X Xeq=+′1 2 and r R Req=+′1 2. Keep in mind, this is the per phase equivalent. If the slip s is zero (i.e. the motor is at synchronous speed ns) then the vari-able load resistor is open. Thus, the OC test is at synchronous speed. On the other hand, if s=1, then the variable resistor is zero (i.e. short circuited). Clearly, s=1 is equiva-lent to zero speed, i.e. locked rotor produces the SC test. Please take a moment to convince yourself that both the OC test and the SC test are “no load” tests. Both take very little electrical power. Thus, be careful to apply a small volt-age in the SC test because the current will be large. Thus, to find the parameters of the simplified model we will need to perform the two tests which were performed on a transformer earlier. These are the SC test (locked rotor) and the OC test (rotor at synchronous speed). Locking the rotor is easy to do: simply insert a pin in a flange on the shaft which will immobilize it. To run at synchronous speed, use a synchronous motor (studied earlier) to drive the IM at ωs. Your bench will be already set up for you with three machines coupled mechanically: On the left will be the IM, in the center the DC Dynamometer and on the right the SM. The pin hole for locking the rotor is at the flange near the DC machine in the center. Experiment Outline (OC and SC tests): 1. Connect the IM to a three-phase supply (do not include the neutral) and place instru-ments to measure line current and line to line power and voltage. The reason we do not include the neutral is that we do not know for sure if the windings are Y or ∆ or some hybrid combination. We will find the equivalent Y per phase. Measuring line to line will give twice the per-phase Y parameters. Line to line power multiplied by 2/3 will give the per-phase Y power (why?). 2. Lock the rotor. Ask the instructor for permission to do this. 3. Be careful, this is the locked rotor test (SC test) and high currents may be present. Be sure to take as short a time as possible in making this measurement. 4. Check your wiring, and increase the source voltage slowly till rated current is reached (what is the value of this full rated current?). Take a reading of all the instruments. 5. Power down. Remove the pin so the shaft is free to rotate. This completes the SC test. 6. Make sure the locking pin has been removed so the IM is free to rotate. 7. Increase the power to the IM slowly, just to get the machine to rotate. Note the direc-tion of rotation.38. Power down and turn the power OFF. 9. Identify (mark) the supply connections to the IM (you will need to connect it exactly the same way later on). Disconnect the IM from the three-phase supply. Connect the three-phase power supply (same source used for the IM) to the stator of the SM. 10. GET PERMISSION OF THE INSTRUCTOR TO GO TO THE NEXT STEP. 11. Increase the supply voltage on the SM till it just begins to rotate. Note the direction of rotation. If it is not in the same direction found in step 7, two of the supply lines to the SM need to be interchanged as directed in step 12 below. 12. Power down and switch all power OFF. If the SM rotation is the same as that in step 7, continue to step 13 below. IF NOT, reverse any two lines of the supply lines to the SM. 13. Connect the three-phase supply to the IM in the same manner as they were identified before (in step 9) so the rotation will stay in the same direction. 14. Connect a DC supply to the field of the SM. Make sure all supplies are set down to zero. 15. Check your wiring. Switch the power on (AC and DC). Increase the AC power slowly. The machines should rotate in the same direction as in step 7, and they should run quietly. If there is any noise, STOP immediately and power down. 16. Continue to increase the AC power slowly till the full rated voltage of the IM is ap-plied (do not exceed 120V). Now the machines should be running close to ωs. 17. Slowly increase

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