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ECE 304
Electrical M achines Lab
Experiment 3

INDUCTION MACHINE CHARACTERISTICS
V 1.3 , Spring 2009

Objectives 1) To measure the torque, current, and power factor characteristics of the induction machine for rated-voltage, rated-frequency operation. 2) To observe the line start-up inrush dynamics. 3) To determine the effectsof reduced voltage operation. Lecture Comments: How does an ac induction machine (IM) work? What is inrush current? How does reduced voltage starting provide soft-start? What is slip, and how do the IM operating characteristics vary as a function of the slip?

230 Vrms (line-to-line) 133 Vrms (line-to-neutral) R I1 1 60Hz

jX1

jX2

I2 R2 (1-slip) R 2 slip

V1

+ -

Stator

ImagjXm

Rotor

R2 slip

slip = 0 to1.0 !e - !r slip = !e RPMe - RPMr = RPMe 120 * Freq RPMe = # Poles

Fig. 1

Single-phase equivalent circuit model. Note: slip = 1.0 (100%) initially when starting from standstill

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pg. 2

ECE 304 Lab 3

A) No-Load Reduced-Voltage Starting (Fixed excitation frequency = 60 Hz) In this part of the lab an inverter (drive) will be used tostart the induction machine at reduced voltages (i.e., less than rated voltage, 230 VRMS line-to-line). Start-up transients (current & rotor speed) will be observed on the scope. Finite element analysis has been performed in order to give you the opportunity to observe the magnetic flux density and ohmic losses inside the machine during the start-up transients. More information about these finiteelement calculations is provided in the Appendix at the end of this document. Set up a) Energize the test stand by turning on the disconnect switch located at the end of the bench. b) Turn on the bench computer and login as ECE304. c) Open the file lab3 and wait for the code to initialize the bench. d) We will excite the machine with varying levels of voltage from the drive, from 25% to 50% of Vrated(60 Hz) in 5% increments. Procedure 1) Enter the desired voltage to be applied to the IM as % of rated voltage, beginning with 25%. 2) Click Start Drive. 3) Observe the machine cross-sections on your display illustrating the calculated magnetic flux density and loss distributions inside the machine during the start-up transient. 4) Wait for the start-up transient to subside. 5) Click Stop Drive.6) Repeat Steps 1-5 until data for 115 VRMS (50% of rated voltage) has been observed. 7) Save and verify the Current vs. Speed, Torque vs. Speed, Speed vs. Time, and Current vs. Time plots for each of the reduced-voltage starting tests. 8) Verify that the files have been saved. Explain plots and features of reduced-voltage starting to the lab TA. Click Go to Part 2 of This Experiment

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ECE 304 Lab 3

B) Constant Voltage and Frequency Operation with Load Variation In this part of Lab 3, the induction machine will be run at constant voltage, and the surface permanent magnet (SPM) will provide the appropriate torque load.
Torque

Fixed V and Freq Operation

Freq = Excitation Freq. [Hz]

!e = 2 * " * Freq [elec rad/sec] !rm = rpm * 2 * "
60 [mech rad / sec] [elecrad / sec]

Speed

!r = Pole # * !rm
2

!r !e

* 2 * 60 [rpm] slip speed = (!e - !r) # Poles 2*" slip =

!e - !r !e

[no units]

Fig 2 Torque-speed relationship for 3-phase IM with fixed voltage and frequency excitation. Procedure 1) Click Start Drive 2) Set IM voltage to 230 VRMS (rated voltage) 3) Adjust the SPM-machine’s torque command over a wide range from –5 N-m to +5 N-m inincrements of 2 N-m. Measure encoder speed, torque (−DAQ Torque), and stator current for both motoring and generating operation of the IM. Notes: a) The torque reading is displayed for the SPM machine, not for the IM. As a result, the polarity of the displayed torque reading will be opposite from that of the IM. b) Shaft speeds below the zero-torque synchronous speed ( = 1800 rpm for 60 Hz...