April 2014 Electrical Apparatu

April 2014 Electrical Apparatus

This is a summary of the Electrical Apparatus April 2014 featured technical article,  by Richard L. Nailen, P.E.    

Whereas a-c motor performance such as full-load current, efficiency, or power factor can be accurately calculated, winding temperature rise is more difficult to predict. It is always of concern to motor users. Overheating is universally considered a major cause of failure. The smaller machines can readily be tested by direct-loading, in which the motor supplies rated power to a coupled dynamometer. This becomes impractical for many large motors because a suitable dynamometer may not be available, and even if it is, the power cost and utility limitations may preclude full-load testing.

Consequently, several methods have been developed over the past century to closely approximate full-load temperature in motors running at full speed without a connected shaft load. One of the most common is the dual-frequency test described in IEEE Standard 112. The motor is connected to two power sources supplying frequencies 15% to 10% apart (such as 50 and 60 Hz), causing the motor to alternate rapidly between induction generator and motor operation. This generates torsional vibrations that can be troublesome during the testing of vertical motors. The power required during the test supplies only motor losses, which closely approximate their full-load values.

Electronic power supplies, as well as motor-generator sets, have been used to create the two frequencies. In the so-called “sweep frequency” procedure, microprocessor control modulates a single applied frequency to produce the rapid motor-generator operation.

Some other methods involve a series of three or four tests in which applied voltages are well below or above the motor rating, so that one temperature rise can be measured when heating is almost entirely from core loss, another when core loss is insignificant, and the two can be combined to closely approximate full-load temperature. One of the simpler variations is useful when a dynamometer load is available but of insufficient capacity for a full-load test.

A three-step test based on IEC Standard 60034-29 combines rated and reduced-voltage tests at rated frequency with a dual-frequency test at reduced voltage, yielding three different temperature rises that are combined to give the estimated full-load value.

Most of these tests are accurate within 5 degrees C. But they cannot be expected to measure efficiency, vibration, or sound level that would be observed in normal operation.

To order a back issue with the full article, “Determining full-load motor temperature from no-load tests” call 312-321-9440 or visit our online webstore.

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