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Summary:
"When (and Why) is Motor Surface Temperature Important?" With
trilingual summary
By
Richard L. Nailen, EA Engineering Editor
Although overheating of electrical machinery is most often considered only as a danger to winding insulation, temperatures of component surfaces (internal and external) are also a concern, for two reasons.
First, in an atmosphere containing combustible gas or dust, high motor surface temperature can cause a fire or explosion. If the motor is explosion-proof, of course, an internal gas explosion will be safely confined. However, many motors for petrochemical service are now of totally enclosed fan-cooled (TEFC) design. During either starting or full-speed running, then, rotor surfaces must remain below the ignition temperature of the atmosphere.
Measuring rotor surface temperature is difficult. Thermocouples, temperature-sensitive paints, and slip comparisons each have disadvantages. Using those and other methods, the IEEE has published Standard 1349 as a guide to safe application of TEFC motors in Division 2 or Zone 2 locations.
In the United States, no specific standards govern external surface temperatures of motors. In Division 2 areas, the National Electrical Code Section 501.8(B) includes only a "Fine Print Note" saying "It is important to consider the temperature of internal and external surfaces that may be exposed to the flammable atmosphere." Although a valid warning, that offers no practical guidance to the motor user.
The second concern with motor surface temperature is its value as a diagnostic indicator of either winding or bearing overheating. Comparison of infrared thermography scans from time to time, and between individual motors, is a useful maintenance procedure. However, comparing single readings to some arbitrary temperature scale is unlikely to be productive. Such readings will be influenced by ambient temperature; by driven machine behavior; and by inaccuracies in the measurements themselves. That is particularly true for motor bearings. Frame surface temperature may also vary from end to end of the machine, with the frame size, and with the manufacturer.
Despite their usefulness, then, surface temperatures of motor and generator components require careful, experienced
interpretation
Copyright 2003, Barks Publications, Inc., Chicago.
Reproduction by any means prohibited.
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