November 2012 Electrical Apparatus

November 2012 Electrical Apparatus

This is a summary of the Electrical Apparatus November 2012 featured technical article,  by Richard L. Nailen, P.E. 

The chief enemy of electrical insulation is moisture. A motor enclosure can protect windings against dripping or windblown moisture, but not against condensation from moisture-laden air. Without free internal air circulation, a tight enclosure worsens the condensation problem within an idle motor.

When not in operation, machines in humid or outdoor environments are therefore commonly equipped with means of adding internal heat to prevent condensation. Such so-called ”space heating” is intended to keep winding temperature at least 5 degrees C above the ambient. Ideally, such heating also serves to prevent condensation in bearings and lubricant.

Heat can be applied in several ways. One method uses separate strip or tubular heaters mounted inside the enclosure. Because heat naturally rises, most heaters are placed in or near the motor base. The location and heater ratings must be chosen carefully so that nearby motor components are not overheated when others farther away are adequately warmed.

A second heating method uses flexible heating elements wrapped directly around stator winding end turns. This warms only those portions of the winding, minimizing the required power. However, it won’t prevent condensation elsehwere within the enclosure. Also, in large motors, such heating elements can obstruct cooling air flow while the motor is running.

In a third warming method, a low d-c or single-phase a-c voltage is applied to part of the idle winding. Because this heats more than just the end turns, it typically requires somewhat more power than flexible heaters, and is best suited to small machines. Interlocking must be provided to disable the heating circuit when the motor winding is energized.

Whatever the method, circuitry is most often arranged to energize heaters automatically whenever the motor is shut down. Heating is not needed in a running motor regardless of load. Heating energy usage can be further reduced by adding a thermostatic control to apply heat only when ambient temperature falls low enough to promote condensation. Because this depends upon relative humidity, it can require careful study.

To order a back issue with the full article, “Keeping idle motors dry” call 312-321-9440 or visit our online webstore.

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