Electrical Apparatus March 2015

Electrical Apparatus March 2015

This is a synopsis of the Electrical Apparatus March 2015 featured technical article,  by Richard L. Nailen, P.E.    Receive this article with your subscription to Electrical Apparatus. To advertise in this issue call 312-321-9440. 

When air-cooling is not an option

Whether of open construction (such as IP21) or totally enclosed (IP55), any electric motor generates internal heat losses that must be dissipated to the environment. The usual coolant is atmospheric air. The amount of air required is typically 2.8 to 3.5 cubic meters per minute. When the air is too hot or too contaminated, or when the air circulation is accompanied by an unacceptably high noise level, the alternative can be cooling with liquid, usually water.

Water is a much more effective coolant than air. Furthermore, either open or totally enclosed motors dependent on air cooling are frequently sources of objectionable noise that is eliminated when water is the coolant. On the other hand, air is available almost everywhere, whereas a suitable water source of controlled flow and temperature may not always be at hand.

An indirect cooling method common for large motors (400 kW and above, depending upon speed) involves an air-to-water heat exchanger mounted within the motor enclosure, through which the rotating parts of the motor circulate internal air that passes over the surfaces of water-cooling tubes to dissipate entrained heat. This cooling method is designated 1C8AW7 by the International Electrotechnical Commission.

Selection of the proper heat exchanger is a complex process involving the developed pressure and flow path resistances throughout the internal air circulation system as well as within the exchanger, and the cooling tube surface area required to dissipate the heat. Two or more heat exchangers may be required for the largest machines, particularly hydroelectric generators, for which this cooling method is widely used.

Many smaller machines use some form of direct cooling, in which the fluid (again, usually water) flows through cooling ducts, pipes, or channels built into the motor enclosure. The flow may take a variety of back-and-forth or spiral paths to entrain as much heat as possible directly from the stator core and bearing brackets.

Output power ratings for direct-cooled machines have ranged as high as 1,500 kW at normal line frequency. For such special applications as electric vehicle service, high power ratings are also available in small frame sizes when the power supply is at high frequency and speeds reach several thousand RPM.

For sizes up to at least 1,750 kW, in deep well pumping service, motors designed for submersion in the well are cooled by the fluid being pumped. They may be water-filled for maximum cooling, but needing special winding insulation, or oil-filled, needing special seals to exclude water but without the risk of contact between water and stator conductors.

The effectiveness of all these cooling methods depends upon designs that control corrosion or contaminant buildup within the liquid circulation system, upon control of coolant temperature, and upon some means of detecting leakage or other failure in the coolant system.

This featured technical article will be published in the March 2015 issue of Electrical Apparatus.  Receive this article with your subscription to Electrical Apparatus. To reserve advertising space in this issue, call Kristine Weller, 312-626-2316 direct line.  Ads close February 15; space is sometimes available after the closing date.  

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