Voltage rating versus horsepower of synchronous and induction motors

EVERY industrial plant of any size today has two or three utilization voltages, any one of which is available for supplying motors. The maximum primary distribution voltage within the plant buildings may be any voltage up to 15 kv, the maximum standard being 13.8 kv. Thus a choice must be made as to the most desirable voltage rating for the various motor ratings. Many factors must be considered if an intelligent selection is to be made. Each of these factors will be discussed to the end that the recommended minimum horsepower for each voltage level may be determined. This paper is concerned with the recommended minimum and not with the minimum horsepower at the various voltage ratings that it is possible to manufacture. Stated otherwise, it is not what can be built but what should be built from the standpoint of the best over-all installation for the purchaser. Likewise, just because a motor is listed in the motor manufacturers price handbook, for example a motor rated 50 horsepower (hp), 2,300 volts, does not mean that it is as good a motor from the standpoint of long life and trouble-free service as a similar motor rated 440 volts. It does indicate, however, that such a motor can be built if absolutely necessary. Maximum horsepower ratings at the various voltage ratings will be discussed also. The natural question then is: why do motor manufacturers list such ratings in their handbooks? The answer is self-evident when considering that all motor manufacturers list these ratings. Whether this practice should be continued is beyond the scope of this paper.