Polyphase Induction Motors A Labor Saving Method of Calculating Performance from Previously Determined Constants

This paper presents a method of calculating polyphase induction motor performance which eliminates a large amount of detail work without making use of approximations which sacrifice accuracy. In any rigorous system for calculating induction motor performance, the determination of the relation between input and current values makes the greater part of the work. By the procedure here presented, the relation of watts to amperes for all cases likely to be encountered in ordinary design work may be accurately calculated once for all and recorded in a set of curves. When making practical calculations tedious detail work may be eliminated by taking the necessary values from the curves, in much the same way that sines are taken from sine tables. The calculation begins with the torque, from which the corresponding secondary input is obtained by a simple formula. Then, by reference to the appropriate curves and a few simple slide rule operations, the primary and secondary currents are determined. With the secondary input and the current values known, the completion of the calculation requires nothing more than a few operations of simple arithmetic. The entire process for one load point may be completed in from five to seven minutes.