The theory and characteristics of the 3:1 pole-changing induction motor

It is shown that the primary winding of a 3-phase induction motor, having p poles, say, may be reconnected to constitute a 3p-pole winding of two or three symmetrical phase windings, the latter being little known and the former being a new type of pole-changing connection with considerable practical possibilities. Thus, a synchronous speed of one-third the normal value is obtained. It is noted that reconnection as an np-pole winding is possible, where n is any odd integer. It is shown also that the performance characteristics of the motor for the 3p-pole connection can be deduced satisfactorily from the design data of the machine, and that the single machine can be designed to provide characteristics which are comparable with those provided by two separate single-speed machines of standard design. Also¿for the first time so far as the authors are aware¿an attempt is made to predict the performance of an induction motor from design data when the performance is appreciably affected by the presence of pronounced space harmonics of m.m.f. A satisfactory prediction is made possible only when the parameters include the ¿skew leakage reactance¿ component, which appears to have been ignored in the published work of this country. It is an important component of the unsaturated leakage reactance in the case of the harmonics of space m.m.f., and an important component also in the case of the fundamental space m.m.f. of a squirrel-cage induction motor having a full-pitch primary winding, and the customary degree of skew of the rotor bars.