A physical conception of the operation of the single-phase induction motor

This paper covers a method of studying the actions of the single-phase induction motor, which the writer has found to be very convenient from the educational standpoint. It is based upon the assumption of two equal and oppositely rotating primary magnetomotive forces combined with a synchronously rotating secondary m. m. f., such as would be produced by direct current excitation. It follows that there is a resultant rotating primary field just as in the polyphase motor, while in the secondary there are two currents, one of low frequency, corresponding to the polyphase motor, and the other of nearly double the primary frequency. Diagrams and descriptions are given to illustrate the magnetomotive forces and fluxes, showing how, among other conditions, two oppositely rotating fields of unequal value may be possible. The next step is a consideration of e. m. f. generation by two oppositely rotating fields, showing how both must be taken into account. The effects upon the counter e. m. f. and excitation, of the reduction or suppression of one field is shown. This illustrates, in a simple manner, why the excitation on single-phase must be practically the same as on polyphase at full speed and falls to one-half value at standstill. The full-load conditions are next considered. A comparison is made between a two-motor unit, consisting of two similar polyphase motors coupled together and connected for opposite rotation, and the straight single-phase induction motor. Various discrepancies are pointed out between the resultant action of the two-motor unit and the single-phase. Modifying conditions are then taken into account which remove the discrepancies. This is followed by a considerable amount of test data which illustrate the principles and actions described in the paper.