Method for measuring torques of induction motors for the slip ranging from 0 to 2 by detecting twists of stator

There is a negative torque at some slip when the locus of a rotating magnetic field is ellipse in a two-phase induction motor. A method for measuring the torque including the negative value continuously for the slip ranging from 0 to 2 is required for evaluating the trial motors and improving them, but there are few papers on this subject. In this paper the method for resolving this subject is proposed. It is based on the principle that the torque generated from the stator and the restoring torque from the springs of the sensors are balanced, and the twists of the stator which are proportional to the torques are transmitted to the sensors. In addition, a new method for determining the forward-field torque and backward-field torque separately are presented. They are obtained by detecting the fluxes linking with the closed circuit on the rotor, and by substituting the fluxes, resistance and reactance for the formulas. These calculated values agree well with the measured values for the forward-field torque vs. slip characteristics of the motor having the circular rotating magnetic field. Same result is obtained for the backward-field torque. Comparing two type of motors having the wound-rotor and the cast squirrel-cage rotor, it is clear that the ratio of the reactance to the resistance is the key factor to obtain the torque versus slip characteristics which a customer requires