Influence of transient switching currents and fluxes on the torque developed by a squirrel-cage induction motor

Theoretical expressions for the instantaneous torque as a function of time are given in terms of the motor constants, its speed and the conditions of simultaneous stator switching.Calculated torque/time curves are shown superimposed on experimental oscillograrns. The correlation in phase, frequency of pulsation and decrement verifies the theoretical analysis, though there is a discrepancy in amplitude for slips greater than 1.0. This difference also exists between the calculated and experimental steady-state torques.It is shown that the transient components carried by the rotor have a longer duration than those due to the stator. The former therefore predominate and produce torque pulsations at a frequency slightly less than slip frequency, except at standstill when this difference disappears. The frequency departure is governed by the short-circuit constants of the motor, particularly the ratio of leakage reactance to resistance of the windings. The stator-borne transients produce torques which last for about one cycle of the supply for the machine investigated, and have a frequency departing slightly from (1−s)f.The time constant of the rotor-produced transients is a maximum at standstill and decreases symmetrically to approximately 10% of the standstill value for slips of 2.0 and 0. The time constant of the stator-produced transient does not vary by more than 30% for slips from 0 to 2.0. The torque components are tabulated with their corresponding frequencies and amplitude decrements.