Measurement and computation of starting torque pulsations of salient pole synchronous motors

Five basic methods of synchronous motor torque measurement are described. The first method is the traditional mechanical measurement using an accelerator; the second method involves an electrical technique using search coils as flux sensors; the remaining techniques employ either terminal voltage and current or input power and speed. Measurements of instantaneous electromagnetic torques of a medium-size 25 HP salient pole synchronous motor using these methods are compared with simulation results obtained from a full-order model as well as with a quasi-linear algebraic approximation. It is shown that torque measurement using search coils in the stator windings gives the best correlation with the simulation results, followed by a modified terminal measurement based on terminal voltage and current. The accelerometer method give the poorest results in that it is unable to capture the low-frequency components of the electromagnetic torques. It is also demonstrated by simulation that the approximate method for calculation of pulsating torques based on a quasi-linear model of the motor gives an estimate comparable to that of the full-order model, particularly for large machines