POSSIBLE ERRORS IN MEASUREMENT OF AIR-GAP TORQUE PULSATIONS OF INDUCTION-MOTORS

The torque produced by adjustable speed induction motors fed by inverters inherently contains harmonics. Most mechanical torque sensors have their own natural frequencies and bandwidths. The shaft of a motor can be considered as a spring. Consequently, the measured pulsating torques obtained by using strain gauges coupled to the motor shaft are different from the air-gap torques. This paper discusses the possible errors associated with the air gap torque measurements. The methods based on the direct measurement of the air-gap fluxes are not error proof. The convenient method using motor-terminal data is further studied for the purpose of understanding the sources of its possible errors.