Universal Torque Estimation Applicable to Both Induction and Permanent Magnet Synchronous Motors without Torque and Speed Meters

This paper presents a method to estimate the torque of an in-service three-phase ac motor. This method entails performing a highly accurate air-gap torque estimation calculation that considers iron loss, mechanical loss, and stray load loss from measured voltage and current values. A new approximation formula is proposed to represent the variation of stray load loss depending on fundamental power source frequency, rotational speed and torque, and is devised so that the information of both the rotation speed and the equivalent circuit parameters on the rotation side is unnecessary. This allows the proposed method to universally apply to both induction motors and permanent magnet synchronous motors. The proposed method is implemented on a squirrel cage induction motor and a line-start permanent magnet synchronous motor. The validity of the proposed method is verified by comparing the estimated and measured values of torque under sine-wave, square-wave, and pulse width modulation drives.