A simple method for detecting torque of an induction motor with a synchronous rectifier circuit

There are many torque measuring methods for an induction motor. Some applications require a simple and easy method to detect torque change rather than a precise method to measure the torque of the motor. The torque of an induction motor is roughly estimated by the input current. This is the simplest method of measuring torque. T he input current of the induction motor is the sum of the torque current and the exciting current. The torque current is proportional to the synchronous wattage and the motor torque. The exciting current has a phase lag of almost 90 degrees behind the torque current, is variable with the input voltage, and is larger than the torque current, especially at a light load. Therefore, it is difficult to find the motor torque from the input current of the motor at a light load. In this paper, we propose a simple and easy torque measuring method with a synchronous rectifier circuit. The synchronous rectifier circuit can find the vector component at the detection angle from the input current of the induction motor and can detect the torque current in the input current of the motor. By adjusting the detecting angle of the synchronous rectifier, the rectified signal is mixed with the torque current and the exciting current in proper proportions and compensated for the fluctuation of the input voltage. This serves as a simple and easy torque measuring method suitable for trouble detection at loads such as in life testing. (C) 1999 Scripta Technica, Electr Eng Jpn, 126(3): 83-90, 1999.