Electronic – Calculate torque from current induced in a single phase of a 3 phase AC Induction servo motor

accontrolmotorservotorque

Example data from the motor, 2 of 3 phases
I'm looking to measure the torque or slip angle on an AC induction (asynchronous) servo motor given the current induced in one of its phases. The servo and drive is yaskawa. Example data is shown above, the scale is in bit count from and ADC in a microcontroller. The picture shows a rapid movement.

I assume there isn't enough information given to calculate this? I've actually got access to two of the three phases in the servo motor sampled at 1000Hz.

My research so far has shown that torque can only be calculated if given the slip angle (or frequency?). However I've added a load to the servo and can see the average RMS current increase. This RMS value is taken for every 128 samples. Does this increase because the servo drive control loop increases the current to reduce the slip angle?

I've also found a rated motor-torque constant, does this apply for all motor speeds?

Thanks for your time!

Best Answer

The current of an induction motor increases with increased load, but the increase is not linear. An induction motor has a significant current with no load applied. Much of the no-load current is magnetizing current that lags the voltage by 90 degrees. At no-load, there is also a real (in-phase) component of the current due to motor losses.

Since the motor losses are small, the real component of the motor current can provide an estimate of motor torque, but the losses reduce the accuracy of the estimate.

I would expect there is a way to get torque information from the drive in the form of a data display or an analog output signal. I would expect that to be more accurate than any measurement that can easily be done by the user.

Slip speed is proportional to torque and the torque/slip rpm will be fairly constant over a wide range of operating speed and torque. Operating rpm can be measured with a tachometer. Synchronous RPM can be calculated from the frequency of the current. Slip RPM is synchronous RPM minus operating RPM.