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!

  • \$\begingroup\$ This probably isn't an AC induction motor, correct? So the concept of "slip" doesn't apply. You could talk about "tourque angle". \$\endgroup\$
    – John D
    Jan 29 '19 at 19:56
  • \$\begingroup\$ An induction motor has a slip speed (RPM or radians per second). It does not have a slip angle. It has a torque angle, but that is difficult to determine. A synchronous motor has a torque angle and no slip. If you can revise your question to provide the information on the rating plate of the motor, it may be possible to answer the question. At minimum, you need to determine if the motor is a synchronous or induction motor. As it stands, the question is unclear and I am voting to close it. \$\endgroup\$ Jan 29 '19 at 21:46
  • \$\begingroup\$ Edited to clarify it's an asynchronous induction motor/ If there's a slip speed I assume that is controlled to some degree by the servo drive, ie increase current proportional to the load and this is measurable? \$\endgroup\$
    – foldone
    Jan 29 '19 at 21:48

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.


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