# Calculate torque from current induced in a single phase of a 3 phase AC Induction servo motor 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?

• This probably isn't an AC induction motor, correct? So the concept of "slip" doesn't apply. You could talk about "tourque angle". – John D Jan 29 '19 at 19:56
• 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. – Charles Cowie Jan 29 '19 at 21:46
• 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? – foldone Jan 29 '19 at 21:48