Can VFD increase torque

Can I drive a three-phase induction motor using this method?

Hobbyist

Over the past 25 years there has been a tremendous amount of evolution and technology invested in controlling 3-phase frequency converters. Discussing details of how to do this with a new method or better is beyond the scope of such a forum, but would be a good basis for a PhD thesis.

A good starting point for your study is to control the vector mode. This is usually one of three standard modes a standard VFD drive can run in.

Constant torque is a standard mode in most VFDs - BUT when you are talking about a 50 or 60 Hz motor - why go for 100 Hz unless you had a reduction in torque demand in your 60 Hz range up to 100 Hz. In my experience, every 3-phase motor with a base speed of 60 Hz has a reduction in torque for speeds above 60 Hz.

You should also read and study the pull-up torque curve which has been a staple of engine theory and engine manuals for 75 years. When you know exactly how vector control modes manipulate this torque curve at different speeds, you have a foundation to start with.

Alper91

Thanks for the answer. But I sense that you are implying that this is not going to work. I actually believe I understood the vector control method and I can see that the inverter in the FOC is getting a response regarding a change in slip. It does this by controlling both frequency and amplitude, but in the FOC the amplitude control is very limited while the frequency control has a wide range. I have a common answer from everyone that even this method works and thermally fails. I want to understand why It seems that the only obstacle to the method is thermal management. And I still haven't got it.

Hobbyist

I am not going into the thermal warmth problem. I doubt your ability to get any significant constant torque at speeds greater than 60Hz. I have set up large horsepower motors to operate in the 60 to 80 Hz range, and these applications always start with the understanding that speeds in these ranges require less torque than is rated for the motor. Can you run a test to graph torque capability in speed ranges above 60 Hz? If the torque in the graph drops at 60-100 Hz, your mode would not be constant torque.

Charles Cowie

The speed of an induction motor can only be controlled by changing the synchronous speed by changing the frequency of the power supplied or by changing the motor slip. The method you described is a means of changing the motor slip. Most of the heat generated in an induction motor is heat generated due to slippage in the rotor. The heat generated in the rotor is the percentage slip multiplied by the power transferred from the stator to the rotor. This is why the speed control with this method has limited uses, mainly for controlling the speed of small fans.

This is illustrated and explained in my answer to: This question

Alper91

Thanks for the answer. But in order to increase the slip, I also have to reduce the torque, i.e. the phase current, i.e. the power transmission to the rotor. Doesn't it compensate for the heat output?

Charles Cowie

See link added to my answer. It depends on how much power the load needs.

Autistic

@ charles cowie If it's a car you need good torque unless you want a clutch to slip through. Otherwise, the car wouldn't peel the skin off a three-day-old rice pudding. Are we all pharesies and are we missing something?

Charles Cowie

@Autistic Yes. Controlling induction motors by increasing the slip is not good for loads that require low speed torque. The only such loads are fans and centrifugal pumps.