# How to predict holding torque for stepper motor?

I have a project where I was going to use a conventional stepper motor and ball screw, but now I want to use SimpleFOC. My question is how to predict holding torque after I change the driver.

My guess is that the zero RPM case is the same for FOC as for a conventional full step driver, assuming both have the same current limit.

Am I right to assume that if the motor is spec’d for 3.0 Newton Meters at full rated current, then the spec is valid independent of the control algorithm?

So I should be able to do a direct controller swap and re-use the same motor.

Hello @ChrisA

Welcome.

Your assumption is correct. The holding torque of a motor is independent of the driver as long as you reach the current at which the holding torque was specified, which is also the max current limit of the motor. As long as the driver can reach that current in a particular configuration, you will achieve the torque.

All that said, the driver’s current is controlled by the algorithm so that’s where you will have to make sure the control could make the driver put out the current in the correct output configuration. Simple swap of the driver must ensure the driver could be driven in the same manner as the old driver, as long as you meet or exceed the current specs.

Cheers,
Valentine