Read motor current using the PXI-7350 controller and MID-7604 drive

[Casey]

I'm using the PXI-7350 controller and MID-7604 drive to run a stepper motor.  I would like to be able to measure motor current while moving the motors.  Is this measurement possible?  If so, what commands are needed to read the motor current?

[Brian_Beal]

Why do you want to read the currents?  Steppers are not like other motors, and the current is really fixed by the drive settings.  If the load increases, the drive won't put out more current to compensate for the load.  If you use a servo motor, the controller command signal is proportional to the motor current if you are running your drive in torque mode.

[Casey]

My major motivation was as some means of determining motor heating.  As the coils heat up they should change resistance and for all other setting being the same (bus voltage, step velocity, microsteps/ step, ...) the current should change.  If some indication of stall were buried in there that would be nice too, but it seems pretty unlikely.

Any ideas if this is possible with this controller and drive?

[Jochen · Accepted solution]

Hi Casey,

the 7604 can limit the current to a user selectable value and you can turn on current reduction if the axis is stopped, but there is no way to read back the motor current. If you want to measure the motor current, you will have to do this with an external device, but due to the current chopping and multi-phase control of stepper motors this is not a trivial task. In micro stepping mode the phase current looks like a rough sine signal with a lot of harmonics, so you will have to do a true RMS measurement which requires quite expensive measurement equipment.

Anyway. If you need to know the motor's temperature you better should measure the temperature directly. The most inexpensive device from NI to do this is the NI USB-TC01, but there are a lot of more options available.

Regards,

Jochen