Mesa 7i76 spinout + Thyristor DC Driver

Hello,

I have a specific question.
A started to retrofit an Emco Compact 5 cnc lathe.
I use 7i92 ethernet card + 7i76 step/dir card.

The spindle motor on lathe is a Baumüller 180VDC motor at 440W.
The original DC motor driver is a non galvanic isolated (220VAC supply) thyristor driver, with a potentiometer adjust speed reference.
My plan was to use 7i76s galvanic isolated digital potentiometer instead of original potentiometer.

But it does`t work. I tested the 7i76s potentiometer with another +5VDC power supply, and it works fine, but when I connect
with thyristor driver, it does`t work, i cant set the output resistance, and fall the potentiometers resistance.

The driver works fine with his original potentiometer.

But I think the problem come from thyristor driver, because the motor negative pole is connected with potentiometers 1. pin.
The thyristors generated noise is about 150VAC measured to GND.

I think this noise make problem with digital potentiometer.

How Mesa solved the galvanically isolated potentiometer?
Has anyone, suggestion how to solve this problem?

Can I Ground the motors negative pole?

Thanks,
Attila.

Its not likely to be a noise issue and more likely that the potentiometer
power is insufficient to run the 7I76's isolated analog output, or the drive does not
use the potentiometer to control a DC voltage.

If you are sure the drive uses a variable DC voltage to control the speed, you might try an
isolated 12V supply . Beware that on many SCR drives, the potentiometer does not simply
vary a DC voltage.

Can I Ground the motors negative pole?

You can if you like explosions

Thanks

It works, without explosion.