Digital Control of an SCR (Thyrister) DC Motor Control Board?

The motor probably already has a PWM controller - more efficient, lighter, and less costly than other types.

You have three options, ditch the controller altogether and add a switching transistor to modulate power using your PWM signal from the computer. (switching transistor plus other parts)

Or you can convert the PWM to a voltage level and use that to set the speed through the original pot input. AND if it is reliable that's what I would do - having seen what happened to a coil winding lathe where the speed control shorted and the out of balance mandrel flew out of it.

Or find some software and D to A board that will output a DC level then use that to directly control a resistive output optical coupler.

This guy shows his circuit for controlling a light using a PWM signal from a controller:

The pwm is converted to a voltage level which is used to adjust the output resistance of an optical coupler which sets the speed (in his case light intensity).

You'd want that sort of isolation to keep from having ground loop problems between the mill and computer.

Usually when you go buy a "90V DC" motor it's a brushed, DC motor with no electronics -- the only silicon will be in the lamination iron.

The OP just said that the pot is floating -- moreover, if it's a typical SCR-controlled circuit there's no isolation from the 120V line, so the experiment of connecting it to the PC will be a short (albeit exciting) one.

This, assuming voltages and resistance levels match, would work.

Place I worked at had several of the (90VDC) controllers and they did have isolated LV transformers and were intended to be run with either a voltage in 0-5 VDC or 10 K potentiometer. These did use PWM and switching supplies similar to computers.

You are quite right about it if he has something with no isolation, essentially a lamp dimmer. Things would smoke if he tried to make a direct connection. SCR controller - should have caught that. SCR implies phase control.

That limits the good choices to an opto isolated control via PWM to DC level to the speed control, or a PWM DC supply from scratch which does sound easy enough.

This is for a mill which may mean it has torque control not just unregulated speed control.

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Look up "digital potentiometer" - several places make them, might do what you want directly, might require a step of opto-isolation somewhere on the way to do what you want.

Thank you all for your advice....

I have attached a link to information regarding this motor drive, which may help in explaining what I am trying to achieve. Page five has the pinout for the pot to the board:

Thanks again..

SRG