3-phase pump on/off detection

On one of my CNC mills I'd like to make a circuit that can detect when the coolant pump (3-phase, 200v) turns on so that I can turn a solenoid valve on/off (basically the solenoid valve will control the air that goes to the spindle's positive pressure "seal"). The circuit will also do a few other things which I would prefer to do with a microcontroller, so I need to be able to detect when the pump turns on.

One way to detect if the pump has turned on is to monitor the input to the solid state relay that turns the pump on/off. Assuming I can't easily get access to that signal, another way might be to monitor the actually wires going to the pump.

My first idea was to monitor one of the phases and use a diode bridge to rectify the AC signal I'd get. Then I'd use a capacitor and resistor voltage divider to get the voltage output to be something compatible w/an optocoupler (I think this would be on the safe side, maybe not necessary). This all seems like it'd work, but I'm only about 70% confident.

Am I on the right path here? Any other suggestions? I don't want to be reinventing the wheel, especially if there's a component I can just buy off the shelf for this purpose.

Cheers,
Matt

A pair of current transformers (or hall sensors) on any two phases will tell you when it is running.

You do require at least two phase monitoring.

Does a light go on anywhere, so you could detect that?

I wanted to detect when the refrigerator's cooling cycle started. I was unable to get at the internal motor. I made a custom extension cord, with split wires. I wrapped one wire a couple turns around a small transformer. Whenever the unit drew large current, the transformer provided sufficient voltage to barely light an led. I detected that.

I believe it's worth a try to place an inductor near the motor. Just being nearby, it may pick up sufficient changing magnetic flux, to generate a slight AC signal.

Brad:
You are right....thinking "outside the box" there are a number of alternatives. A cheap electret microphone attached to the motor's case will easily detect the rumble of the motor running.

I believe an automation practioner would use an auxilary contactor connected in parallel to the motor. But yes, there are many other ways.

You could use a non-invasive current sensor like this one:

https://www.seeedstudio.com/depot/Noninvasive-AC-Current-Sensor-50A-max-p-2056.html?cPath=25_28

You can mount in around any current carrying wire (live/neutral) without any electrical intervention. There are sensors for different current ranges (i.e. 0 - 5A, 0 - 100A). Choose a suitable one, depending on your application power rating.