I need to monitor a piece of 3ph equipment and detect (via microcontroller) when the motor is running. I do not need to measure the current - I just need to close an unpowered circuit.

I need to minimize invasiveness on the machinery. I've found plenty of fancy split core current sensing switches but none are inexpensive. In my ignorance, it seems like a loop in one of the hot leads would be enough to use a hall effect sensor or reed switch ... then I could debounce the data programatically and get away for >$5/ea rather than $100+.

Why would this not work? any details in the explanation will be appreciated - I'm pretty ignorant, but trying to learn - Thanks!

 

3 phase implies industrial, and in most industrial applications, sensing a motor 'run' condition is done through the starter aux contacts. It's a two state question, is it running or not.

Current sensing is typically engaged to protect the motor from overload/currents or to measure torque.

If you need a definitive answer to 'is the motor running', then you might want to measure the results, ie; hydraulic pressure built, mechanical motion incurred, etc.

 

Why would this not work? any details in the explanation will be appreciated - I'm pretty ignorant, but trying to learn - Thanks!

I think it would work just fine. The principle seems sound and I believe that people often use Hall-effect current sensors to measure 3-phase currents in motors and generators. The accuracy, while not ideal, is usually sufficient to allow operational current feedback control.

Unfortunately, I'm not up on the exact devices that are appropriate for your application, but I have used Allegro Hall sensors to measure magnetic fields. They seem to be a good company and perhaps a call into their technical support will give you better information.

 

As you suspected, it is an industrial application - in this case it needs to be as non-invasive as possible (for reasons of liability and convenience). I need to get the information without wiring anything to the machine.

It sounds like a hall effect sensor might work ANY comments or alternatives welcome.

Jor