What kind of contactor or device do I need to keep a contactor closed until power draw of the primary circuit is complete?

Question

I have an application where I need to signal a start by sending a 120V signal to the contactor, so it will close a 208V 3 pole circuit. The load on the circuit will run for an unspecified amount of time, until its work is complete.

At that time, I want the contactor to open again, so that the load will not restart until signal is sent again. I can have no feedback from the load to send a second pulse, and the time may range from 20 minutes to several hours.

The load will cease consuming power on its own at the end of its task, but I want to disable power back to it until I send a signal back to the contactor.

Any ideas? I though of a latching contactor, but that does not work for my application as I can not send an "I am done" signal to disengage the latch.

Answer 1

Seems like you want a current-sensing relay that enables the contactor. As long as the remote load is drawing current the contactor remains energized. When it drops below the threshold the contactor is released.

In order to start the process you would use another contact in parallel with the current-sensing relay, and only energize it briefly (you could use a time delay relay).