Potential Relay Sticking

Hi there,
I built myself a rotary phase converter for powering my workshop. I’m UK based, so the rotary converter runs off 240v Single phase live and neutral. I run L1 to U, Neutral to V, and caps to W. Once started I use a generator change over switch and it moves the neutral to the star point giving me 440v P-P. The start caps originally ran off the a momentary push button, but because I wanted to automate it I bought a potential relay, normally closed and then when the volts on the generated phase build up it opens the switch and brings them out of circuit. The circuit will work once, then when the converter is turned off, the relay will close as the volts drop and you can hear the caps discharge back into the windings and it almost slows the motor. Once it does this, the switch is welded shut and the potential relay won’t actuate with the magnet and it requires a screwdriver to break it free. Any ideas why this is happening?
Thanks

Henruu said:

Hi there,
................. because I wanted to automate it I bought a potential relay, normally closed and then when the volts on the generated phase build up it opens the switch and brings them out of circuit. The circuit will work once, then when the converter is turned off, the relay will close as the volts drop and you can hear the caps discharge back into the windings and it almost slows the motor. ..............
Thanks

Click to expand...

I am making a number of assumptions about how you have it set up, but........

There is a very high current when the caps are nearly shorted by the relay. That probably is quite a bit higher than the relay rating. Also possible that there is some "circulating current" as the motor slows down.

You can use the relay to trigger a contactor which IS rated for that current, for instance.

Or wire it so that the caps are connect via a spare pole on your main power contactor, so that the caps aren't connected unless the mains is also connected.

My Start caps are wired to the change over switch, so when I change it from start to run, the live that feeds the caps is completely bought out of the circuit, however there is the feed from L3 going back to the caps which is used to start it. I can’t quite get my head around it, but it keeps welding the relay shut.

Could you provide a schematic of your setup.

Do you have a resistor on each to discharge any stored charge?

Is the relay rated for the capacitive connection to begin with? *

Is the relay rated for the final voltage? **

It does not have to be an issue of stored charge, it can (and easily may be) an issue of the voltage still being generated by the motor that induces a high current when the relay re-closes.

Relays that weld are generally being used outside their ratings, current, voltage, or both. But, using a potential relay for starting is not an unusual method of running the RPC. With a higher power setup, it is common to use a higher rated contactor for the capacitors instead of just the potential relay.

* When connecting capacitors relays need to be rated for capacitors, and for the AMOUNT of capacitance involved. That is one of the 30 or more different rating types that european relays may have. AC-6b is the IEC "utilization category" for switching capacitor banks. An IEC relay would need a suitable rating of that category to be suitable for switching capacitors.

** The usual US potential relay is rated for the purpose of switching start capacitors of a certain size, and works at 240V. It is generally used for refrigeration/air conditioning, to start single phase motors. Yours would be similar. There would be no reason for it to be rated for 415V, since that is a 3 phase voltage. So it may not be rated for the actual voltage it sees in your setup.