PhaseQuest OMPQ-50 RPC blowing start capacitors

Mark L Tucker · Jan 31, 2018

Hi
I have had a PhaseQuest OMPQ-50 RPC wired in my shop and for the last 6 months it has been doing its job great but 2 weeks ago it was idling while I was setting up the next job I heard a crackling noise from inside the panel, then the idler motor bogged down and within a couple of seconds it blew the end off of one of the start capacitors. I have has an electrician out 2 times now and each time we think its right it seems to overload the start capacitors.
I'll list everything that's happened and been done so far;
1. start capacitor blew and I noticed a wire on the run capacitors was completely burnt out and not making a connection. Changed the start capacitor, fixed any damaged wires and re-crimped any loose connection
2. started the RPC, the idler motor was slower to spin up and before it got up to speed it blew the start capacitors again. At this point I figured it would be a god time to call the electrician in.
3. the electrician checked the unit and found that I had 7 blown start capacitors and the contactor was welded shut. He checked the run capacitors and they all tested within their operating range. He also checked and changed out some wiring and tested for resistance between the phases on the idler motor.
4. this time when we started the unit the idler motor spun as it should have but within 2 seconds it
let out a bang and lit up the inside of the panel (Im sure where the start capacitors are). We opened the cover and found that 3 of the start capacitors had been weakened and were now testing at the low end of their rage.
5. at this point the electrician figured it must be the starter relay and ordered a new one up. He fitted that and just to be sure there wasn't an issue after the phase converter we disconnected the lines going to the transformer. When we started it the same happened where within 2 seconds it let out a bang and a flash of light.
At this point the electrician is also scratching his head and so I was hoping someone on here may be able to offer some guidance.
Could the flash and bang have come from the start capacitors discharging as there was zero load with the transformer being disconnected?
Although I know the electrician was very careful is it possible that some wires have been crossed?
I should also say that when it flashes and bangs I immediately shut it down and so I don't know if it would successfully switch on to run capacitors and continue to run.

Any advice would be greatly appreciated

Thanks in advance
Mark

Vovan · Apr 6, 2018

Mark L Tucker said:
Hi
I have had a PhaseQuest OMPQ-50 RPC wired in my shop and for the last 6 months it has been doing its job great but 2 weeks ago it was idling while I was setting up the next job I heard a crackling noise from inside the panel, then the idler motor bogged down and within a couple of seconds it blew the end off of one of the start capacitors. I have has an electrician out 2 times now and each time we think its right it seems to overload the start capacitors.
I'll list everything that's happened and been done so far;
1. start capacitor blew and I noticed a wire on the run capacitors was completely burnt out and not making a connection. Changed the start capacitor, fixed any damaged wires and re-crimped any loose connection
2. started the RPC, the idler motor was slower to spin up and before it got up to speed it blew the start capacitors again. At this point I figured it would be a god time to call the electrician in.
3. the electrician checked the unit and found that I had 7 blown start capacitors and the contactor was welded shut. He checked the run capacitors and they all tested within their operating range. He also checked and changed out some wiring and tested for resistance between the phases on the idler motor.
4. this time when we started the unit the idler motor spun as it should have but within 2 seconds it
let out a bang and lit up the inside of the panel (Im sure where the start capacitors are). We opened the cover and found that 3 of the start capacitors had been weakened and were now testing at the low end of their rage.
5. at this point the electrician figured it must be the starter relay and ordered a new one up. He fitted that and just to be sure there wasn't an issue after the phase converter we disconnected the lines going to the transformer. When we started it the same happened where within 2 seconds it let out a bang and a flash of light.
At this point the electrician is also scratching his head and so I was hoping someone on here may be able to offer some guidance.
Could the flash and bang have come from the start capacitors discharging as there was zero load with the transformer being disconnected?
Although I know the electrician was very careful is it possible that some wires have been crossed?
I should also say that when it flashes and bangs I immediately shut it down and so I don't know if it would successfully switch on to run capacitors and continue to run.

Any advice would be greatly appreciated

Thanks in advance
Mark

Good morning Mark.
I was going to get 20hp rpc from phase quest, but found your post.
On their website rpc units have 1 year warranty.
So what do they say about your problem?
Volodymyr

Sent from my SM-G935W8 using Tapatalk

JST · Apr 6, 2018

If the thing was idling, then the start capacitors SHOULD HAVE BEEN OUT OF CIRCUIT.

It appears from what you way, that the start caps got connected back into the circuit, and that WILL blow them rather quickly. Presumably the start relay (voltage sensitive relay) closed.

The voltage sensitive relay may be poorly adjusted so it can close on a slightly low line voltage, or the RPC output on the generated leg dropped for some reason, which would allow the relay to close again.

Could even be a fault in the control system, if it is more complex than a regular voltage sensitive relay.

rons · Apr 6, 2018

The lower left has an identified module. There is also an identified module to the right of the start relay. Identify those. Better to know the entire design before going further.

JST · Apr 6, 2018

To the right of the start relay looks an awful lot like a voltage relay, that may be operating the contactor next to it (presumably the start contactor). Of course it might be something else, there is a hint of something next to it.

The other one looks like a relay also, type or use unknown, it has a pilot light off of it that is on the side panel. (there are also terminal blocks that look similar to that).

I really like the tilted capacitor, and the terminal block at top right that appears to be hanging there at an angle.

rons · Apr 6, 2018

That's what I was thinking about the block to the right of the capacitor start relay. A potential relay? I wouldn't doubt that it is questionable and can be the cause of the problem.

The wire gauge (blue wires) looks pretty small to me. And it would be nice if there were bleed resistors.

JST · Apr 7, 2018

yeah, the wire is thin, but probably will stand the current quite a bit longer than the capacitors can..... it's short term duty. There are some pretty fat wires elsewhere that seem to be for the motor itself.

I don't see a bleed resistor anywhere in the box.... good point.

Hopefuldave · Apr 7, 2018

As RonS says, the potential / current relay could be the culprit, if it's not operating and taking the start cap's out of circuit it'll definitely blow them, they're not rated for continuous duty. I've had issues with them on compressors etc. and they're a popular replacement part on air conditioners, fridges and freezers, which is why my RPC needs a fat finger on and off a button to start

If one of the start cap's (left in circuit when it shouldn't have been) went short before its death throes it would have put that short across two of the phases and that would definitely have the motor bogging down...

As a quick bodge/test, disconnect the potential/current relay and wire in a push-button to control the start contactor and try starting manually, if all spins up and runs it points to that being the cause?

Dave H. (the other one)

JST · Apr 7, 2018

The relay is likely running the contactor next to it. In that case it has fairly easy duty, and should not stick.

It could have failed.

There could be an un noticed loose wire. The potential relay is normally closed, and opens when the voltage rises above its setting.

a bad relay will not open.

A loose wire may not apply any volts to the relay, so it cannot open.

The contactor seems to be under rated, if it welds shut. Did it weld again?

rons · Apr 7, 2018

Part of the design appears to be under-built. Not a good thing if your main intention is to have a lasting relationship with it. It could be upgraded with not much work.

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