Faulty capacitors and potential relay

[Pykester1]

Hello all.
I am having repeated capacitor failures on a multi deck system and I am running out of options. The condensing unit in question has a 2HP, single phase Prestcold reciprocating compressor with 2 x 10 MF capacitors, 1 x 150 MF capacitor and 1 x 70 MF capacitors plus one APR5 adjustable potential relay. The compressor failed a few months ago after many capacitor and potential relay changes and a fully reconditioned compressor of the same type was fitted. Since then it ran fine for three months and has recently suddenly started killing capacitors and potential relays again. I have replaced just about every thing I can think of in the control circuit; contactor, overload, anti short cycle timer, LP/HP switch. Once I change the faulty capacitors and relay it runs fine again for a week or two. The system ran fine for many years with the old compressor before it failed. I realise that there has always been issues with these single phase compressors and with the Copeland 2HP compressors, but I would be very grateful if any one has any ideas.
Cheers.

[multisync]

Sounds like low voltage..?

[Magoo]

I agree with Multisync that it sounds like a voltage versus current problem.
Seems like a strange arrangement of varying capacitor ratings. Are they paralelled or seriesed connected.
As an aside I came across a 2 HP single phase compressor with a soft start black box and a large run capacitor. Started smooth and easy, low inrush. Maybe concider up-grading starter functions on compressors.
magoo

[Yuri B.]

My opinion would be your system had shown the first sign of some its illness "few monthes ago", but overcame it then and now the ailment is in full swing.

Maybe it is that simple as, for instance, oil become viscous and is adding load on crankshaft ? Cylinders walls not smooth? Or the pressures every time have not been equalized properly preceading the starting moment?

[Pykester1]

Capacitors are in parallel. Unit starts fine but does run about a couple of amps over name plate FLA during pull down and runs at about one amp over name plate when down to temperature. Compressor sounds very slightly rattly. But only just. Compressor is reconditioned and only a couple of months old.

[Pykester1]

Voltage is fine.

[Brian_UK]

If motor is running above nameplate amperage then you have a problem that needs resolving before you lose another compressor.

[tonyhavcr]

normally I would think cpr for high amps
@ pull down but I started to think how much work is this system doing, my goodness does it have a lot of caps did you check with the compressor oem on the mf and the Voltage of the Capacitors needed for the compressor.

I know you say the Voltage is fine
did you record it over a week.

and what is your sub cooling and superheat I bet its high.

[Pykester1]

Did not record voltage for any length of time.

[Peter_1]

Why you use single phase? We always use for this a 220VAC 2 phase to 3 phase frequency inverter. This is not that expensive and you can use a normal 3 phase compressor.

[tonyelian]

hi
with potential relay capacitors never be in parallel that explain why ur loosing start cap
can u draw a wiring diagram so we can help

[Gary]

Compressor manufacturers do not use adjustable potential relays, multiple start capacitors, or multiple run capacitors.

Potential relays are matched to the motor. You MUST use the right potential relay, the right start capacitor, and the right run capacitor.

[Frikkie]

I agree with Gary. The only reason you get capacitors in parallel is if the right size is no available and you use 2 x 20mf in parallel to make a 40mf equivalent for example. Did you check the discharge voltage rating on your capacitors is what it should be by the manufacturer information. If this is okay then most likely current is too high or starting is too slow and too long or even rapid cycles of starting and too many starts per hour.

[Pykester1]

The Supco adjustable potential relay I used is designed to substitute a whole range of potential relays in the field up to a certain maximum size of compressor. I have used them many times in the past with no problem. The capacitors are arranged as they were originally arranged and the unit ran like that for years with no problem. Having researched the problem more thoroughly, I believe the failures of the capacitors are being caused by the start winding staying in for too long probably due to an occasional low voltage situation. However I thank everyone for their suggestions.

[multisync]

Compressor manufacturers do not use adjustable potential relays, multiple start capacitors, or multiple run capacitors.

Potential relays are matched to the motor. You MUST use the right potential relay, the right start capacitor, and the right run capacitor.



Prestcold would often use multiple capacitors as anyone who has actually worked on one could testify. Although those adjustable relays are s.h.i....t :p

[Pykester1]

I am beginning to agree with you about these adjustable relays. The problem occured again today and after having replaced the faulty start capacitor (again!!), I was wathcing the contacts of the relay and they denergised while the unit was running fine, (with 223 volts onto the live and neutral of the compressor). The start winding came back on line immediately as the contacts made and the new start capacitor overheated and popped after about 5 seconds. What I don't understand is what made the relay drop out. I lowered the pick up voltage, replaced the capacitor and it didn't happen again. (Until next time!!!)

[tbirdtbird]

"If motor is running above nameplate amperage then you have a problem that needs resolving before you lose another compressor."
Has this been addressed?

"What I don't understand is what made the relay drop out."
Me neither but I wouldn't trust it. I assume no utility drop-outs