Run capacitor failure

[chillyblue]

Hi

What would be the results of a failed run capacitor on a voltage type relay??

The compressor is a Prestcold PK50 (240 volt capacitor start and capacitor run)
When i start the compressor a current is detected on the start cappacitor for a short time (as normal)
but i cannot detect a current draw through the run cappacitor.
I have checked the run capacitor (5uf) and it was open circuit, i have replaced it with a new one but the compressor is struggling, the amps gradually rise from 4amps upwards and eventually (within 5 mins) re-energises the start relay and trips the clixon.

Please advise if a current reading should be measured through the run capacitor and what if any damage may have occur due to the failed run capacitor.

many thanks

CB

[Brian_UK]

I know that this isn't relating to your compressor directly but Bristol Compressors have quite good instructions for checking and restarting compressors - with diagrams.

Have a look at this one -
http://www.bristolcompressors.com/InfoDocs/200022.pdf

[monkey spanners]

I believe there should be a current reading on the start and/or run capacitor depending on whether the comp is starting or running.
Is it a current or potential relay on these?
I'd check the wiring on the unit to make sure all is as it should be and also the winding resistances to see how they add up.
Have you replaced the 5uf with a motor run rated capacitor? i've seen once a intermitant capacitor fitted as a run and it made the compressor sound and pull current like it was siezing up.


Does sound like the comps had it though to me.

Jon

[chillyblue]

Hi

i've been back today and run the compressor from a cold start, started and run fine getting a reading of about 0.5amps on the start winding when running.The system is running on R409A and was running with a discharge pressure of around 100psi (30deg C) suction pressure of around 5psi (-20deg C) cabinets should be around -20deg C they started to pull down in temperature, amps around 5 amps ran well until i stopped it to set pressure switches and then failed to restart, i left it for 30 mins restarted ok but then would not run consistantly (kept tripping on the klixon, or the start relay would energize also causing it to fail on the klixon (locked rotor reading of 25amps) left it for a further 30mins but a repeat of the above occureed, and would not run for any period of time, for the next few hours.
I measured the windings resistance before i left, the casing was warm, i had a reading of around 7ohms (common to start, 2.5-3ohms common to run and 10.5 run to start so comp seems ok.
Temp on suction line was above zero deg C as no frosting was occuring so the superheat is quite high (so i don't believe there is a threat of liquid flood back), the only thing i'm not sure of is if there is oil flooding back (i don't how i would be able to detect this. i'm wondering if the oil (as the compressor is mounted above the evaporators by approx 6mtrs) could be being pulled up the suction line and starting to hydraulic the compressor casuing a high load on the motor windings causing the relay to re-energize etc, etc.
The other possibility is due to the high suction superheat, could this be causing the the windings to overheat and the klixion trip that way, i have constantly felt the compressor head and body and would not say it has been excessively hot causing a trip or excssively cold threat of liquid.
Can't seem to keep this thing running.
Thanks monkey spanners/ Brian UK
The relay is not a current relay (i think these are for smaller compressors), it's the voltage relay (the little box with 5 or 6 terminals remotely mounted from the compressor, not mounted on the compressor terminal pins.

Thanks

CB

[Electrocoolman]

Have you tried replacing the relay?
It could be an intermittent coil, causing start caps to switch back in circuit whilst running.
Best bet is to try a new set of electrics (relay and caps)....a lot cheaper than a compressor!

[chillyblue]

Have you tried replacing the relay?
It could be an intermittent coil, causing start caps to switch back in circuit whilst running.
Best bet is to try a new set of electrics (relay and caps)....a lot cheaper than a compressor!

Thanks, but i've already fitted a new starter pack. this is happening after that!!

CB

[Gary]

Potential relays are specific to the compressor model... and require exact replacements.

[Electrocoolman]

Have you checked the condition of the crimp terminals on the various connecting wires? I've had a poor crimps giving trouble before now.

Is 5 mfd the correct value for the run capacitor? I would have expected 10 - 15 mfd. Will try and check up in some old manuals.

[chillyblue]

Thanks Lads

The relay/ starter pack was supplied via a compressor remanufacture.

CB

[monkey spanners]

5uf sounds about right for a half horse prestcold compressor, its roughly 10uf a horsepower (with a few exceptions)

How old is the compressor?
Has it got a oil level sightglass?
Some of the smaller ones don't but you can make a dipstick out of some wire and check the level that way through the oil fill plug once pumped down. I think they hold between 0.5L and 0.6L depending on the age/body style .

If it has got oil or liquid coming back in sufficient quantities to stall it it would sound like a sds or hammer drill when you hit a steel lintel.

Jon

[Gary]

Have you checked the voltage across the compressor with the compressor running? You could have burnt contacts somewhere in the circuit, providing low voltage when the compressor is under load.

[chillyblue]

Thanks guys

I have checked the oil level it's fine, there is no noises when running,
I have also checked the voltage when running that is also fine around 242volts.
The compressor was a new (reman) which i fitted.
What would happen or what damage would occur to the start windings with the following sinareo.
The compressor was fitted with only a 100uf start compressor and a 5uf run cappacitor, the compressor was started with a partial refrigerant charge, and fully charged when up and running, it was then found (a few days latter)that the run cappacitor had failed (open circuit), this was replaced and i was also advised that the start cappacitor should have been a value of 160uf not 100uf, so it had been trying to start or starting with a smaller than required starting cappcitance. could this or would this have caused some damage to the start windings???

CB

[paul_h]

I've come across plenty of 3-10 year old systems with a failed capacitor. While the capacitor is faulty, it trips on overload for countless times before I get called there.
I put in new capacitors and the system runs for years afterwards without problems.
A cap failing does not seem to do instant damage if the system has adequate fail safes like an overload (external or internal)
This year alone I am replacing a run cap every week across the city, all the systems are still running fine months later.

Being so far off the start cap value would have been bad, but then again it's not the start winding/cap you are having a problem with and as long as the start cap is a lot higher than the run cap, I can't see any long term damage being done.
If the run winding has the right cap, and the relays good, the only reason why it's drawing too much current when running is mechanical fault IMHO.

[Pyr0Beast]

...could this or would this have caused some damage to the start windings???

No, unless it overheated.

Any capacitor you install should be of appropriate rating.
Too much or too little, and you may burn windings, cause excessive damage to the relays and similar stuff. Yes, it will work .. but for how long ?

[chillyblue]

Hi

I think it may have damaged windings, (not burnt out or shorted, but when run body gets hot and runs with high amps then trips!!!)

CB

[Pyr0Beast]

At that point it could be anything. But, probably the cause is an oversized capacitor or internally shorted windings, so it is not completely shorted out.

Watch the current (after it starts) and monitor it, if it starts rising (low at start but then goes thru the roof) it probably isn't compressor's fault and can be a pressure issue.

If it is more or less constant, it may be something wrong with compressor in which case you should look for ohmic resistance of windings (when unplugged/off-line of course), then check the capacitor again, you have the right one. They are different for 60Hz/50Hz versions.

From the first post it would seem that capacitor was indeed damaged, not necessarily open circuit, but it has lost much of its capacitance and thus it didn't allow much juice to pass thru, making compressor struggle. Same can be happening with some washing machines, most noticably when centrifuge looses it's -strenght-. As the capacitor warms up, it's capacity increases, that's why it was drawing more and more current.

If you get a RCL meter, you can check the capacitor's capacity, you can get them for a few bucks anywhere. A maximum of 200uF measuring range should be more than enough for all tipes of motor run-capacitors.

Difference between 160 and 100uF capacitor shouldn't cause any harm if it started properly. If it didn't ..

[chillyblue]

Did you forget the end?? if it did'nt??

[monkey spanners]

When starting both the start and run capacitors are connected to the start winding, when started the relay switches the start capacitor out of circuit leaving the run cap still connected to the start winding.

So the compressor has been starting/trying to start with 100uf instead of 165uf, thats quite a bit below what it should have been...

Jon

[chillyblue]

I've come across plenty of 3-10 year old systems with a failed capacitor. While the capacitor is faulty, it trips on overload for countless times before I get called there.
I put in new capacitors and the system runs for years afterwards without problems.
A cap failing does not seem to do instant damage if the system has adequate fail safes like an overload (external or internal)
This year alone I am replacing a run cap every week across the city, all the systems are still running fine months later.

Being so far off the start cap value would have been bad, but then again it's not the start winding/cap you are having a problem with and as long as the start cap is a lot higher than the run cap, I can't see any long term damage being done.
If the run winding has the right cap, and the relays good, the only reason why it's drawing too much current when running is mechanical fault IMHO.

Hi Paul
You say the under sized cap was bad, if it did cause damage to the start winding, would that not cause problems when running although the start cap disconnects after startup the run cappacitor still feeds the same winding permanenlty when running.
Do you know what sort of current i should read through the run cap during normal operation??

Thanks for the help guys

CB

[Pyr0Beast]

If it didn't .. start properly, then it would overheat and trip clixon. Clixon's are the last (sometimes the only) measure of overheat protection, meaning, once they trip, there is something VERY VERY wrong, and can happen, when they trip, they burn a winding or two out ..

If it started successfully, meaning rotor was spinning and pumping gas then it shouldn't be damaged. If it didn't and made only that network-humm noise with rotor not spinning, it may be damaged. Once clixon trips it is fine ... but two, three times ..

Also, don't cool it with a blast of water. Windings can sustain a temperature of around 200 degrees C, not for long, but they will, but cooling them rapidly may cause damage because of different temperature expansions of materials.
Cooling for 1 degree C per minute is fine. After it is 'cooled' let it sit for a while (10 minutes) so temperatures in the compressor equalize.
Not doing so and immediately re-energising compressor will cause trouble, because inner windings may still be very hot and if clixon is on the outer side, the inner side may now reach even greater temperatures than before before it trips.

Do you know what sort of current i should read through the run cap during normal operation??

Normal operating currend IMO.

[Pyr0Beast]

Normal operating currend IMO.
Actually 1/10th or less.

[Gary]

If I understand correctly, the start winding keeps pulling back in... and that points to the start kit.

[GHAZ]

Hello CHILLY BLUE prestcold k50 has 5uf run capacitor, and 160 uf start capacitor with AA1 relay. THERE is a copeland compressor which is 1/2 horse power compressor and it has 10uf run cap and 80uf for start capacitor, looking at a pk50 and copeland dkm 50 compressors is one is black and the other is grey but apart from that you cant tell them apart . the terminal pins are u=run ,v=common, w=start, Your relay is numbered it has 1,2,4,5, connect U=4 ,V=5, W=2, Connect run capacitor to 4 and 2, and start capacitor to 4 and 1, your klixon should be connected between V and 5 , your power supply 240v live wire on 4 and neutral wire on 5 ,when the compressor is starting you,ll have high current reading,after a couple of seconds the amps should drop on cable w to 2 and a higher current reading on u to 4 , good luck

[GHAZ]

Hi i forgot to mention its 5 fla and 23 lra

[chillyblue]

Thanks for all your help lads.

I fitted another compressor yesterday wired it exactly as the other one with new starting gear, runs, stops, starts fine, i even struggle to catch the start load reading. Touch wood every thing is fine, so there must either have been a problem with the windings or the missing start cap and failed run cap.
I know this isn't the answer but fingers crossed, i don't see it again for along while.

CB