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Dave 123
12-12-2016, 06:04 PM
Hi,
I have a customer with a Searle evap paired to a l'unite single phase condensing unit. The control is simple mechanical setup, ie;- 240V switched across mechanical room stat, up to the condensing unit, through the HP/LP then onto the overload then onto L1 of the contactor relay. The compressor and electric pack (Start cap/run cap and potential relay have been replaced) I have tested that the stat switches the 240V many times, I replaced the stat 3 months ago and I'm confident that the 240V reaches the 240V relay on the contactor (hp and lp switches are made, system full of R134a. The system runs perfectly, 6A on the compressor, 25psi/90psi, full sight glass 6 degrees evap superheat and 10 degrees cond subcooling. However, the compressor sometimes doesn't start. Once it hasn't started, it then wont start until either power is turned off and on or the stat fiddled with. If the stat was not making closed circuit, then the compressor wouldn't start if power was turned off and on again, as it is just a switch. The contactor and overload have also been replaced when I fitted the new comp last week. I was sure the stat was OK so I turned the condenser isolator on and off until the fault occurred (this was the old contactor and overload) Eventually, the compressor failed to start so I checked the contactor and found T2-240V L2-0V (the contactor is linked out to spread the load across three poles so it doesn't fail on overload) I replaced the contactor and overload and thought that was the end of that, but the customer has called today and stated the cellar is high in temp again. My question is; - the contactor is wired as follows; L1 240V in T1 link to T2, L2 link to L3 then 240V out to compressor on L3. The condenser fans are also fed from T1. Is the balancing wrong? I know it should be L1, T1, loop should go from T1 to L2, then T2 to L3, could this be the fault? how much difference is the fact that the wiring is looped slightly wrong across the contactor and the fact that the condenser fans are linked into T1. Obviously this will pull slightly more amps across T1 than the T2,T3. I isn't failing on overload as it restarts everytime the isolator is turned on and off. I have checked all wiring for damage and am getting a good neutral. Any help or suggestions with the contactor/overload wiring appreciated?
cheers,

Dave

chemi-cool
12-12-2016, 06:33 PM
Compressor will not start if the pressure has not equalized.
All you need is a timer between the starting contactor and control circuit.

Look for TD69 or TD73, set it for 3 minutes and the compressor will thank you.

Dave 123
12-12-2016, 06:53 PM
Thanks for the reply. I understand what you are saying. The condenser is off for around 15 minutes after the stat opens, it then intermittently wont start once the stat has closed again. this should be plenty of time to equalize the pressures? The condenser isn't short cycling unfortunately.

monkey spanners
12-12-2016, 08:03 PM
If the fan is also reluctant to start, i think i would try a new contactor, or at least strip the current one and check for bugs etc in it.

Dave 123
12-12-2016, 08:57 PM
Thanks for the response. I'll look into it tomorrow. I'm pretty sure it's the way that the contactor and overload is looped though I'll rewire it with the loops t1-L2 and T2-L3 also I'll move the feed to the fans from t1 and put it on the common terminal in the starter box.

chemi-cool
12-12-2016, 09:20 PM
To increase contactor life, do not run the "0" through its contacts, have it permanently connected.

cadwaladr
12-12-2016, 11:38 PM
My simple wiring would be a2 on the contactor to neutral,then a feed from the incoming to the hp/ lp then thru the overload 95 and 96 then thru the stat to a1 on the contactor that would be the control circuit,if the condensor fan has a circuit for protection wire that in series.

mikeref
13-12-2016, 07:35 AM
My 2 Cents worth. Somewhere in the control Circuit, contacts are closed so it appears 240V is passing through.
Sometimes Contacts close but fail to have a solid connection for Current to flow.

frank
13-12-2016, 06:45 PM
If you suspect the contactor and overload, bypass it and see what the results are when you do your switching on and off to create the fault.

cadwaladr
13-12-2016, 08:33 PM
It sounds like a lp fault to me,if it's set incorrectly this fault may occur or there is a drop in suction pressure happening a moisture issue or restricted filter drier/expansion valve.

Neddy
15-12-2016, 07:49 AM
Have you checked the electrical connections in the isolator as the fault seems to reset when you switch off the isolator and switch it back on. Also has the evaporator got a frost stat?

niceman
15-12-2016, 09:42 AM
Have never come across a l'unite unit that had cond fans wired off unit, always wired on unit. Are the overloads set to auto or manual reset? If set to auto, reset to manual, so if it is the overloads at fault you will then know. Could well be you have a dodgy lp/ hp causing multiple quick starts, check pressure switch operation using your gauges, not a pump down system so lp should only be for safety, stop evap icing up, and should be set to have a good diferential. Worst fault to find is the one like you got, best of luck.