Author Topic: transformer burning out  (Read 5132 times)

Offline stew

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transformer burning out
« on: June 30, 2010, 09:33:52 AM »
i have a icp (pgaa18c) outdoor unit that keeps burning out the primary of a 240v primary and 24v secondary transformer.the transformer gets it power from the system control board.i replaced the contactor it was burnt and would not start the unit. the voltage comming to the primary is 234v and and 24 volts comes out the secondary which is connected to the contactor on one side.the contactor will not energize and the transformer primary burns out after a very short periocd of time.i checked all low voltage and high voltage wiring everything is to specs. and no wiring problems. the control borad has no visible problems but could it be acting up internally? everything else in the system is good but i do not want to guess because the control is a $200.00 part. 

thanks stew

Offline TECH X

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Re: transformer burning out
« Reply #1 on: June 30, 2010, 05:27:24 PM »
You said the 24 volt wire connects to one side of the contactor.  Where does the other 24 volt wire connect to?  Is there a pressure switch or low ambient control in your a/c, wired in series?  You may have two seperate problems.  If you measure 24 volts across the 2 wires entering the outdoor unit, then the contactor should pull in.  If it doesn't, then there is another problem in that a/c.  (pressure switch open, low ambient control stuck open ect..)

Is this replacement transformer rated for 208 volts and 230 volts?  Did you match up the VA rating?  Is there 4 or 6 wires on this transformer?  You may be mis-wiring the primary side if you have 6 wires.  Most universal transformers have the 2 wires for 24 volts, and 4 wires for the primary, but you only use 2 of them.  The common is black for 120 volts, 208 volts and 240 volts.  Use the white and black for 120 volts, the black and red for 208 volts and the black and orange for 240 volts.

It's unlikely a 24 volt short can cause the primary side to fail, but I would still wire a 3 amp fuse on the 24 volt load side of the transformer, and see if it blows.  If it does, then test the 24 volt 2 wire for continuity, and make sure there is no continuity when the wires are seperated.

If you disconnect the 24 volt wiring from the transformer, will it still burn out?  Bring a new contactor down to the transformer, wire the 24 volt wires directly to the contactor coil and see if it pulls in.  That way you eliminate the low voltage wiring feeding the outdoor unit, and the controls inside.

Also if you manually push the contactor coil in, at the outdoor unit, does the compressor and fan start?

Let me know what you find.

Offline Admin

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Re: transformer burning out
« Reply #2 on: June 30, 2010, 05:45:16 PM »
Quote
i replaced the contactor it was burnt

How so?  Was the 24 volt coil bad, or was the contactor literally burnt?

Offline stew

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Re: transformer burning out
« Reply #3 on: July 04, 2010, 07:12:12 PM »
the contactor was closing but the contacts were all burnt and it would not always allow the system to start up! i then replaced the contactor and the transformer burnt out at the primary winding. i checked the voltage at the primary which is fed from the system control board,and found 237v going into the primary winding so why did it burn out? if the control board was feeding more than 240v or had a dead short i could understand what was happening but this makes no sense?

Offline Admin

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Re: transformer burning out
« Reply #4 on: July 06, 2010, 06:40:17 PM »
The 240V that feeds the indoor transformer should be a different 240V circuit than the one feeding the outdoor contactor, so we can rule out a line voltage problem.  The outdoor contactor is fed by the indoor transformer's 24V side, so if you're transformer failed I would expect to see the secondary side of the transformer shorted.  Did you throw a couple wires and stakeons on a 3A fuse and wire it in series with the 24V load side?

I would eliminate the low voltage altogether.  Disconnect the 24V wires at the furnace that feed the outdoor unit.  Set the thermostat to cool and manually start the outdoor unit, and see what's going on with the compressor and refrigerant charge.  Something caused that contactor to burn out.

If the original transformer was still good when you got there, and only fried after you replaced the contactor, I would look at the contactor as being the cause.  Did you replace a single or double pole contactor?

Offline stew

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Re: transformer burning out
« Reply #5 on: July 07, 2010, 08:59:45 AM »
i found the problem late yesterday! the problem was the y wire from the t'state was shorting out intermittenly against the cabinet of the outdoor unit. the low voltage wiring from the t'state was done very poorly and when i pulled it out (decided to change it) the problem was evident. the contractor that installed this wiring did a real patch job and had added wiring at three different locations (must have run out of wire) then taped it all up to look good. the wiring extensions were just wound together it was a real mess. it was run inder ground as well and just taped with dirt thrown on it,i could not believe what i found and could not believe a contractor would stoop this low. thanks for your reponse and really appreciate your help and i think this web site is excellent!!!!!!!!!!!