Blown Contactor Coil in A/C Control Circuit

[dminunni]

We have a relatively new Aaon HVAC system (2 Years) with a 4 stage compressor control circuit. We have a problem where the contactor coil for the 4th stage fries about every 2 months. The control circuit for each compressor is fairly simple. The only interlocks in the circuit are the low and high pressure switches for the compressor and a 5 minute delay on timer. The circuit is powered by three 480 to 24VAC, 75VA transformers wired in parallel for a total of 225 VA. The unit was originally supplied with 2 xfrmrs, but somebody added a third one shortly after the unit was installed. My feeling is that the xfrmrs may be unbalanced which may be causing the coil to fail on the 4th stage. The first 3 stages pretty much are on all the time with the 4th stage cycling on/off as needed. All 4 stages are wired identically except that someone upgraded the contacts on the 4th stage contactor from 25amps to 40amps. Others remain at 25amps.

Some other info: we usually melt the TON relay also, since about 9.6 amps start flowing through it and the three xfrmrs have internal 3.2A circuit breakers that never trip when the coil melts.

Anyone have any other ideas as to what might be causing this.

Thanks

Dave

 

[itsmoked]

I believe your unbalanced transformers idea is correct. I have attempted to use multiple transformers in projects and NEVER had success. I would not hesitate to yank all of them out and replace the whole fetid mess with one correctly sized larger one.

You understand that these AC contactors/relays have a very large current requirement when first energized? Once you have 3 of them powered your fourth one is probably not getting enough current to *make it* whenever your line voltage is lower than other times (e.g. just low enough).

An AC coil that can't make it in will without-a-doubt fry the first time it doesn't make it in.

 

[dminunni]

Dear itsmoked,

Sorry for the slow reply on this. You have confirmed exactly what I suspected. The replacement xfrmr is currently on order.

Thanks

 

[itsmoked]

Atta boy! Your AC coils will love you for it.

Thanks for getting back.