View Full Version : control voltage transformer nuisance tripping
02-14-2011, 08:19 PM
I've recently had a sevice call on a Lennox heat/cool packaged unit where the step down transformer, 600V-24V, has been tripping off the control voltage/secondary side. I've traced through all the low voltage wiring and found no shorting. The transformer has been replaced. The heating unit may run fine for days without any problems. Any ideas?
02-14-2011, 08:59 PM
Most of the time that I have had a transformer trip or control fuss blow on a consistant bases and find no shorts or bad wiring. Its been the contactor. Mostly with the compressors. But in heating you may try the blower motor. If you check the coil on the contactor, it will check out fine. Its only when it runs and heats up, then the coil shorts. I have had many do this. So thats the first thing I check. I cycle the unit and see at what point it blows and what was calling at the time. A relay coil can do the same thing if the relay is grounded to the cabinet like a contactor base is.
02-14-2011, 11:11 PM
Ive found when i have trips like that sometimes its the defrost board that has a direct short to ground. The only time it trips is when the unit goes into defrost mode. Id check all the low voltage wire insulation to ensure it hasnt rubbed through and also check the duct dectector too. good luck
02-15-2011, 04:26 PM
I concur with RF. Check your major low voltage loads (contactors and relays) for high resistance at the coil. Compare ohms to a new one. I have seen bad/old contactors do this quite often. You may find it upon start up of the compressor(s), or it may be intermittent until the contactor goes south. Of course I assume that since your in Arizona that you may still be operating in cool mode. Happy Hunting!
02-15-2011, 04:28 PM
Sorry, your in Canada. Check the heating circuit.
02-15-2011, 10:49 PM
Here is one more thing to check. This happened to me once. Drove me nuts.
I had a unit that had a compressor overamping. Not enough to trip a breaker or a fuse, but pulling high amps. When the compressor started there was a voltage drop to the entire unit due to the high current draw/borderline line voltage wire gage. The voltage drop to the transformer caused higher output amperage and would occasionally trip the control voltage circuit breaker. Basically, when the compressor started the output voltage dropped from 24 to the mid teens and caused the relays to draw more amperage tripping the control circuit breaker. It's just not something I thought about right away in the middle of Fall. In a nutshell, an 06E compressor change out solved the control transformer tripping problem.
03-07-2011, 08:00 AM
Thanks for responding, guys.
Just thought I'd post an update. The indoor room thermostat was faulty. (possibly one of the transistors on the circuit board of the thermostat was shorting) The thermostat was replaced and everything's operating fine now.
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