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Posted by Bubba on June 29, 2008, 9:46 am
On Thu, 12 Jun 2008 09:17:30 -0400, "John Gilmer"
>
>
>>>
>> Defective transformer, transformer voltage or current improperly rated for
>> the service, mis-connected, or overloaded. These transformers do not
>> present enough load, even when the secondary is shorted, to trip the main
>> breaker.
>>
>> Try another transformer, making sure it is for 240 volts and you have it
>> so connected. Some transformers have different leads for different
>> voltages.
>
>"Some?" Try ALL.
>
>He should:
>
>1) Ensure that the "next" transformer is nameplated for 240 (or whatever
>the air handler is wired for) volts input or if it's dual voltage that he
>uses the right wires and tapes off the unused wire(s). Many air handlers
>use both 120 and 240 volts so he should determine which voltage the
>transformer sees. It's quite common to use 120 volt fans even when all
>the heaters are for 240 volts. If the fan motor is 120 volts, it's likely
>that the control transformer is also 120 volts but nothing is certain.
>
>2) Install a fuse on the transformer output rated for 80% of the nameplace
>output current of the transformer.
>
>Clearly "something is wrong" here.
>
>The clamp on amp meter work just as well at 24 volts as they do at line
>voltages. That might be a start.
>
>It's quite possible that the transformer is underrated. It might have
>enough juice to power the fan relay and the changover relay and the
>compressor contactor. But if it's an especially COLD day and the system
>calls for "suplemental" heat, it may have the energy to power the thermal
>relays that bring on the heat strips. The supplement heat strips and
>"sequenced" and the load (including the load on the 24 circuit) increases in
>steps.
You really dont have a clue, do you Don?
This happened in the A/C mode and the guy has a short........a simple
short. Finding it is the fun part.
Bubba
> If there is a short on the input of the last thermal relay in the
>sequence you would never know unless the demand for supplement heat lasts
>long enough to sent 24 volts to that last relay.
>
>But I would definitely put a fuse on the 24 volt circuit while figuring
>things out. It's better to replace a $.50 fuse than a $25 transformer.
>
>
>> Also make sure the VA or current rating is at least what the original
>> transformer was. Monitor it carefully and if it seems to be overheating
>> check the secondary current to determine why it is overloaded.
>>
>> Don Young
>>
>
>
>** Posted from http://www.teranews.com **
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