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Old 2nd December 2011, 01:22 AM   #1
Cassiel is offline Cassiel  Libya
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Default Power transformer primary is open.

Winding is open somewhere between the 125V tap and the 150V tap. I have bridged both taps and secondary voltages seem OK. It does buzz a little, very little....I have to bend down and get my ear close to it to hear the buzz. That may be normal - transformer is old. It doesn't overheat (unloaded). Primary sees 230V (240 tap). Should I worry? Will it fail on me?
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Old 2nd December 2011, 02:44 AM   #2
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the buzz may indicate a shorted winding. I'd take a cautious approach and rebuild or ditch it... Fires are expensive.
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Old 2nd December 2011, 02:51 AM   #3
Cassiel is offline Cassiel  Libya
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I don't know how some PT's work, I mean I don't understand how the primary can work with all those voltages with only one winding. Am I right if I assume that the 240V tab now it is a 215V one?


Quote:
I'd take a cautious approach
Yeah, that's why I'm asking, to be on the safe side. Rebuild it it's not an option.
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Old 2nd December 2011, 03:06 AM   #4
cerrem is offline cerrem  United States
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Quote:
Originally Posted by Cassiel View Post
Winding is open somewhere between the 125V tap and the 150V tap. I have bridged both taps and secondary voltages seem OK. It does buzz a little, very little....I have to bend down and get my ear close to it to hear the buzz. That may be normal - transformer is old. It doesn't overheat (unloaded). Primary sees 230V (240 tap). Should I worry? Will it fail on me?
Most likely it can be fixed.....The most likely place for the break is in the lead outs from the windings....there will be 2 wires coming out at the 125 tap, the 110 finish and the 125 Start, based on your description it would be the 125 Start lead that is broken.... Or it will be at the finish lead of the 125 where it joins the 150V start lead...
You can look into the side of the paper sections to see if you find this break...you may get lucky and be able to repair it...

Chris

Last edited by cerrem; 2nd December 2011 at 03:11 AM.
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Old 2nd December 2011, 03:21 AM   #5
Cassiel is offline Cassiel  Libya
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Yes, I did that before posting. No luck - no access to the break point.
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Old 2nd December 2011, 12:18 PM   #6
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Quote:
Originally Posted by Cassiel View Post
I don't know how some PT's work, I mean I don't understand how the primary can work with all those voltages with only one winding. Am I right if I assume that the 240V tab now it is a 215V one?.
Yes. The slight buzz may be caused by having 230v on what is now a 215v primary. The magnetic core may be saturating on the peak voltage. Do you have access to an oscilloscope? If you do, check the waveform on the secondary winding with a light load. If the sinewave is flat topped then you need to reduce the input voltage somehow to 215v. A wirewound resistor will do the trick. The value depends on the load to the transformer. Once done, it'll be fine.
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Old 2nd December 2011, 12:20 PM   #7
Cassiel is offline Cassiel  Libya
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payitforwardeddie - Thank you very much.
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Old 2nd December 2011, 12:26 PM   #8
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payitforwardeddie - Thank you very much.
My pleasure. That's what we're all here for.
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