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Old 25th October 2003, 10:07 AM   #1
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Default Sizzling torroids.

I have two 100 VA torroids that ended up in the waste bin. The magnetising current was very low, less than 1 mA. That is normal.
The secondary is 240-0-240 O.1A and 15-0-15 1A. It was connected to a valve pre amp and worked well. All voltages were fine and after 10 minutes the torroid was hardly warm.
I played some music for about an hour when I smelt something burning. Then I walked pass the amp and found the transformer looked funny. I could see the enamel wire. Normally it is covered with silvery plastic tape. The tape had cooked and melted over the transformer. The amp was still playing fine and the transformer was sizzling . Subsequently I tested a second transformer I had and tested it for magnetising current. All fine. I left it on without any load for over an hour. Then again I discovered that it had heated up quite a bit and one part of the transformer had already melted the plastic foil wrapped around it.

A third identical transformer is now on for the past 6 hours without any problem. All were brand new and all came from the same manufacturer at the same time.

Would anyone know why this could have happened. We can rule out shorted turns because that would have showed up when checking the magnetising current. Or is that correct?
I think the core is heating up slowly for what ever reason and then rising rapidy . There is no noise until the plastic starts melting. Does anyone know what could have happened?
Thanks.
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Old 25th October 2003, 04:35 PM   #2
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are you sure you were not drawing too much current?
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Old 26th October 2003, 04:14 AM   #3
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Default Load .

Hi Mattcatt,
The load current was well below the rating. In any case the second torroid that broke down the same way had 'no load' at all. So loading does not seem to be the cause.
I now think it could be a shorted coil internally. This would probably not show up as too much initially till it heated up the surrounding coils and broke down their insulation. Just an idea !
It really bugs me. I also know that torroids are very bad at handling small amounts of dc on the mains. Could this have been the reason? I guess the dc component comes up due to a poor mains waveform(?).
Cheers.
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Old 26th October 2003, 05:15 AM   #4
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yah...I guess so...you could have DC on ur mains...but shouldn't happen twice...I mean within such short notice...you got any dimmers on the same line you are using??
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Old 26th October 2003, 08:59 AM   #5
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Default Dimmers

Hi Li,
No I don't have any dimmers on line - none at home !
In any case I was wondering how dc could last so long. The heating took place over a long time.
Cheers.
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Old 26th October 2003, 09:21 AM   #6
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Yeah that was what I was wondering too...if there are no dimmers...hmmm...wonder if it's ur grid...oir it might just be the trafos themselves...
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Old 26th October 2003, 09:29 PM   #7
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Default Re: Load .

Quote:
Originally posted by ashok
Hi Mattcatt,
The load current was well below the rating. In any case the second torroid that broke down the same way had 'no load' at all. So loading does not seem to be the cause.
I now think it could be a shorted coil internally. This would probably not show up as too much initially till it heated up the surrounding coils and broke down their insulation. Just an idea !
It really bugs me. I also know that torroids are very bad at handling small amounts of dc on the mains. Could this have been the reason? I guess the dc component comes up due to a poor mains waveform(?).
Cheers.
i dont know if it is safe to try this, but have you tried testing the mains for DC?
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Old 27th October 2003, 01:51 PM   #8
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we where talking aboiut it in the previous posts...but if there are no dimmers or anything...how can DC be present for so long??
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Old 27th October 2003, 03:04 PM   #9
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Default maybe shorted turn.

I think this may be a shorted turn that slowly heats up the surrounding till more turns get shorted and eventually leads to a melt down. I came to this conclusion because the second transformer which also started melting - melted the plastic wrap only at one point. I am sure it would have spread if I had not shut it off. I will test them again now and see what the magnetising current is compared to the good one.
Thanks to all for taking the interest to figure it out.
Cheers.
Ashok.
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Old 27th October 2003, 03:35 PM   #10
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yeah...and becareful what you do to those trafos...seems a big waste to me...breaking 2 wiithin a few hours...
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