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ostie01 17th May 2006 07:47 AM

Bad transformer rewinding
 
HI, the transformer on my power amp is shorted on the primary side. could it be possible to rewind it. the secondary is O.K. Is it a big job, never done this before but have some magnet wire of the same gage.
Thanks for any good advice.

AndrewT 17th May 2006 08:13 AM

Hi,
try unwinding the secondary.
Once you've done this try to imagine the system you need to set up to control the LENGTH of the primary wire you will have to cope with.

Personally, I would avoid primary rewinding before even considering the safety issue.

ostie01 17th May 2006 08:22 AM

Hi, why there's safety issue in primary rewinding. What is the difference between primary and secondaty rewinding. Thanks

AndrewT 17th May 2006 08:47 AM

Hi,
glad to hear you're awake enough to note my caution.

The rewinding of the primary will involve a very long length of enamelled wire. You will have to remove it from the spool to transfer it to the bobbin (or what else you come up with to pass the correct length through the hole in the toroid) then you have to wind the primary without kinking the wire. The modern enamels are quite flexible but kinks and repeated transfer from spool to bobbin to core may crack the insulation. have you been able to calculate how long a length to wind onto the bobbin? Too much and the bobbin may be too big to pass through the hole and too little and you need to joint your primary!

Next you have to consider the voltage between turns. Adjacent turns will only have a few tenths of a volt or so between them but you always have more turns on top of each other at the inside edges of the core. Here the voltage is not between turns but across many turns. This should still not be a problem if you can avoid those cracks in the enamel coinciding with each other.

Now you have wound about 70% (or any number) of turns and you find that you have returned to the start point. Oh dear! the voltage between the latest few turns and the beginning of the winding is going to be about 70% of the mains voltage. A short between turns here will be a disaster and this voltage mismatch will continue to occur all the way around the last 30% of the turns you have to finish the double layering on.

There is a solution, but it consumes more volume. Wind on an inter layer of insulation before beginning the second over layer.

Use 55% overlap in the inslation so that you can GUARANTEE two layers of insulation between windings with a significant potential difference. This also applies to the primary to secondary separation insulation.
Remember the 55% overlap applies at the outside of the core, the overlap at the inside is more likely to be 80% meaning you have about 4 to 5 layers of insulation (even more volume wasted) at the inside of the core.

If you are very clever/lucky you will just finsh the primary winding before you reach the beginning, but theoretically you want this beginning to end gap to be zero or as close as physically possible. The manufacturers will set up their winding machines to just achieve this with a small gap they have found does not reduce performance. The gap can be used to solder and insulate the flexible lead out wires you need to attach.

Are you beginning to appreciate the problem?

ilimzn 17th May 2006 08:54 AM

Quote:

Originally posted by AndrewT
...pass the correct length through the hole in the toroid
How do you know it's a toroid?

AndrewT 17th May 2006 09:03 AM

Hi,
OOPS, thanks for pointing out my oversight.

Interlayer insulation still applies, all else pales into insignificance for an EI.

jaycee 17th May 2006 06:04 PM

To be fair it's a pain in the *** rewinding any transformer, primary or secondary.... especially a toroid where you have to feed the wire through the middle and the spacing is critical to get it around the toroid neatly and without overlaps or huge gaps. If it's a toroid you would be easier and cheaper just getting a replacement custom made.

Bazukaz 17th May 2006 06:13 PM

Hi,
Even if not a toroid , you will need a winding machine with counter.
It is possible to build a counter from calculator with magnet contactors and little magnet rotating. The contactors would short and release "+" and "1" of calculator buttons(would have to solder through a resistor).
A better way is to find a mechanical counter from old tape players.
Once you have build a machine , tranformer winding is much simplified :).
Having no ideas how toroid winding machines look like...

Regards,
Lukas.

ostie01 17th May 2006 06:39 PM

Hi, don't know it was so complicated to rewind a transformer. but should have add to my question that this is not a toroidal transformer, it's an ordinary iron core transformer. I tought that I only have to remove the bad wire, counting the turns and rewind it the same way it was.It is not a very big transformer, this is part of a Denon POA8300 triple mono power supply. There're 3 transformer in this power amplifier and two have no reading on the primary side.

ilimzn 17th May 2006 09:55 PM

Are you sure there isn't a thermal breaker in there somewhere, that has decided to become open?


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