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Registered Member #3324
Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276
i have access to a ~5Kw variac, its very old and the enamel on the copper is not on there anymore, would it be a good idea to strip it all off and wind a 5Kw Isolation transformer on it?
Registered Member #187
Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140
Steve Conner wrote ...
Having said that, the wire in a variac is supposed to have a strip of missing enamel. How else would the brushes make contact?
My thoughts exactly. If you are referring to the upper surface where the carbon contact slides across the windings, the enamal is not missing... it was purposely removed.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Physical examination
Inspect mains fuse if any.
Inspect state of carbon rotor brush.
Inspect windings and their terminations and leads very carefully for signs of charring, insulation failure etc (other than on rotor track!)
Sniff windings - the odour of burnt insulation can still be detected years after the event if you're not a cigarette smoker.
Test for any wonkiness or catching when you turn the spindle.
Electrical tests:
Check the resistance across the whole winding for continuity using your ohm meter. A measurement of a few ohms suggests health.
Measure the resistance between the slider and the cold end of the winding. Again this should be a few ohms, and vary as you turn the spindle across its range.
Put your ohm meter on its maximum range and measure the resistance between the winding and the centre spindle and/or case/Earth connection. The resistance should be ultra high - heading towards infinity. It should show as open circuit on an ordinary ohm meter.
If your variable transformer passes all these tests, then it is probably working.
Connect a ballast resistor such as a tungsten filament light bulb or electric fire element in series with the input and give it a try!
Registered Member #3324
Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276
all the enamel on the sides of the wire is stripped off, i was thinking of winding a isolation transformer on it, ive already got a 7.5Kva variac thats in "decent" condition
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Tom Williamson wrote ...
all the enamel on the sides of the wire is stripped off, i was thinking of winding a isolation transformer on it, ive already got a 7.5Kva variac thats in "decent" condition
Probably best to put it in the bin, Tom, rather than waste any further time and money on it.
Registered Member #3324
Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276
yeh, i havent brought it yet, but im really after a nice large isolation transformer (if anyone has a large one going cheap in the south UK pm me, same goes for PTs and Pole pigs)
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Variac Rewind?
Just look on ebay for a second-hand one in working order. You should be able to get it cheap if you're prepared to collect it.
