Rewinding an Auto Transformer.

Thread Starter

R!f@@

Joined Apr 2, 2009
9,802
I have had good experience with a guy who rewound a toroid for me.
So I am thinking of asking him to rewind my auto TX which has a bad spot where the wiper touches at around 120V.
It's rated at 2 Amps.

I dunno if I need to coat the winding with carbon or not so that the wiper slides smoothly.
Any hints are most welcome.

Primary question is to beef it up.
I am thinking of asking him to use a bit thicker wire so I can increase the current.
I dunno how much or what the limit is.

Any pointers or advice any one ?
 

Dodgydave

Joined Jun 22, 2012
10,366
The thickness will be limited to the size of the core and Va rating, just use enamelled copper wire, with a new carbon wiper brush, and remove the enamel at the point of contact for the wiper.
 

Thread Starter

R!f@@

Joined Apr 2, 2009
9,802
The thickness will be limited to the size of the core and Va rating, just use enamelled copper wire, with a new carbon wiper brush, and remove the enamel at the point of contact for the wiper.
How thick I can use is the question.
Can I increase the size a bit ?
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
You can increase the Gauge but you will still be restricted to the same Va rating.
You will also need to keep the amount of turns as close as possible to the original.
Max.
 

crutschow

Joined Mar 14, 2008
30,131
If you use thicker wire, that will allow for fewer turns which could cause the core to saturate and the transformer to overheat.
Stick with the same gauge wire as the original.

Have you looked at the bad spot on the winding to see what the problem is?
 

Thread Starter

R!f@@

Joined Apr 2, 2009
9,802
I see the bad spot. It is almost sorta corroded. It sparks when wiper moves over it.

I see @MaxHeadRoom idea ain't half bad.

If I am rewinding I will use a bit thicker wire. This might loose a few turns but it might be OK.
What do you think ?
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
Slightly larger gauge may work as long as you don't get dead spots between conductors, if less turns however, you could lose VA capacity slightly.
You cannot go smaller without decreasing the contact size of the brush due to short from adjacent turns.
Max.
 

Thread Starter

R!f@@

Joined Apr 2, 2009
9,802
Slightly larger gauge may work as long as you don't get dead spots between conductors, if less turns however, you could lose VA capacity slightly.
You cannot go smaller without decreasing the contact size of the brush due to short from adjacent turns.
Max.
I do not understand why you are saying that VA rating will be less.

I am trying to increase the current form 2 Amps to say 3 Amps.
Using thicker wires is the way, isn't it ?
 

Kjeldgaard

Joined Apr 7, 2016
461
I am trying to increase the current form 2 Amps to say 3 Amps.
Using thicker wires is the way, isn't it ?
It is true that there should be a thicker wire for a larger output current.
But there must also be a greater core to the larger total output power.
 
If you decide to replace the bad winding's you'll need to flatten the wire were the brushes contact them. If there is mica between them it will need to be undercut around 0.010" or so. The new winding's will need to be the same height as the original winding's. With the brush assembly off you can take a flat file holding it flat across the winding's and lightly dress them until they are the same height as the old. Be sure to remove any fillings - shop vac if possible, compressed air could blow copper under the winding's.
Also take a look at the brushes. If they need dressing slip a piece of fine (400 or higher) sandpaper between them and the winding's and pull it through until the brushes seat. Be careful not to sand too much. Also check that the brushes still have good spring tension against the winding's. I've seen this start the problem you have.

Worked on a bunch of these in he past.
 
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