Stumped by welder exciter generator

[OperaHouse]

A friend had me look at his old Lincoln gas powered welder that his repair guy can't fix. It has a two pole 120V DC exciter on the end. It went bad when a worker pluggrd in a transformer type battery charger to the 120V DC convenience outlet.

When I first saw it the armature was pulled out. Dusty but no signs of windings burning, overheating or commutator arcing. Tapped a screwdriver to the field coils and almost no sign of residual magnetism. The 145 ohm field coil was isolated and connected to 12V. That gave me 5V at the brushes with the engine running. Reversing the field polarity gave me -5V at the brushes. Seemed simple enough, just needed to flash the coils.

Next day connected 140V DC to the field to flash it. With the field reconnected and a 100W lamp connected as a load I started the engine. Output was only a little over a volt. Then I powered the field independently with 80V DC. That gave it a really strong magnetic field. Still only a couple volts output. The commutator has been cleaned. Armature has continuity all around. Brushes make good contact. Resistance from brush to brush is around 1.5 ohms. There is just no sign that anything is wrong. Field does have an additional low ohm winding which I assume is to boost voltage to compensate for IR loss. It was not connected durring testing. Welder section works normally when independently powered with 100V DC. I can't imagine an armature failure mode that would cause this. Appreciate any ideas.

 

[ScottyUK]

Rotating rectifier diodes ok?


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If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

Brushes in original position?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[rasevskii]

Hmmm...as Skogs has said, it sounds like the brushes are a long way off from the neutral position. As you said it was all taken apart when you got it...?

rasevskii

 

[OperaHouse]

Brushes are two separate assemblies with no way to move or adjust.

 

[waross]

Apply AC to the field. Connect an AC voltmeter to the brushes. Watch the voltage as you rotate the armature. Crank the engine with the ignition off. You should see zero or low volts at the brushes. If the voltage varies as the armature rotates the armature is bad.
If the field is bad, this may make it worse and easier to find the failure.

Bill
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"Why not the best?"
Jimmy Carter

 

[dicer]

Many years ago I had a miller big 40 welder I used at a place of work. For some reason the exciter always lost its magnetizim. It was so bad I ran wires out so I could flash it at the start of the day, after that it usually did just fine the rest of the day. I had it apart many times trying to figure out why? Never found a problem.

 

[waross]

dicer: Possibly too large an air gap.

Bill
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"Why not the best?"
Jimmy Carter

 

[OperaHouse]

Had a chance to work on it again last weekend. Did a variation of the AC test. Aplied AC to the field and looked at the output on a scope referenced from one brush to each commutator bar. This requires repeating after several partial rotations. Sure enough there was a major change in voltage between two adjacent bars. A resistance check also indicated motr than double the resistance there. Removed the armature and still couldn't find any obvious efect. connections were fine at those two bars. They found a guy that may have a spare one.

40 years ago I rewound by hand a generator for slower speed by hand. An experience I never want to repeat. Forget how the coils went. With mltiple paths I still would have expected some output.

 

[waross]

You probably have an open circuit in the armature. You may be seeing the resistance of parallel path(s).

Bill
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"Why not the best?"
Jimmy Carter