Re: Generator troubleshooting

Posted by Bob Carabbio on December 20, 1997 at 17:51:29

In Reply to: Generator troubleshooting
posted by Jack Metcalf on December 20, 1997 at 14:49:09

Depends on the Volt-Ohm meter.
The resistances of the windings are pretty low, so most measurements will look pretty much like shorts unless you have an instrument capable of getting down into the 0.1 or ideally the 0.01 Ohm range.
As a quick test, the generator should "Motor" when connected to a battery (+ to the frame, - to the terminal), and draw 5 amps with 6 volts applied. The armature must turn freely for this test to be valid.If it does, then it may just need polarization to generate.
Resistance checks:
Pull the back cover, and disconnect the wires from it.THEN check with an ohmmeter on the field wire (that you disconnected from the third brush)that the field is connected to the frame.Sometimes the field ground has corroded - it's held by the ternimal bolt. The field should show 1.11 ohms resistance to the frame. If it doesn't check the ground conection.
Unhook the field connection to ground which is under the terminal post so that everything is floating. Then test the field to frame resistance.It should be high but may not be infinite due to 60 year old insulation leakage.It shpould be infinite resistance, but anything greater than 10,000 ohms is workable, but you should consider disassembly and re-laquering the windings at the minimum.The winding itself should be 1.11 Ohms from end to end. Since it's 200 turns, a single shorted turn would be hard to see (being only 0.005 Ohms/turn).But, if the resistance is real close, it should still function.If you apply 6 volts across the field, it should draw 5.2 amps.
The armature is much lower in resistance, and consists of windings that are interconnected so an ohmmeter check wouldn't tell a lot except that all the commutator segments should be connected to each other with as little as 0.09 Ohms resistance.There should be NO connection to the shaft or laminations. ANY connection between the commutator and the armature shaft/lams is fatal.
To check for shorted windings, you need an fixture called a "Growler" the subjects the armature to an A.C. magnetic field.A piece of magnetic steel will be attracted to, and buzz against the armature laminations of there's a shorted turn.
Check the disconnected brush assembly.Two of the brush holders (The "Third brush" which is adjustable, and one of the stationary brushes) should be insulated from the frame with balelite shoulder washers. The remaining brush is grounded to the end cover.Check with the ohmmeter that the insulated brushes are actually insulated from the end cover, and that the grounded brush really is.
Bob Carabbio

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