How to check field windings???

[john11139]

Just purchased a 8.5 kw gen head, sight unseen, fellow swore on his mothers grave that it was working when taken off (engine blew)  I have not mounted it yet as it is very labor intensive to mount it on my engine. But upon examening it very closely, I find one of the field windings that I am questioning.  It looks like it is burnt some what and looks like the insulation is sort of flaked off in places. (other windings are ok.  Is there a way to check it to see if it is shorted?  I think the book says 0.33 ohms of resistance.  Would that little of resistance show up on a ohm meter?  If I decide to use it, should I coat the windings with varnish or shellac or purchase a can of spray on winding insulation, and coat it good??

[Thob]

It depends on the meter you have.  My digital meter reads about 0.2 ohms when I short the leads, so I assume that's the lead resistance.  If I measured your windings, I'd expect to read about 0.5 ohms.  You can also compare one winding to the next.  One of these days I'm going to get around to the project of building a millli-ohm meter (1 amp current source and a DVM).

I'd be equally concerned about the resistance to ground.  You can check to be sure it's way up in the meg-ohms with your meter, but you really need a device called a "megger" to make sure the coil isn't leaking to ground.  The megger applies a high voltage (higher than a standard volt meter).

A trip to a motor shop might be in order - they would have the equipment to test it and can properly dip the windings in the right stuff and then bake them dry.

[Ronmar]

Yep a megger applies upwards of 1000V to coil and ground and looks for leakage current.  I use them all the time to test HF antennas for signs of electrical breakdown...

I agree, if you don't have access to the tools, a motor shop should be able to electrically evaluate the windings quickly and realitively inexpensively...

[SteveU.]

EEK! John!
These are bad windings! Do not put into service.
First winding test - do they smell bad, burnt, fishy? If so they are bad and will, or will shortly fail. Should be NO smell at all.
Second winding test - do they in comparison to each other, or another known good set appear darkened or burnt? ONLY overheating to the point of insulation failure will cause this. (Sometimes one set versus the other may be different color based to help with the winding set up)
Third winding test - feel. Any looseness or flaking and they will move/vibrate under power; rub wire to wire with each other and short out.

Spray on insulation will only mask these problems.

Only after the windings pass these first steps then do you even bother to electrically test.

Smell, see, feel comes before that - it will tell you before the problem will show up on a meter.

Regards
Steve Unruh

[john11139]

Thanks, that was what I was afraid of.