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Tractor Talk Discussion Forum

Generator repair ?

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jCarroll

05-01-2007 12:03:46




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To allow access for another fix-it job, I removed the generator from my 1956 John Deere 420W (6v, + grnd, 2 brush generator). Thinking this would be a good time to check the generator, I test motored it by grounding the frame to the + post, and jumpering the - post to the A terminal. The generator motored about 80 or 90 rpm, and I thought that's kind of slow, so I'll tune-up the genny.
I cleaned it, put in new brushes and a pulley bearing, cleaned the commutator, and undercut the mica.
Put 'er back together, and tested for motoring like above, and it only drew a spark. I additionally grounded the F terminal, and it ran 300-400 rpm.
Now the question:
Does the F terminal have to be grounded for the motoring test?
If YES, how did it run when I first took it off the tractor?
If NO, why did I have to ground the F terminal to get it to run after repair?

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Bus Driver

05-01-2007 13:03:13




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 Re: Generator repair ? in reply to jCarroll, 05-01-2007 12:03:46  
After the motoring test, be sure to polarize the unit before starting the engine on which it is installed.



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John T

05-01-2007 12:47:55




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 Re: Generator repair ? in reply to jCarroll, 05-01-2007 12:03:46  
JC, When I motor test them I FIRST have the Field post grounded (frame also of course) and run her n she ought to spin thennnnn nnnnn when you unground the Field, she ought to run FASTER. If the speed dont increase when you unground the Field I suspect the field windings may be faulty.

If she motors with no field connection that tells me the armature n brushes are okay but she ought to slow down once the field is grounded or else I suspect theres a problem.

YES she ought to run with no Field connection at all (even faster then when grounded)

John T

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