Farmall C generator polarity

mhb@ufe · Jul 20, 2013

I'm working on a C that had been run negative ground and after replacing the voltage regulator and checking all wiring I polarized the generator for positve ground but it continues to charge for negative ground. When doing a motor test on the generator with the field grounded it will spin counter clocwise when looking at the pully and when the ground is removed from the field it reverses direction and speeds up.
What am I doing wrong?

Mark

Janicholson · Jul 20, 2013

Check battery voltage. If 7 volts or so, and positive is positive / negative is negative using a meter, the amp meter might have been changed so that it worked correctly when it was set up for negative ground. Just reverse all those amp meter leads. If the lights work, and you turn them on with the tractor off, and it moves to the charge side of the meter, that is the issue. Jim

mhb@ufe · Jul 20, 2013

The battery is new and correct polarity and without the amp meter connected testing the generator output is negative ground. With everything connected it is like it has a dead short when running. I don't understand why I can't change the polarity in the generator to positive ground. It will put out 7.8 volts but when connecting the multimeter I get a negative reading unless I test as a negative ground system.

Mark

Owen Aaland · Jul 20, 2013

This same problem was discussed a couple weeks back:

C charging problems

John T posted an explanation of your problem

mhb@ufe · Jul 20, 2013

Thanks, I had found and followed John T's trouble shooting procedure but missed that post. Sounds like a may have a problem in the field windings.

Mark

John T · Jul 21, 2013

In order to change the Polarity of a genny, its necesary that BOTH the Field windings be continuous (no breaks and good middle splice) and theres a good and continuous circuit from them starting at the ARM post (Fld 1) then terminating out the FLD post (Fld 2). If thats not all in place even if you try to polarize the genny it will remain the same it was before. If it runs and the genny polarity is opposite from the battery, that can trash a VR.

Sounds like a Field(s) problem but I cant say exactly what it is, open or no splice or a short or wrong orientation????????

John T

Janicholson · Jul 21, 2013

I agree with John T.
With the gen disconnected from the wiring, if you lift the armature brush (or the third brush if a 3 brush gen it is the one not 180 from the others) and put a strip of plastic under it (milk jug thickness cut to a brush wide strip) it will isolate the field from everything.
It then should have infinite ohms (no connection) to the generator frame (more than 100K ohms). It should also have about 2.5 to 3.5 ohms measured between the F terminal and the Arm terminal. If either of these are different the Field is improperly connected.
Jim

John T · Jul 21, 2013

Jim, you may know the answer to this, if you buy a new set of fields is the geometry and mechanics such that its impossible (or maybe possible) to get them on backwards or the wrong ends in the wrong place which could reverse the magnetic North/South Polarity???????? Sounds like he has some sort of a field problem and unles you can pass current through both them in the right direction and achieve the correct resultant North/South magnetic polarity in the Field Poles, YOU CANT EVER CHANGE THE INITIAL START UP POLARITY

Whatch think Professor?? Youre still active in the "field" while I,m gettin old n worn out lol Maybe Bob or Bob M knows also??

John T

Janicholson · Jul 21, 2013

Yes no maybe.
A set of field coils that are one continuous wire (no splices) would need to have been made very wrong. One would need to have been wound counter to the correct magnetism. A hand wound set from a private rewinder with a flat learning curve in a new employee might do that.
In this case he indicates the generator makes 7.8 (IIRC)so that would not be a reasonable guess.
A set that were installed with one up side down would be the same as above, but require the curvature of the coil to be reshaped. Nah!
A set made to be soldered into the housing between the coils (separated at birth) would work correctly due to the winding direction being a given.
One winding shorted to ground would still allow polarization of one pole, and doubling of field current and it would have output all the time.
Detected by the test I put above.
That is the best I have. Jim

mhb@ufe · Jul 21, 2013

It is a three brush generator and when I insulate the armature brush I get 2.5 ohms through the fields but I also get 3.1 ohms from field terminal to the generator frame. If I disconnect the field wire from the brush I still have the 2.5 through the fields and no connection to the frame of the generator.

Mark

Janicholson · Jul 22, 2013

The brush may be conducting to ground check it. Jim

mhb@ufe · Jul 22, 2013

(quoted from post at 07:24:25 07/22/13) The brush may be conducting to ground check it. Jim

I disconnected the field wire from the brush and I get 1.0 ohms from the brush to generator body. If I lift the field brush or the ground brush from the comutator there is no continuity from the field brush to ground. Is it the armature that is causing my proplem?

Mark

Janicholson · Jul 22, 2013

No that is normal. the armature is grounded through the other (grounded) brush. Jim

Farmall C generator polarity

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