I have just rebuilt a model t generator and I am having problems setting the brushes. This procedure is outlined in the MTFCA electrical manual on page 7. When I apply 6 vdc it run as a motor and moving the brushes has no noticible effect.
I am sure I wired the field correctly, crossed the wires and I lifted the third (small) brush just as the manual instructs. I have done this several times before and I have always found the so called null point.
I have recovered the field winding and removed the pole pieces, reinstalled with new screws
What have I done wrong?
(Message edited by Tony_bowker on September 30, 2014)
Did you disengage the third brush from the armature ?
Yes. See third line above.
It can't be the field winding as they are not connected, by lifting third brush, so it has to be the commutator or the residue magnetism in the pole pieces. I will compare the magnetic fields to a known good generator when I set up again on Thursday..
I am traveling in Argentina and will return Oct 5.
If you are still having trouble give me a call.
Ron the Coilman
Are the brushes making good contact with the commutator? Weak springs, or commutator turned down too much could cause a problem. However, since it runs like a motor, I don't think that is the problem. If you loosen the 4 screws that hold the brush plate, and you move the plate in each direction, you should find a point where the generator rotates to the right or to the left. The point where it doesn't rotate in any direction is the "null" point and that is where it should be set. From your post, it seems as though you did all the right things. I don't know why you can't find the null point. Unless, the plate might be rotated in the cover. Are you sure the plate is on correctly? Check the position of the brushes in relation to the output contact in a working generator and see if the ones you are working on are in the same relative position.
"It can't be the field winding as they are not connected, by lifting third brush"
Is the gen connected like this:
Yes. When I lift the small brush, the one on the right of the sketch above, the circuit to the field coil is open. If the circuit is open, no current can flow, there will be no magnetic field caused by the field, therefore the lack of a null point can't be caused by the presence of the field winding....
Did you polarize the generator field after you replaced the field coils? You need some residual flux in the generator to set the brushes for armature reaction. Mike
There must be some residual magnetism for it to run as a motor with the third brush lifted, therefore no field magnetism.
This afternoon it checked the magnetic field by the four screws, seems to be N S N S just like you want. Very strange. Now I am going to change the rotor...
Well.... if the armature turns, the must be current flowing in the armature, the main brushes must be passing current and there must be residual magnetism in the field. How far can you turn the brush rigging? Can you get nearly 90°? Does the speed of the rotation change as you turn the rigging? Is it possible that the brush rigging is getting caught up on something and not letting you turn it far enough?
Methinks that drawing may be wrong, I'm thinking that the ground should be on the brush on the RH side of the drawing, not the third brush.
Here's is one of my rebuilt brush plates. The large brush holder are the main or armature brushes. The small is the 3rd brush holder.
The output is on the top, the grounded brush is on the left and the 3rd brush is on the other side of the main brushes.
Did you ever find out why you couldn't find the null point on your generator?
Last week I started up a car in which I had rebuilt the engine. In the process of the work, I installed a new gear on the generator. When I got in and drove the car, the generator started charging irratically and then quit charging. I changed the cutout, but that didn't fix the problem. I cleaned the commutator and that didn't fix it. So I took out the generator and it wouldn't run as a motor. Anyway I checked all the connections at the brushes and found one screw loose. I tightened it up and the generator ran like a motor. I installed it and now it charges. The connection was probably loose before I worked on it but when I installed the gear, the pounding on the pin must have moved the wire just enough to cause the problem.
The reason I am posting this is that maybe you too have a loose connection of some kind.
Anyway, I hope you find the source of the problem and are able to fix it.
Someone suggested I NOT cross the wires from the field coils, it had no effect. Actually it does not surprise me as when the third brush is lifted, there is no current thru the field coil.
I have given up for now and started to try a different rotor, I'll report back when I have completed the rebuild.
When I changed the rotor, the problem was still there....
Now I am guessing what to change next, so I changed the brush holder. It now has a null point and then when I lowered the third brush, it generates...
So the problem is in the brush holder. It looks physically correct but there is a leakage on the output brush. I have no idea how this would effect the null point, but there it is, solved.
"So the problem is in the brush holder." ..... or was the insulator broken shorting out the third brush ????
I'll bet if you conducted a detailed resistance test on the brush-plate you would find a partial short between ground and the output brush.
There are three main electrical components in the generator; brush-plate, armature and their failure rate is in that order.
A "rebuilt" generator will always include a fully rebuilt brush-plate.
Some people cut the brush-plate insulators in half repairing only the third brush area. That is a mistake because all three brush holders may have electrical faults that, thought not visible, are critical to obtaining a working generator.
It is the difference between "patching it up" and "rebuilding" the generator.
Ron the Coilman
Ron is correct, there is a partial short between the output brush and ground. The insulation "looked" OK so I didn't replace it. Big mistake. The new brush holder i used on the second rebuild, has new insulation and works. I will pull the bad one apart, it will probably work just fine, maybe next week....
I see I misspoke in my message above.
That should read "There are three main electrical components in the generator; brush-plate, armature and field winding and their failure rate is in that order"