Home Forums General Discussion Technical Questions Composite MG issuse

I have a building with 4 Dover composites, 11 stops. One of the cars had a very noisy MG that went to the motor shop for work. I suspect the armature was coming off of the shaft. Our office had the commutator cut, armature cleaned and fields cleaned to clear all grounds.

MG set was re-installed today. Ran the car on inspect and found that the car was not suiciding properly, jumper off of the terminals and the ran through the brake. I found that the back contact of BR was dirty, cleaned the contact and no more driving through the brake. Suicide circuit appears intact.

Generator still noisy (office did not spec work correctly), I started checking the same stuff all over again. I found that I have a 5.5-6 volt output at the generator armature(bad) and 2-2.5 volts on my hoist motor armature with an idle car. Output doesn't appear to to build past 6 volts. I am concerned because when I run empty car up, I read only 3.5 volts on my hoist motor armature. The other cars have zero voltage on the generator or hoist motor when idle.

I have two generic sets of prints that are not job specific. I am grabbing a tach tomorrow morning to check speed in both directions on auto (doors out). I am wondering if improper compounding can cause this issue? I should also mention that I have checked all connections, installed and sanded in new brushes.

car is over compounded. decrease the series field strength by taking out some of the turns in the series field. you want to keep the voltage at the generator armature below 2 volts after stopping. if it is near 6 volts as you say, it will only take a few more volts to start it pulling through the brake. you will see that the car speed in leveling is quite a bit faster at level speed in the down direction. keep adjusting the turns in the series field until you get up and down speed close to equal

Double check Neutral first. Many motor shops use the AC way of setting neutral which is not adequate. Grab your battery and bi-directional ammeter and get it as close to 0 deflection as possible.

Hey Tarth, can you explain the procedure on using this method? I never have had to go to this degree(our Westinghouse gennies are stamped for neutral plane and you can generally tell by your levelling speeds and im pretty sure the Otis ones are too)

Thanks in advance U.

the dover generators that i have worked on, have the brush rigging pinned. you are not able to move the rigging to adjust neutral. I have removed the pin once to try and get a generator that would not compound properly, i thought that i could adjust the rigging like i have on many other generators that i have worked on, It did not go well. i compounded the car, and let it run, the first few runs were ok, then we picked up a full load of people, the very next stop, the car pulled thru the brake violently and headed up with the doors wide open. luckily i was in the car and kept the people from exiting. very hairy. had to put the rigging back to where it was originally pinned. ultimately i had to pull the generator for repair, as i could not get the car to compound. just thought i would pass this on, in case anyone decides to remove that pin.

The issue ended up being commutation. With the commutator being freshly cut, I needed to build a new film on the surface of the comm. Even after sanding the Crap out of all of the brushes yesterday, it still looked like a compounding issue.

Today I noticed that I was only building an output of 3.5-4 volts. After running the car for a while on inspection and then on high speed, my brushes got nice and shiny and the comm got a nice chocolate color; didn't have to move any taps. Up and down speeds were within 10fpm.

Thank you everybody for your responses.

If you play with adjusting the compounding, note that there is both a high speed and low speed compounding adjustment. On each pole piece, there is either a high speed fixed and a low speed adjustable winding, or a low speed fixed and a high speed adjustable winding. Also, the spiral shunt will let you get to less than 10fpm difference. Also, shimming the interpoles in toward the armature will reduce the residual voltage. Use brass shims, not steel ones.

A good solid pine 2x4 held on the comm will help build you up a good film to help with a freshly turned and undercut comm.

Wood on the comm was suggested to me (maple was the wood species recommended), anybody heard of using a capful of peroxide? I have spoken with a few mechanics that swear by it.

Whenever I've had a repair or a brush changeover, I have just used a com stone/chalk over the com with the brushes in and left the lift on inspection, to let them bed in overnight or at least during the day.

Motor shops use a seating (chalk)stone, I've always sanded brushes and let them run on hand overnight and/or for a day or two like uppo72 suggested. I've heard of older Dover and Otis adjusters making helpers seat generator brushes with sandpaper and ONLY pull in the rotation of the commutator direction. They sell commutator sandpaper at Martindale I think, for seating brushes. It's not supposed to get embedded in the brush.

Helpful trick, I don't know how well know this is but works great. Even with neutral set right brush holders can be set at different angles so brushes are not on proper comm segment at the right time. So take a piece of adding machine paper and wrap it around the comm. Cut it off when it overlaps in a valley between segments. Now remove paper and fold in half then half again. Now wrap back around comm and tape ends together. Now you can set the leading edge of each brush to the lines made when you folded the paper and you will have a perfectly quartered comm.

Excellent advice. Same idea. Ive heard to the paper all the way around the comm and tape it. Then take a pencil and stick it down in the brush holders (at the same identical spot in each holder) and make a mark. Then pull out the paper and measure between the marks, they should all be the exact same dimension from each other. May help if you label the holders like a clock, 2:00, 4:00 etc.