Generator Brushes and Adjustment and How It Works
 

I'm asked quite often about brush adjustment and how they work, so hopefully these pictures will help.
The first picture is a cutaway taken at the Model A museum at Gilmore Auto Museum in Hickory Corners, Michigan. The second picture shows the 3 brushes. This happens to be the style with a ball bearing in the rear, as used until about March 1930. At that time the bearing was changed to a bushing and the output stud was moved from the front of the case to the rear.

The brush on the left is the output ground brush and has no insulators on the brush holder.
The top brush is the adjustable brush (third brush) and it sets the generator output amperage.
The brush on the right is the output brush and connects to the output stud in the generator case.
Notice all 3 brushes have a long and short side, and the short side is always closest to the brush holder pivot post.
Also notice the adjustable field brush is thinner than the 2 output brushes.

The adjustable brush connects to the field windings, and the other end of the windings is connected to ground. So, consider the ground brush as 0 potential and the output brush as 6 volts potential. Now you can see that as you move the adjustable brush (field brush) closer to the output brush, you will increase the voltage feeding the field coils. This will increase the magnetism of the field poles, which increases the generator output.

So, you can see this also becomes a runaway situation. As the field gets stronger, the output increases, and this also increases the output to the adjustable brush, and it becomes a vicious circle. However since the output is connected to the 6 volt battery, this keeps the output under control. If you have a poor connection in the charging circuit, then the battery is no longer connected and the voltage can rise to as high a 40 volts. This can smoke the field windings and cook the armature. If the lights are on they will also burn out.

With the generator on the car, as you face it from the front of the car, the adjustable brush will be about in the 2 o'clock position. Moving the brush UP lowers the output, and of course moving the brush DOWN will increase the output. For most daytime driving a couple amps should work fine. The more amps, the more the water is evaporated from the battery. If you do a lot of starts and short drives, or night drives, then you will need to move the brush down for more output.

This picture shows too much grease for the rear bearing, and I removed about half of it before the final assembly. I also put a thin coat of grease on the adjustable brush ring and 2 tension holders, and a drop of oil on each of the 3 brush pivot posts.

Re: Generator Brushes and Adjustment and How It Works
 

Your generator knowledge is superb and always helpful to the rest of us Thomas, thank you for sharing with us!!!

Re: Generator Brushes and Adjustment and How It Works
 

Tom, I have never had my cover off yet. To adjust the third brush one way or the other, do you just push it with your finger/thumb one way or the other? Then start her up and check the amp gauge? Is the third brush attached to a ring that will pivot?

Re: Generator Brushes and Adjustment and How It Works
 

Yes, see my second picture showing the ring that pivots around the center of the end plate. Unless it's been taken apart, cleaned and lubed, you'll have to use a screwdriver to move the ring. I pry against the post for the brush holder. Do NOT pry against the brush itself.

Re: Generator Brushes and Adjustment and How It Works
 

3 brush generators are a mystery to many, including most auto electricians thee days. I've even had one or two who thought they knew about generators argue with me that it can't work. Showing them one even didn't convince one guy who accused me of rigging up a fake just to fool him!:D
Tow, I'm sure this will be a useful reference for many on this forum.

Re: Generator Brushes and Adjustment and How It Works
 

Thank you guys. Another "A" mystery solved for me!

Re: Generator Brushes and Adjustment and How It Works
 

Great post, and great information . I never really understood how it worked, and never took the time to research it. Th asks for the education. Jim.

Re: Generator Brushes and Adjustment and How It Works
 

Fantastic explanation and pictures to go with. Thank you so much Tom for the learning experience. You are really a great resource for beginners like me. Can't thank you enough.

Bob Anthony

Re: Generator Brushes and Adjustment and How It Works
 

Tom; thank you, this has been a "black hole" for me. This information is incredibly helpful

Thank you very much

Re: Generator Brushes and Adjustment and How It Works
 

Very interesting and informative! Thanks for posting;)

Re: Generator Brushes and Adjustment and How It Works
 

Thanks, excellent as usual.
Bob

Re: Generator Brushes and Adjustment and How It Works
 

Tom, excellent post, very informative. A slightly off topic question, on my car at low rpm ( 25 mph in high gear), the generator will charge 9 amps or so. As the speed increases, the charging rate goes down to about 2 amps at 50 mph. The ammeter is very steady, the armature is smooth and round and the brushes look good. It has worked this way for 15000 miles or so, not a problem, just curious as to why it is that way.

Re: Generator Brushes and Adjustment and How It Works
 

Thank you Tom! As always you take allot of time to help the rest of us out with well written explanations and photos. I love the EVR that you make and I purchased from you 3 years ago. It works great.
I used to adjust my 3rd brush with a wooded dowel while the car was running so it was easier to see the changed amperage on the gage. Do you see any issues with this practice?

Re: Generator Brushes and Adjustment and How It Works
 

You might consider battery oil or use mineral oil on a three brush system.
It floats on top of the "water" and greatly reduces evaporation.

Re: Generator Brushes and Adjustment and How It Works
 

Loved it. Thanks, Tom!

Your description also inspired me to go back to the Les Andrews Blue book for a system description of the generator. All good info for a newbie. :)

Thanks!

Re: Generator Brushes and Adjustment and How It Works
 

Tom, as usual you have come through with an excellent post and pictures to resolve a mystery to some..me included. Thanks for taking the time and effort to help clear this up.

Re: Generator Brushes and Adjustment and How It Works
 

If your commutator has a good undercut, then I'd guess you may have a weak brush spring. As long as the commutator bars aren't being burnt from arcing you should be OK. Output will fall off very slightly once the generator speed passes it's peak output point, but it won't fall off as much as you are showing

Re: Generator Brushes and Adjustment and How It Works
 

As Tom mentioned, the output will fall of slightly after peak output. The reason for this is quite interesting...

While the voltage from a generator depends on how fast the flux is changed (d phi /dt or faraday's law) and should increase as the speed increases, the designer of the Model A generator added some features that would reduce the "magnetic flux" as the field current increased thus reducing the effect of the faster flus change. For example, the shape of the iron in the field around the gap between the poles was changed to "self limit" or saturate as the field as it got too strong.

What the engineers were trying to do was make the output as constant as possible without using a mechanical voltage regulator.

Marc

Re: Generator Brushes and Adjustment and How It Works
 

Tom's pic, showing the "slip" ring that the brush is mounted to, helps clear up some "mystery"! His tip about prying against the mounting post would eliminate damage, if the "slip" ring was a little too tight. He also mentioned that the 3rd brush was smaller, which helps to identify it. THANKS AGAIN, TOM!!:D:D:D
Bill W.

Re: Generator Brushes and Adjustment and How It Works
 

Tom thanks for your wisdom, last summer this was the first thing I had an issue with and after discovering a spir on my ring which prevented the 3rd bush from moving, a small file on the spir won the day -- was well as cleaning the ring from rust etc.