For you electrical gurus in the audience...

M

Marshall V. Daut

Well-known member
I have fully or partially disassembled 14 Model A and T starters and generators with the intention of rebuilding them for my cars. I am NOT an electrically-minded or experienced person and have danced around addressing generator/starter issues for decades. But now I am going to man-up and face them head-on with these 14 generators and starters. No exchanges! And yes, I have the Model A starter/generator rebuild book. But it does not answer my question. I have looked on-line for an answer to what I consider a basic question - no success. Apparently my question is TOO basic to be addressed by the experts.

Here it is: If an armature passes a multimeter test and a growler test, does that mean it is up to snuff for re-use as-is and will perform its function correctly? In other words, is an armature either good or bad, no in-between? I realize that if the insulation is flaking off or burned, that is a different issue. That's an obvious problem. But I am concerned that my armatures test o.k. on the bench, but don't function satisfactorily in the car.

So, are there degrees of functionality hidden inside an armature, running from strong to weak that don't show up in bench testing, or is an armature either good or bad, nothing in-between?

'Sorry for the near-moronic question, you electrical gurus. But first things first. :)
Marshall
 
Marshall V. Daut said:
So, are there degrees of functionality hidden inside an armature, running from strong to weak that don't show up in bench testing, or is an armature either good or bad, nothing in-between?

'Sorry for the near-moronic question, you electrical gurus. But first things first. :)
Marshall
Click to expand...

I don't find that a "near-moronic question and would really be interested to hear what the electrical gurus have to say. I'm certainly not in that category.
 
The distance between the armature and the field coils is critical to the number of amps generated. If the armature has been significantly worn/ground down, even by a fraction of an inch, it will max out at less than 10A and be useless for a generator. So you need to know the factory dimensions of the armature and be able to check yours against the spec.
 
I would add that the commutator needs to be shinny copper, round, and with no runout. A polished surface is best. The gaps between the copper pieces should be cleaned out to slightly below the copper.

Alex, can the field winding be spaced out to make up excess gap, with iron or steel spacers that will conduct the magnetic flux?
 
If the armature passes the growler and commutator continuity checks as well as cleaning/truing and removing the mica between the segments, its good to go.
 
nkaminar said:
Alex, can the field winding be spaced out to make up excess gap, with iron or steel spacers that will conduct the magnetic flux?
Click to expand...

I'm not sure, my info on this is courtesy of TomW. When he hit this problem, he would just swap in a known-good armature. I think it was uncommon relative to other armature problems, he only mentioned it happening a few times. He was in a production mindset, though. If Marshall encounters this, maybe he can experiment to find solutions.
 
old31 said:
Curious.

If an armature passes a growler test, is there a need to do a multimeter test?
Click to expand...



yes, I do. Its quick so its mostly for peace of mind.



A clarification that I didn't think of at the time. I was thinking we were talking about generators. The mica should be cut on generators but not starters.
 
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I wouldn’t clean between the segments on the commutator, it can wear the brushes down, I recently read about that. Don’t clean the rust off of the steel segments or the gap between the armature and fields will be there great to work, learned that from Tom Wesenberg. Use a growler to test and no continuity from commutator to the shaft, the armature should be good. When you reassemble make sure the brushes are centered in the holders and move smoothly, I just found that on my starter, one brush would hang up, the spring was pushing off center. My starter would work for awhile and than acted dead making me think it was the starter switch. Good luck on your new endeavor!
 
I was told many moons ago that it is not necessary to remove mica between the segments as manufacturers use a softer materials in later motors. In the distant past they used a hard material that needed to be below the surface of the copper segments.
 
The general rule I've read is Generator commutators need the mica undercut, Starter commutators, it is not necessary nor even desirable.

I have a few generator and starter "bones", parts, and carcasses laying around and wondered about testing as well. From drilling down into the various forum archives a bit, the guys who used to work in the motor shops say that if they pass the growler you're most likely good to go.

Armatures most definitely fail now and then but the odds are good they will be just fine. Of course that was then, and this is now. You want commutators with some meat left on them, and they need to be (almost) perfectly round, with no runout. Good bearings/bushings & brushes/brush holders &c. and they should give good service. A lot of the housings and bearing plates and cases and such have thick coats of paint on them by now, that can interfere with current transfer. Reflowing solder joints on armatures seems to be common practice too.
 
Make sure starter shafts are straight. A lot of them are bent but can be straitened at a rebuild shop. They really take a beating.
 
Hi Marshall, Crankster (post #12) has it right on mica. His comment on paint as an insulator is also important.
Replace the bushing in the Bendix end plate.
Good luck you can do it!!
 
To cut the mica or not depends on the type of brushes, the hard sintered copper brushes of the A starter do not get the mica cut, the hard brush can wear the mica along with the copper bars,many European starters had soft more carbon based brushes, they need undercutting, the mica would chew them up.
Passing a growler test and shorting to ground is not a guarantee that it is good, I found that a output testing that proves each winding can generate its share.
Some growlers have a meter for this.
A real simple teas is to short between the bars and look for sparking, that between each bar sparks about the same.
In the picture is a piece of coat hanger, cotter pin and bent piece of steel copper coated welding wire( all chosen because they were laying nearby on the bench ),usually the 3/9 o'clock position has the highest output, but as long as you test in the same position (rotate armature) you are looking more for dead spots, the cotter pin gets hot in seconds, like 2, the welding wire piece about 5 seconds gets warm with a good growler and armature.
I have a 1 foot long piece of lamp cord, it is a short, the tip bent to touch the bars, and I shoved the voltmeter in( yes it is crude but I wanted to show simple cheap ways, I have been working towards making a video), there's enough amperage going through that "short" to make a loss of .737 volt --- the wire does get warm, the ampmeter is showing that 18.77 amps are being made, some armatures are wound that every other bar is a little different-- if you get a 5 amp, or 0 in one spot you have a problem . ---- I learned to resolder every armature
 

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