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Rewinding a DC Starter Motor

03-14-2020, 11:40 PM
Hi guys,
I have a small starter motor that actually belongs in a single cylinder 200cc 2 stroke engine that is failing.

Symptoms, even with brand new charged batteries, the starter is barely turning over the engine...and after a couple of tries actually stops working altogether. Cleaning the commutator and brushes results in a repeat of the above symptom...initially barely turning over then stops completely.

The positive (active) brush is burned quite badly even after cleanup when I attempt to start engine.

I performed some tests on the commutator and armature.

the armature is fine (shows open line on digital multimeter as it should)

The commutator on the other hand seems to have a problem. When I check resistance across 180 degrees and also on adjacent poles, the resistance initially is about 2 ohms but then after a second or two, drops to zero.

my understanding is that resistance dropping to zero means that there is a short in the wiring coils running from the commutator and around the armature.

So, I have decided to remove all of the wiring coils and rewind the motor. The existing coils were 7 winds across 4 armature poles (there are 10 commutator and 10 armature poles).

-The copper wire used for the winding is 0.8mm
-I believe polyurtherane insulation
-The existing wiring was glued (a polyurethane or epoxy adhesive?)
-The Armature is also coated with a green coating prior to winding that I assume is to ensure that the wiring and armature are well insulated from each other. I have made some repairs (with 24 hour epoxy) to that insulation in that when it was removed the old wiring and glue, some of the original green coating came away from the armature.

1. For a starter motor, is it critical that I use a polyurethane coated wire over an enamel one?

2. The existing wire that failed was 0.8mm, in what way would increasing the thickness of the new copper wire to 1.0mm affect the performance of the starter motor?

3. Do i need to reglue the winding (ie must i use an insulated wire that contains and adhesive coating that is heated to cure the glue?)

03-15-2020, 03:07 AM
I used to work at an electric motor repair shop, but I didn't do rewinds, only assisted on larger ones when needed. Some of my advice should be taken lightly at best.

1. I'm not 100% sure how (or if) different coatings affect the way the winding works.

2. You need to stay with the same thickness wire, as well as same amount of turns and same wiring hookup, unless you know the formulas for how each affects a motors performance.

3. If there are stacked windings (ie more than one winding per slot) you have to use an insulating paper between the windings. After the final winding is in you need to use an insulating "stick" to hold it all in place. After it is all in place, and your connections checked and verified you need to use an epoxy to seal it.

For larger armetures and stators we had a large dip tank with an epoxy that we would dip them into until they saturated then bake them in a large oven. For very small ones, especially ones that needed a quick turn around we would use a 2 part epoxy and pour it over the armeture.

Once it sets you'll need to sand the epoxy off each "rib" that separates each coil.

The whole process is tedious. For very small armetures we actually sent them out to another company, in Texas I think, because for the price they did them it didn't make any sense for us to do it ourself.

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