Starter was fine... Now A Slow Whirr...

[Doctor Shifty]

OK, got the starter out, took it to the shop, the guy said that the bushings are worn, and the rotor is making contact with the magnet and thats the short right there.

Electric motor lesson coming up. :-)

This (the contact between armature and magnets) is not an electrical short and is not the reason for the heating up. The leads heat up because they are drawing so much current because the motor is not turning fast enough.

A motor runs on EMF (ElectroMotive Force) or voltage. When the armature spins in the magnetic field it also acts as a generator, making another EMF in the opposite direction (called back EMF). This controls the amount of current that the battery voltage can send through the wires.

When you prevent a motor shaft from spinning it is still drawing the supply EMF but not generating any back EMF. The result is high current and the wires will either melt themselves or burn the insulating varnish off. When they melt you have an open circuit, when they burn the varnish off you have an electrical short.

The reason for understanding what is happening is this:- If you are going to replace your bushes, all you have to do is fit them with close enough tolerances so that the spindle doesn't hit the body and slow its rotation. That is all you need.

Having said that, remember that as soon as you load up the motor the spindle will try to push to one side. It will spin easily when out of the bike, but as soon as it is in under load the tolerances have to be close enough to hold against that sideways effort.

And the good thing about all this is ... When a motor is side-loaded as the starter motor is, then the bushes tend to wear more on one side. It might be that you can turn the existing bushes through 180 degrees and you have a temporary fix. I haven't seen the motor and don't know if this is possible, but it's worth a look.

You have now survived DC Machines 101. :-)

Wow, this takes me back. It was 1968 that I started studying Electrical Engineering, failed most exams because I was out riding dirt bikes, and in less than 3 years I was kicked out. 8O

Kim