8 replies to this topic

[chasbeeman]

I'm curious about how to test an armature for shorts with a basic multimeter.

[zipper]

I just put some input, a couple of volts and measure the amperage going through, adjusting so that it is some 2-3 amps and measure all poles. They should be equal and nothing may go when measuring through the axle. I do have a similar cheap multimeter which is unable to straight measure the low resistance of arm windings.

[Dave Crevie]

Set it for 200 ohms (at bottom of the dial) and check two adjacent comm segments. You should get a reading at each pair of segments. If you get OL, the pole is dead. Ideally, each pole should read close to the same ohms.

[havlicek]

To test for shorts, you really don't even need a multimeter. If you rig a cheap/small battery-powered buzzer with two contacts, you will get a closed circuit and the resulting "BUZZ" when you touch the positive and negative leads to the comm and the shaft or the comm and the stack (as long as the stack isn't coated). Some multimeters have a built-in buzzer to check for continuity (electrical connectivity across two or more pieces) and a setting for continuity, which will do the same thing... just touch the two leads (it doesn't matter which ones to which parts) to the comm segments and/or the outside of the stack or the shaft. If you get the "signal" then the arm is toast.

[chasbeeman]

Thanks, guys! Sounds simple enough.

[MSwiss]

All the above is good info.
 
The best way to check for shorts is turning your power supply all the way up, clip one lead to the arm shaft, and then touch each comm segment with the other lead.
 
If the arm is shorted, it will peg the amp meter.
 
By using a lot of power, you occasionally can get lucky, and blow out the short. You'll hear a pop, like a fuse popping, and when tested subsequently, it won't peg the amp meter.
 
What Dave is describing in post #3 is checking for continuity, pole to pole.
 
You can do that, also with your power supply.
 
Set the voltage at 3V or 4V, and touch the 3 combinations of segments, with the two leads. The voltage will drop, and as he said, you should get similar amp readings all three times.
 
If you have a bad weld, or a broken wire you will probably get two different readings than the normal one, and one, double that.

[idare2bdul]

The really fun ones are the arms that don't show a short till they get hot under racing conditions. Don't know how common that is now but back in the '70s it could be very annoying.

Some of the reasons for this happening was that contrary to popular myth, team guys sometimes used arms that could be described as blems(for blemished with what looked like cosmetic problems). Sometimes the problems weren't all that cosmetic. We took back armatures from time to time that had lifted wires, rebalanced them and used them for practice arms or giveaways to young racers. Sometimes the wire would lift more and sometimes after the balancing it wouldn't.

These arms were more prone to the heat short problem. Likely the insulation had been scratched during the winding process or the wire had a defect for the factory.

The irony here is that a few of these arms turned out to be really fast. Most of these armatures were G7 arms but some were G27. I don't think any of those were ever torn down or even remember if any were the fast ones, but you would wonder if they were if they had the proper number of turns of wire.

[Dave Crevie]

The intermittent short described above is not so uncommon in a motor with a loose winding. The loose loop can vibrate against the arm laminations and wear through the shellac insulation of the wire. If wires from two separate poles do this, you can have a "sometimes" short. Also, if the arm is grounded to one brush, you can have the same thing. A tiny drop of superglue will lock the wire down, and keep it from moving around.

 

That tiny drop can throw off the balance, though. So be careful when you add it. 

[zipper]

Swede Anders Gustafson made a lot qualifying records on Group 27 late '90s in Europe with a partially=burned 27 arm. He was quite overwhelming - until the comm wore out.