Testing magneto windings

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Model T Ford Forum: Forum 2012: Testing magneto windings
Top of pagePrevious messageNext messageBottom of page Link to this message  By Anthony Bennett on Sunday, May 06, 2012 - 12:08 pm:

G'day blokes,

I was having a quiet little thought of testing unknown magneto windings the other day, as one I recently removed from a car has sustained some damage.

Though it seemed to be working quite well, the plate in question has suffered an impact from a loose magneto clamp.

Being that most windings adjacent the starter motor seem to be victimised in a similar way, I was interested to see if I can test for windings that are shorted internally.

If I were to apply say 2 volts DC to the plate as an assembly, the voltage drop across each winding would be hard to measure accurately, however there would have to be a discernable difference in magnetic strength I would think. Were I to use a spring scale to pull a test piece from each pole, that could be measured, or at least compared from one bobbin to another.

I think I'll have a crack when it's back from the motor rewinders with a fresh coat of varnish.

any other ideas welcomed;)

cheers

Anthony


Top of pagePrevious messageNext messageBottom of page Link to this message  By Hal Davis-SE Georgia on Sunday, May 06, 2012 - 12:24 pm:

I put a flashlight battery across the one I rewound then checked the magnetic draw of each pole piece with a hacksaw blade. Not real scientific, as I was just using my perception to determine if the draw was about the same from one to another. It would at least tell you if any were shorted to ground as none of the others between the short and the last coil (Which is grounded) would have any magnetism. I don't suppose it would tell you if any coils were shorted internally unless the short was across several windings. I doubt you could tell the difference in magnetism of a coil with one less winding with a hacksaw blade in your hand.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Thode Chehalis Washington on Sunday, May 06, 2012 - 12:45 pm:

As Hal said, "I don't suppose it would tell you if any coils were shorted internally unless the short was across several windings. I doubt you could tell the difference in magnetism of a coil with one less winding with a hacksaw blade in your hand."

It also follows that that you would likely not notice any decrease in voltage output or performance in the car under the same circumstance. Probably the killer would be if it was shorted to ground, especially closer to the live end.

Jim


Top of pagePrevious messageNext messageBottom of page Link to this message  By James A. Golden on Sunday, May 06, 2012 - 03:25 pm:

The white area in this photo is smoke from a shorted coil ring. It only took about two seconds of applying 6 volts to the terminal to make the smoke appear.

Smoke


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Monday, May 07, 2012 - 07:43 am:

James:

Your picture is why I have repeatedly told people to NOT apply power for more than a split second when doing an "in-the-car" recharge of the magneto. Most of the procedures call for holding the power connected for a few seconds to "burn off any shorts" which is an absurd idea as you have witnessed by this picture. It takes less than .040 seconds to actually charge the magnets in the magneto once you apply the power. After that time interval you are merely heating up the coil winding and possibly damaging it. Shorts are far more likely to be installed rather than burned off which is why you don't do an "in-the-car" magneto recharge unless you have nothing to lose by trying it.


Top of pagePrevious messageNext messageBottom of page Link to this message  By James A. Golden on Monday, May 07, 2012 - 08:30 am:

John, you will notice those coils were recently redone without the insulation pieces between coils and the frame. I was actually able to remove that coil and clip two small 3/8th inch pieces from the ends hidden on the back side and glue them over the short area with Devcon 5 minute expoxy and then glue the coil back on the frame and re-solder the connections. It no longer smokes or shows any shorted coils, although the two coils near the starter notch had been damaged and painted over. That coil ring appears to work fine on my HCCT, but I would not put it back in a vehicle, due to the amount of work required to change it out again.

I suspect that often a person could remove those coils one at a time and wrap a fresh covering around them and replace them without even unwinding them. The real secret is finding a source for that wrapping material.


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