Residual Magnetism

[ian Simpson]

From the Electrical Engineering courses I sat through more than 50 years ago, I dimly remember that generators need residual magnetism to start generating the desired EMF. In my 1928 Tudor I have a rebuilt generator supplied by Snyders some five years ago. All was well until this past month!

Now every time I start up the old dear after more than a couple of days the generator does not perform its function, and I run on battery capacity alone. I can start up the generation by shutting off the engine and shorting out the regulator with a length of wire connected across the two sides of the regulator.

Note the use of the word "regulator", I have a regulator in place of the cut-out.


Any ideas why the generator loses its magnetism?

[Willie Krash]

I have seen a cracked armature winding do this but very rare. You'd need a scope to see it.
when you polarize the generator you are of course strengthening the magnetism in the pole shoes.
As it takes lines of force to turn the generator on, the point that it will it do so is predicated upon a number of things, generator speed being one, the faster it turns the more lines of force.
Look at the commutator and make sure you have good brush contact. As the brushes shorten the spring tension decreases. The other is armature to pole shoe airgap.
All these variables play in to the ability of the generator to self excite.
if it will not turn on, in neutral rap the engine and see if it will. Perhaps this generator has been on the edge to excite.

[Mike V. Florida]

Originally Posted by ian Simpson

From the Electrical Engineering courses I sat through more than 50 years ago, I dimly remember that generators need residual magnetism to start generating the desired EMF. In my 1928 Tudor I have a rebuilt generator supplied by Snyders some five years ago. All was well until this past month!

Now every time I start up the old dear after more than a couple of days the generator does not perform its function, and I run on battery capacity alone. I can start up the generation by shutting off the engine and shorting out the regulator with a length of wire connected across the two sides of the regulator.

Note the use of the word "regulator", I have a regulator in place of the cut-out.


Any ideas why the generator loses its magnetism?

The instructions for the "cutout" style regulator state:

"Never short the voltage regulator terminals together as with a jumper wire to test the operation of the regulator or to flash the generator - The voltage regulator will be damaged. Remove the voltage regulator from the generator before performing any tests on the generator."

Could that be the problem, the regulator is no longer working correctly?

[Tom Wesenberg]

Yep, what Mike just said.
I would install a stock cutout or diode cutout and see if the generator self-excites.

[mshmodela]

Originally Posted by ian Simpson

From the Electrical Engineering courses I sat through more than 50 years ago, I dimly remember that generators need residual magnetism to start generating the desired EMF.

Ugh, 30 years since I took EMF as an EE undergrad.. However, before that I in high school I worked motors and generators (vocational school) and I do not recall one that lost it magnetism --most failures were burnt windings, bad end connections, or shaft bearing issues... I think the suggestions on cutouts etc. is likely but if you took the generator off the car and mated it to either an A/C motor or even a shop drill, with variable speed, you should likely see output or not...

[ian Simpson]

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Thanks everyone for your suggestions. I will investigate further.

All the best