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Re: Weak dynamo magnets

A long time ago I refitted a rewound armature into it's stator and found it was rubbing on the pole pieces which I hadn't had out. The pole piece securing screws were tight, however with the aid of two G clamps I was able to squeeze the pole pieces into the stator and get another turn on each screw which resolved the problem. So the pole pieces were removed everything cleaned and refitted. I often wonder if poor contact between the pole pieces and the stator has any negative effect on the ability of the dynamo to retain it's residual magnetism. It is this residual magnetism which generates a small voltage which is fed to the field to increase the field strength and so the dynamo self excites. I love that term. Alternatively a poor connection between the armature and field will prevent this from happening.
Finally one of our cars had a non operative ignition warning light and the dynamo wouldn't generate if the car was left for a couple of months, in the end I fixed the ignition warning light and that cured the dynamo failing to generate. The ignition warning light provides a path for a small voltage to be applied to the dynamo and may help in it coming to life.

Re: Weak dynamo magnets

AdrianPayne
Evening All
The iron cores on my CAV DFL dynamo loose all their magnetism if not used for a couple of weeks. I have to resort to revving the engine and manually closing the cut-out points to get some current in the field coils. After this the dynamo and cut-out work perfectly until the next layup. Would a strategically placed rare earth Neodymium magnet do any good or harm.
Or are there any other fixes for this complaint.
Many thanks.
Adrian.


I'm Harking back to my memory of theory I was taught during my apprenticeship here, so hopefully this has not been scrambled by the (many) passing years since. The field shoes in your dynamo would probably be some kind of steel rather than pure iron. Iron takes, but doesn't retain magnetism well, steel doesn't take magnetism so well, but does retain it.

None of this theorising helps with your problem though. But when I had a similar problem with a dynamo -losing it's residual magnetism- the fault disappeared after I changed the slack bearings. if it wasn't coincidental, I can only put this down to the armature rattling about between the pole faces before I changed them.

Location: N W Kent

Re: Weak dynamo magnets

Thanks everyone for your replies.

The ignition light is not the problem as my car would never of had one.
This problem does put into question the usual test for a dynamo of connecting the field and dynamo wires together and spinning with a drill etc. If there is insufficient residual magnetism then no output will be measured although all the windings may be ok.
The only issue I see using a small permanent magnet might be that the polarity might be wrong, similar to flashing the field coils when changing from positive to negative earth etc.

Adrian

Location: The new forest

Re: Weak dynamo magnets

Adrian ,I don't know your depth of knowledge regarding vehicle electrics but if you take the dynamo off the car then you could connect to a battery and run it as a motor , which is in effect what you do if you "flash" the field .

Bob , regarding lack of residual magnetism , I was thinking along the lines of " de-gaussing " . Not knowing how the car is stored etc means there may be something in the vicinity doing that .

Merv

Re: Weak dynamo magnets

In normal running, esp if adjusted for modest output, or when prodding the cutout, the field magnetising current is modest. Possibly a full polarisation with the 3rd brush lifted off the armature and earthed directly, and battery applied to the isolated field terminal for a short while will make things more permanent. If the paint is not pristine hammering on the dynamo body at the same time may assist (Rivetted ships became very magnetised). 12v briefly would be OK.

(If anyone is referring to the Williams Special building book for polarising, the description there is wrong)

Merv emphasises an important point. On the car, most tinkerers press the cutout at some stage, but certainly a point to note off the car. Care necesary that polarity matches that in the car.

Location: Auckland, NZ

Re: Weak dynamo magnets

Dave Mann writes:

For those who really want to do the job right, here is a man sized tool for the job just a couple of hours to put together. Regards Dave Mann

 photo Pole spreader - screwdriver_zpscyar4jwk.jpg