Positive or negative earth Lucas dynamo

[Vanden Perre]

Hi to all,

I just finished placing a new Lucas replacement generator. I was very pleased until a noticed it's written on it with big characters "Negative Earth'. How did I noticed it before working 1h30 to install it !!! My S3 is still in positive earth.
Now my question is: Is there really a difference between a negative and a positive earth dynamo? I just read this on the net:

"To polarize a Lucas dynamo for POSITIVE earth.
Your battery's POSITIVE terminal should be connected to the chassis or frame.
Disconnect the D & F wires from the dynamo. Run a wire from the unearthed terminal on the battery (NEGATIVE) and "flash" it onto the F terminal on the dynamo. By flash I mean brush it over the F terminal 2 or 3 times until a small blue spark can be seen.
Your dynamo's field is now polarized for positive earth and ready to safely connect to an electronic regulator."

Any advice?

Thanks.

Olivier.

[PeterK]

No physical differences between positive & netative arthed dynamos, just how the field windings are polarised.
Good instructions (just as I recall when doing the opposite and changing to negative earth) and everything should work nicely afterwards.
Peter

[billwill]

That description is correct as far as I know.

A dynamo can be either positive earth or negative earth, provided that it does not contain any permanent-magnets.


The polarity is determined by the residual magnetic field in the soft iron core of the field coils. This is sufficient to get a small current created, which then flows through the field coils, strengthening the magnetic fields & so the dynamo output climbs up to the point at which the iron magnetic field is saturated which is the max output design condition. Generally the dynamo controller will switch the field current off & on rapidly to achieve the average output needed under given condition.

So brushing the field coil with a battery in the desired polarity can change the polarity of the dynamo.

They need to mark the dynamo to show what polarity it left the factory, which is what you saw.

[AHM]

With a Lucas Dynamo/regulator I always do it at the regulator - A to F - no need to unplug anything and you can't get it wrong as it is the same for Positive or negative.

http://www.youtube.com/watch?v=PPUfQtVEJGg

[Vanden Perre]

Thanks for the confirmation.
Olivier.

[billwill]

<later> NO I checked and found that this is wrong... the dynamo controller does not connect it to the battery until AFTER it has generated enough voltage and the polarity of that voltage depends on the polarization of the field coil </later>

WRONG: I suspect that the process is not actually essential, because I think as soon as you try to start the engine you will be feeding current from your car battery into the field coil in the correct direction and it will polarise it anyway.

But heck it's nice to do it first and maybe spin the rotor in the correct direction and see the nice new volts on a multi-meter.

[AHM]

Bill,

I think it is disconnected by the regulator until it produces enough volts?..... which it might not do.

Also if it was the other polarity wouldn't the output also be the oposite polarity to the battery?

Looking at the regulator in my hand F gets powerd from D so the dynamo needs to produce some volts in order to power the field coil. I'm guessing that is why it needs residual magnetism. Makes sense or it couldn't controll the cut-in voltage.

[billwill]

Bill,

I think it is disconnected by the regulator until it produces enough volts?..... which it might not do.

Also if it was the other polarity wouldn't the output also be the oposite polarity to the battery?

Looking at the regulator in my hand F gets powerd from D so the dynamo needs to produce some volts in order to power the field coil. I'm guessing that is why it needs residual magnetism. Makes sense or it couldn't controll the cut-in voltage.

Aye I was wrong, there... Bad me...
I found a very good description of what a dynamo controller does here from the MG guys:
http://www.mgb-stuff.org.uk/controlbox.htm

From that the only initial current from the battery would be through the ignition warning light.


The full info on Lucas dynamo controllers is at:
http://www.google.com/url?sa=t&rct=j&q= ... 4Pva070y-A

[AHM]

The Lucas manual was a good find Bill