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Wiring a CAV dynamo

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vintage engineer05/10/2019 21:00:30
259 forum posts
1 photos

Anyone know how to wire this dynamo. It has 3 terminals Plus, Minus and an unmarked one in the middle.dynamo

Brian Sweeting05/10/2019 21:23:21
453 forum posts
1 photos

Stroke of luck whilst searching brought up this pdf which should help...


Redsetter05/10/2019 22:01:43
205 forum posts
3 photos

Interesting PDF which does look about right, though not too easy to follow!

Before seeing that, I would have said that it is a "3 brush" dynamo. Positive and negative terminals are the output from the armature via the main brushes. The unmarked terminal is the field winding. The field winding is energised from the armature via a third brush, and the unmarked terminal is connected to a cut-out and possibly a switched resistance to control the output. The field circuit is then completed by connecting to either the positive or negative terminals, depending on the polarity/direction of rotation required.

Howard Lewis06/10/2019 03:01:38
6316 forum posts
15 photos

It is more likely that it is a two brush machine, meant for use on a commercial vehicle with two pole wiring (not using the chassis as the return ).

C A V were part of the Lucas group, but tended to cater for the commercial market, where Lucas dealt more with the car / light van market.

The + and - terminals are just that, the third terminal would be the field winding which is fed from the voltage control box. The control box controls the output by effectively providing the field wings with a pulse width modulated supply.

The output from a third brush machine would be controlled by the third brush being wired internally. Moving the third brush around the commutator, over a limited range, will change the excitation voltage applied to the field.


Redsetter06/10/2019 06:27:39
205 forum posts
3 photos

Not all three brush dynamos have a moveable brush

Given the age of the machine it may pre-date the use of a voltage control box.

I could give examples, but I'll shut up now.

Reasonably easy to look inside it,rather than speculating.

Howard Lewis06/10/2019 16:45:23
6316 forum posts
15 photos

My first car was a Singer 9. The charge rate was controlled by the headlamp switch.

In the OFF position, the field windings were fed through two resistances. With Sidelights selected, one of the resitances was shorted out. When the Headlamps were turned on, the second resistance was shorted, so the dynamo then delivered its maximum charge. In this way, to compensate for the higher electrical load of the lights.the output voltage increased, and so the charge rate.


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