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

CAV NE

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.

Howard

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.

Howard