braided strap amp capacity for dynamo earth brush

[JonE]

I've noticed that the little braided straps for the earth brush on the late Seven dynamos are often quite old and deficient.
The braid seems to be 15A - about the 3mm width the originals might have been when new as its quite tight for space in there. If one is going to dispense with third brush at 6V, should this wire be upped in size or will that 15A spec do fine just to restore what should have been?

[John Mason]

Hi Jon E, The dynamo on my Mk1 Ruby was converted to a two brush dynamo about 12 years ago by a fellow PWA7C member. He supplied a diode type of arrangement that he had made that worked with the existing cut out on the car. This works well and gives better charging and longer battery life (no over charging.) I now constantly run on winter setting without problems. There was no alteration made to the braided strap. He is quite well up on electrickery and if needed I am sure he would have addressed this.

John Mason

[JonE]

ok, thanks - I'll just buy this 15A stuff which looks about the same.
Was just wondering what the ideal relationship between max output and actual physical spec would be.

[Bob Culver]

The earth wire takes all dyn output so is much same whether 2 or 3 brush. In free space could merrily run very hot so mechanical considerations the main factor determinng thickness.

[JonE]

Bob - thanks, but what does that last sentence mean please? is it.... No point in making larger bore because won't dissipate heat as well? Or.... if you have the space, put 30A in as it will help?

[Bob Culver]

Mechanical strengthe the main factor. Very thin wire will just run hot but as it is not touching anything and surrounded by air of little consequence.

[Ian Williams]

It runs hot because or resistance i.e it is too small for the amount of current trying to force its way through the wire. On such a short bit of wire this may be if little consequence, but in a wider context resistance in a 6v system is an important consideration.