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MG MGB Technical - Generator output

Hi all,

Just thought I'd throw a concept out there for comment. Could you conceivably increase the output of a generator in the early rev range by fitting a larger diameter pulley?

I imagine that the peak output is just what the peak output is however could you get their earlier with a bigger pulley?

Thinking mainly around slow moving traffic at night in winter while raining and indicating to turn!!!!!

Thoughts appreciated.

Regards,
C Brennan

A bigger pulley on the generator will reduce the running speed.
Dave O'Neill 2

You would be able to increase the output at low engine speeds by fitting a smaller pulley, but would need to be careful you didn't exceed the rpms for the generator/dynamo at high engine speeds.

You won't increase the maximum current unless you fitted a more powerful dynamo. Or an alternator.
Paul Hunt

But do you need it? The dynamo/alternator runs electrical stuff, only calling on the battery when the requirement exceeds the dynamo/alternator output. Your battery is XX amp hrs ?70. So even if it did pull some charge out of the battery and the ignition light came on, as soon as you rev'd it awhile it would be back to normal. Even the headlights are only about 10 Amps so you have quite some lee-way.
Michael Beswick

If you are on a dynamo, don't do anything to increase it's speed, they are not happy at high speed because of the way they are made. Alternators will take higher speed running, bearing in mind Paul's point about recommended Max rpm.
Allan Reeling

Dave et al,

Absolutely all good points, and too right about the big pulley vs small pulley!

I have worked on all of the earth points so this may well assist anyway.

Thanks to all.

Cheers,
C Brennan

Generators do not take kindly to high rpm's
Many moons ago I raced a Triumph Tiger 100.I blew up two generators when the dealer informed me that this was the last one under warranty.
The high generator speed was blowing the copper segments off the armature.
I simply removed the generator while racing as it used a magneto for ignition.
Sandy
SANDY

In the dynamo era the 2-6 volt batteries in series gave a lot of amp-hr reserve. Of course that is much less with the common single 12V setup.
Robert McCoy

The Workshop Manual quotes the original battery capacity as 51 Ampere-hours at the 10-hour rate or 58 Ampere-hours at the 20-hour rate.

Depending on supplier you can get up to 88Ah in 6v, but only 50 or 52 Ah at the 20 hour rate for a 12v that fits a 6v hole.

That's really only an issue if you park with lights on, which is why manufacturers these days more often quote capacity in 'cranking amps' or 'cold cranking amps'. One source quotes the original MGB 6v batteries as 360CCA, but the above modern construction 52Ah 12v battery to fit a 6v hole is said to have 520CCA.

Incidentally, unless you insist on the original tar-top exposed link 6v batteries, and opt for modern single-fill 6v instead, you get the benefits of modern construction.
Paul Hunt

This thread was discussed between 06/05/2015 and 24/05/2015

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