Getting back to your original question.....

I don't think a bigger alternator is less efficent and it might be
more efficient, but I freely admit I'm speculating. Consider:

-- The bigger alternator will have bigger windings and therefore less
I^2 R loss inside. This is a small factor, as by far the biggest heat
producer in an alternator is the voltage drop across the output diodes
and that won't be affected. Nonetheless, it should be a plus for the
bigger unit.

-- The bigger alternator may have a bigger case and should therefore
run cooler, always a good thing.

I must add that I'm not quite sure why you ask the question, Glenn --
your various posts make it clear that you have an excellent
understanding of DC systems..... What do you think?

Jim Woodward
www.mvfintry.com


Glenn Ashmore wrote in message ...
It is not a rule of thumb. It is an engineering fact. All the belt
drive makers have web sites and they all have pages about sizing belts.
I used the Martin sheave formulas and double checked on the Gates Belt
site. "A" and "AX" belts which are normally used on boat engines have a
maximum of about 4 HP each at the speeds and pully sizes we use. (6-7"
drive at 1500-3000 RPM and 2.5" to 3" driven at 3200-7000 RPM)

There IS a rule of thumb involved though. Counting efficiency losses
and the ever present safety factor, the rule is that an alternator
requires an average of 1 HP per 25 amps of output at 13.8V. So when you
get past about 90-100 amps a single AX belt just can't handle the power.

Joe Wood wrote:
Rule of thumb that I heard was that anything over 90 Amps should have
dual belts.

Joe Wood

Glenn Ashmore wrote:

Put another way, is a large alternator operating at 60% of capacity
more or less efficient than a smaller one operating at close to
maximum output?