Alternator speeds

[veggie]

What rpm is necessary to get the rated amperage for a given alternator?

For instance, my accessory drive sheave on the listeriod is 5.5" dia.
The v-belt sheave on the alternator I'm looking at (older Ford F150) is 2.5" dia.
With the listeroid at 650 rpm the alternator would spin at 1430 rpm.
How does one estimate the output of such a setup?

I have seen many alternators running off the 5.5" water pump pulley on a roid. I would prefer to do it this way for reasons of easy setup on my particular base.
Besides, if I drive an alternator with a 3" sheave from my 23" flywheel, it will spin at ~4300 rpm.(Too fast??)

Veggie

[mobile_bob]

many if not most automotive alternators don't produce much of any amperage at ~1400rpm
its just too slow, and of those that are designed to really kick butt at low rpm they suffer from
lack of cooling from the fan being turned too slow.

you mention 4300rpm, that is not at all too fast in my opinion.

you get much better airflow/cooling, and if you aren't drawing many amps (which is likely anyway)
the field current will be so low that the hp required to drive the thing will be almost unmeasureable.

its much more trickey to get something working that will be useful now and have room for expansion
later if you insist on driving with the 1400rpm limit.

driving at 4300rpm opens the doors for other capabilities later should you wish to explore them.

why paint yourself into a corner is my motto.

btw, most automotive alternators are turning ~6000rpm at highway cruise speed anyway, to give
you some perspective, and most all mid and heavy truck alternators also turn in the 5500-6000rpm
range at rated engine speed as well, and they run for a very long time with little effort in one of the
most demanding, hostile and downright miserable environments one can imagine. (think well over 200
degree's F. ambeint temps, vibration, dust/dirt/oil fumes, water spray, salt etc)

if it were me and i were to build a listeroid, i would use the same belt (serpentine) that i drive my st head
with to also drive a hd truck alternator using a switchback scheme and a tensioner, this would allow for the
use of a common belt and provide for nearly a 180 degree belt wrap on the alternator, as well as increasing
the belt wrap on either the flywheel or the st head pulley or possibly both, which would aid in keeping down
the belt chirp issues some report.

bob g

[veggie]

Good ideas Bob.
Thanks for that.
I thought that 4300 rpm was a bit to high, but from what you tell me, it's just fine.
I will look into incorporating the alternator drive into the 6 groove serpentine ST head belt drive.

Cheers,
Veggie