Get bit complicated but the limiting factor for an alternator is the
internal resistance of the windings. Since it is really in series with
everything if you have 90 amps with a drop of 12 volts you are dissipating
1,080 watts as heat. At 750 watts/HP you have 1.44 HP right there. For the
alternator rpm it is the ratio of the circumference of the pulleys. You'd be
surprised how fast that thing spins at 4000 engine rpm.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
David Beierl
Sent: Sunday, September 23, 2012 2:27 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Alternator continuous-duty rating?

>Just some context - with 90 amps at 13.6 volts, assuming 85% efficiency
>the alternator will be sucking down about two horsepower from the belt
>and dissipating about 225 watts as heat.

Hmmm...Leece-Neville show this 90-A alternator using almost four horsepower
at full output:
http://www.prestolite.com/productinfo/alternators/8AR2112T/8AR2112T_curve.jp
g

I guess 85% is a bit optimistic.

I've also been seeing alternators rated full output at 6,000 rpm, with max
rpm of 6,500.  Our alternator spins a lot faster than that - just measured
the sheaves, and alternator runs twice engine rpm or a bit more.

And you're right, the big three-groove crank pulley is tapered, at least on
the engine I just looked at (random 2.1l core).  I've been looking at that
thing for twenty years now...in the engine bay, mostly.  Not that that's an
excuse. <shakes head>

Yrs,
d