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[Dr. Gerald N. Johnson, electrical engineer]

You don't generally need a special alternator to give 24 volts, just a
pulley to be sure to turn it faster and a regulator set for 24 volts.
E.g. a 2:1 voltage divider ratio at the sampling input.

There was a fairly good regulator circuit using an LM-723 in the Holiday
'76 issue of 73 magazine. I didn't prefer its use of the internal Zener
to drop the drive voltage to the pass transistor, I preferred to move
that power dissipation out of the chip, but otherwise the one I built
about that time was identical. I used mine to replace faulty Ford
factory regulator.

Several years ago I was experimenting with a Delco alternator and found
it needed to turn about 1400 RPM to self excite and then it produced
about 14 volts. With 12 volts on the field it would need to turn 1800
RPM to do 28 volts. Increasing the field voltage may eventually fry the
field, but would keep the speed requirement down.

Or find an alternator with both positive and negative terminals isolated
from ground, (more or less common in Leece-Neville alternators made for
trucks) and use it with a 13 volt regulator with negative to the truck's
original 14 volt system. E.g. two alternators in series. The effect of
two batteries, but using only one additional battery.

Some models of JD diesel farm tractors used 24 volt systems with
floating generators (two output terminals, two field terminals, nothing
grounded) and grounded the center of the battery. Those generators were
only good for 10 amps which took all day to recharge the battery from
starting. 

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.