On May 4, 3:09 pm, Cydrome Leader wrote:
jeff_wisnia wrote:

I'm thinking that all else left alone, it probably doesn't matter which
way the shaft of an automobile alternator is rotated since the machine
produces ac which is then rectified to create a polarized dc output.

Am I right about that?

The only issue should be the cooling fan. If it spins the wrong way, it
won't cool anything.

I built an inboard boat. Chev 283. The alternator hung off one
side too far, interfering with the sidewalls of the compartment, so I
made a new bracket and turned the alternator around, facing the engine
and closer to centerline. The angled-blade fan I replaced with a
straight-blade fan (blades directly radiating from shaft) from the
junkyard, some cars had them and it doesn't matter which way they
turn. That alternator ran for hundreds of hours before I sold the
boat. The brushes in all the alternators I've had apart point straight
at the shaft and they don't care about rotation unless they're badly
worn and the holder has let them cock and wear at an angle. The output
will be the same in either direction.

If the battery is totally dead the alternator won't pick it up.
Its rotor won't hold enough magnetism to generate enough voltage to
overcome the diode drops, especially if there's any load present, as
there always is. Leaned this the hard way, towing a dead truck to try
to start it. Engine turned merrily but would not fire. Had to jump it.
Generators, on the other hand, have field pole shoes that will carry
enough flux to bootstrap the system. No diodes there.

In about 1977 I bought, for $27.95, a black box that, once wired
into my pickup, would generate 110VDC for series-wound power tools and
the like. Before I put it in I took it apart to see what I had paid
$28 for. It was: One plastic box; one 110VAC household duplex outlet;
one DPDT toggle switch, one neon lamp assembly (NE-2H, IIRC), and one
resistor of something like 33K. Total cost in those days of about $4.
The outlet was wired through one pole of the DPDT switch to the
alternator output. The other throw of the same pole went to the
battery +. The other pole of that switch simply switched the
alternator's field between the regulator and the battery +. The neon
assembly, with its suitable resistor to make it fire at 110V, was
across the duplex outlet's terminals. So with the switch "off,
everything worked normally. With it "on," the field got full battery
voltage and the alternator's output, freed of the battery via the
switch, generated increasing voltage as the throttle was opened until
the light lit up and you wnt to work for ten of 15 minutes, then
switched back to normal for a minute or so to recharge the battery.
The diodes in the alternator took this for as long as I had the truck.
My only peeve with it was being hosed so badly for a handful of
cheap parts. No magic in that box at all. Disappointing. $28 in 1978
was worth something. I could rent an airplane for an hour for not much
more than that.

Dan