George White wrote in a message to Greg Mayman:

 GW> That was before diodes were reliable enough. The three coil systems
 GW> were for dynamos (you only need two on an alternator). They were:-
 GW> 1) an isolator relay, to cut the dynamo off untill it produced
 GW> enough output.
 GW> 2) a voltage regulator trembler relay.
 GW> 3) a current limiter trembler relay.

 GW> Two coil regulators for dynamos used a current bucking coil on the
 GW> voltage regulator coil to limit the output current.

Even though it's been *ages* since I've actually seen one of those,  now
that you mention this I can remember that a couple of the coils on those
things (the 3-coil units) were wound with some *really* thick wire.  :-) 
Those must be the "current" windings.

The other thing that just comes to mind with those is that underneath the
mounting base were wire-wound "resistors",  maybe 2 or 3 of them.

 GW> One thing to remember is that _all_ dynamos needed a current 
 GW> limiter as they are not intrinisically self limiting. Most 
 GW> alternators intrinsically self limit the current output, so all 
 GW> that is needed is a voltage control. The only automotive 
 GW> alternators that we built (I worked for CAV) that had current 
 GW> limiting in the regulator were sealed marine and military versions 
 GW> where there was none of the usual air cooling of the windings.

Hm.  I wasn't aware of that.



 RJT>> I don't think it's much heavier than that,  though I could find 
 RJT>> out when I get my hands on those wiring diagrams.  And it's not
 RJT>> like it's carrying that current continuously.  The ammeter in
 RJT>> the truck is actually marked with a "40" at each
end,  and it's
 RJT>> *very* seldom I've ever seen it go the whole way over,  and then
 RJT>> only for a fairly short period of time.

 GM> Yes, that's right. The leads usually aren't very long as the 
 GM> battery is usually near the alternator.

Actually in that vehicle they're on opposite sides of the engine.  And in
my car as well.

 GM> And 5000A/squ.inch isn't all that heavy, certainly not when that 
 GM> is only reached occasionally.

 GW> The duty on most car batteries and the rating of the alternators is
 GW> such that for most people by the time they've pulled out the drive,
 GW> and certainly by the time they've reached the end of the street all
 GW> the power taken from the battery to start the engine has been
 GW> replaced, and that is about the only time the full alernator 
 GW> current can go to the battery.

That depends.  I've known of some people who did *lots* of fairly short
trips,  and that ran their battery down because they weren't running long
enough to put the charge back into it.  This may have been in colder
weather,  may have been with the heavy use of a lot of accessories,  too, 
I don't recall.

 GW> In a modern car the alternator has a high current rating to drive 
 GW> all the extra things in the car (windscreen wipers, lights, 
 GW> heater/air conditoning fans, engine cooling fan, heated screens, 
 GW> heated mirrors, heated seats, in car entertainment, etc...), most 
 GW> of which only work when the enging is running anyway.

Just so.

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