Hi Roy,

On 30-Sep-03, Roy J. Tellason wrote to George White:

 GW>> That was before diodes were reliable enough. The three coil
 GW>> systems were for dynamos (you only need two on an alternator).

In fact, come to think of it, you usually only needed _one_ on an
alternator system. But I don't think I ever came across trembler
controled alternators (but I know they did exist).

 GW>> They were:-
 GW>> 1) an isolator relay, to cut the dynamo off until 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
 GW>> the voltage regulator coil to limit the output current.

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

One was the current winding, the other a bucking winding on the isolator
relay in series with the main output to force the isolator to drop out
when the engine stopped (or if the output voltage at idle fell below
the battery voltage). Otherwise the battery would just discharge
through the dynamo... The main winding on the isolator relay was just
a voltage sensative winding connected to the output of the dynamo so
once it was "on" it would stay "on" without that current bucking
winding to cancel it under those conditions. The field circuit was
isolated by the "ignition" switch.

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

If you had looked closely you would have spotted that they are across
the relay contacts. They're there to reduce (kill) arcing in the
contact as they make and break to get a decent life out of the unit.

 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
 GW>> versions where there was none of the usual air cooling of the
 GW>> windings.

 RJT> Hm.  I wasn't aware of that.

Unless you've worked in the industry, why should you be?

 RJT> 

 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
 RJT>>> then 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.

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

Normally they are placed near the _starter_, since that is the
heaviest draw, _always_ several times the alternator output. It can be
up to several hundred amps, depending on the engine size and ambient
temperature (bigger engine/lower temperature means higher current).
It's typically (2L engines) 250 or so Amps.

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

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

Yes, that can happen. Usually in cold climates where you have heated
screens, fan blowers, lighting all running all the time. This can
limit the current available to charge the battery to relatively few
amps even when the engine is running fast enough to produce full
output. If the vehicle was only used for school runs and shopping and
in heavy traffic (choose any big city - I'm used to London (UK), New
York (NY), Boston (Mass), Bristol (UK)) the car spends so much of it's
time in heavy traffic with the engine idling the the alternator output
falls below the accessory drain and the battery is being discharged in
normal use. (On most systems I'm used to the alternator reaches full
output at about 25% max revs). The traditional solution is to up the
engine idle speed - but that only works for stick shift systems. For
automatics all you can do is cut down the power use (lower heater fan
speed, no extra lights) or put the battery on charge overnight...

 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.

 RJT> Just so.

But you do still need to be moving along...


George

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