> But why does it work? I don't understand why increasing the resistance
> across the 'charge light' circuit makes it kick in sooner.
>
> Right after start up, there's enormous demand for juice, as the battery
> has just shot the wad starting the car. I didn't know that the charge
> light circuit had anything to do with when the regulator engaged.
<
Since I spent quite a few years designing voltage regulators for AC
generators I feel I must weigh in on this issue.
A generator (or alternator) produces electric current through the "moving a
wire through a magnetic field" principle. Except in the rare case where
permanent magnets are used, the magnetic field is generated using an
electro-magnet (field winding). To enhance the generator's performance
steel poke pieces are used to collect the magnetic flux and concentrate it
in the areas most desired. These pole pieces are ferro-magnetic and as
such will retain a bit of magnetism after the generator is shut down and
when the generator is spun again this residual magnetism will cause the
generator to produce electrical power even in the absence of current in the
field coil. This voltage is used to power the field coil which will
increase the power from the generator which will increase the field coil
current... This is called bootstrapping. If, through some miscarriage of
control system, the residual magnetism is removed, this boot straping will
not occur and "flashing the field" will "kick start" the whole operation.
The difference between the older dynamo and the more modern alternator is
in the regulator and the rectification process. The old dynamo used a
comutator and brushes to "steer" main winding current to produce the
polarity desired and the regulator would connect the dynamo output to the
field via a pair of contacts. The modern alternator uses diodes (a six
diode bridge) to steer the winding output to the desired polarity and a
transistorized regulator provides current to the field. The diode bridge
will loose about 1.5 volts and it takes a couple of volts to make the
regulator start to work whereas the old dynamo regulator will start
providing current at a much lower system voltage.
Will an alternator start without this "idiot" light? Yes it will. Here's
some empirical evidence: On day, as an experiment, I disconnected the wire
to the charge light on my TR6 (which had sat idle for at least a week) and
started it up. I slowly increased the engine speed until the charging
circuit kicked in (as indicated on my ammeter). The alternator started
working at about 1700 RPM. With the light connected it usually kicks in at
1200.
Why is there a light? Mostly to indicate a problem with the chargine
system since the alternator will eventually bootstrap once the engine is
started (who drives their car keeping the revs below 1700?). But this
light will also automatically flash the field in the event of loss of
residual magnetism. A terciary benefit is it will get the charging system
active more quickly.
That's enough theory for now.
Dave Massey
57 TR3 (dynamo)
71 TR6 (alternator)
80 TR8 (alternator)
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