Alternator drain

Steve Baldwin steveb at kcbbs.gen.nz
Wed Apr 2 22:57:23 GMT 1997


>  - the magnetism required to operate the alternator or
> generator comes from current passing through wires/windings
> in the device.  (Could use a permanent magnet -- I know of
> no commercially available, automotive unit that does).
> 
> As some of you might surmise from my other posts, I date far enough 
> back to remember when ALL cars had generators (no alternators).  The 
> fields in the generator, being energized under pretty much steady 
> state DC conditions became (although weak) permanent magnets.  Turn a 
> generator, and you get DC, whether you have the battery hooked up or 
> not.  If you have a battery hooked up, the field is stronger, 
> permitting greater amounts of current to drawn.
> 
> 

Close, but not quite. Once everything is up and spinning and doing its'
thing, both the alternator and the generator provide their own field
energy. The difference, other than the working principle (rectified AC,
pulsed DC, etc) which has already been covered, is in the startup
mechanism.
One of the jobs of the regulator is to give precedence to powering the
field windings. With a generator, there is sufficient residual magnetism
to get 'something' from the output. This is immediately fed into the
field windings, which increases the output, which is fed back into the
field windings,... And so on, until there is enough output.

The alternator doesn't have enough iron to do that, so it has another
set of windings or an alternative means of powering the main windings at
startup or low speed. This only needs to be enough to get things going,
as above.

Some of the older members of the list may recall 'polarising' the
generator when you installed a new one, or the car had sat for some
time. This is where you quickly brush a wire from the positive terminal
of the battery across the field terminal of the generator. This is to
provide that initial magnetism. I'm sure this was used by some
auto-electricians to earn a few extra dollars in the past. 
Show the customer that the generator didn't work. Polarise it and charge
for a new one.

Steve.




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