Re: 120A alternator: hp drain (a little electrical theory)

["Bradley Peet" <btpeet@effervescent.com>]

As long the efficiency is the same (which it probably is), the 120 Amp
alternator shouldn't be any more of a burden on the engine than the 90 Amp
under normal operating conditions.  The load on the engine is directly
related to the current the alternator is delivering.  What Julie said is
absolutely correct, although the formulas she cited don't take frictional
losses and resistance in the wire into account.  (I can only speculate at
how efficient a typical alternator is but I would hope 80% or more.)

If you're curious as to how/why the alternator can dynamically load the
engine down when current is being drawn from it, I'll try to explain.  The
coils of wire are moving through the magnetic field created by the permanent
magnets which induces a current in the wire (according to Faraday's law),
but the current flow is limited by the effective "resistance" (load) that's
connected across the alternators terminals.

As many of you may know, any time there is current is flowing in a wire, it
creates a magnetic field of it's own.  So, as you load the alternator and
current starts flowing, a magnetic field is created that is OPPOSITE the
static magnetic field that the wire is passing through (as the armature
spins).   This effect is best described by Lenz's Law which states:

"The induced voltage must have a polarity such that any current produced by
it will set up a [magnetic] flux in opposition to any change in the original
flux."

This dynamically generated magnetic field opposes the field established by
the permanent magnets inside the alternator.  Imagine the resistive force
you feel when you put two magnets near each other in the "wrong" way and
they repel each other.

Clear as mud?

Bradley
Alpine White 1988 Scirocco 16V
Cosmos Blue  1981 Scirocco S
R.I.P: '86 Scirocco 16V 2.0 "T"
http://effervescent.com/vw/



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