Bill,

Okay, I'll admit I was oversimplifying the description a little.  The rotor will always "cog", meaning that it will tend to line up with the iron poles in the stator and stay in one position.  with the field fully energized the cogging torque will certainly be significant, as you state below.  But if you force it past one pole it will the pull itself to the next and attempt to stop there.  What I'm trying to say is that once it is spinning even slowly the cogging effect evens out and the net (average) torque will be very low.  The voltage capability of the alternator is proportional to rpm and with full field excitation it will be able to produce battery voltage well below idle rpm, but probably not at cranking speed, which might be something like 100 to 200 rpm.  I've not tested this, so I'm not sure of the numbers.  If it can't reach battery voltage it can't produce power and therefore won't absorb power.  If it did absorb as much power as you suggest and it can't put out any power, where does all that power go?  It would have to go into heat and it certainly doesn't overheat at low rpms.  Another point - with a fixed field current and at a fixed output voltage (like when the battery is absorbing the current and limiting the output voltage) the alternator will produce current proportional to rpm until it gets to its internally-limited maximum current (which is independent of rpm).  It usually will reach that condition at about 4,000 rpm, or something less than 2,000 engine rpm.  A rule of thumb:  Assuming a fixed field current at a fixed voltage the current is proportional to rpm (up to a limit) and at a fixed current the voltage is proportional to rpm.  The internal inductance of the stator will limit the output current - regardless of rpm and field current the alternator cannot put out more than a given current which is approximately the rating (a "60-amp" alternator cannot put out more than about 60 amps no matter what). 

Hopefully that clarified things a little.

Gary

Bill previously posted:

Gary,

Here is something you can try.  Take the belt off the alternator and using your hand, spin the alternator pulley,  Then turn on the master and spin the pulley.  Then turn on the alternator and spin the pulley.  Tell us again what you think happens when that “3 amps” of field current hits that alternator.

If you can turn that pulley with your hand with the alternator on, I apologize profusely!  Not because I am wrong, but because you will have to be one Gorilla!

Bill B