Posted by David Sherman on Monday, October 11, 2004 at 12:21PM :
In Reply to: Re: Nope, it's the field posted by Tom Petroff on Monday, October 11, 2004 at 4:30AM :
I've always polarized the field, because that's what people told me and that's what I read in some book once. The purpose of polarizing is to put a permanent weak magnetic field in the iron of the pole pieces (the part that doesn't spin). This ensures that when the generator starts up it will begin producing enough current to build up its own electromagnetic field. Anything that sets up a magnetic field through the iron in the right direction will serve to polarize it. That's one nice feature of an old-fashioned generator -- it will work without a battery, unlike an alternator. Some old machinery used a generator to run lights, even though the motor itself had a magneto and started with a crank or rope. You can't do that with an alternator.
I can see that putting a current through the armature will, by the action of the commutator, make a magnetic field of the correct polarity, and most of that field will go through the pole pieces, so it would also polarize the generator. I would just be concerned about the heavy current doing some damage, but since one thing everybody agrees on is that you just momentarily touch the wire to "flash" the field, it's probably fine.