RE: (ET) field weakening with series-wound motors

["Elie, Larry (L.D.)" <lelie ford com>]

I have been away from my e-mail for a few days, but I think a 'short' 
description of what field weakening means and why are in order.

This is the way I recall the situation.  I hope I don't over simplify, but 
I may.  DC motors (and to a very lesser degree, AC as well) have strong 
issues with speed and field.  I am going to ignore issues of switching 
position (for induction that's just phase angle) and speed, although they 
are valid; variable high-speed motors actually change the switching 
frequency to get to high speeds.  We can't do that.  If you think about 
what determines the speed of a DC motor under no or light load, it is 
largely due the reverse torque due to the field itself; at some speed the 
field is doing about as much pushing as pulling of the armature.  The 
designer has a catch 22; you want high field for high torque, but the 
field itself will limit the speed at low torque.  One fix is to include 
multiple windings, with a winding to match the torque/speed requirements 
of any load. That's always a good idea.  It's a stepwise improvement (hard 
to do 1.3 windings without some external circuitry) b!
ut it works.  You also are very limited because there is only so much 
real-estate to use in the field of the motor for more combinations of 
wire.  To make the area bigger requires more iron, mass, and so on.  So 
you end up with a compromise.  Field weakening gives you the equivilant of 
another winding(s), without adding the space for another winding.

I think that description is fair.

That said, you can indeed do all sorts of things to reduce the field, but 
all MUST lower the AVERAGE current to the active winding(s) in the field.  
PWM circuits are very nice for this as they don't throw away too much 
power to heat (FET's are ON or OFF almost all the time) and the average 
current is just the duty cycle.  You just have to make sure the field 
never gets very close to zero while current is still in the armature-- 
ever.

Larry Elie