On 9/15/2012 6:49 PM, David Roden wrote:
"Compound winding means there's a 'light series' winding on top of the main shunt field winding which adds flux proportional to armature amps to help the main field produce torque as load goes up. Unless you change its polarity in reverse, it will do the opposite with load - decrease main field flux and motor speed increases. That's why no field weakening in reverse and higher speed under load. The reason it's used is to compensate for armature reaction (current in the armature makes flux which cancels field flux and causes speed to rise inherently under load)."
Indeed. There is another reason; because a shunt field motor can't draw a lot of amps through the field, starting torque and power is limited. On the E-20, putting a series field in there allows the motor to start from a stall under load more like a series motor.
But if the E15 really had a series field, it should have done something without the little field. The fact that it didn't do anything, not even cogging, makes me think it is *not* a compound motor.
I'm trying to remember when I took it apart if both of the armature studs went straight to the brushes. I think so actually.
Chris