Long shunt DC motor speed control characteristics?

Question

So from what I can see based on stuff published online:

Series wound motors:

• Are not good for variable speed applications
• Can achieve very high starting torque
• Speed is almost entirely dependent on load

Shunt motors:

• Are great for variable speed applications
• Have lower starting torque
• Can maintain speeds, to some degree, under varying loads

For a short shunt motor, the series field comes first, then the armature and shunt field come after it. For a long shunt motor, the shunt field is in parallel to the series field and armature.

My question is: How do long-shunt compound wound motors compare to (straight) shunt motors in terms of speed control characteristics? Assuming that the shunt field is larger than the series field, can a long shunt motor work well in a variable speed/torque application?

Answer 1

DC motor speed control does not normally use series, shunt compound or any other connection of the field to the same supply as the armature. The controller provides a separate regulated power supply for the field. The field current is maintained at a constant value unless field weakening is used to extend the operating speed above the normal base speed of the motor. There might be some other configurations used in special circumstances.

The term "variable torque" is usually applied to loads that have a torque requirement that is low at low speed and increases with speed. Fans and centrifugal pumps have that characteristic. DC motors are not used for such applications except when battery operated. In that case a permanent magnet motor would be preferable.