Why is the armature control method not employed above the rated speed in DC shunt motors?

And similarily why is the field control method not employed below the rated speed ?

I was trying to think the reason. What I thought was

If field flux is kept constant, Speed is proportional to armature voltage. So, is preferred for below rated speed. And in above rated case if used, voltage will be higher which is avoided.

If armature voltage is kept constant, Speed is inversely proportional to field flux. So, is preferred for above rated speed. And in below rated case if used, field flux will be higher(ie high field current) which is avoided.

Is my explanation ok. Please verify it.

  • \$\begingroup\$ Just make a different invention and forget this one. \$\endgroup\$ May 13, 2018 at 13:53
  • \$\begingroup\$ Why are you running something above its rated speed, anyway? \$\endgroup\$
    – Hearth
    May 13, 2018 at 14:22
  • \$\begingroup\$ I am a student. So, for theoretical understanding I am asking this question. \$\endgroup\$ May 13, 2018 at 14:43

1 Answer 1


What you have described is a separately-excited motor, not a shunt motor. A shunt motor has the field connected in parallel to the armature while a separately excited motor has the armature and field connected to to two independent power supplies.

You are essentially correct in your description of the operation of the motor. The rating point of the motor is the point where the armature voltage and the field current/flux are at their maximum design points. Reducing the armature voltage below that point reduces the motor speed while maintaining a constant torque capacity. The power capacity reduces proportional to speed. Reducing the field current below that point increases the motor speed while reducing the torque capability at a rate that maintains a constant power capability.


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