Speed control methods of DC shunt motor

The speed of a DC motor is N = (V − I_aR_a) / (KΦ), i.e. N ∝ E_b / Φ. So the speed can be changed by changing the flux Φ, the resistance in the armature circuit, or the applied voltage V.

1. Field control (flux control)

A rheostat R_f is connected in series with the shunt field.

 (+) o----+------------------+
          |                  |
         [Rf] rheostat      (M) armature, Ra
          |                  |
      [Shunt field]          |
          |                  |
 (-) o----+------------------+

• Increasing the field resistance decreases I_f and Φ, so the speed rises above the base speed.
• Advantages: simple, cheap, small power loss (the field current is small), and smooth control.
• Limits: only speeds above normal; the range is about 3:1; commutation gets worse and the motor may become unstable at a very weak field.

2. Armature control

(a) Armature resistance control: a variable resistance R_e is inserted in series with the armature.

• Speed falls as R_e increases, because E_b = V − I_a(R_a + R_e) decreases while the flux stays at full value.
• Gives speeds below normal at constant torque.
• Drawbacks: a large I_a²R_e loss, poor efficiency, speed that depends on the load, and a resistance that must carry the full armature current. Used for short-duration duty or small motors.

(b) Armature voltage control (Ward-Leonard system)

 AC motor -> DC generator (variable field) -> DC motor (field const.)

• The motor armature is fed from a separate generator whose output is varied smoothly from zero to maximum by its field rheostat.
• Speed varies smoothly in both directions, even through zero. Combined with field control, it gives a wide range (up to about 30:1).
• Disadvantages: high initial cost (three machines), lower efficiency and large floor space. Modern drives use thyristor or chopper control instead.