DC shunt motor characteristics

In a shunt motor the field winding is connected across the supply, so the flux Φ is nearly constant. The three characteristics follow from the relations T = KΦI_a and N = (V − I_aR_a)/(KΦ).

  I_a ----->[Ra]---(Eb)---+
  Ish ----->[Rsh]---------+--- V (constant)

1. Speed-current characteristic (N–I_a)

• N ∝ E_b = V − I_aR_a with constant Φ.
• With load, I_a rises and the small drop I_aR_a reduces the speed slightly. So the curve is a nearly flat, slightly drooping straight line.
• Armature reaction weakens the flux a little, which tends to hold the speed up.
• Hence a shunt motor is a constant-speed motor (the speed falls only 5 to 10 % from no load to full load).

2. Torque-current characteristic (T–I_a)

• T ∝ Φ I_a, and Φ is constant, so T ∝ I_a: a straight line through the origin.
• At no load, a small current I_a0 is needed to supply the rotational losses, so the shaft-torque line starts a little to the right of the origin.
• The starting torque is adequate at the permitted starting current, but less than that of a series motor.

3. Speed-torque characteristic (N–T)

• Since I_a ∝ T, the curve is similar to the N–I_a curve: a slightly drooping line.
• Speed changes very little with torque, so the motor is used where constant speed is needed.

 N |-----------            T |            /
   |           ----\         |          /
   |                \        |        /
   +----------------> Ia     +------/--------> Ia
   speed vs Ia           torque vs Ia

Applications: lathes, fans, blowers, centrifugal pumps, machine tools.