Characteristics of Shunt Motors

Author: E.E. Kimberly

Shunt motors are called constant-speed motors, although the speed may change as much as 12 per cent from full load to no load on motors developing 3/4 to 5 hp, and 10 per cent on motors of larger size. When a shunt motor runs without load, its armature current is just large enough to produce the torque required to drive the armature against the frictional resistance of air, bearings, and brushes and the retarding drag (load) of armature-iron loss. The current is limited by the armature-circuit resistance and by the counter electromotive force in the armature conductors. The counter electromotive force of a motor is the generated emf which would appear as terminal voltage if the motor were being operated as a self-excited generator in the same direction, at the same speed, and with the same field excitation. Then,

(11-3)

where V = voltage applied to the motor;

IaRa = resistance voltage drop in armature circuit; Eg = counter electromotive force.

In an unloaded motor the relative magnitudes of quantities in equation (11-3) may be about as follows:

The motor power equation may be obtained by multiplying each member of equation (11-3) by I0. Thus,

(11-4)

The term I₂aRa represents the power lost in the armature because of its resistance. The term Egla represents the remainder of the power delivered to the armature, and is the sum of the iron loss, the windage and friction loss, and the power output at the shaft if there is a load on the motor. Inasmuch as EQ is a function of speed, its magnitude is a fairly good indication of the motor speed for any given load. Hence,