Question

A 100 hp, 220 V, 1200 rpm DC shunt motor with an armature resistance of 0.05 ? and a field resistance of 50 ?. The motor has compensating windings, so armature reactance can be ignored. Mechanical and core losses may be ignored also. The motor is driving a drilling machine, TL ? (1/n). The motor is driving a load with a line current of 120 A and an initial speed of 1200 rpm. Assume Linear Magnetization curve
i. What is the motor speed if the field resistance is increased to 55?? While supplying a constant-torque load
ii. Calculate the motor speed as a function of the field resistance, assuming a constant-Load Torque. Plot the motor speed as a function of the field resistance.

Answer 1

for, the given initial operating condition of shunt motor (as shown in the figure):   and,

Filed current

Armature current

Back Emf

Now, the field resistance is changed to,.so new value of field current is,

Let the new value of armature current be and the new speed be . Since load torque remains constant. So we can write: and as mechanical and core losses are neglected. These load torque can be equated to electromagnetic torque i.e.

Now the new value of back emf is:

For dc shunt motor back emf . so we can write