QUESTIONS ON DC MOTOR
QUESTION 1
The armature of DC shunt motor has a resistance of 0.5 Ω and the armature is connected to a
240 V supply. Calculate the internal generated voltage if the armature current is 55 A.
QUESTION 2
A DC shunt motor runs at 1200rpm at internal generated voltage of 415 V. Calculate the
approximate speed when the internal generated voltage is 220 V. Assume the new flux to be
0.5 of the original flux.
QUESTION 3
A series DC motor runs at 1100 rpm when the armature current is 150 A from a 220 V supply.
The resistance of the armature circuit is 0.15 Ω and series resistance is 0.02 Ω. The useful flux
per pole for 150 A is 0.025 Wb and that for 60 A is 0.05 Wb. Calculate the speed when the
armature current has fallen to 60 A.
QUESTION 4
A 240 V, DC shunt motor delivers power to a load at 1000 rpm. The armature
current drawn by the motor is 150 A. The armature resistance of the motor is
0.5 Ω. If the rotational losses are 700 W, calculate the load torque and
efficiency of the motor.
QUESTION 5
A series connected DC motor has an armature resistance of 0.7 Ω and the series
field resistance of 1.2 Ω. In driving a certain load at 1200 rpm, the armature
current drawn by the motor is 40 A from voltage source of VT = 215 V. The
rotational loss is 250 W. Calculate the output power and efficiency of the motor.
QUESTION 6
A 250V DC shunt motor with 𝑅𝑎 =0.22 Ω, 𝑅𝑓 =170 Ω. The motor’s speed is 1200 rpm
for armature current of 3 A. What is the motor’s speed for armature current of 55 A.
Hint: Check if 𝐼𝑓 is equal for both case. If 𝐼𝑓 is equal than ø is equal as well.
QUESTION 7
A 230V DC shunt motor delivers 30 hp. If the motor has an efficiency of 87% ,
determine the following.
I.
Input power
II.
Armature current
Hint: 1hp = 746 W
QUESTION 8
A separately excited motor runs at 1045rpm, with a constant field current, while
taking an armature current of 50A at 120V. The armature resistance is 0.1 Ω . If the
load on the motor changes such that it now takes 95A at 120V, determine the motor
speed at this load.
THE END