Q1. Performance curves of a DC motor show relation between:
A. Speed vs flux
B. Power factor vs load
C. Torque, current, efficiency vs output power
D. Voltage vs current
Correct Option: C
Explanation:
Performance curves of a DC motor represent how torque, armature current, speed, and efficiency vary with output power. These curves help in analyzing motor performance under different loading conditions.
Q2. Shunt motor is generally called:
A. Variable speed motor
B. Constant speed motor
C. Low efficiency motor
D. Zero-load motor
Correct Option: B
Explanation:
In a shunt motor, flux remains nearly constant. Since speed N ∝ Eb/Φ, speed variation from no-load to full-load is very small, hence it is called a constant speed motor.
Q3. Series motor must never be run at no load because:
A. Armature current becomes zero
B. Flux becomes very small causing very high speed
C. Torque becomes zero
D. Core loss increases excessively
Correct Option: B
Explanation:
In a series motor, flux Φ ∝ Ia. At no load, armature current is very small, hence flux reduces sharply. Since speed N ∝ 1/Φ, speed becomes dangerously high.
Q4. Which motor has highest starting torque?
A. Shunt motor
B. Series motor
C. Cumulative compound motor
D. Differential compound motor
Correct Option: B
Explanation:
In a series motor, Φ ∝ Ia, therefore torque T ∝ ΦIa = Ia2. This gives very high starting torque.
Q5. In performance curves of DC motors, no-load input power is mainly spent in:
A. Armature copper loss only
B. Field copper loss only
C. Iron, friction and windage losses
D. Stray losses only
Correct Option: C
Explanation:
At no-load, output power is zero. Input power is consumed in constant losses such as hysteresis loss, eddy current loss, friction loss, and windage loss.
Q6. Drop in speed from no-load to full-load in shunt motor is:
A. Very high
B. Moderate
C. Negligible
D. Zero
Correct Option: C
Explanation:
In a shunt motor, flux is almost constant and voltage drop IaRa is small. Hence speed drop from no-load to full-load is negligible.
Q7. Which application is NOT suitable for series motor?
A. Crane
B. Hoist
C. Traction
D. Centrifugal pump
Correct Option: D
Explanation:
Series motors can run at dangerously high speed under light-load conditions. Centrifugal pumps may operate at light load, making series motors unsafe for such applications.
Q8. A shunt motor is preferred when:
A. Load torque varies as square of speed
B. Load requires constant speed
C. Load requires very high starting torque
D. Load torque is zero at start
Correct Option: B
Explanation:
Shunt motors provide nearly constant speed irrespective of load variation and are therefore preferred where constant speed operation is required.
Q9. For the same current input, starting torque of series motor compared to shunt motor is:
A. Same
B. Less
C. Greater
D. Zero
Correct Option: C
Explanation:
Shunt motor torque T ∝ Ia, whereas series motor torque T ∝ Ia2. Hence series motor develops higher starting torque.
Q10. In DC motor, maximum efficiency occurs when:
A. Iron loss = mechanical loss
B. Armature copper loss = constant loss
C. Copper loss = zero
D. Flux is maximum
Correct Option: B
Explanation:
Efficiency of a DC motor is maximum when variable losses (armature copper loss) are equal to constant losses (iron, mechanical, and shunt field losses).
Q11. Which of the following losses are constant losses?
A. Armature copper loss
B. Field copper loss
C. Core loss + friction and windage loss
D. Stray load loss only
Correct Option: C
Explanation:
Constant losses are independent of load and include hysteresis loss, eddy current loss, friction loss, and windage loss.
Q12. Power stages of DC motor are represented as:
A. Mechanical → magnetic → electrical
B. Electrical input → armature power → mechanical output
C. Output → losses → input
D. Mechanical input → electrical output
Correct Option: B
Explanation:
Electrical input power is first converted into armature power and then into mechanical power available at the shaft.
Q13. Overall efficiency of DC motor (ηo) is:
A. B/A
B. A/B
C. C/A
D. B/C
Correct Option: C
Explanation:
Overall efficiency is defined as Output/Input = Shaft output power (C) divided by electrical input power (A).
Q14. Input power = 10 kW, Total losses = 2.5 kW, Efficiency = ?
A. 60%
B. 65%
C. 70%
D. 75%
Correct Option: D
Explanation:
Output power = 10 − 2.5 = 7.5 kW.
Efficiency = 7.5 / 10 = 0.75 = 75%.
Q15. A 220 V shunt motor has Eb = 210 V at full load, Ia = 20 A. Armature copper loss is:
A. 50 W
B. 100 W
C. 200 W
D. 400 W
Correct Option: C
Explanation:
Armature copper loss = Ia(V − Eb) = 20 × (220 − 210) = 200 W.
Q16. Mechanical efficiency of DC motor is:
A. Shaft output / armature input
B. Electrical input / shaft output
C. Copper loss / armature input
D. Developed power / shaft power
Correct Option: A
Explanation:
Mechanical efficiency is the ratio of shaft output power to armature developed power.
Q17. Difference between developed power and shaft output power is due to:
A. Armature copper loss
B. Stray loss
C. Friction and windage loss
D. Hysteresis loss only
Correct Option: C
Explanation:
The difference between developed power and shaft output power is due to mechanical losses such as bearing friction and windage loss.
Q18. Which motor is preferred for electric traction?
A. Shunt
B. Series
C. Cumulative compound
D. Differential compound
Correct Option: B
Explanation:
Electric traction requires very high starting torque, which is provided by series motors as T ∝ Ia2.
Q19. Efficiency curves of DC motor are:
A. Straight line
B. Always increasing
C. Parabolic
D. Same general shape as generators
Correct Option: D
Explanation:
Efficiency of a DC motor increases with load, reaches a maximum, and then decreases, similar to the efficiency curve of a DC generator.
Q20. A DC motor takes 15 kW input, constant losses = 2 kW, armature Cu loss = 1 kW. Output power is:
A. 10 kW
B. 11 kW
C. 12 kW
D. 13 kW
Correct Option: C
Explanation:
Total losses = 2 + 1 = 3 kW.
Output power = 15 − 3 = 12 kW.