Q1. Speed of a DC motor is given by:
A. N ∝ ϕ/Eb
B. N ∝ Eb/ϕ
C. N ∝ Eb/ϕ
D. N ∝ V/ϕ
Correct Option: C
Explanation:
From EMF equation Eb = (PϕZN)/(60A). Rearranging gives N = K·Eb/ϕ. Hence speed is directly proportional to back EMF and inversely proportional to flux per pole.
Q2. For a DC motor:
A. Eb = V + IaRa
B. Eb = V − IaRa
C. Eb = IaRa
D. Eb = V
Correct Option: B
Explanation:
Applying KVL to the armature circuit: V = Eb + IaRa.
Therefore, Eb = V − IaRa. Back EMF controls armature current and speed.
Q3. Speed regulation of DC motor means change in speed from:
A. No-load to full-load
B. Full-load to half-load
C. Half-load to no-load
D. Starting to full-load
Correct Option: A
Explanation:
Speed regulation is defined as the change in speed between no-load and full-load conditions, expressed as a percentage of full-load speed.
Q4. % speed regulation is:
A. (NFL − NNL)/NFL × 100
B. (NNL − NFL)/NFL × 100
C. NFL/NNL × 100
D. NNL/NFL × 100
Correct Option: B
Explanation:
Since no-load speed is higher than full-load speed, speed regulation is given by
(NNL − NFL)/NFL × 100.
Q5. Torque equation of DC motor is:
A. T ∝ Eb
B. T ∝ ϕ
C. T ∝ ϕIa
D. T ∝ Ia2
Correct Option: C
Explanation:
Electromagnetic torque T = kϕIa.
For shunt motor (ϕ constant) → T ∝ Ia.
For series motor (before saturation) → T ∝ Ia2.
Q6. Speed of DC motor decreases when:
A. Flux increases
B. Flux decreases
C. Back EMF increases
D. Supply increases
Correct Option: A
Explanation:
From N ∝ Eb/ϕ, increasing flux increases the denominator, thereby decreasing speed. This principle is used in field control.
Q7. In series motor, torque is proportional to (before saturation):
A. Ia
B. Ia2
C. 1/Ia
D. 1/Ia2
Correct Option: B
Explanation:
In series motor, flux ϕ ∝ Ia.
Therefore, T ∝ ϕIa = Ia2, giving very high starting torque.
Q8. In shunt motor, flux is approximately:
A. Proportional to armature current
B. Constant
C. Proportional to torque
D. Proportional to speed
Correct Option: B
Explanation:
Shunt field current If = V/Rf is nearly constant. Hence flux remains practically constant.
Q9. Shunt motor is considered:
A. Variable speed motor
B. Constant speed motor
C. Low speed motor
D. High slip motor
Correct Option: B
Explanation:
Due to constant flux, speed variation from no-load to full-load is very small (about 5–10%), so shunt motor is called a constant speed motor.
Q10. In DC motor, speed is inversely proportional to:
A. Flux
B. Torque
C. Current
D. Resistance
Correct Option: A
Explanation:
From N ∝ Eb/ϕ. At constant supply, Eb is nearly constant, hence N ∝ 1/ϕ.
Q11. In series motor, speed is:
A. Directly proportional to Ia
B. Inversely proportional to Ia
C. Independent of Ia
D. Proportional to Ia2
Correct Option: B
Explanation:
In series motor, ϕ ∝ Ia and N ∝ 1/ϕ. Hence N ∝ 1/Ia.
At light load, speed becomes dangerously high.
Q12. Series motor must not be started without load because:
A. Large current flows
B. Torque becomes zero
C. Speed becomes dangerously high
D. Flux becomes zero
Correct Option: C
Explanation:
At no load, current and flux are very small. Since N ∝ 1/ϕ, speed becomes excessively high and may damage the motor.
Q13. In shunt motor, which characteristic is almost straight line?
A. Ta − Ia
B. N − T
C. N − Ia
D. Ia − V
Correct Option: C
Explanation:
Because flux is constant, change in speed with armature current is very small. Hence N–Ia characteristic is nearly a straight horizontal line.
Q14. Mechanical characteristic means relation between:
A. Torque & armature current
B. Speed & armature current
C. Torque & speed
D. Flux & current
Correct Option: C
Explanation:
Mechanical characteristic is the N–T curve showing variation of speed with load torque.
Q15. Compound motor characteristic lies between:
A. Shunt only
B. Series only
C. Shunt & series
D. None
Correct Option: C
Explanation:
Compound motor has both shunt and series field windings. Hence its characteristics lie between shunt and series motors.
Q16. Cumulative compound motor has:
A. Series field opposing shunt field
B. Series field aiding shunt field
C. No series field
D. No shunt field
Correct Option: B
Explanation:
In cumulative compounding, series field aids the shunt field, giving higher starting torque and stable operation.
Q17. Differential compound motor is generally:
A. Stable
B. Unstable
C. High starting torque motor
D. Traction motor
Correct Option: B
Explanation:
In differential compounding, series field opposes shunt field. Flux decreases with load, causing speed to rise and making the motor unstable.
Q18. Shunt motor should not be started with:
A. Low voltage
B. No load
C. Heavy load
D. Field resistance
Correct Option: C
Explanation:
At starting, back emf is zero and armature current is high. Starting with heavy load increases current further, causing overheating and commutator damage.
Q19. For shunt motor:
A. T ∝ Ia2
B. T ∝ Ia
C. T ∝ ϕ2
D. T ∝ 1/Ia
Correct Option: B
Explanation:
In shunt motor, flux is constant. Therefore T ∝ ϕIa ≈ constant × Ia.
Q20. A 220 V shunt motor has Ra = 0.5 Ω and back EMF 210 V. Armature current is:
A. 5 A
B. 10 A
C. 20 A
D. 2 A
Correct Option: C
Explanation:
Ia = (V − Eb)/Ra = (220 − 210)/0.5 = 20 A.