Q1. Speed of DC motor is given by:
A. N ∝ Ï•
B. N ∝ Ia
C. N ∝ Eb/Ï•
D. N ∝ Ï•/Eb
Correct Option: C
Explanation:
Speed of a DC motor is directly proportional to back EMF and inversely proportional to flux. From Eb = V − IaRa and N ∝ Eb/Ï•, increasing back EMF increases speed, while increasing flux decreases speed.
Q2. Factors controlling DC motor speed are:
A. Flux only
B. Armature resistance only
C. Applied voltage only
D. V, Ï• and Ra all
Correct Option: D
Explanation:
From N = (V − IaRa)/(kÏ•), speed depends on supply voltage, armature resistance drop, and flux. Hence all three parameters control DC motor speed.
Q3. Flux control method of shunt motor gives:
A. Only below rated speed
B. Only above rated speed
C. Constant speed
D. Zero speed
Correct Option: B
Explanation:
Flux control weakens the field flux. Since N ∝ 1/Ï•, decreasing flux increases speed, giving speeds only above rated. Flux cannot be increased beyond rated due to saturation.
Q4. In shunt motor field control, speed increases because:
A. Armature current increases
B. Flux decreases
C. Back EMF decreases
D. Supply increases
Correct Option: B
Explanation:
Adding resistance in the field circuit reduces field current and hence flux. As N ∝ 1/Ï•, reduction in flux causes an increase in speed.
Q5. Armature resistance control is primarily used for:
A. Above base speed
B. Below base speed
C. Constant speed
D. Regenerative braking
Correct Option: B
Explanation:
Armature resistance reduces the effective armature voltage and back EMF, causing a reduction in speed. Hence it is suitable only for below base-speed control.
Q6. Main drawback of rheostatic control method is:
A. Low starting torque
B. Complex design
C. High power loss in resistance
D. Sparking at brushes
Correct Option: C
Explanation:
In rheostatic control, large I2R losses occur in the external resistance, resulting in poor efficiency and excessive heat dissipation.
Q7. Rheostatic control gives:
A. Speeds above normal only
B. Speeds below normal only
C. Both above and below
D. Constant speed only
Correct Option: B
Explanation:
External resistance reduces armature voltage, which lowers back EMF and speed. Hence rheostatic control provides only below-normal speeds.
Q8. Rheostatic method is suitable for:
A. Continuous duty
B. Frequent speed change at light load
C. Short-time low-speed operation
D. Traction continuously
Correct Option: C
Explanation:
Because of high power losses, rheostatic control is economical only for short-duration, low-speed operations such as cranes and hoists.
Q9. Speed control of shunt motor by armature rheostat is:
A. Efficient
B. Inefficient
C. Most efficient
D. Lossless
Correct Option: B
Explanation:
Armature rheostat control wastes large power as I2R loss in resistance and provides poor regulation, making it inefficient.
Q10. Speed control of DC series motor by field diverter gives:
A. Increase in flux
B. Reduction in flux
C. Same flux
D. No effect on speed
Correct Option: B
Explanation:
Field diverter diverts part of armature current away from the series field. Field current reduces, flux decreases, and speed increases.
Q11. In series motor tapped field control, speed varies by:
A. Varying applied voltage
B. Adding external resistance
C. Changing number of field turns
D. Changing brushes
Correct Option: C
Explanation:
Tapped field control changes the effective number of series field turns, altering flux and hence controlling speed.
Q12. Series–parallel control is mainly used in:
A. Fans
B. Elevators
C. Traction
D. Machine tools
Correct Option: C
Explanation:
Series–parallel control provides high starting torque and smooth speed control, making it suitable for traction applications like trains and trams.
Q13. In series–parallel control, when motors are in series:
A. Same current, half voltage per motor
B. Same voltage, half current
C. Half current, half voltage
D. Same current, same voltage
Correct Option: A
Explanation:
When two motors are connected in series, the same current flows through both and the supply voltage divides equally across each motor.
Q14. When two similar series motors are shifted from parallel to series, speed becomes:
A. Double
B. Half
C. One-fourth
D. Four times
Correct Option: C
Explanation:
Changing from parallel to series increases flux and reduces voltage per motor. Combined effect reduces speed to one-fourth of the previous value.
Q15. In series–parallel control, torque in series mode compared to parallel mode is:
A. Same
B. Half
C. One-fourth
D. Four times
Correct Option: D
Explanation:
Torque of a series motor is proportional to I2. In series mode, current doubles compared to parallel mode, so torque becomes four times.
Q16. A 220 V shunt motor runs at 900 rpm. Flux is reduced to 75% of rated. New speed is:
A. 675 rpm
B. 900 rpm
C. 1100 rpm
D. 1200 rpm
Correct Option: D
Explanation:
Since N ∝ 1/Ï•, reducing flux to 0.75Ï• gives N2 = 900 / 0.75 ≈ 1200 rpm.
Q17. A 250 V shunt motor with Ra = 0.5 Ω runs at 1000 rpm with Ia = 40 A. If Ia becomes 20 A (flux constant), speed ≈:
A. 800 rpm
B. 900 rpm
C. 1000 rpm
D. 1050 rpm
Correct Option: D
Explanation:
At reduced current, IaRa drop decreases, back EMF increases slightly, and speed increases slightly above rated, giving approximately 1050 rpm.
Q18. Which control is most efficient for shunt motors?
A. Armature resistance
B. Flux control
C. Rheostatic
D. Series–parallel
Correct Option: B
Explanation:
Flux control wastes negligible power and does not use armature resistors, making it the most efficient and commonly used method for shunt motors.
Q19. Series motor should never be run at no-load because:
A. Armature burns
B. Flux becomes zero
C. Speed becomes dangerously high
D. Commutator fails
Correct Option: C
Explanation:
At no-load, armature current and flux are very small. Since N ∝ 1/Ï•, speed rises dangerously high, which may cause mechanical damage.
Q20. Ward–Leonard speed control system is preferred when:
A. Narrow control range required
B. Very fine control over wide speed range required
C. Cheapest method desired
D. Losses must be minimum
Correct Option: B
Explanation:
Ward–Leonard system provides smooth, stepless, and precise speed control over a wide range with good torque characteristics, hence preferred where fine control is required.