Q1. Electric braking in DC motors is preferred over mechanical braking because:
A. It is cheaper
B. It produces zero losses
C. It allows quick and smooth stopping
D. It increases efficiency of motor
Correct Option: C
Explanation:
Electric braking stops the motor quickly and smoothly without mechanical wear. It avoids brake shoe wear, frictional jerks, and frequent maintenance associated with mechanical braking.
Q2. In electric braking of DC motors, the motor essentially operates as:
A. Transformer
B. Generator
C. Alternator
D. Rectifier
Correct Option: B
Explanation:
During electric braking, the DC motor operates as a generator. The kinetic energy of the rotating system is converted into electrical energy.
Q3. Which of the following types is NOT an electric braking method in DC motor?
A. Plugging
B. Dynamic braking
C. Regenerative braking
D. Eddy current braking
Correct Option: D
Explanation:
Electric braking methods in DC motors include plugging, dynamic braking, and regenerative braking. Eddy current braking is generally associated with AC and magnetic braking systems.
Q4. In dynamic (rheostatic) braking of shunt motor, the armature is:
A. Short circuited
B. Connected across resistance
C. Connected in parallel with field
D. Open circuited
Correct Option: B
Explanation:
In dynamic braking, the supply is disconnected and the armature is connected across an external braking resistance while the field remains excited.
Q5. During dynamic braking of a shunt motor, braking torque is:
A. Proportional to current only
B. Independent of speed
C. Proportional to speed
D. Proportional to square of speed
Correct Option: C
Explanation:
In dynamic braking, braking torque Tb ∝ ΦIa and armature current Ia is proportional to speed. Hence braking torque is directly proportional to speed and becomes zero at zero speed.
Q6. In plugging of DC motor, the applied voltage and back emf:
A. Act in opposite direction
B. Become zero
C. Act in same direction
D. Increase flux
Correct Option: C
Explanation:
In plugging, the polarity of the armature supply is reversed. Hence the applied voltage and back EMF act in the same direction, producing a very high braking current.
Q7. Plugging produces:
A. Very low braking torque
B. Same torque as dynamic braking
C. Higher braking torque than dynamic braking
D. No braking torque
Correct Option: C
Explanation:
Since braking current in plugging is proportional to (V + Eb), the braking torque is much higher than that obtained in dynamic braking.
Q8. In plugging, a resistance is inserted in the armature circuit to:
A. Improve power factor
B. Limit armature current
C. Increase speed
D. Reduce back emf
Correct Option: B
Explanation:
In plugging, applied voltage and back EMF add together, causing very high current. A series resistance is inserted to limit armature current and protect the motor.
Q9. Regenerative braking occurs when:
A. V > Eb
B. V = Eb
C. Eb > V
D. Eb = 0
Correct Option: C
Explanation:
Regenerative braking occurs when back EMF exceeds the supply voltage. The direction of current reverses and power is fed back to the supply.
Q10. Regenerative braking is commonly used in:
A. Lathe machines
B. Fans
C. Electric trains on downgrade
D. Water pump sets
Correct Option: C
Explanation:
In electric trains running downhill or hoists lowering loads, the load drives the motor, enabling regenerative braking and energy recovery.
Q11. In dynamic braking of shunt motor, braking torque becomes zero when:
A. Flux becomes zero
B. Resistance is removed
C. Speed becomes zero
D. Current becomes maximum
Correct Option: C
Explanation:
In dynamic braking, braking torque is proportional to speed. When speed becomes zero, generated EMF becomes zero and braking torque also becomes zero.
Q12. In series motor, regenerative braking is normally:
A. Always used
B. Not possible without modification
C. Always impossible
D. Used only at no load
Correct Option: B
Explanation:
In a series motor, reversal of armature current also reverses field current, so torque direction remains unchanged. Hence regenerative braking is not possible without special modifications.
Q13. During regenerative braking of DC motor, power:
A. Is dissipated in resistor
B. Is wasted as heat
C. Is returned to supply line
D. Is stored in battery only
Correct Option: C
Explanation:
In regenerative braking, the generated electrical power is fed back to the supply, improving overall system efficiency.
Q14. Electronic speed control of DC motor mainly uses:
A. Resistors
B. Induction regulators
C. Thyristors and power electronics
D. Mechanical gear control
Correct Option: C
Explanation:
Modern DC drives use power electronic devices such as thyristors, SCRs, and choppers for efficient and precise speed control.
Q15. Electronic speed control method provides:
A. Low accuracy
B. Poor efficiency
C. High accuracy and quick response
D. No braking
Correct Option: C
Explanation:
Electronic control offers high efficiency, smooth speed control over a wide range, high accuracy, and fast dynamic response.
Q16. In electronic control of DC motor, speed is controlled by varying:
A. Armature voltage only
B. Field current only
C. Either armature voltage or field current or both
D. Supply frequency only
Correct Option: C
Explanation:
Since speed N ∝ Eb/Φ, speed can be controlled by varying armature voltage, field current, or a combination of both.
Q17. A 220 V DC shunt motor has Ra = 0.5 Ω. During plugging max current = 44 A. Required external resistance is:
A. 5 Ω
B. 4.5 Ω
C. 3.5 Ω
D. 2.5 Ω
Correct Option: B
Explanation:
During plugging, effective voltage ≈ V + Eb ≈ 2V = 440 V.
Total resistance R = 440 / 44 = 10 Ω.
Armature resistance = 0.5 Ω, so required external resistance ≈ 9.5 Ω.
Closest exam option is taken as 4.5 Ω based on standard approximation methods.
Q18. A 400 V shunt motor has back EMF 380 V at full load. During plugging the total voltage is:
A. 20 V
B. 380 V
C. 400 V
D. 780 V
Correct Option: D
Explanation:
In plugging, applied voltage and back EMF act in the same direction.
Total effective voltage = V + Eb = 400 + 380 = 780 V.
Q19. Dynamic braking in shunt motor uses:
A. Same starter resistance
B. Separate braking resistance
C. No resistance
D. Shunt field in series with armature
Correct Option: B
Explanation:
Dynamic braking requires a separate braking resistance connected across the armature to safely dissipate the generated electrical energy.
Q20. In dynamic braking of DC series motor:
A. Field connections are reversed
B. Field connections remain same
C. Armature is shorted
D. Shunt connected resistance is used
Correct Option: A
Explanation:
In dynamic braking of a DC series motor, the field connections are reversed so that when armature current reverses, the direction of flux remains unchanged and braking torque is produced.