Q1. In a Permanent-Magnet DC motor, the field flux is produced by:
A) Series field winding
B) Shunt field winding
C) Permanent magnets
D) Armature reaction
Correct Option: C
Explanation: In a PMDC motor, permanent magnets mounted on the stator produce the magnetic field. No field winding is required.
Q2. The main advantage of PMDC motor over shunt DC motor is:
A) High starting torque
B) No field copper loss
C) Better commutation
D) Higher overload capacity
Correct Option: B
Explanation: PMDC motors do not use field windings, so field copper loss is eliminated, improving efficiency.
Q3. PMDC motors are most commonly used in:
A) Large rolling mills
B) Electric traction
C) Servo and control applications
D) Power plants
Correct Option: C
Explanation: PMDC motors provide linear torque-current relation and good speed control, ideal for servo applications.
Q4. Speed control of a PMDC motor is mainly achieved by controlling:
A) Field current
B) Armature voltage
C) Field resistance
D) Flux weakening
Correct Option: B
Explanation: Since flux is constant, speed is controlled by varying armature voltage.
Q5. The back emf equation of a PMDC motor is:
A) Eb = ΦZN / 60
B) Eb = V − IaRa
C) Eb = IaRa
D) Eb = PΦN
Correct Option: B
Explanation: Back emf of a DC motor is given by Eb = V − IaRa.
Q6. A low-inertia DC motor is preferred in:
A) Constant speed drives
B) High torque at steady load
C) Rapid acceleration and deceleration
D) High power applications
Correct Option: C
Explanation: Low inertia allows quick speed changes, making such motors suitable for fast response applications.
Q7. The moment of inertia of a motor mainly depends on:
A) Supply voltage
B) Rotor mass and its distribution
C) Armature current
D) Flux density
Correct Option: B
Explanation: Moment of inertia depends on rotor mass and its distance from the axis of rotation.
Q8. Low-inertia DC motors are widely used in:
A) Lifts
B) Fans
C) CNC machines
D) Transformers
Correct Option: C
Explanation: CNC machines require rapid acceleration and precise positioning.
Q9. In shell-type low-inertia DC motors, the armature winding is:
A) Distributed in slots
B) Drum type
C) Printed on a disc
D) Placed on shell-shaped rotor
Correct Option: D
Explanation: Shell-type motors have a shell-shaped rotor to minimize inertia.
Q10. The main feature of shell-type low-inertia DC motor is:
A) High inertia
B) Large air-gap
C) Very small rotor inertia
D) High iron loss
Correct Option: C
Explanation: Extremely small rotor inertia enables fast dynamic response.
Q11. Printed-circuit DC motor is also known as:
A) Drum motor
B) Disc motor
C) Universal motor
D) Stepper motor
Correct Option: B
Explanation: Printed-circuit DC motors have disc-shaped armature.
Q12. In printed-circuit DC motor, the armature conductors are:
A) Wound in slots
B) Embedded in iron core
C) Printed on an insulating disc
D) Made of aluminum bars
Correct Option: C
Explanation: Conductors are printed on an insulating disc, eliminating iron core.
Q13. The absence of iron in printed-circuit DC motor results in:
A) High hysteresis loss
B) No eddy current loss
C) Large armature reaction
D) Poor commutation
Correct Option: B
Explanation: Eddy current and hysteresis losses occur only in iron cores.
Q14. Which motor has the least rotor inertia?
A) DC shunt motor
B) PMDC motor
C) Shell-type low-inertia DC motor
D) Printed-circuit DC motor
Correct Option: D
Explanation: Printed-circuit motors have no iron core and very low rotating mass.
Q15. Printed-circuit DC motors are best suited for:
A) Heavy duty drives
B) Constant speed loads
C) Precision control systems
D) Power transmission
Correct Option: C
Explanation: Low inertia and fast response make them ideal for precision control.
Q16. A PMDC motor is supplied with 200 V and draws 5 A. Armature resistance is 2 Ω. Find back emf.
A) 180 V
B) 190 V
C) 200 V
D) 210 V
Correct Option: B
Explanation: Eb = V − IaRa = 200 − (5 × 2) = 190 V.
Q17. A PMDC motor develops constant flux. If armature voltage is reduced by 10%, speed will:
A) Increase by 10%
B) Decrease by 10%
C) Remain same
D) Decrease by 20%
Correct Option: B
Explanation: Speed is directly proportional to armature voltage for constant flux.
Q18. A low-inertia motor has rotor inertia reduced by 50%. Acceleration will:
A) Reduce
B) Double
C) Remain same
D) Become zero
Correct Option: B
Explanation: Acceleration α = T/J. Halving inertia doubles acceleration.
Q19. In printed-circuit DC motor, eddy current loss is negligible because:
A) High speed
B) Low resistance
C) No iron core
D) Small air-gap
Correct Option: C
Explanation: Eddy current loss occurs only in iron cores.
Q20. Torque equation of PMDC motor is:
A) T ∝ Ia²
B) T ∝ Φ²
C) T ∝ ΦIa
D) T ∝ Ia / Φ
Correct Option: C
Explanation: Torque of DC motor is proportional to flux and armature current. In PMDC motor, flux is constant.