Q1. Unexcited single-phase synchronous motors are also known as:
A. DC synchronous motors
B. Self-excited motors
C. Reluctance and hysteresis motors
D. Capacitor motors
Correct Option: 3
Explanation:
Unexcited single-phase synchronous motors do not require DC excitation on the rotor. Reluctance motors and hysteresis motors operate based on magnetic properties of the rotor material and run at synchronous speed after pull-in.
Q2. Which of the following motors can start on single-phase supply without auxiliary winding?
A. Reluctance motor
B. Hysteresis motor
C. Single-phase induction motor
D. Repulsion motor
Correct Option: 2
Explanation:
A hysteresis motor is inherently self-starting because hysteresis torque exists at standstill due to lag of rotor magnetization behind the rotating magnetic field.
Q3. In a reluctance motor, torque is produced mainly due to:
A. Interaction of stator and rotor flux
B. Rotor current
C. Variation of reluctance with rotor position
D. Hysteresis loss
Correct Option: 3
Explanation:
Reluctance motor torque is produced because the rotor tends to align along the path of minimum magnetic reluctance.
Q4. The rotor of a reluctance motor is made of:
A. Laminated silicon steel with uniform air gap
B. Squirrel cage
C. Salient pole soft iron
D. Hard magnetic steel
Correct Option: 3
Explanation:
Reluctance motor rotor is made of salient pole soft iron to create different reluctances along direct and quadrature axes.
Q5. Which motor operates at synchronous speed without DC excitation?
A. Induction motor
B. Reluctance motor
C. Hysteresis motor
D. Both (B) and (C)
Correct Option: 4
Explanation:
Both reluctance and hysteresis motors run at synchronous speed without DC excitation and are classified as unexcited synchronous motors.
Q6. The synchronous torque in a reluctance motor is proportional to:
A. Rotor resistance
B. Difference between direct-axis and quadrature-axis reluctance
C. Supply frequency only
D. Rotor current
Correct Option: 2
Explanation:
Reluctance torque is proportional to the difference between direct-axis and quadrature-axis reluctances.
Q7. Hysteresis motor rotor is made of:
A. Soft iron
B. Laminated steel
C. Hard magnetic material
D. Aluminium
Correct Option: 3
Explanation:
Hysteresis motor rotor is made of hard magnetic materials having high coercivity and large hysteresis loop area.
Q8. The torque in hysteresis motor is independent of:
A. Speed
B. Rotor diameter
C. Supply voltage
D. Hysteresis loop area
Correct Option: 1
Explanation:
One key feature of hysteresis motor is that torque remains nearly constant from standstill to synchronous speed.
Q9. Which motor produces smooth and ripple-free torque?
A. Reluctance motor
B. Hysteresis motor
C. Split phase motor
D. Universal motor
Correct Option: 2
Explanation:
Hysteresis motor produces smooth and ripple-free torque due to constant hysteresis torque.
Q10. Hysteresis torque depends mainly on:
A. Rotor current
B. Slip
C. Area of hysteresis loop
D. Air-gap flux frequency
Correct Option: 3
Explanation:
Hysteresis torque is directly proportional to the area of the hysteresis loop of the rotor material.
Q11. A 4-pole hysteresis motor is supplied with 50 Hz. Find its synchronous speed.
A. 750 rpm
B. 1000 rpm
C. 1500 rpm
D. 3000 rpm
Correct Option: 3
Explanation:
Synchronous speed Ns = 120f / P = (120 × 50) / 4 = 1500 rpm.
Q12. A reluctance motor has direct-axis reluctance of 3 and quadrature-axis reluctance of 5. The motor torque depends on:
A. Sum of reluctances
B. Difference of reluctances
C. Product of reluctances
D. Ratio of reluctances
Correct Option: 2
Explanation:
Reluctance motor torque depends on the difference between direct-axis and quadrature-axis reluctances.
Q13. If supply frequency of a hysteresis motor is doubled, synchronous speed will:
A. Become half
B. Remain same
C. Become double
D. Become four times
Correct Option: 3
Explanation:
Synchronous speed is directly proportional to supply frequency.
Q14. A 6-pole hysteresis motor runs on 60 Hz supply. Calculate synchronous speed.
A. 1000 rpm
B. 1200 rpm
C. 1500 rpm
D. 1800 rpm
Correct Option: 2
Explanation:
Ns = 120f / P = (120 × 60) / 6 = 1200 rpm.
Q15. Which loss is responsible for torque production in hysteresis motor?
A. Copper loss
B. Eddy current loss
C. Hysteresis loss
D. Mechanical loss
Correct Option: 3
Explanation:
Torque in hysteresis motor is produced due to hysteresis loss in the rotor material.
Q16. Reluctance motor torque equation is mainly derived from:
A. Lorentz force
B. Power balance
C. Energy stored variation with rotor position
D. Rotor emf equation
Correct Option: 3
Explanation:
Reluctance torque is derived from variation of magnetic field energy with respect to rotor position.
Q17. Why hysteresis motor can start without starting winding?
A. Rotor current is induced
B. Rotor has permanent magnets
C. Hysteresis torque exists at standstill
D. Centrifugal switch is used
Correct Option: 3
Explanation:
At standstill, hysteresis torque exists due to delayed magnetization of rotor material.
Q18. Which motor has the lowest efficiency among these?
A. Induction motor
B. Reluctance motor
C. Hysteresis motor
D. DC motor
Correct Option: 3
Explanation:
Hysteresis motors have higher losses and low power rating, resulting in poor efficiency.
Q19. In reluctance motor, synchronous speed is achieved when:
A. Slip = 1
B. Slip = 0
C. Rotor speed > synchronous speed
D. Rotor speed < synchronous speed
Correct Option: 2
Explanation:
When the rotor locks with the rotating magnetic field, slip becomes zero and synchronous speed is achieved.
Q20. Which application prefers hysteresis motor?
A. Pumps
B. Fans
C. Electric clocks
D. Elevators
Correct Option: 3
Explanation:
Hysteresis motors are preferred in electric clocks and timing devices due to constant speed and silent operation.