# Electrical Machines Questions and Answers – Reluctance Motor

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Reluctance Motor”.

1. What is the angle between stator direct axis and quadrature axis?
a) 90°
b) 0°
c) 45°
d) 60°

Explanation: In reluctance motor, direct axis is horizontal axis and quadrature axis is at 90° to the direct axis.

2. Space angle, θr is measured between stator d-axis and _______________
b) direct d-axis
c) long rotor axis
d) none of the mentioned

Explanation: Space angle is measured between stator direct axis and long rotor axis.

3. The reluctance offered to the stator flux by two very large air gaps in series with high permeability iron, in reluctance machine is maximum, when the space angle θr = ___________
a) 0°
b) 45°
c) 90°
d) 180°

Explanation: Only when θr = 90°, the long rotor axis is perpendicular to the stator d-axis, and thus reluctance is maximum.
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4. The reluctance offered to the stator flux by two small air gaps in series with high permeability iron, in reluctance machine is minimum, when the space angle θr = ___________
a) 0°
b) 45°
c) 90°
d) 270°

Explanation: Only when θr = 0°, the long rotor axis is coincident with the stator d-axis, and thus reluctance is minimum.

5. The variation of reluctance Rl with space angle θr depends on the shape of __________
a) stator poles
b) rotor poles
c) stator or rotor poles
d) both stator and rotor poles

Explanation: Also, the variation is assumed to be a function of space angle θr.

6. Reluctance motor can produce torque at ________
a) any speed less than synchronous speed
b) synchronous speed only
c) any speed greater than synchronous speed
d) all of the mentioned

Explanation: Te(av) = -1/4 ∅max2(Rlq – Rld)(1/2 sin⁡(-2δ)), w = synchronous speed
if wr≠w then Te(av)=0, and if wr=w then Te(av)≠0, as the last term in the equation 1/2 sin⁡(-2δ) does not vary with time, and hence average torque is not equal to zero.

7. For a reluctance motor, the maximum average torque occurs when δ = __________
a) 45°
b) 90°
c) 0°
d) 180°

Explanation: The equation for average torque is Te(av)=1/8 ∅max2(Rlq – Rld)sin⁡(2δ), and it is maximum when δ=45°.

8. For a given reluctance motor, Rld and Rlq are ________
a) constant
b) varying
c) zero
d) all of the mentioned

Explanation: Rld and Rlq are constant for a given motor, as they depend on the geometry of the magnetic circuit.

9. The single phase reluctance machine acts as a generator when angle δ is _______
a) positive
b) negative
c) zero
d) any of the mentioned

Explanation: If δ is positive, then the machine acts as a motor and if δ is negative, the machine acts as a generator.

10. Single phase reluctance motors are extensively used in ________
a) grinder applications
b) driving electric clocks and other timing devices
c) welding applications
d) lifts/ elevators

Explanation: The single phase reluctance motors operate at constant synchronous speed, in case the supply frequency remains constant, and hence timing devices mostly use these motors.

11. If the salient pole rotor in a single phase reluctance motor is replaced by a cylindrical rotor, then Which of the following statements are true?

```(i) reluctance offered to stator flux remains constant for all rotor positions
(ii) no reluctance torque will be developed
(iii) reluctance torque will be developed
(iv) reluctance offered to stator flux changes for all rotor positions
```

a) (i), (ii)
b) (ii), (iii)
c) (iii), (iv)
d) (i), (iv)

Explanation: The reluctance torque is developed only when the reluctance is seen but the stator flux varies with rotor movement, and it depends on the air gap. For a cylindrical rotor, the air gap remains same, and hence no change in reluctance and no torque is developed.

12. Which of the following are applications of singly excited magnetic systems?
a) electromagnets, relays
b) moving-iron instruments
c) reluctance motors
d) all of the mentioned

Explanation: All of the applications mentioned above needs singly excited magnetic systems.

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