# Electrical Machines Questions and Answers – Production of Torque in Non-Salient Pole Machines

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Production of Torque in Non-Salient Pole Machines”.

1. The electromagnetic torque in non salient pole machines depends on _________
a) number of poles
b) machine dimensions at the air gap
c) peak value of stator and rotor MMF
d) any of the mentioned

Explanation: Te also depends on the sine of the angle between the axis of the two fields(or MMFs) Te = -(P/2)(μoΠrl/g)(FsFr)sinλNm.

2. The torque equation for non salient pole machine is Te = -(P/2)(μoΠrl/g)(FsFr)sinλNm. What does the negative sign in the torque expression indicates?
a) stator and rotor MMF tend to reduce angle λ
b) stator and rotor MMF tend to increase angle λ
c) stator tend to reduce the air gap g
d) rotor tend to reduce the air gap g

Explanation: The torque acts in such a direction as to align the two MMFs Fs and Fr and thereby reduce the angle λ between them.

3. Which of the following statements are correct regarding the torque equation in non salient pole machines, Te = -(P/2)(μoΠrl/g)(FsFr)sinλNm?
a) stator experiences less torque than rotor and in opposite directions
b) rotor experiences less torque than stator and in same directions
c) stator and rotor experiences equal and opposite torques
d) stator and rotor experiences equal torque and in same directions

Explanation: Both stator and rotor experience equal and opposite torques.
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4. In practice, electromagnetic torque acting on the stator _________
a) rotate in opposite direction to rotor
b) is transmitted to ground
c) makes it rotate in same direction as of rotor
d) none of the mentioned

Explanation: As stator is fixed, Teis transmitted to ground through its foundation.

5. Torque angle λ is the angle between ____________
a) stator MMF Fs and resultant MMF FR
b) stator MMF Fs and rotor MMF Fr
c) rotor MMF Fr and resultant MMF FR
d) any of the mentioned

Explanation: The electrical space angle λ between stator and rotor MMFs is called the torque angle.

6. Load angle δ is the angle between ____________
a) stator MMF Fs and resultant MMF FR
b) stator MMF Fs and rotor MMF Fr
c) rotor MMF Fr and resultant MMF FR
d) stator MMF Fs and resultant MMF FR (or) rotor MMF Fr and resultant MMF FR

Explanation: The electrical space angle δs between Fs and FR ; and δr between Fr and FR are called load angles.

7. In the design of electromagnetic devices, the maximum value of MMF is limited from the considerations of ____________
a) temperature rise
b) limitation on flux density in teeth
c) torque production rise
d) power rating

Explanation: The maximum value of MMF is limited from a consideration of temperature rise.

8. Consider the principle that the torque in a rotating machine is proportional to peak of stator MMF(Fs), rotor MMF(Fr) and the angle between them(δ). Following statements relate to Fs,Fr and δ in different machine. Which of these statements are correct?

```(i) δ is fixed in DC machine
(ii) Fs is fixed in DC shunt machine
(iii) δ is variable in induction machine
(iv) Fr is fixed in synchronous machne
(v) Fr is variable in DC machine
(vi) Fr is fixed in induction motor
(vii) resultant of Fs and Fr is fixed in synchronous motor
```

a) (i), (ii), (iii), (iv) and (vi)
b) (i), (ii), (iii), (v) and (vii)
c) (ii), (iii), (v), (vi) and (vii)
d) (i), (ii), (iv), (v) and (vii)

Explanation: Fr is not fixed in synchronous and induction machine.

9. In all rotating electrical machine, electrical torque is developed when relative speed between stator field and rotor field is ____________
a) zero
b) equal to rotor speed
c) equal and opposite to rotor speed
d) dependent upon the type of electrical machine

Explanation: If the relative speed between stator field and rotor field is not zero, then the load angle δr varies with time.
From equation Te = -(Π/8)P2ΦFrsinδr Nm, we can say that average torque over a complete cycle is zero, implies Te=0.

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