# Electrical Machines Questions and Answers – Torque Production and Dynamic Equations

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This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Torque Production and Dynamic Equations”.

1. The magnitude of electromagnetic or interaction torque, in all rotating machines, is given by ____________
a) Te ∝ (stator field strength)sinδ
b) Te ∝ (rotor field strength)sinδ
c) Te ∝ (stator field strength)(rotor field strength)sinδ
d) Te ∝ sinδ

Explanation: The tendency of two fields (stator field and rotor field), to align themselves in the same direction, is called interaction torque, and it also depends on the torque angle & delta;.

2. In a rotating electrical machine with 2 poles on the stator and 4 poles on the rotor, spaced equally, the net electromagnetic torque developed is ____________
a) maximum
b) zero or no torque is developed
c) minimum
d) none of the mentioned

Explanation: In a machine with 2 stator poles and 4 rotor poles spaced equally, the force of attraction will be cancelled due to the force of repulsion, and hence the net electromagnetic torque will be zero.

3. For the development of electromagnetic torque in a rotating electrical machine, the number of rotor poles must be ____________
a) greater than the stator poles
b) less than the stator poles
c) equal to the stator poles
d) either greater or lesser than the stator poles

Explanation: If the rotor poles are not equal to the stator poles, the net electromagnetic torque will be zero as the force of attraction will be cancelled by the force of repulsion.

4. Torque angle ‘δ’ is the angle between ____________
a) rotor field axis and resultant field axis
b) stator field axis and rotor field axis
c) stator field axis and mutual field axis
d) stator field axis and resultant field axis

Explanation: The angle between the stator field axis and rotor field axis is called as torque angle.

5. The interaction torque in a rotating electrical machine depends on which of the following components?

```(i) sinδ
(ii) cosδ
(iii) torque angle δ
(iv) stator field strength
(v) rotor field strength
```
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a) (i), (iv), (v)
b) (ii), (iii), (v)
c) (i), (iii), (v)
d) (ii), (iv), (v)

Explanation: Interaction torque Te∝(stator field strength)(rotor field stregth)sinδ.

6. The basic torque and EMF expression of rotating electrical machines are ____________
a) applicable to DC machines only
b) applicable to AC machines only
c) applicable to both AC and DC machines
d) none of the mentioned

Explanation: Basic torque and EMF expressions are same for both AC and DC machines as the fundamental principles underlying the operation of AC and DC machines are same.

7. The final forms of the expression for generated EMF and torque,for AC and DC machines differ, because ____________
a) the fundamental principles underlying their operation are same
b) the fundamental principles underlying their operation are different
c) their construction details are same
d) their construction details are different

Explanation: Though AC and DC machine have same operating fundamental principles, their construction details differ.

8. The voltage equation for the electrical system, in an electromechanical energy conversion device is as follows:
vt=ir+Ldi/dt+i(dL/dθrr = (i)+(ii)+(iii)
What does the (ii) and (iii) terms represent respectively?
a) transformer voltage term and speed/rotational voltage term
b) speed/rotational voltage term and transformer voltage term
c) force voltage term and transformer voltage term
d) none of the mentioned

Explanation: (ii) term = Ldi/dt is known as transformer voltage because it involves the time derivative of current.
(iii) term = i(dL/dθrr is the speed/rotational voltage term because of the presence of speed ωr in it.

9. For an inductor made from magnetic core, with two air gaps of equal length g, exciting coil with 1000 turns, A=5cm∗5cm and g=1cm, what will be the coil inductance?
a) 0.314H
b) 0.157H
c) 0.078H
d) 0.628H

Explanation: Reluctance Rl=2g/μ0A and coil Inductance L=N2/Rl = N2μ0A/2g = (1000)2(4π∗10-7)(0.05)2/(2∗0.01) = 0.15708H.

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