# Electrical Machines Questions and Answers – Analysis of the Equivalent Circuit

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This set of Electrical Machines Questions and Answers for Experienced people focuses on “Analysis of the Equivalent Circuit”.

1. Below is the torque slip characteristic of the induction machine. What can be inferred from it?

a) R22 > R21
b) R22 < R21
c) R22 = R21
d) R22 = -R21

Explanation: As rotor resistance increases, the starting torque will increase but not the maximum torque. But the slip at which the maximum torque occurs will change.

2. Choose the most appropriate from the following characteristics of 3 phase induction motor torque-slip characteristics.
a)
b)
c)
d)

Explanation: As rotor resistance increases, the starting torque will increase but not the maximum torque. But the slip at which the maximum torque occurs will change.

3. If the reactance of the 3-phase slip ring induction motor is increased then the torque-slip characteristic varies as?
a)
b)
c)
d)

Explanation: Torque is inversely proportional to reactance
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4. The reactance vs slip is given by?
a)
b)
c)
d)

Explanation: Slip is inversely proportional to the slip.

5. Induction generator is equivalent to _________
a) underexcited alternator
b) dc generator
c) over excited alternator

Explanation: It is similar to underexcited alternator.
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6. When an induction generator is connected to an infinite bus, it _________
a) requires no capacitor bank
b) requires star connected capacitor bank
c) requires delta connected capacitor bank
d) requires any capacitor bank

Explanation: There is no requirement of the capacitor bank when induction generator is connected to infinite bus.

7. Torque produced by induction generator is negative.
a) True
b) False

Explanation: Because the developed torque is produced opposite to the rotating field.

8. Slip greater than unity is obtained practically by _________
a) plugging
b) changing phase sequence
d) any of the mentioned

Explanation: The direction of the rotating field can be reversed by any one of the methods.

9. Changing the rotor resistance will also affect _________
a) starting torque, speed, current
b) maximum torque, speed, current
c) maximum torque, speed, current, starting torque
d) speed and current

Explanation: Rotor resistance does not alter the maximum torque of the induction machine.

10. At starting, developed mechanical power is _____ and developed torque is ______
a) zero, not zero
b) zero,zero
c) not zero, zero
d) not zero, maximum

Explanation: At start, torque is not zero.
P α (1-s)/s; so it is zero at s = 1.

11. Mechanical power developed in the braking region of the induction motor is _________
a) negative
b) positive
c) zero
d) any of the mentioned

Explanation: slip is more than one, so Pm will be negative.

12. Developed torque during plugging is _________
a) negative
b) positive
c) zero
d) any of the mentioned

Explanation: Torque will not be negative in braking mode in the induction motor.

13. In the 3-phase induction motor developed torque during plugging is ______ mechanical power developed in the braking region of the induction motor is ______
a) positive, negative
b) negative, positive
c) positive, positive
d) negative, negative

Explanation: Tem is positive while Pm will be negative in braking or plugging operation.

14. The power slip characteristic is given as?
a)
b)
c)
d)

Explanation: Mechanical power developed is negative in braking and generating mode and it is zero at starting.

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