# Electrical Machines Questions and Answers – Determination of Equivalent Circuit Parameters

This set of Electrical Machines Questions and Answers for Aptitude test focuses on “Determination of Equivalent Circuit Parameters”.

1. Blocked rotor test comprises of ____________
a) rotor and stator copper losses.
b) rotor copper and stator core losses.
c) rotor core losses
d) stator copper losses

Explanation: Blocked rotor test consists of copper losses of stator and rotor winding copper losses.

2. A 10kW,400V, 4-pole delta connected induction motor gave the following test results:

```No-load test: 400V, 8A, 250 W
Blocked rotor test: 90V, 35A, 1350 W
```

The dc resistance per phase is 0.6 ohms. The no load rotational losses is?
a) 204W
b) 304W
c) 196W
d) 186W

Explanation: Pr = 250-3*64/3*(0.6*1.2) = 204W.

3. A 10kW,400V, 4-pole delta connected induction motor gave the following test results:

```No-load test: 400V, 8A, 250 W
Blocked rotor test: 90V, 35A, 1350 W
```

The dc resistance per phase is 0.6 ohms. The effective stator resistance is?
a) 0.72 ohms
b) 0.6 ohms
c) 0.3 ohms
d) 1.2 ohms

Explanation: Reff = 1.2*r; Due to delta connected rotor.
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4. While conducting the tests on the 3-phase induction motor, the variation of rotational losses with respect to voltage can be obtained as?
a)
b)
c)
d)

Explanation: Core loss is zero for, zero applied voltage but the friction and windage losses will be present in the machine which need to be overcome.

5. The external rotor resistance is ____________
a) gradually cut out as motor picks up the speed
b) fixed for a stable point
c) removed instantalLy as the motor is started
d) added or reduced as per the efficiency requirement

Explanation: External resistance is gradually cut out after picking up the speed.

6. The speed of the squirrel-cage induction motor can be changed by varying slip.
a) True
b) False

Explanation: For squirrel-cage induction motor, the rotor get shorted and it can not be altered.

7. The speed characteristic of the induction motor is analogous to ____________
a) dc shunt motor
b) synchronous motor
c) cumulative compound motor
d) stepper motor

Explanation: As the speed practically remains constant and lesser than than the rated speed, it is similar to dc shunt motor.

8. If the squirrel-cage induction motor is replaced by a solid cylinder in an induction motor its slip will ____________
a) increase
b) decrease
c) be same
d) zero

Explanation: Reactance of the rotor will decrease as the leakage wil reduce, so the slip will increase.

9. Number of the slip rings required for the satisfied operation of an induction motor is?
a) number of phases
b) number of poles
c) halve of number of poles
d) double the phases

Explanation: Slip rings number is same as the phase of the supply.

10. I. An induction can never operate at leading power factor.
II. Speed of the induction motor can be made more than synchronous speed and still it will work as motor.
a) I is true, II is false
b) I is true, II is true
c) I is false, II is true
d) I is false, II is also false

Explanation: Speed more than Ns will be an induction generator not motor.

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