# Electrical Machines (Test 4)

## Gate Exam : Ee Electrical Engineering

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Electrical Machines
| Electrical Machines |
Q.1
The single phase, 50Hz, iron core transformer in the circuit has both the vertical arms of cross sectional area 20cm2 and both the horizontal arms of cross sectional area 10cm2. If the two windings shown were wound instead on opposite horizontal arms, the mutual inductance will

A. double
B. remain same
C. be halved
D. become one quarter
Answer : Option A
Explaination / Solution:
No Explaination.

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Q.2
In a constant V/f control of induction motor, the ratio V/f is maintained constant from 0 to base frequency, where V is the voltage applied to the motor at fundamental frequency f. Which of the following statements relating to low frequency operation of the motor is TRUE?
A. At low frequency, the stator flux increases from its rated value.
B. At low frequency, the stator flux decreases from its rated value.
C. At low frequency, the motor saturates.
D. At low frequency, the stator flux remains unchanged at its rated value.
Answer : Option B
Explaination / Solution:

During constant V/f control, at low frequency, the voltage also applied to the induction motor is low. Hence the stator flux also decreases from its rated value.

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Q.3
A 3-phase squirrel cage induction motor supplied from a balanced 3-phase source drives a mechanical load. The torque-speed characteristics of the motor (solid curve) and of the load (dotted curve) are shown. Of the two equilibrium points A and B, which of the following options correctly describes the stability of A and B?

A. A is stable B is unstable
B. A is unstable B is stable
C. Both are stable
D. Both are unstable
Answer : Option A
Explaination / Solution:
No Explaination.

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Q.4
A three-phase slip-ring induction motor, provided with a commutator winding, is shown in the figure. The motor rotates in clockwise direction when the rotor windings are closed.

If the rotor winding is open circuited and the system is made to run at rotational speed fr with the help of prime-mover in anti-clockwise direction, then the frequency of voltage across slip rings is f1 and frequency of voltage across commutator brushes is f2. The values of f1 and f2 respectively are
A. f + fand f
B. f - fand f
C. f - fand f + fr
D. f + fand f - fr
Answer : Option A
Explaination / Solution:

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Q.5
A 20-pole alternators is having 180 identical stator slots with 6 conductors in each slot. All the coils of a phase are in series. If the coils are connected to realize single-phase winding, the generated voltage is V1. If the coils are reconnected to realize three phase star connected winding, the generated phase voltage is V2. Assuming full pitch, single-layer winding, the ratio V1/V2is
A.
B. 1/2
C.
D. 2
Answer : Option D
Explaination / Solution:

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Q.6
For a single phase, two winding transformer, the supply frequency and voltage are both increased by 10%. The percentage changes in the hysteresis loss and eddy current loss, respectively, are
A. 10 and 21
B. -10 and 21
C. 21 and 10
D. -21 and 10
Answer : Option A
Explaination / Solution:

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Q.7
Find the transformer ratios a and b that the impedance (Zin) is resistive and equal 2.5Ω when the network is excited with a sine wave voltage of angular frequency of 5000 rad/s.

A. a = 0.5, b = 2.0
B. a = 2.0, b = 0.5
C. a = 1.0, b = 1.0
D. a = 4.0, b = 0.5
Answer : Option B
Explaination / Solution:

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Q.8
A 4-pole, separately excited, wave wound DC machine with negligible armature resistance is rated for 230 V and 5 kW at a speed of 1200 rpm. If the same armature coils are reconnected to forms a lap winding, what is the rated voltage (in volts) and power (in kW) respectively at 1200 rpm of the reconnected machine if the field circuit is left unchanged?
A. 230 and 5
B. 115 and 5
C. 115 and 2.5
D. 230 and 2.5
Answer : Option B
Explaination / Solution:

In wave wound, no. of parallel paths is 2. In lap wound, no of parallel paths is A = p = 4. Rated voltage = (230 × 2)/4 = 115 V As no. of parallel paths is divided into four from two, the voltage reduces to half but power remains the same.

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