Electrical machines miscellaneous Easy Questions and Answers | Page - 42

Electrical machines miscellaneous

In a transformer fed fr om a fundament al frequency voltage source, the source of harmonics is the

1. overload
1. poor insulation
1. iron loss
1. saturation of core

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If B-H curves were linear, there would not have been harmonics in the magnetising current.

Correct Option: D

If B-H curves were linear, there would not have been harmonics in the magnetising current.

Two 3-limb, 3-phase delta-star connected transformers are supplied from the same source. One of the transformers is of Dy 1 and the other is of Dy 11 connection. The phase difference between the corresponding phase voltages of the two secondaries would be

1. 0°
1. 30°
1. 60°
1. 120°

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The connection D of 1 signifies that secondary phase voltage lags the corresponding primary phase voltage by 30°; and connection Dy 11 signifies that secondary phase voltage leads the corresponding primary phase voltage by 30°. Thus phase difference between corresponding phase voltages of the two secondaries would be 60°.

Correct Option: C

The connection D of 1 signifies that secondary phase voltage lags the corresponding primary phase voltage by 30°; and connection Dy 11 signifies that secondary phase voltage leads the corresponding primary phase voltage by 30°. Thus phase difference between corresponding phase voltages of the two secondaries would be 60°.

To prevent maloperation of differentially connected relay while energising a transformer, the relay restraining coil is based with

1. second harmonic current
1. third harmonic current
1. fifth harmonic current
1. seventh harmonic current

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NA

Correct Option: A

NA

If the frequency of input voltage of a transformer is increased keeping the magnitude of voltage unchanged, then

1. both hysteresis loss and eddy current loss in the core will increase
1. hysteresis loss will increase but eddy current loss will decrease
1. hysteresis loss will decrease but eddy current will increase
1. hysteresis loss will decrease but eddy current will remain unchanged.

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P_h = k_n ƒ B_m^1.6 = k_h ƒ B_m B_m^0.6
P_e = k e ƒ² B_m²
Also, V= 4.44 ƒ B_m A₁ N
If frequency is increased, while V remains constant, B_m will decrease; however, ƒ B_m remains constant. Thus, eddy current loss P_e remains unchanged.
P_h1 = B_m1 ^0.6 = ƒ₂ ^0.6
P_h2 B_m2 ƒ₁

Hence, as frequency increases. P_h decreases.

Correct Option: D

P_h = k_n ƒ B_m^1.6 = k_h ƒ B_m B_m^0.6
P_e = k e ƒ² B_m²
Also, V= 4.44 ƒ B_m A₁ N
If frequency is increased, while V remains constant, B_m will decrease; however, ƒ B_m remains constant. Thus, eddy current loss P_e remains unchanged.
P_h1 = B_m1 ^0.6 = ƒ₂ ^0.6
P_h2 B_m2 ƒ₁

Hence, as frequency increases. P_h decreases.

For successful parallel operation of two singlephase transformers, the most essential condition is that their

1. percentage impedance are equal
1. polarities are properly connected
1. turns ratios are exactly equal
1. kVA ratings are equal.

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NA

Correct Option: D

NA