Electrical machines miscellaneous Easy Questions and Answers | Page - 41

Electrical machines miscellaneous

A single -winding single-phase motor has

1. low starting torque
1. zero starting torque
1. high starting torque
1. starting torque equal to full-load torque.

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NA

Correct Option: B

NA

If a single-winding single-phase motor is running in a particular direction

1. both the rotating fields (rotating in opposite directions) have the same strength.
1. forward rotating field is slightly stronger than the backward field.
1. forward rotating field is much stronger than the backward field.
1. backward field is stronger than the forward field.

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NA

Correct Option: C

NA

An iron-cored choke with 1 mm air-gap length, draws 1 A when fed from a constant voltage ac source of 220 V. If the length of air-gap is increased to 2 mm, the current drawn by the choke would

1. become nearly one half
1. remain nearly the same
1. become nearly double
1. become nearly zero

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NA

Correct Option: C

NA

In a 3-phase Δ/ Y transformer shown in the figure, the phase displacement of secondary line voltages with corresponding primary line voltages will be

1. zero
1. 30° lag
1. 30° lead
1. 180°

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Consider any pair of line terminals on primary and secondary sides.
Consider pair AB.
Let V_A1B1 be reference on primary side
V_A1B1 = V ∠0°
V_B1C1 = V ∠– 120°
V_C1A1 = V ∠120°
V_A2n = k V_A1C1 = − k V_C1A1 = − k ∠120°
Vn_B2 = k V_A1B1 = k V ∠0°
V_A2B2 = V_A2n − Vn_B2 = k V( ∠−60° + ∠0°)
= √3 kV ∠−30°
Thus line voltage VA2B2 on secondary side which equals √3 kV ∠−30°, lags corres-ponding line voltage V_A1B1 on primary side, which is V ∠0°; by 30°.

Correct Option: B

Consider any pair of line terminals on primary and secondary sides.
Consider pair AB.
Let V_A1B1 be reference on primary side
V_A1B1 = V ∠0°
V_B1C1 = V ∠– 120°
V_C1A1 = V ∠120°
V_A2n = k V_A1C1 = − k V_C1A1 = − k ∠120°
Vn_B2 = k V_A1B1 = k V ∠0°
V_A2B2 = V_A2n − Vn_B2 = k V( ∠−60° + ∠0°)
= √3 kV ∠−30°
Thus line voltage VA2B2 on secondary side which equals √3 kV ∠−30°, lags corres-ponding line voltage V_A1B1 on primary side, which is V ∠0°; by 30°.

Two single-phase transformers with turns ratio 1 and 2 respectively are connected to a 3-phase supply on the primary side as shown in the figure. The voltmeter V₂, will read

1. 100 V
1. 173 V
1. 200 V
1. 265 V

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Voltmeter V₂ read rms magnitude of the sum of two voltage phasors,
viz., 100 ∠0° + 2 × 100 ∠ – 120° υ₂
= | 100 ∠0° + 200 ∠ – 120° |
= | 100 – 100 – j 100 √3 |
= | – j 100 √3 | = 173.2 V

Correct Option: B

Voltmeter V₂ read rms magnitude of the sum of two voltage phasors,
viz., 100 ∠0° + 2 × 100 ∠ – 120° υ₂
= | 100 ∠0° + 200 ∠ – 120° |
= | 100 – 100 – j 100 √3 |
= | – j 100 √3 | = 173.2 V