9 + Interview Questions in GENERAL QA IN HARMONICS Page 1 InterviewSolution

1.	In a 3-phase system, ___th harmonic has negative phase sequence of RBY.(a) 9 (b) 13 (c) 5 (d) 15
Answer» Correct option (c) 5

Discussion

2.	The r.m.s. value of the complex voltage given by v = + 16√2sinωt + 12√2 sin3ωt is(a) 20√2 (b) 20 (c) 28√2 (d) 192
Answer» Correct option (b) 20

2.

The r.m.s. value of the complex voltage given by v = + 16√2sinωt + 12√2 sin3ωt is(a) 20√2 (b) 20 (c) 28√2 (d) 192

Answer»

Correct option (b) 20

Discussion

3.	When a pure inductive coil is fed by a complex voltage wave, its current wave (a) has larger harmonic content (b) is more distorted(c) is identical with voltage wave (d) shows less distortion.
Answer» (d) shows less distortion.

Discussion

4.	Non-sinusoidal wave forms are made up of (a) different sinusoidal wave forms (b) fundamental and even harmonics (c) fundamental and odd harmonics (d) even and odd harmonics only
Answer» (a) different sinusoidal wave forms

Discussion

5.	The positive and negative halves of a complex wave are symmetrical when (a) it contains even harmonics(b) phase difference between even harmonics and fundamental is 0 or π (c) it contains odd harmonics (d) phase difference between even harmonics and fundamental is either π/2 or 3π/2.
Answer» (c) it contains odd harmonics

Discussion

6.	A complex current wave is given by the equation i = 14sinωt + 2sinωt. The r.m.s. value of the current is ___ ampere. (a) 16 (b) 12 (c) 10 (d) 8
Answer» Correct option (c) 10

6.

A complex current wave is given by the equation i = 14sinωt + 2sinωt. The r.m.s. value of the current is ___ ampere. (a) 16 (b) 12 (c) 10 (d) 8

Answer»

Correct option (c) 10

Discussion

7.	Which of the following harmonic voltage components in a 3-phase system would be in phase with each other? (a) 3rd, 9th, 15th etc. (b) 7th, 13th, 19th etc. (c) 5th, 11th, 17th etc. (d) 2nd, 4th, 6th etc
Answer» (d) 2nd, 4th, 6th etc.

Discussion

8.	A complex voltage wave is applied across a pure capacitor. As compared to the fundamental voltage, the reactance offered by the capacitor to the third harmonic voltage would be (a) nine time (b) three times (b) one-third (d) one-ninth
Answer» Correct option (c) one-third

8.

A complex voltage wave is applied across a pure capacitor. As compared to the fundamental voltage, the reactance offered by the capacitor to the third harmonic voltage would be (a) nine time (b) three times (b) one-third (d) one-ninth

Answer»

Correct option (c) one-third

Discussion

9.	Determine whether the following two waves are of the same shape e = 10sin ( ωt + 30º) – 50 sin(3ωt – 60º) + 25 sin (5ωt + 40º) i = 1.0 sin (ωt – 60º) + 5 sin (3ω t – 150º) + 2.5cos (5ω t – 140º)
Answer» Two waves possess the same wave shape (i) if they contain the same harmonics (ii) if the ratio of the corresponding harmonics to their respective fundamentals is the same (iii) if the harmonics are similarly spaced with respect to their fundamentals. In other words, (a) the ratio of the magnitudes of corresponding harmonics must be constant and (b) with fundamentals in phase, the corresponding harmonics of the two waves must be in phase. The test is applied first by checking the ratio of the corresponding harmonics and then coinciding the fundamentals by shifting one wave. If the phase angles of the corresponding harmonics are the same, then the two waves have the same shape. In the present case, condition (i) is fulfilled because the voltage and current waves contain the same harmonics, i.e. third and fifth. Secondly, the ratio of the magnitude of corresponding current and voltage harmonics is the same i.e. 1/10. Now, let the fundamental of the current wave be shifted ahead by 90º so that it is brought in phase with the fundamental of the voltage wave. It may be noted that the third and fifth harmonics of the current wave will be shifted by 3 × 90º = 270° and 5 × 90º = 450º respectively. Hence, the current wave becomes i' = 1.0sin(ωt - 60º + 90º) + 5sin(3ωt - 150º + 270º) + 2.5cos(5ωt - 140º + 450º) = 1.0sin(ωt + 30º) + 5sin(3ωt + 120º) + 2.5cos(ωt + 310º) = 1.0sin(ωt + 30º) - 5sin(3ωt - 60º) + 2.5sin(5ωt + 40º) It is seen that now the corresponding harmonics of the voltage and current waves are in phase. Since all conditions are fulfilled, the two waves are of the same wave shape.

9.

Determine whether the following two waves are of the same shape e = 10sin ( ωt + 30º) – 50 sin(3ωt – 60º) + 25 sin (5ωt + 40º) i = 1.0 sin (ωt – 60º) + 5 sin (3ω t – 150º) + 2.5cos (5ω t – 140º)

Answer»

Two waves possess the same wave shape

(i) if they contain the same harmonics

(ii) if the ratio of the corresponding harmonics to their respective fundamentals is the same

(iii) if the harmonics are similarly spaced with respect to their fundamentals. In other words,

(a) the ratio of the magnitudes of corresponding harmonics must be constant and

(b) with fundamentals in phase, the corresponding harmonics of the two waves must be in phase.

The test is applied first by checking the ratio of the corresponding harmonics and then coinciding the fundamentals by shifting one wave. If the phase angles of the corresponding harmonics are the same, then the two waves have the same shape.

In the present case, condition (i) is fulfilled because the voltage and current waves contain the same harmonics, i.e. third and fifth.

Secondly, the ratio of the magnitude of corresponding current and voltage harmonics is the same i.e. 1/10.

Now, let the fundamental of the current wave be shifted ahead by 90º so that it is brought in phase with the fundamental of the voltage wave. It may be noted that the third and fifth harmonics of the current wave will be shifted by 3 × 90º = 270° and 5 × 90º = 450º respectively. Hence, the current wave becomes

i' = 1.0sin(ωt - 60º + 90º) + 5sin(3ωt - 150º + 270º) + 2.5cos(5ωt - 140º + 450º)

= 1.0sin(ωt + 30º) + 5sin(3ωt + 120º) + 2.5cos(ωt + 310º)

= 1.0sin(ωt + 30º) - 5sin(3ωt - 60º) + 2.5sin(5ωt + 40º)

It is seen that now the corresponding harmonics of the voltage and current waves are in phase. Since all conditions are fulfilled, the two waves are of the same wave shape.

Discussion

Explore topic-wise InterviewSolutions in .

In a 3-phase system, ___th harmonic has negative phase sequence of RBY.(a) 9 (b) 13 (c) 5 (d) 15

The r.m.s. value of the complex voltage given by v = + 16√2sinωt + 12√2 sin3ωt is(a) 20√2 (b) 20 (c) 28√2 (d) 192

When a pure inductive coil is fed by a complex voltage wave, its current wave (a) has larger harmonic content (b) is more distorted(c) is identical with voltage wave (d) shows less distortion.

Non-sinusoidal wave forms are made up of (a) different sinusoidal wave forms (b) fundamental and even harmonics (c) fundamental and odd harmonics (d) even and odd harmonics only

The positive and negative halves of a complex wave are symmetrical when (a) it contains even harmonics(b) phase difference between even harmonics and fundamental is 0 or π (c) it contains odd harmonics (d) phase difference between even harmonics and fundamental is either π/2 or 3π/2.

A complex current wave is given by the equation i = 14sinωt + 2sinωt. The r.m.s. value of the current is ___ ampere. (a) 16 (b) 12 (c) 10 (d) 8

Which of the following harmonic voltage components in a 3-phase system would be in phase with each other? (a) 3rd, 9th, 15th etc. (b) 7th, 13th, 19th etc. (c) 5th, 11th, 17th etc. (d) 2nd, 4th, 6th etc

A complex voltage wave is applied across a pure capacitor. As compared to the fundamental voltage, the reactance offered by the capacitor to the third harmonic voltage would be (a) nine time (b) three times (b) one-third (d) one-ninth

Determine whether the following two waves are of the same shape e = 10sin ( ωt + 30º) – 50 sin(3ωt – 60º) + 25 sin (5ωt + 40º) i = 1.0 sin (ωt – 60º) + 5 sin (3ω t – 150º) + 2.5cos (5ω t – 140º)