A.C. Fundamentals Interview Question - Day 1

[Q:-01] what is alternating quantity?

Ans :

      An alternating quantity either current or e.m.f. is one which acts in alternate positive and negative directions, whose magnitude undergoes a definite series of numeral changes in definite intervals of time and in which the sequence of changes while positive is identical with the sequence while negative.

[Q:-02] define 'wave form' ?

Ans :

      the wave form or wave shape of an alternating quantity is the shape of the curve obtained by plotting the instantaneous values of current or e.m.f. as ordinate against time as abscissa.

[Q:-03] define 'cycle'

Ans :

      cycle is the combination of two similar halves of an alternating wave either current or e.m.f. consisting of the growth and decay of the current or e.m.f. in direction together with its growth and decay in the reverse direction.

[Q:-04] Define 'time period'

Ans :

      Time period of an alternating wave is the time taken to complete one cycle which is denoted by 'T'.

[Q:-05] Define 'frequency'

Ans :

      Frequency of an alternating wave ( current or e.m.f. ) is defined as the number of complete cycles performed by the wave in one second which is denoted by 'f'.

[Q:-06] define 'amplitude' or 'maximum value'.

Ans :

      Amplitude or maximum value of an alternating wave is the highest value attained by the current or e.m.f. in a half cycle.

[Q:-07] define 'instantaneous value'

Ans :

      the value at any particular instant is called instantaneous value.

[Q:-08] define 'average value'.

Ans :

      The average of the instantaneous values in a half cycle is called the average value.

[Q:-09] define 'R.M.S. value'.

Ans :

      The root mean square (R.M.S.) value of alternating current or voltage is defined to be the particular steady value which acting through a resistance in a.c. circuit for a given time produces the same heating effect as will be produced by the direct current or voltage acting through the same resistance for the same time.

This d.c. equivalent of a.c. r.m.s. value is also known as effective or virtual value used in purposes.

[Q:-10] define 'form factor'.

Ans :

      Form factor of an alternating wave (e.m.f or current ) is defined as the ratio of r.m.s. value to its average value.

Form-factor  =  E_r.m.s/E_average

=(E_m/√2)/((2/Π)E_m)

=Π/2√2=1.11

[Q:-11] define 'peak factor'.

Ans :

      Peak factor is the ratio of maximum value of an alternating wave to its r.m.s. value.

peak factor = I_max/I_r.m.s = (√2I_r.m.s )/I_r.m.s = √2 = 1.414

[Q:-12] what is the use of maximum value?

Ans :

      Maximum value is used in calculating the r.m.s and average values. It is also used to determine the amount of shock received by a person.

[Q:-13] what value of A.C. measures the ammeter or voltmeter ?

Ans :

R.M.S value

[Q:-14] what is the relation between

(i)Maximum value & R.M.S value

(ii)maximum value & average value

(iii)R.m.s. Value & Average value

Ans :

(i) R.M.S value = 0.707 x Maximum value

(ii) Average value = 0.636 x maximum value

(iii) R.M.S value =1.11 x average value

[Q:-15] what is electrical degree ? how many electrical degrees make one cycle ?

Ans :

      Electrical degree means 180th division of one pole pitch . 3600 (electrical degree) make one cycle.

[Q:-16] what is the relation between mechanical degree and electrical degree ?

Ans :

      electrical degrees = P/2  X mechanical degrees ,

where P is the number of poles.

[Q:-17] on what factors does the frequency depend ? what is the standard frequency in India ?

Ans :

      Frequency is directly proportional to the number of poles and speed of the generator .

Standard frequency in India is 50 Hz.

[Q:-18] what is sine wave ?

Ans :

      Sine wave is that alternating quantity (e.m.f of current) in which all the instantaneous values are proportional to the sine of the angle moved.

[Q:-19] what are the instantaneous values of current and voltage at any instant ?

Ans :

      The instantaneous value of current , i = Imax sin ωt, and the instantaneous value of e.m.f, e = E_max sin ωt.

Where w = angular velocity and t = time taken by the vector from zero to the instant .

[Q:-20] what do you mean by vector ?

Ans :

      Any quantity which has both the magnitude and the direction is called vector .

[Q:-21] is alternating quantity a vector ?

Ans :

      Alternating quantity is a vector quantity as it has both the magnitude and the direction .

[Q:-22] what do you mean by phase ?

Ans :

      The phase of an alternating quantity is that which indicates its position at any particular instant and its measures either in terms of the angle in radian or time in second with respect to some reference point

[Q:-23] what do you mean by phase difference ?

Ans :

      The time difference or angular difference between two alternating vectors ( between two voltage vectors or one voltage and one current vector or two current vectors ) having the same frequency is called the phase difference between them .

[Q:-24] what do you mean by "in phase"?

Ans :

      When the angular phase difference between two sinusoidal alternating current vectors is zero i.e. two vectors reach their maximum or minimum values at the same time in the same direction, they are said to be in phase.

[Q:-25] what do mean by 'out of phase' ?

Ans :

      When two alternating vector quantities reach their maximum values at different times keeping an equal phase angle between them they are said to be out of phase .