14 March 2020

AC Fundamental Interview Question Answer - 1


1
Give reason : The alternating waveform is called as sinusoidal waveform.


Sinusoidal waveform
  • The induced emf in the coil varies with the sine function of the time angle ωt therefore the alternating waveform is called as sinusoidal waveform.


2
Explain the following terms : Waveform , Amplitude  and Frequency


 Waveform
  • The shape of the instantaneous value of alternating quantity      ( voltage or current ) is represented on Y – axis and time on X – axis is called waveform of the alternating quantity.

 Amplitude
  • The maximum ( Positive or Negative ) value of an alternating quantity is called as amplitude.

 Frequency (  f  )
  • The numbers of cycle per second of an alternating quantity is called as frequency.
  • Its unit is cycle per second or Hertz ( Hz ).


3
Explain the following terms : Cycle , Instantaneous value and  Time period  


Cycle
  • One complete set of positive and negative cycle of an alternating quantity is called as cycle.

 Instantaneous value
  • The value of alternating quantity at given instant (or time) is called as instantaneous value.

 Time period (T)
  • It is defined as the time required to complete one cycle of an alternating quantity.
  •  It is measured in second.


4
Explain the following terms : Peak value , Average or mean value , Effective or RMS value and Form factor


Peak Value
  • The maximum value of positive or negative half cycle of symmetrical waveform of an alternating quantity is called as peak value.

 Average or mean value
  • It is DC quantity which transfers the same charge in a given circuit as is transferred by the alternating quantity during the same time.

 Effective or RMS value
  • The RMS value of an alternating quantity is defined as the value of direct current which when flowing through a given circuit for a given time produces the same heat as that is produced by alternating quantity when flowing through the same circuit for the same time.

 Form factor ( Kf )
  • It is defined as the ratio of the RMS value to the average value of an alternating quantity.

       Form factor = RMS value / Average value                 
  • The value of form factor for sinusoidal alternating quantity is 1.11


5
Explain the term : Complex waveform


Complex Waveform
  • It is composed of fundamental waveform and number of other harmonics waveform.


6
What is frequency of the fundamental harmonic?


50 Hz

7
At which condition, the complex waveform does not contain distortion?


Distortion in the Complex waveform
  • When the two halves of the complex wave are identical in shape there is no distortion in the output waveform. It is only possible when only odd harmonics ( 3rd , 5th , 7th etc ) are present.
  • When the two halves of the complex wave are not identical in shape, there is distortion in the output waveform. It is only possible when only even harmonics ( 2nd , 4th , 6th etc ) are present.


8
State the frequency of the 5th harmonics if the fundamental frequency 50 Hz?


The frequency of the 5th harmonics is 250 Hz.

9
State the speed of the 5th harmonics if the fundamental waveform rotates at Ns RPM.


The speed of the 5th harmonics is Ns / 5 RPM.

10
State other name of the RMS current.


Effective / virtual current

11
Which method is used to find out RMS value of the alternating quantity?


Method to find out RMS quantity
  • Mid – ordinate method
  • Analytical method


12
State the average value of the complete symmetrical alternating voltage.



Zero

13
Write the equation of the RMS voltage, average voltage and form factor for sinusoidal and half wave rectified voltage


Sinusoidal voltage
  • RMS voltage :       Im / √ 2
  • Average voltage :  2Im / π
  • Form factor : 1.11

Half wave rectified voltage
  • RMS voltage : Im / 2
  • Average voltage : Im / π
  • Form factor : 1.57


14
Describe the importance of the amplitude or crest factor.


Importance of amplitude factor
  • The dielectric stress on the insulation depends upon maximum voltage therefore the amplitude factor is importance during testing of dielectric insulation.
  • The iron loss depends upon maximum flux density not RMS flux density therefore the amplitude factor greatly affect the value of the iron loss.


15
Why alternating supply is not used for charging batteries?


The alternating supply consists of both positive and negative half cycle therefore the charging of battery is done during positive half cycle and it will discharge during negative half cycle.

16
State the RMS and average value of symmetrical rectangular waveform.


The RMS value and average value of symmetrical rectangular waveform is one.

17
An alternating current of peak value is 50.0 Amp. If this current is passing through ( a ) Moving coil ammeter ( b ) Moving iron ammeter. What is indication on these meters?


  • The Moving coil ammeter indicates average value of an alternating quantity. Therefore the meter indicates Zero reading for sinusoidal alternating current of peak value 50.0 amps.
  • The moving iron ammeter indicates RMS value of an alternating quantity.
  • Therefore the meter indicates =  RMS value of current =  0.707 × Maximum value of current  = 0.707 × 50 = 35.35 Amp


18
Which parameters affect the value of self inductance or back emf of the inductive coil?


The self inductance of the inductive coil is affected by
  • Rate of change of current and
  • Inductance of the coil : E = – L ( di / dt )


19
Give reason : The power loss in the pure resistive circuit never becomes zero.


The instantaneous value of voltage and current are always either either negative or both positive therefore the power loss never becomes zero.

20
Explain : Pure inductive coil


Pure inductive coil
  • It is coil in which no ohmic resistance and no power loss. It is only consider for theory point of view.
  • Practically the pure inductance is not possible.





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