14 March 2020

AC Fundamental Interview Question Answer - 2


21
Describe the effect of harmonics on voltage and current in the pure inductive circuit.


Effect of harmonics in Pure Inductive circuit
  • The harmonic in the current waveform is much smaller than the voltage waveform. The magnitude of the 3rd harmonic current is 1 / 3 times to that of harmonic voltage.
  • The self inductance of the coil smoothen the current waveform due to presence of harmonics.


22
Explain : Pure capacitor


Pure capacitor
  • The pure capacitor is one in which has neither resistance nor dielectric loss.


23
Describe the significance of complex operator j.


Significance of Complex operator j
  • It indicates counter clockwise rotation of vector through 900. The value of j is equal to √ ( - 1 ).


24
What is meaning of the conjugate complex number?


Conjugate Complex number
  • The two numbers are conjugate to each other if the algebraic sign of its quadrature component is differ.
  • If one number is ( 5 + j 3 ), the other conjugate number is ( 5 – j 3 ).


25
Describe the significance of 1200 operator a.


Significance of 1200 operator a
  • It indicates counter clockwise rotation of vector through 1200.
  • It does not affect the magnitude of the vector. 
  • It is used in the three phase circuit.


26
In which direction the – a operator rotates? At what angle?


Direction of – a operator
  • The – a operator does not rotates at – 1200.
  • The – a operator rotates in the counter clockwise direction at angle of 3000 or – 600. [ The – a operator means ( j ) × ( j ) ×    ( a ). 
  • It means the rotation of angle through 900 + 900 + 1200 = 3000 or – 600 in the counter clockwise direction. ]


27
State the power factor of the pure resistive load, pure inductive load and pure capacitive load.


Power factor of the load
  • Pure inductive  : Zero
  • Pure capacitive : Zero
  • Pure resistive    : Unity


28
Explain the following terms : Active power , Reactive power , Apparent power and Power factor


Active Power ( P )
  • It is defined as the product of the RMS voltage, RMS current and cosine of the angle between them. 
  • It is also called as wattful power or true power.

          P = V × I × Cos Ф
Reactive Power ( Q )
  • It is defined as the product of the RMS voltage, RMS current and sine of angle between them.
  • It is also called as watt less power.

           Q = V × I × Sin Ф 
Apparent Power ( S )
It is defined as the product of the RMS voltage and RMS current.
           S = V × I
           Therefore S = P + j Q
Power factor ( Cos Ф )
  • It is defined as the cosine of angle between voltage vector and current vector.

                             OR
  • It is ratio of the active power to the apparent power. ( Power factor = Active power / Apparent power )

       Cos Ф  = ( V × I × Cos Ф ) / ( V × I )
                            OR
  • It is ratio of the resistance to the reactance of a given circuit parameter.

        Cos Ф  = R / Z

29
Explain the term : Q factor of coil


Q factor of coil
  • It is defined as the reciprocal of the power factor of the coil.
  • Q – factor = Z / R  = ωL / R ( If resistance of the coil is small as compare to reactance )


30
State the power factor of the series RLC circuit for the following cases : XL > XC , XL <  XC and XL  = XC


The power factor of the series R – L – C circuit when
  • XL > XC    :  Lagging power factor
  • XL  <  XC  :  Leading power factor
  • XL  = XC   :  Unity power factor


31
Describe the condition to be satisfied in the series RLC circuit to behave as ( 1 ) Resistive circuit ( 2 ) Inductive circuit and ( 3 ) Capacitive circuit


The series R – L – C resonance circuit behaves as
  • Resistive circuit when XL  = XC
  • Inductive circuit when  XL > XC
  • Capacitive circuit when XL  <  XC 


32
Compare Series Resonance and Parallel Resonance circuit


Salient Features of the Series Resonance Circuit
  • Minimum impedance
  • Maximum current
  • Unity power factor
  • Net reactance is zero
  • Q factor zero
  • When the supply frequency less than the resonance frequency, the load power factor is leading.
  • When the supply frequency greater than the resonance frequency, the load power factor is lagging.
  • Voltage magnify
  • Resonance frequency = 1 / 2π√ ( LC )

Salient Features of the Parallel Resonance Circuit
  • Maximum impedance
  • Minimum current
  • Net susceptance is zero. It means that admittance is equal to conductance at resonance.
  • Dynamic impedance L / CR
  • Unity power factor
  • Net reactive power is zero.
  • Current magnify
  • Resonance frequency = √ [( 1 / LC ) – ( R2 / L2 )]


33
Describe the effect of frequency on inductive and capacitive reactance.


Effect of frequency on reactance
  • As the supply frequency increases, the inductive reactance increases. ( XL = 2πfL )
  • As the supply frequency increases, the capacitive reactance decreases. ( XC = 1 / 2πfC )


34
What is resonance curve?


Resonance Curve
  • It is a graphical representation of circuit current and frequency of the applied voltage.


35
Define the term : Selectivity


Selectivity
  • It is ability of the resonance circuit to discriminate between one particular frequency and other frequency.


36
Describe the effect of resistance on the resonance curve.


Effect of resistance on the resonance curve
  • Smaller value of resistance, the circuit is said to be sharply resonant or highly selective.
  • Higher value of resistance, the circuit is said to be flat or poor selectivity.


37
Define : Half power bandwidth


Half Power Bandwidth
  • It is band of frequencies which lies between two points on either side of the resonance frequency when current falls to I0 / √ 2.
  • The I0 indicates current at resonance frequency.


38
Describe the effect of bandwidth on the selectivity of the series resonance circuit.


Narrow the bandwidth, higher the selectivity

39
Explain : Half power points


Half Power Points
  • The power at the two corner frequencies is becomes half to that of power at resonance. 
  • Therefore these two points are called as half power points.
  • The bandwidth corresponding to these points is called as half power bandwidth.
  • It is also called as – 3 dB bandwidth.


40
Describe the importance of Q factor in the series resonance circuit.


Importance of Q factor in the series resonance circuit
  • High value Q factor means high voltage magnification and higher selectivity of the coil.




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