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 3^{rd} 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 90^{0}. 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 120^{0} operator a.


Significance
of 120^{0} operator a
 It
indicates counter clockwise rotation of vector through 120^{0}.
 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 – 120^{0}.
 The
– a operator rotates in the counter clockwise direction at angle of 300^{0}
or – 60^{0}. [ The – a operator means ( j ) × ( j ) × ( a ).
 It means
the rotation of angle through 90^{0 }+ 90^{0} + 120^{0}
= 300^{0} or – 60^{0} 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 : X_{L} >
X_{C} , X_{L }< X_{C}
and X_{L} = X_{C}


The power factor of the series
R – L – C circuit when
 X_{L
}> X_{C} : Lagging power factor
 X_{L }<
X_{C} : Leading power factor
 X_{L }= X_{C} : 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 X_{L }= X_{C}
 Inductive circuit when X_{L }> X_{C}
 Capacitive
circuit when X_{L }< X_{C}

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 ) – ( R^{2} / L^{2} )]

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. ( X_{L} = 2πfL )
 As the supply frequency
increases, the capacitive reactance decreases. ( X_{C} = 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 I_{0} / √ 2.
 The I_{0} 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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