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Electric Circuit Analysis - I Unit - 2: Power Factor & Its Significance

Power factor is a measure of how effectively electric power is being used in a circuit. It is the ratio of the active power to the apparent power and is represented by the cosine of the angle between the voltage and current. A low power factor is undesirable as it causes poor voltage regulation, lower efficiency, higher power losses, and higher operating temperatures. In an AC circuit, power can be categorized as apparent power, active power, or reactive power depending on whether it is the total power, power actually used, or power that oscillates back and forth in the circuit.

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117 views3 pages

Electric Circuit Analysis - I Unit - 2: Power Factor & Its Significance

Power factor is a measure of how effectively electric power is being used in a circuit. It is the ratio of the active power to the apparent power and is represented by the cosine of the angle between the voltage and current. A low power factor is undesirable as it causes poor voltage regulation, lower efficiency, higher power losses, and higher operating temperatures. In an AC circuit, power can be categorized as apparent power, active power, or reactive power depending on whether it is the total power, power actually used, or power that oscillates back and forth in the circuit.

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meeravali_sn
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ELECTRIC CIRCUIT ANALYSIS – I

UNIT – 2

Power Factor & Its significance:

 The term ‘cos φ’ is called power factor. i.e


cosine of the angle between Vrms and Irms
and its value is always between + 900.
PF = cos φ
 It is also the ratio of Active power (P) to
the Apparent Power (S).
PF = Active Power / Apparent Power = P/S

 Power factor is also equal to the ratio of


resistance (R) to the impedance (Z) of the
circuit.
PF = R / Z
Significance:

 The range of Power factor is 0 to 1. Under


ideal conditions, the power factor is unity.
In a purely resistive circuit, PF = Unity
 The PF of an AC system should be as high
as possible so that current and applied
voltage is brought as near in phase as
possible.
 PF is lagging, when the current (I) lags the
supply voltage (V) and leading, when the
current (I) leads the supply voltage (V).
Low PF causes following drawbacks:
 Causes poor voltage regulation
 Efficiency of the system decreases
 More power loss in lines
 The current needed to obtain a given power
is very high. Thereby temperature also
increases.
AC Power / Instantaneous Power:

Let steady state voltage in an AC circuit is given Pav = Vrms. Irms watts
as,
v = Vm sinωt Generally rms values are represented by capital
and steady state current is given as,. letters.
i = Im sinωt
Hence,, Pav = V. I watts = I2R watts
Then, instantaneous value of power is,
p = vi = Vm sinωt . Im sinωt
p = Vm. Im sin2ωt Power drawn by an AC circuit can be 3 types
1  cos 2t
= Vm. Im
2 1. Apparent Power (S): It is the product of
V I rms value of the applied voltage and circuit
= m m (1-cos2ωt) current. It is also known as ‘wattles’ (or
2
V I V I idle) component of power.
P= ( m m ) – (( m m ). cos2ωt)
2 2 Apparent Power(S)=Vrms.Irms= V.I=(IZ).I= I2.Z
From the above equation, it is clear that the Units: volt-amp (V.A)
instantaneous power consists of two components.
2. Active Power (P) : It is the power which is
V I actually dissipated in the circuit resistance.
1. Constant Power component : m m
2 It is also known a ‘wattful’ component of
V I power (or) True Power (or) Real Power
2. Fluctuating Component: ( m m ).cos2ωt,
2 (or) Power
having the frequency, double the frequency of the
applied voltage. Active Power(P)= I2.R = V.I.cosφ
Units: watts
Now, the average value of the fluctuating cosine
component of double frequency is zero, over one 3. Reactive Power (Q): It is power
complete cycle. developed in the inductive reactance of the
circuit. Thus,
So, average power consumption over one cycle is Reactive Power(Q)= I2.XL = V.I.sinφ
equal to the constant power component only i.e.
Vm I m Units: volt – ampere reactive (VAR)
.
2
** Relationship between ‘S’, ‘P’ and ‘Q’
Vm I m Vm . Im
is given by
Hence, Pav = =
2 2 2 S =P+jQ (or) S2 =P2+Q2

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