BFF1303: ELECTRICAL /
ELECTRONICS ENGINEERING
Alternating Current Circuits :
Power Analysis
Ismail Mohd Khairuddin , Zulkifil Md Yusof
Faculty of Manufacturing Engineering
Universiti Malaysia Pahang
Alternating Current Circuit
(AC)-AC Power Analysis
BFF1303 ELECTRICAL/ELECTRONICS ENGINEERING
Contents:




Outcome
Instantaneous and Average
Power
Apparent Power
Power Factor


Complex Power
Power Factor Correction
Faculty of Manufacturing
Universiti Malaysia Pahang
Kampus Pekan, Pahang Darul Makmur
Tel: +609-424 5800
Fax: +609-4245888
BFF1303 Electrical/Electronic Engineering
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Draw the power
triangle, and compute
the capacitor size
required to perform
power factor
correction on a load.
Convert time-domain
sinusoidal voltages
and currents to phasor
notation, and vice
versa; represent
circuit using
impedance
Learn complex power
notation; compute
apparent power, real,
and reactive power for
complex load.
Solve steady-state ac
circuits, using phasors
and complex
impedances.
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Instantaneous Power
The instantaneous power 𝑝 𝑡 absorbed by an element is the
product of instantaneous voltage 𝑣 𝑡 across the element and the
instantaneous current 𝑖 𝑡 through it.
p t   v t  i t 
The instantaneous power (in watt) is the power at any instant of
time.
It is rate at which element absorbs energy.
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Let
v  t   Vm cos t   v 
Peak value
Phase angle of
voltage
i  t   I m cos t  i 
Peak value
Phase angle of
current
p  t   v  t  i  t   Vm I m cos t   v t  i  
Apply trigonometric
identity
1
cos A cos B  cos  A  B   cos  A  B  
2
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Sinusoidal
function
1
1
p  t   Vm I m cos  v  i   Vm I m cos  2t   v  i 
2
2
Time
independent
The instantaneous power
changes with time and is
therefore difficult to measure
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Average Power
The average power (in watts) is the average of the instantaneous
power over 1 period.
1 T
P   p  t  dt
T 0
1
P  Vm I m cos 
2
P  Vrms I rms cos 
   v  i
A resistive load 𝑅 absorbs power
at all times, while reactive load
𝐿 or 𝐶 absorbs zero average
power
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Given that
v  t   120 cos  377t  45  V
i  t   10 cos  377t  10  A
Find the instantaneous power and the average power absorbed.
Solution
Instantaneous power
p t   v t  i t 
p  t   120 cos  377t  45   10 cos  377t  10  
p  t   344.2  600 cos  754t  35  W
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For the circuit shown, find the average power supplied by the
source and the average power absorbed by the resistor.
Average power
supplied by source
Solution
V 530
I 
Z 4  j2
I  1.11856.57 A
1
P  Vm I m cos 
2
1
P   5 1.118  cos  30  56.57 
2
P  2.5 W
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Average power supplied by resistor
1
P  Vm , R I m cos 
2
Vm , R  4I
Vm , R  4.47256.57 V
1
P   4.472 1.118  cos  56.57  56.57 
2
P  2.5 W
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Determine the average power generated by each source and the
average power absorbed by each passive element in the circuit
shown
P3  0 W
Answer
P1  367.8 W
P4  0 W
P2  160 W
P5  207.8 W
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The apparent power is the product of the rms values of voltage
and current.
S  Vrms I rms
 VA 
The unit is volt-amperes or VA to distinguish it from the average
power, which is measured in watts.
We know
P  Vrms I rms cos 
Then
P  S cos 
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pf  cos   cos  v  i 
P
pf 
S
The power factor is the cosine of the phase difference between
voltage and current. It is also the cosine of the angle of the load
impedance.
The value of pf ranges between 0 – 1.
For a purely resistive load, the voltage and current are in phase,
so that 𝜃𝑣 − 𝜃𝑖 = 0 and 𝑝𝑓 = 1
For a purely reactive load 𝜃𝑣 − 𝜃𝑖 = ±90° and 𝑝𝑓 = 0
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For 0 < 𝑝𝑓 < 1, the pf is said to be leading or lagging.
Leading pf means that current leads voltage, which implies a
capacitive load.
Lagging pf means that current lags voltage, implying an inductive
load.
Leading and lagging pf can be determined by using a phasor
diagram.
Power factor affects the electric bills consumers pay the electric
utility companies.
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Steps to determined leads or lags using phasor diagram:
Make sure the amplitude of the expression is +ve value. Use
the following to change from –ve to +ve.
 sin A  sin( A  180)
 cos A  cos( A  180)
Make sure the function is in sine function. Change the cos
function into sine function by using
cos A  sin( A  90)
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Draw the phasor diagram.
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From the phasor diagram, rotate (anti-clockwise) both of the drawn expression
to determined the leading or lagging pf. From the rotation if the current follows
the voltage then it is said that the current lags voltage, thus the pf is lagging.
V(t)
i(t)
f
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Given that current and voltage for series-connected load. Find the
apparent power and the power factor of the load. Determine the
element values that form the series-connected load.
i  t   4 cos 100 t  10  A
v  t   120 cos 100 t  20  V
Solution
Apparent power
S  Vrms I rms
 120  4 
S 

  240 VA
 2  2 
Power factor
pf  cos 
pf  cos  v  i   0.866
The pf is leading because the current leads the voltage.
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V 120  20
Z 
I
410
Z  25.98  j15 
The load impedance 𝐙 can be modeled by a 25.98 Ω resistor in
series with a capacitor with
1
XC  
C
1
15  
C
1
C
100 15
C  212.2  F
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Determine the pf and average power delivered by the source
Solution
The total impedance is
 j2  4
Z  6  4 ( j 2)  6 
 6.8  j1.6  7  13.24
4  j2
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Thus
I rms
Vrms
300


 4.28613.24A
Z
7  13.24
The power factor,
pf  cos  v  i   cos  0  13.24   0.9734
Which is leading power factor
P  Vrms I rms cos  v  i 
The average power
supply by the source is
 30  4.286  0.9734 
 125 W
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Determine the pf and average power delivered by the source
Answer
pf  0.936
lagging
P  2.008 kW
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Complex power is the product of the rms voltage phasor and the
complex conjugate of the rms current phasor. As a complex
quantity, its real part is real power P and its imaginary is reactive
power Q
Consider the following
v  t   Vm cos t  v   Vm v
i  t   I m cos t  i   I m i
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*
Complex power  Vrms I rms
 Vrms I rms   v  i 
 VA 
 S   v  i 
 P  jQ



P 2  Q 2   v  i 
Where Q is the reactive power (VAR)
If
Q = 0 for resistive loads (unity pf).
Q < 0 for capacitive loads (leading pf).
Q > 0 for inductive loads (lagging pf).
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Power triangle:
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Given that
v  t   60 cos t  10 
i  t   1.5cos t  50 
Find the complex and apparent powers, the real and reactive
powers and the power factor.
Solution
Complex power  Vrms I rms   v  i 
60 1.5

  10  50 
2 2
 45  60 VA
 22.5  j 38.97 VA
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Power factor,
𝑝𝑓 = cos −10° − 50° = 0.5
Thus the apparent power, 𝑆 = 45 VA
real power, 𝑃 = 22.5 W
reactive power, 𝑄 = −38.97 VAR
i(t)
From the phasor diagram 𝑖(𝑡)
leads 𝑣(𝑡), thus the pf is
leading.
Power triangle
50
10
V(t)
P = 22.5 W
60
Phasor diagram
Q = -38.97 VAR
S = 45 VA
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Given that
Vrms  11085 V
I rms  0.485 A
Find:
i. The complex and apparent powers.
ii. The real and reactive powers.
iii. The power factor and load impedance.
Answer
i. 4470 VA, 44 VA
ii. 15.05 W, 41.35 VAR
iii. 0.342 lagging, 94.06  j 258.4 
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Most domestic loads (such as washing
machines, air conditioners, and refrigerators)
and industrial loads (such as induction
motors) are inductive and operate at a low
lagging power factor.
Although the inductive nature of the load
cannot be changed, we can increase its power
factor.
The process of increasing the power factor
without altering the voltage or current to the
original load is known as power factor
correction.
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A load’s power factor is improved
or corrected by deliberately
installing a capacitor in parallel
with the load
The effect of adding the capacitor
can be illustrated using either the
power triangle or the phasor
diagram of the currents involved
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P  S1 cos 1
Q1  S1 sin 1  P tan 1
If we desire to increase the power
factor from cos 𝜃1 to cos 𝜃2 without
altering the real power, then the
new reactive power
Q2  P tan  2
The reduction in the reactive
power is caused by the shunt
capacitor;
QC  Q1  Q2  P  tan 1  tan  2 
The value of the required shunt
capacitance C
P  tan 1  tan  2 
QC
C

2
2
Vrms
Vrms
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Note that the real power P dissipated by the load is not affected
by the power factor correction because the average power due to
the capacitance is zero.
Although the most common situation in practice is that of an
inductive load, it is also possible that the load is capacitive; that is,
the load is operating at a leading power factor.
In this case, an inductor should be connected across the load for
power factor correction.
QL  Q1  Q2
2
2
Vrms
Vrms
QL 

X L L
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2
Vrms
 L
QL
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When connected to a 120-V (rms), 60-Hz power line, a load absorbs
4 kW at a lagging power factor of 0.8. Find the value of capacitance
necessary to raise the pf to 0.95.
Solution
The apparent power
pf  0.8
cos 1  0.8  1  36.87
P
4000
S

cos 1
0.8
S  5000 VA
The reactive power
Q1  S1 sin 1  5000sin 36.87
Q1  3000 VAR
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The new apparent power
pf new  0.95
cos  2  0.95   2  18.19
P
4000
S2 

cos  2 0.95
S 2  4210.5 VA
The new reactive power
Then the value of
capacitor
Q2  S 2 sin  2  4210.5sin18.19
Q2  1314.4 VAR
QC
C
2
Vrms
QC  Q1  Q2  1685.6 VAR
BFF1303 Electrical/Electronic Engineering
C  310.5  F
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Find the value of parallel capacitance needed to correct a load of 140
kVAR at 0.85 lagging pf to unity pf. Assume that the load is supplied
by a 110-V (rms), 60-Hz line.
Answer
C  30.69 mF
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