1
CHAPTER 5
Exercises
E5.1 (a) We are given
)30200cos(150)(
ttv
. The angular frequency is
the coefficient of
t
so we have
radian/s 200
. Then
Hz 1002/
f
ms 10/1
fT
V 1.1062/1502/
mrms VV
Furthermore,
v
(
t
) attains a positive peak when the argument of the
cosine function is zero. Thus keeping in mind that
t
has units of
radians, the positive peak occurs when
ms 8333.0
180
30 maxmax
tt
(b)
(c) A plot of
v
(
t
) is shown in Figure 5.4 in the book.
)30300cos(100)60300sin(100
tt
The period corresponds to
360
therefore 5 ms corresponds to a phase
angle of
108360)67.16/5(
. Thus the voltage is
)108377cos(6.155)(
ttv
V
)45cos(14.14)sin(10)cos(10
ttt
(b)
330.45.25660.86053010
1
jj
I
44.318.11670.016.11
j
)44.3cos(18.11)30sin(5)30cos(10
ttt
(c)
99.125.70206015020
2
jj
I
28.2541.3099.125.27
j
)28.25cos(41.30)60cos(15)90sin(20
ttt
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2
E5.5 The phasors are
4510 and 3010 3010 321 VVV
v
1 lags
v
2 by
60
(or we could say
v
2
leads
v
1 by
)60
v
1 leads
v
3 by
15
(or we could say
v
3
lags
v
1 by
)15
v
2 leads
v
3 by
75
(or we could say
v
3
lags
v
2 by
)75
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3
13507.7 IV
R
R
4507.7 IV
Lj
L
(b) The phasor diagram is shown in Figure 5.17b in the book.
(c)
i
(
t
) lags
vs
(
t
) by
.45
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4
E5.12 (a) For a power factor of 100%, we have
,1)cos(
which implies that
the current and voltage are in phase and
.0
Thus,
.0)tan(
PQ
Also
 
A. 10)]0cos(500/[5000]cos/[
rmsrms VPI
Thus we have
.4014.14 and 14.142
I
rmsm II
implies that the current lags the voltage by
.46.78)2.0(cos 1
Thus,
.kVAR 49.24)tan(
PQ
Also, we have
 
A. 0.50]cos/[
rmsrms VPI
Thus we have
.46.3871.70 and A 71.702
I
rmsm II
of part (b) than for that of part (a). Wiring costs would be lower for the
load of part (a).
903.265)/(1
CjZC
90
C
A 770.3/
C
rms
Crms ZVI
0)cos(
CCrms
rms
CIVP
kVAR 770.3)sin(
CCrms
rms
CIVQ
power factor of 80% lagging from which we have
.87.36)8.0(cos 1
2
Notice that we select a positive angle for
2
because the load has a
lagging power factor. Thus we have
kW 0.8)cos( 222
rms
rms IVP
and
kVAR 6)sin(
22
rms
rms IVQ
.
kW 8
2
PPP C
s
kVAR 23.2
2
QQQ C
s
kVA 305.8
22
sssrmsrms QPIV
A 305.8/
rmssrmsrmssrms VIVI
%33.96%100)/(factor power
srmsrmss IVP
E5.14 First, we zero the source and combine impedances in series and parallel
to determine the Thévenin impedance.
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5
50502550
100/1100/1
1
2550
jj
j
jZt
04.141.103j25100
4571.70
100100
100
100
j
oct
VV
04.596858.0/
tt ZV
n
I
453536.0
2510025100
4571.70
jj
I
The load power is
W 25.6)2/3536.0(100 22
rms
LL IRP
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6
(b) For a purely resistive load, maximum power is transferred
for
. 1.10325100 22
t
LZR
The Thévenin equivalent with the
load attached is:
98.373456.0
251001.103
4571.70
j
I
The load power is
W 157.6)2/3456.0(1.103 22
rms
LL IRP
Van is zero. Then we have
1204.577 1204.577 04.577
cn
bn
an
VVV
The circuit for the
a
phase is shown below. (We can consider a neutral
connection to exist in a balanced Y-Y connection even if one is not
physically present.)
02.37610.4
40.75100
04.577
jZ L
an
aA
V
I
The currents for phases
b
and
c
are the same except for phase.
98.82610.4 02.157610.4
cC
bB
II
kW 188.3)02.37cos(
2
610.44.577
3)cos(
2
3
L
YIV
P
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7
kVAR 404.2)02.37sin(
2
610.44.577
3)sin(
2
3
L
YIV
Q
04.57703/1000
an
V
The phase impedance of the equivalent Y is
. 67.163/503/
ZZY
A 063.34
67.16
04.577
Y
an
Z
V
IaA
Similarly,
A 12063.34
bB
I
and
A. 12063.34
cC
I
Finally, the power is
kW 00.30)2/(3 2
y
aA RIP
601
805030100
211
jj
VVV
302
805050
122
jj
VVV
302
601
8050
1
50
1
8050
1
8050
1
8050
1
30100
1
2
1
V
V
jjj
jjj
Y = [(1/(100+j*30)+1/(50-j*80)) (-1/(50-j*80));…
(-1/(50-j*80)) (1/(j*50)+1/(50-j*80))];
I = [pin(1,60); pin(2,30)];
V = inv(Y)*I;
pout(V(1))
pout(V(2))
21.10698.79
1V
and
30.11613.124
2V
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8
Problems
P5.1 The units of angular frequency
are radians per second. The units of
frequency
f
are hertz, which are equivalent to inverse seconds. The
radian measure of angle is length divided by length. In terms of physical
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9
P5.5
 
 
30500cos50120500sin50
tttv
 
W 25
V 36.352502
ms 41
Hz 250
rad/s 500
2
RVP
VV
fT
f
rms
mrms
ms 3333.0
06500
1
1
t
t
ππ
The first positive peak after
0
t
is at
ms 667.3
1
Tttpeak
The peak voltage is
28.282022 rms
VVm
V. The frequency is
Tf
/1
10 kHz and the angular frequency is
4
1022 ππω
f
radians/s. The phase corresponding to a time interval of
t
Δ
20
s is
72360)/(
Tt
Δθ
. Thus, we have
)72102cos(28.28)( 4
ttv
π
V.
28.282202
T
20002
f
rad/s
108360 max
T
t
)1082000cos(28.28)(
tti
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10
P5.8
 
 
V 250sin10
ttv
   
 
 
W 500sin15.2250sin5 22
ttRtvtp
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11
values divided by the square root of two. However, they are for
sinusoids, which are important special cases.
P5.12*
 
A 808.3425
4
11 2
0
4
20
2
 
dtdtdtti
T
I
T
rms
P5.13
 
V 61.1015
2
11
0
2
0
2
dtdttv
T
i
V
rms
P5.14
 
 
1
0
1
0
2
0
2)2exp(100)exp(10
1
dttdttdttv
T
V
T
rms
 
V 575.6)]2exp(1[50)2exp(50 1
0
t
t
t
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12
P5.15
 
 
1
0
2
0
2)2sin(2)2cos(
1
dttBtAdttv
T
V
T
rms
 
2
4
2
4
0
2
)2(sin4)2sin()2cos(4)2(cos
22
1
0
22
1
0
2222
BA
t
B
t
A
dttBttABtAV
t
t
rms
syms Vrms t A B
Vrms = sqrt((int((A*cos(2*pi*t)+2*B*sin(2*pi*t))^2,t,0,1)))
Vrms =
(A^2/2 + 2*B^2)^(1/2)
 
 
025.0
025.0
025.0
025.0
2
2/
2/
2)40cos(8820)20cos(420
1
dttdttdttv
T
V
T
T
rms
V 228)40sin(
40
160
160
025.0
025.0
t
t
rms ttV
P5.17
 
 
V 774.5
3
25
2
1
5
2
11 2
0
3
2
0
2
0
2
t
t
T
rms
t
dttdttv
T
V
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13
P5.18
 
dttv
T
i
V
T
rms
0
2
 
V 5
06.109.0
1.0
1
)]40cos(1616)20cos(2249
1.0
1
)]20(cos32)20cos(2249
1.0
1
)]20(cos32)20cos(2249[
1.0
1
)]20cos(243[
1.0
1
1.0
0
1.0
0
1.0
0
1.0
0
1.0
0
1.0
0
2
1.0
0
1.0
0
2
1.0
0
2
 
 
dttdtdttdt
dttdttdt
dttt
dttVrms
complex number whose magnitude equals the peak amplitude of the
sinusoid and whose phase is the phase angle of the sinusoid written as a
cosine function.
2. Add the phasors and convert the sum to polar form using complex
arithimetic.
3. Convert the resulting phasor to a sinusoid.
counterclockwise. If phasor
A
points in a given direction before phasor
B
by an angle
, we say that
A
leads
B
by the angle
or that
B
lags
A
by the
angle
.
(b) Examine plots of the sinusoidal waveforms versus time. If
A
reaches a
point (such as a positive peak or a zero crossing with positive slope) by an
interval
t
before
B
reaches the corresponding point, we say that
A
leads
B
by the angle
360)/(
Tt
or that
B
lags
A
by the angle
.
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14
P5.21* We are given the expression
)sin(4)75cos(3)75cos(5
ttt
ωωω
Converting to phasors we obtain
904753755
4)8978.27765.0(8296.42941.1
jjj
09.82763.37274.35176.0
j
Thus, we have
)09.82cos(763.3
)sin(4)75cos(3)75cos(5
t
ttt
ω
ωωω
P5.23*
 
 
 
 
45cos4.141
454.141100100
10090100
1000100
90cos100sin100
21
s
2
1
2
ttv
j
j
tttv
s
ω
ωω
VVV
V
V
45 by leads
45 by lags
90 by lags
2
s
1
s
12
VV
VV
VV
 
 
 
 
 
   
   
   
60 by leads
60 by lags
120 by lags
90400cos10
)150400cos(5
30400cos10
4002
32
31
21
3
2
1
tvtv
tvtv
tvtv
ttv
ttv
ttv
f
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15
P5.24 The magnitudes of the phasors for the two voltages are
212
and
27
V. The phase angles are not known. If the phase angles are the same, the
phasor sum would have its maximum magnitude which is
219
. On the
other hand, if the phase angles differ by
180
, the phasor sum would
have its minimum magnitude, which is
25
. Thus, the maximum rms value
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16
66.5982.22
Thus, we have
)66.59cos(82.22
)120cos(10)30cos(15)45sin(5
t
ttt
3015
1V
V
10071.7
1I
A
 
 
10cos071.7
1
tti
A
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17
P5.33*
   
90 by leads
tvti CC
409040605.230100
jZ
I
V
IV
Because
Z
is pure imaginary and positive, the element is an inductance.
200
mH200
Z
L
25902530460100
jZ
I
V
IV
Because
Z
is pure imaginary and negative, the element is a capacitance.
200
F200
1
Z
C
Because
Z
is pure real, the element is a resistance of 20 .
and
.500
Also, we see that the current lags the voltage by 1 ms or
 
 
 
 
 
ttti
Z
j
C
j
Z
ttv
C
C
C
C
ππ
ω
πω
π
2000sin6283.0902000cos6283.0
906283.0
010
9092.1592.15
2000
2000cos10
CC
C
VI
V
Ω
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication
is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system,
or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to:
Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication
is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system,
or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to:
Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
18
90, so we have an inductance. . Finally,
, 5000/
mm IVL
from
which we find that
L
3.183 H.
(b) From the plot, we see that
16
T
ms, so we have
5.62/1
Tf
Hz
and
.125
Also, we see that the current leads the voltage by 4 ms or
90, so we have a capacitance. Finally,
, 2500//1
mm IVC
from
which we find that
C
1.019 F.
Because
Z
is pure imaginary and negative, the element is a capacitance.
1000
F50
1
Z
C
Because
Z
is pure imaginary and positive, the element is an inductance.
1000
mH10
Z
L
(c)
505
I
V
Z
Because
Z
is pure real, the element is a resistance of 5 .
1. Replace sources with their phasors.
2. Replace inductances and capacitances with their complex impedances.
3. Use series/parallel, node voltages, or mesh currents to solve for the
quantities of interest.
All of the sources must have the same frequency.
45 by lags
s
VI
V 45071.7
V 45071.7
mA 4571.70
100100
010
IV
IV
Lj
R
j
L
R
ω
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication
is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system,
or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to:
Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication
is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system,
or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to:
Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
19
P5.39
mA 43.6372.44
200100
010
jLjR
s
V
I
V 43.63472.4
IV
R
R
V 57.26944.8
IV
Lj
L
43.63 by lags
s
VI
43.63 by leads s
VI
V 45071.7
IV
R
R
 
V 45071.7
IV
Cj
C
I leads Vs by 45
V
s
I
V
R
V
L
63.43
 
V57.26944.8
V 43.63472.4
mA 43.63472.4
20001000
010
IV
IV
Cj
R
j
CjR
C
R
ω
ω
V
s
I
V
R
V
C
45
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication
is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system,
or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to:
Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication
is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system,
or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to:
Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
20
P5.42*
C
jRLjZ
ω
ω1
57.711.158 01550
2005050 :500
Ω
j
jjZ
ω
050 50
10050100 :1000
Ω
jjZ
ω
57.711.158 15050
5050200 :2000
Ω
j
jjZ
ω
 
31.5647.55153846769230
01.0005.0501
1
:500
.j .
jj
Z
01000100 :1000
jZ
31.5647.55153846769230 :2000
.j .Z
:500
57.26944.848
05.0)55/(1
1
j
jj
Z
57.2636.221020 :1000
jZ
60.85519.65.65.0 :2000
jZ
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication
is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system,
or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to:
Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.
© 2014 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This publication
is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system,
or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to:
Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.