366 Sinusoidal Steady-State Power Calculations Example 10.1 Calculating Average and Reactive Power a Calculate the average power and the reactive power at the terminals of the network s
Trang 1366 Sinusoidal Steady-State Power Calculations
Example 10.1 Calculating Average and Reactive Power
a) Calculate the average power and the reactive
power at the terminals of the network shown in
Fig 10.6 if
v = 100 cos (tat + 15") V,
i = 4sin(W - 15°) A
b) State whether the network inside the box is
absorbing or delivering average power
c) State whether the network inside the box is
absorbing or supplying magnetizing vars
'-*•
+
V
Figure 10.6 A A pair of terminals used for calculating power
Solution
a) Because i is expressed in terms of the sine func-tion, the first step in the calculation for P and Q
is to rewrite i as a cosine function:
i = 4cos(o>f - 105°) A
We now calculate P and Q directly from
Eqs 10.10 and 10.11 Thus
P = -(100)(4) cos [15 - (-105)] = -100 W,
Q = -100(4) sin [15 - (-105)] = 173.21 VAR
b) Note from Fig 10.6 the use of the passive sign convention Because of this, the negative value
of -100 W means that the network inside the box is delivering average power to the terminals c) The passive sign convention means that, because
Q is positive, the network inside the box is
absorbing magnetizing vars at its terminals
^ A S S E S S M E N T P R 0 B L E
Objective 1—Understand ac power concepts, their relationships to one another, and how to calcuate them in a circuit
10.1 For each of the following sets of voltage and
current, calculate the real and reactive power
in the line between networks A and B in the
circuit shown In each case, state whether the
power flow is from A to B or vice versa Also
state whether magnetizing vars are being
trans-ferred from A to B or vice versa
a) v = 100 cos (at - 45°) V;
i = 20cos(wr + 15°) A
b) v = 100 cos (cot - 45°) V;
i = 20cos(&rf + 165°) A
c) v = 100 cos (at - 45°) V;
i = 20 cos (w* - 105°) A
d) V = 100 cos at V;
i = 20 cos (art + 120°) A
A
i
—^-+
Answer: ( a ) P
Q
500 W -866.03 VAR
(b) P = -866.03 W
Q = 500 V A R
(c) P = 500 W
Q = 866.03 VAR
(d) P = -500 W
Q = -866.03 V A R
A to B),
B to A);
B to A),
A to B);
A to B),
A t o B ) ;
B t o A ) ,
B t o A )
10.2 Compute the power factor and the reactive
fac-tor for the network inside the box in Fig 10.6, whose voltage and current are described in Example 10.1
Hint: Use -i to calculate the power and
reac-tive factors
Answer: pf = 0.5 leading; rf = -0.866
NOTE: Also try Chapter Problem 10.1,
Trang 2Appliance Ratings
Average power is used to quantify the power needs of household appliances
The average power rating and estimated annual kilowatt-hour consumption
of some common appliances are presented in Table 10.1 The energy
con-sumption values are obtained by estimating the number of hours annually
that the appliances are in use For example, a coffeemaker has an estimated
annual consumption of 140 kWh and an average power consumption during
operation of 1.2 kW Therefore a coffeemaker is assumed to be in operation
140/1.2, or 116.67, hours per year, or approximately 19 minutes per day
Example 10.2 uses Table 10.1 to determine whether four common
appliances can all be in operation without exceeding the current-carrying
capacity of the household
| Making Power
The branch circuit supplying the
Calculations Involving Household Appliances
: outlets in a typical Solution
home kitchen is wired with #12 conductor and is
protected by either a 20 A fuse or a 20 A circuit
breaker Assume that the following 120 V
appli-ances are in operation at the same time: a
cof-feemaker, egg cooker, frying pan, and toaster Will
the circuit be interrupted by the protective device?
From Table 10.1, the total average power demanded
by the four appliances is
P = 1200 + 516 + 1196 4- 146
The total current in the pro
4058
e f r" 120 '
= 4058 W ective device is 33.82 A
Yes, the protective device will interrupt the circuit TABLE 10.1 Annual Energy Requirements of Electric Household Appliances
Appliance
Food preparation
Coffeemaker
Dishwasher
Egg cooker
Frying pan
Mixer
Oven, microwave (only)
Range, with oven
Toaster
Laundry
Clothes dryer
Washing machine, automatic
Water heater
Quick recovery type
Comfort conditioning
Air conditioner (room)
Dehumidifier
Fan (circulating)
Heater (portable)
Average
Wattage
1,200 1,201
516 1,196
127 1,450 12,200 1,146
4,856
512 2,475 4,474
860
257
88 1,322
NOTE: Assess your understanding of this
Est kWh Consumed Annually 3
140
165
14
100
2
190
596
39
993
103 4,219 4,811
860 b
377
43
176
Appliance Health and beauty Hair dryer Shaver Sunlamp
Home entertainment
Radio Television, color, tube type Solid-state type
Housewares Clock Vacuum cleaner a) Based on normal usage When using such factors as the size of the specific
Average
Wattage
600
15
279
71
240
145
2
630 hese figure appliance, I
Est kWh Consumed Annually 3
25 0.5
16
86
528
320
17
46
; for projections
he geographical area of use, and individual usage should be taken into considera-tion Note that the wattages are not additive, since all units are normally not in operation at the same time
b) Based on 1000 hours of operation pe r year This "igure will vary widely depending on the area and the specific size of the unit See EEI-Pub #76-2, "Air Conditioning Usage Study," for an estimate for your location
Source: Edison Electric Institute
material by trying Chapter Problem J0.2
Trang 3368 Sinusoidal Steady-State Power Calculations
K, „cos (tot + 6 r ) | R
Figure 10.7 • A sinusoidal voltage applied to the
terminals of a resistor
10.3 The rms Value and Power
Calculations
In introducing the rms value of a sinusoidal voltage (or current) in Section 9.1, we mentioned that it would play an important role in power calculations We can now discuss this role
Assume that a sinusoidal voltage is applied to the terminals of a resis-tor, as shown in Fig 10.7, and that we want to determine the average power delivered to the resistor From Eq 10.12,
h+T
Vl 1 cos 2 (cot + $ v )dt (10.18)
Comparing Eq 10.18 with Eq 9.5 reveals that the average power
deliv-ered to R is simply the rms value of the voltage squared divided by R, or
If the resistor is carrying a sinusoidal current, say, I,,, cos (cot + <£,-), the
average power delivered to the resistor is
The rms value is also referred to as the effective value of the
sinu-soidal voltage (or current) The rms value has an interesting property:
Given an equivalent resistive load, R, and an equivalent time period, T, the rms value of a sinusoidal source delivers the same energy to R as does
a dc source of the same value For example, a dc source of 100 V delivers
the same energy in T seconds that a sinusoidal source of 100 Vnns delivers, assuming equivalent load resistances (see Problem 10.12) Figure 10.8 demonstrates this equivalence Energywise, the effect of the two sources
is identical This has led to the term effective value being used inter-changeably with rms value
The average power given by Eq 10.10 and the reactive power given
by Eq 10.11 can be written in terms of effective values:
V I
P = cos (0 V - 0,-)
V I
= —•=—7= cos (0,, - Bs)
= K-ff'eff cos (fl„ - 0 , ) ; (10.21)
lOOV(rms) R V s = 100V(dc) R
Figure 10.8 A The effective value of v, (100 V rms) delivers the
same power to R as the dc voltage V s (100 V dc)
Trang 4and, by similar manipulation,
The effective value of the sinusoidal signal in power calculations is so
widely used that voltage and current ratings of circuits and equipment
involved in power utilization are given in terms of rms values For
exam-ple, the voltage rating of residential electric wiring is often 240 V/120 V
service These voltage levels are the rms values of the sinusoidal voltages
supplied by the utility company, which provides power at two voltage
lev-els to accommodate low-voltage appliances (such as televisions) and
higher voltage appliances (such as electric ranges) Appliances such as
electric lamps, irons, and toasters all carry rms ratings on their nameplates
For example, a 120 V, 100 W lamp has a resistance of 1202/100, or 144 ft,
and draws an rms current of 120/144, or 0.833 A The peak value of the
lamp current is 0.833 V2~, or 1.18 A
The phasor transform of a sinusoidal function may also be expressed
in terms of the rms value The magnitude of the rms phasor is equal to the
rms value of the sinusoidal function If a phasor is based on the rms value,
we indicate this by either an explicit statement, a parenthetical "rms"
adja-cent to the phasor quantity, or the subscript "eff," as in Eq 10.21
In Example 10.3, we illustrate the use of rms values for calculating power
Example 10.3 Determining Average Power Delivered to a Resistor by Sinusoidal Voltage
a) A sinusoidal voltage having a maximum
ampli-tude of 625 V is applied to the terminals of a
50 fl resistor Find the average power delivered
to the resistor
b) Repeat (a) by first finding the current in the
resistor
Solution
a) The rms value of the sinusoidal voltage is
625/V2, or approximately 441.94 V From
Eq 10.19, the average power delivered to the
50 Cl resistor is
P = (441.94)2
50 = 3906.25 W
b) The maximum amplitude of the current in the resistor is 625/50, or 12.5 A The rms value of the current is 12.5/V2, or approximately 8.84 A Hence the average power delivered to the resistor is
P = (8.84)250 = 3906.25 W
i / A S S E S S M E N T PROBLEM
Objective 1—Understand ac power concepts, their relationships to one another, and how to calculate them in a drcuit 10.3 The periodic triangular current in Example 9.4,
repeated here, has a peak value of 180 mA
Find the average power that this current
deliv-ers to a 5 kQ, resistor
Answer: 54 W
NOTE: Also try Chapter Problem 10.15
Trang 5370 Sinusoidal Steady-State Power Calculations
10.4 Complex Power
Complex power •
TABLE 10.2 Three Power Quantities and
Their Units
Quantity
Complex power
Average power
Reactive power
Units volt-amps watts var
|.V| - apparent power
reactive power
P - average power
Figure 10.9 • A power triangle
Before proceeding to the various methods of calculating real and reactive power in circuits operating in the sinusoidal steady state, we need to intro-duce and define complex power Complex power is the complex sum of real power and reactive power, or
As you will see, we can compute the complex power directly from the volt-age and current phasors for a circuit Equation 10.23 can then be used to
compute the average power and the reactive power, because P = !R {S}
and<2 = 3 { S } Dimensionally, complex power is the same as average or reactive power However, to distinguish complex power from either average or reactive power, we use the units volt-amps (VA).Thus we use volt-amps for complex power, watts for average power, and vars for reactive power, as summarized in Table 10.2
Another advantage of using complex power is the geometric
interpre-tation it provides When working with Eq 10.23, think of P, Q, and \S\ as
the sides of a right triangle, as shown in Fig 10.9 It is easy to show that the
angle 6 in the power triangle is the power factor angle 0 I} — 0, For the
right triangle shown in Fig 10.9,
Q
But from the definitions of P and Q (Eqs [10.10] and [10.11 J, respectively),
Therefore, 0 = 0 V - 0,- The geometric relations for a right triangle mean
also that the four power triangle dimensions (the three sides and the power factor angle) can be determined if any two of the four are known The magnitude of complex power is referred to as apparent power Specifically,
Apparent power, like complex power, is measured in volt-amps The apparent power, or volt-amp, requirement of a device designed to convert electric energy to a nonelectric form is more important than the average power requirement Although the average power represents the useful output of the energy-converting device, the apparent power represents the volt-amp capacity required to supply the average power As you can see from the power triangle in Fig 10.9, unless the power factor angle is 0°
(that is, the device is purely resistive, pf = 1, and Q = 0), the volt-amp
capacity required by the device is larger than the average power used by the device As we will see in Example 10.6, it makes sense to operate devices at a power factor close to 1
Many useful appliances (such as refrigerators, fans, air conditioners, fluorescent lighting fixtures, and washing machines) and most industrial loads operate at a lagging power factor The power factor of these loads sometimes is corrected either by adding a capacitor to the device itself or
Trang 6by connecting capacitors across the line feeding the load; the latter
method is often used for large industrial loads Many of the Chapter
Problems give you a chance to make some calculations that correct a
lag-ging power factor load and improve the operation of a circuit
Example 10.4 uses a power triangle to calculate several quantities
associated with an electrical load
An electrical load operates at 240 V rms The load
absorbs an average power of 8 kW at a lagging
power factor of 0.8
a) Calculate the complex power of the load
b) Calculate the impedance of the load
Solution
a) The power factor is described as lagging, so we
know that the load is inductive and that the
algebraic sign of the reactive power is positive
From the power triangle shown in Fig 10.10,
P = \S\ cos 0,
Q = \S\ sin 0
cos 0 - 0.8, sin 0 = 0.6
Now, because
Therefore
Q =
SkW cos0
10 sine
0.8
6 kVAR,
= lOkVA,
and
5 = 8 + /6 kVA
b) From the computation of the complex power of
the load, we see that P = 8 kW Using Eq 10.21,
= K>ff4ffcos(0t;
= (240)/eff(0.8)
= 8000 W
0i)
Solving for /efr,
/cff = 41.67 A
We already know the angle of the load imped-ance, because it is the power factor angle:
0 = cos-1(0-8) = 36.87°
We also know that 0 is positive because the power factor is lagging, indicating an inductive load We compute the magnitude of the load impedance from its definition as the ratio of the magnitude of the voltage to the magnitude of the current:
\Z\ = lK-a-1
l/effl
240 41.67 5.76
Hence,
Z = 5.76 /36.87° D, = 4.608 + y'3.456 O
Figure 10.10 • A power triangle
10.5 Power Calculations
We are now ready to develop additional equations that can be used to
cal-culate real, reactive, and complex power We begin by combining Eqs 10.10,
10.11, and 10.23 to get
VI VI
S = ~Y~cos (0 V - 6,) + j—^—sm(e v - $i)
V I
r in 1 m [cos ( 0 , , - 0i) + ; sin (0,, - 0,)]
i g g e f f l r t t o \v m I m /{6 n - 9d (10.27)
Trang 7372 Sinusoidal Steady-State Power Calculations
If we use the effective values of the sinusoidal voltage and current,
Eq 10.27 becomes
S = K f f W ( 0 , ~ 0/) (10.28)
Equations 10.27 and 10.28 are important relationships in power calcula-tions because they show that if the phasor current and voltage are known at
a pair of terminals, the complex power associated with that pair of terminals
is either one half the product of the voltage and the conjugate of the cur-rent, or the product of the rms phasor voltage and the conjugate of the rms phasor current We can show this for the rms phasor voltage and current in Fig 10.11 as follows:
= V ea ei e >-I cii e-! 9 >
Note that left = h&eJ$i follows from Euler's identity and the trigonomet-ric identities cos(—0) = cos(fl) and s i n ( - 0 ) = — sin (#):
I^e* - /cff cos (Si) + //e f fsin (-0,-)
= / e f f cos (0/) - jl ei{ sin (6i)
= Ifo
The same derivation technique could be applied to Eq 10.27 to yield
S = -VI* (10.30)
Both Eqs 10.29 and 10.30 are based on the passive sign convention If the current reference is in the direction of the voltage rise across the termi-nals, we insert a minus sign on the right-hand side of each equation
To illustrate the use of Eq 10.30 in a power calculation, let's use the same circuit that we used in Example 10.1 Expressed in terms of the pha-sor representation of the terminal voltage and current,
V = 100 / 1 5 ° V,
I = 4 / - 1 0 5 ° A
Therefore
S = -(100 /15°)(4 / + 105°) = 200 /120°
•ell
•
-
4-V t ff
Circuit
Figure 10.11 • The phasor voltage and current
associ-ated with a pair of terminals
= - 1 0 0 + /173.21 V A
Trang 8Once we calculate the complex power, we can read off both the real and
reactive powers, because S = P + jQ Thus
P = -100 W,
Q = 173.21 VAR
The interpretations of the algebraic signs on P and Q are identical to those
given in the solution of Example 10.1
let +
vc f f
—
z
1 Figure 10.12 A The general circuit of Fig 10.11 replaced with an equivalent impedance
Substituting Eq 10.31 into Eq 10.29 yields
S - ZIefrIe[f
= Heff|2Z
= |Ieff|2(fl + IX)
= HeffPi? + /|Icff|2X = P + jQ, (10.32)
from which
In Eq 10.34, X is the reactance of either the equivalent inductance or
equivalent capacitance of the circuit Recall from our earlier discussion of
reactance that it is positive for inductive circuits and negative for
capaci-tive circuits
A second useful variation of Eq 10.29 comes from replacing the
cur-rent with the voltage divided by the impedance:
S = V e f f ( ^ y = ^ = P + jQ- (10.35)
Alternate Forms for Complex Power
Equations 10.29 and 10.30 have several useful variations Here, we use the
rms value form of the equations, because rms values are the most common
type of representation for voltages and currents in power computations
The first variation of Eq 10.29 is to replace the voltage with the
prod-uct of the current times the impedance In other words, we can always
rep-resent the circuit inside the box of Fig 10.11 by an equivalent impedance,
as shown in Fig 10.12 Then,
Trang 9374 Sinusoidal Steady-State Power Calculations
Note that if Z is a pure resistive element
P =
and if Z is a pure reactive element,
Q =
R '
|V eff | 2
(10.36)
In Eq 10.37, X is positive for an inductor and negative for a capacitor
The following examples demonstrate various power calculations in circuits operating in the sinusoidal steady state
Example 10.5 Calculating Average and Reactive Power
In the circuit shown in Fig 10.13, a load having an
impedance of 39 + /26 O is fed from a voltage
source through a line having an impedance of
1 + /4 O The effective, or rms, value of the source
voltage is 250 V
a) Calculate the load current IL and voltage VL
b) Calculate the average and reactive power
deliv-ered to the load
c) Calculate the average and reactive power
deliv-ered to the line
d) Calculate the average and reactive power
sup-plied by the source
Thus the load is absorbing an average power of
975 W and a reactive power of 650 VAR
in J T Y Y \ » _ /4 a
6 D 250/0°
V(rms)
Line Source
Figure 10.13 • The circuit for Example 10.5
3912
I I
/26 ft; Load
Solution
a) The line and load impedances are in series across
the voltage source, so the load current equals the
voltage divided by the total impedance, or
I, 250 / 0 °
40 + /30 = 4 - /3 = 5 / - 3 6 8 7 ° A (rms)
Because the voltage is given in terms of its
rms value, the current also is rms The load
volt-age is the product of the load current and load
impedance:
VL = (39 + /26)1 L = 234 - / 1 3
= 234.36 / - 3 1 8 ° V (rms)
b) The average and reactive power delivered to the
load can be computed using Eq 10.29 Therefore
S = \Jl = (234 - / 1 3 ) ( 4 + / 3 )
= 975 + /650 VA
c) The average and reactive power delivered to the line are most easily calculated from Eqs 10.33 and 10.34 because the line current is known Thus
P = (5)2(1) = 25 W,
Q = (5)2(4) = 100 VAR
Note that the reactive power associated with the line is positive because the line reactance is inductive
d) One way to calculate the average and reactive power delivered by the source is to add the com-plex power delivered to the line to that delivered
to the load, or
S = 25 + /100 + 975 + /650
= 1000 + / 7 5 0 V A The complex power at the source can also be cal-culated from Eq 10.29:
S s =
Trang 10-250IL-The minus sign is inserted in Eq 10.29 whenever
the current reference is in the direction of a
volt-age rise Thus
5, = -250(4 + /3) = -(1000 4- /750) VA
The minus sign implies that both average power and magnetizing reactive power are being deliv-ered by the source Note that this result agrees with the previous calculation of 5, as it must, because the source must furnish all the average and reactive power absorbed by the line and load
Example 10.6 Calculating Power in Parallel Loads
The two loads in the circuit shown in Fig 10.14 can
be described as follows: Load 1 absorbs an average
power of 8 kW at a leading power factor of 0.8 Load
2 absorbs 20 kVA at a lagging power factor of 0.6
0.05 n
• V W
-+
/0.50 a
Figure 10.14 • The circuit for Example 10.6
a) Determine the power factor of the two loads in
parallel
b) Determine the apparent power required to
sup-ply the loads, the magnitude of the current, Iv,
and the average power loss in the transmission
line
c) Given that the frequency of the source is 60 Hz,
compute the value of the capacitor that would
correct the power factor to 1 if placed in parallel
with the two loads Recompute the values in (b)
for the load with the corrected power factor
Solution
a) All voltage and current phasors in this problem
are assumed to represent effective values Note
from the circuit diagram in Fig 10.14 that
Iv = l { + I2 The total complex power absorbed
by the two loads is
S = (250)i;
= (250)(1, + I2)8
= (250)1^ + (250)I2
= sx + s2
We can sum the complex powers geometrically,
using the power triangles for each load, as shown
in Fig 10.15 By hypothesis,
8000(.6)
= 8000 - /6000 VA,
S 2 = 20,000(.6) + /20,000(.8)
= 12,000 +/16,000 VA
-36.87°
16kVAR
10 kVAR
Figure 10.15 • (a) The power triangle for load 1 (b) The power triangle for load 2 (c) The sum of the power triangles
It follows that
S = 20,000 + /10,000 VA,
and , 20,000 + /10,000
Therefore
Iv = 80 - /40 = 89.44 / - 2 6 5 7 ° A
Thus the power factor of the combined load is
pf = cos(0 + 26.57°) = 0.8944 lagging
The power factor of the two loads in parallel is lagging because the net reactive power is positive b) The apparent power which must be supplied to these loads is
\S\ = |20 + /10| = 22.36 kVA
The magnitude of the current that supplies this apparent power is
II.sl = 180 - j40| = 89.44 A