Chapter
8
Power Factor Correction
There has been a growing interest in power factor correction(PFC). In fact,the European Union implemented a directive,EN61000-3-2, which controls the harmonic content and power factor of many prod-ucts that are sold to European countries.There are veral important reasons for this control.Poor power factor results in reduced efficiency, which increas the cost of electricity.More importantly,many devices suffer from harmonically rich waveforms.A good example of this is motors,which may overheat as a result of harmonics.In the ca of three-pha motors the harmonics can result in significant neutral cur-rent,which can also result in overheating and ultimately in motor failure.
Typical switching power supplies rectify the input power and uti-lize a capacitorfilter in order to provide a DC bus voltage.The typical power factor of such a conversion is approximately0.6.Linear regu-lated power supplies generally u a transformer to step down the AC input voltage and rectify the condary voltage and then utilize a capac-itorfilter to create the DC voltage to the input of the regulator stage. The transformer improves the power factor of the input just slightly from the typical swit
ching power supply.Pha-controlled power sup-plies utilize either SCRs or triacs to control the conduction angle of the input,which is thenfiltered using an LC-typefilter.This can result in a power factor that is even lower than the typical switching power supply.
Power factor(PF)is defined as the ratio of watts to volt-amperes:
PF=
watts
volts×amperes
183
184Chapter Eight
X1
KBPC810
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.TRAN 10u 104m 54m .1m UIC .FOUR 60I(v1)
X12140KBPC810V121SIN 016560C1431000uF IC=100R140150R2300.1.END
电脑休眠Figure 8.1
Single-pha rectifier filter schematic and netlist.
Single-Pha Transformer Rectifier
Figure 8.1shows a typical single-pha rectifier filter circuit.The re-sults of the input current,input voltage,and input watts are shown in Fig.8.2.
感谢作文500字In this example the power factor is calculated as
PF =
175.45
3.501×116.75
=0.429
预期理论The power factor is largely dependent on the input source impedance and the characteristics of the output capacitor.The poor power factor also results in a very rich harmonic content of the input current.The spectral plot is shown in Fig.8.3.
If the single-pha circuit were to have an LC output filter rather than a capacitor filter,the input current would have a much larger conduction angle.In the extreme ca,where an infinite inductance would be prent,the conduction angle would be 180◦for each rectifier and the input current would approach a square wave.In this ca the
Power Factor Correction
185
i(v1)雷雨时
product
difference
Time in Secs
p r o d u c t i n w a t t s
-720-520-320-12080.0d i f f e r e n c e i n v o l t s
-42.0-22.0-2.0018.038.0i (v 1)
i n a m p e r e s
P l o t 1Figure 8.2Single-pha rectifier filter waveforms.
mag(fft(temp))
Frequency in Htz
我成长我快乐作文
m a g (f f t (t e m p )) i n a m p e r e s
P l o t 1_f Figure 8.3Single-pha rectifier filter input current spectrum.
186Chapter Eight
power factor would increa to unity and the input current harmonic content would be that of a square wave
I harmonic=
孕妇缺钙吃什么食物补充最快I fundamental harmonic number
for the odd harmonics.
Three-Pha Transformer Rectifier
There are many configurations of three-pha rectifiers.In the simplest ca a full-wave rectifier is ud in conjunction with a single three-pha bridge rectifier.The resulting ripple frequency is six tim
es the input AC frequency and the power factor is generally approximately0.7.
A better configuration,using both delta and wye condaries,results in a ripple frequency that is12times the AC frequency.The power factor of this configuration is approximately0.8.More sophisticated approaches utilize even more condaries in order to produce higher ripple frequencies and better power factor.An example of a three-pha delta-wye configuration is shown in Fig.8.4.
The results in Fig.8.5show the input voltage,input current,and out-put voltage for the three-pha delta-wye rectifier.The ripple frequency is12times the input frequency,and the power factor is approximately
0.82.The harmonic content is greatly reduced compared with that of
a single-pha rectifier circuit,as can be en in the Fourier results below.
FOURIER COMPONENTS OF TRANSIENT RESPONSE I(VA)
DC COMPONENT=6.282716E-02
HARMONIC FREQUENCY FOURIER NORMALIZED PHASE NORMALIZED NO(HZ)COMPONENT COMPONENT(DEG)PHASE(DEG)
1 6.000E+01 3.974E+01 1.000E+00-1.771E+020.000E+00
2 1.200E+02 5.768E-02 1.451E-03-1.277E+02 2.266E+02
3 1.800E+02 1.352E+01 3.401E-018.444E+00 5.399E+02
4 2.400E+02 3.121E-027.854E-04-7.946E+01 6.291E+02
5 3.000E+02 4.405E-01 1.108E-02 4.597E+008.903E+02
6 3.600E+02 1.389E-01 3.496E-03 2.320E+01 1.086E+03
7 4.200E+02 4.967E-01 1.250E-02 1.796E+02 1.420E+03
8 4.800E+02 1.634E-01 4.111E-03-1.346E+02 1.283E+03
9 5.400E+02 4.799E+00 1.208E-01 2.251E+01 1.617E+03 TOTAL HARMONIC DISTORTION=3.613245E+01PERCENT
Power Factor Correction187
D17
THREEPHASE.cir
.TRAN10U100M20m100u
.PROBE
.PRINT TRAN V(19)
.PRINT TRAN V(24)
Figure8.4Three-pha delta-wye rectifierfilter and netlist.
.PRINT TRAN V(23)V11A
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读后感250字
V3C 0SIN 01636011.111m V4B 0SIN 0163605.555m D16177DN1188D17520DN1188D18620DN1188D19820DN1188D20920DN1188D211020DN1188D221120DN1188D23127DN1188D24137DN1188D25147DN1188D26157DN1188D27167DN1188.END
Figure 8.4
(Continued )
1i(va)2
v(a)4
p(r1)
Time in Secs
p (r 1) i n w a t t s
-1.40k -1.00k -600
-200
200v (a ) i n v o l t s
-400-2000200
400i (v a ) i n a m p e r e s
P l o t 1Figure 8.5Three-pha delta-wye rectifier filter waveforms.
188
