Transcript Slide 1

AC POWER CALCULATION
Power Factor Correction
SEE 1023 Circuit Theory
1
Power Factor
Irms
+
Vrms

Vrms = Vrmsv
Load
Irms = Irmsi
As we have seen before (for sinusoidal voltage and current),
p.f. = cos (vi)
P = Vrms Irms cos (vi)
The REAL power (or average power) is transformed into useful energy
e.g. heat, mechanical, light, sound, etc
For a given Vrms and P, loads with high power factor draw LESS current
compared with loads with low power factor
2
Power Factor
I2, rms
I1, rms
Vrms
Pave
p.f. = cos 1
Pave
p.f. = cos 2
Vrms
cos 1 > cos 2
1 < 2
S1
1
Pave
S2
Q2
Q1
I1,rms 
S1
2
Vrms
I2,rms 
Pave
S2
Vrms
For a given Vrms and P, loads with high power factor draw LESS current
compared with loads with low power factor
I1,rms  I2,rms
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Power Factor
For a given Vrms and P, loads with high power factor draw LESS current
compared with loads with low power factor
Less current results in LESS losses during transmission
I2 R
+

Source
Load
Utility company (TNB) charge more to loads with LOW power factor
Therefore, it is desirable to increase the power factor
4
Power Factor Correction
Process of increasing the power factor without altering the
voltage or current to the original load
IL
QC
+
+
Vs
QL
1
(rms)
VL
QT
2

PL

Before C added, S = PL + jQL
After C added, S = PL + j(QL – QC)
p.f. = cos 1
p.f. = cos 2
i.e. increased
(voltage and current to original load retained)
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Power Factor Correction
How do we calculate C?
QC
Voltage across C = Vs (rms)
QL
1
1
Impedance
of C =  j
QT
C
2
PL
V
Qc  s
1
C
2
C
Qc
 Vs
2
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