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AC POWER CALCULATIONAC POWER CALCULATIONPower Factor CorrectionPower Factor Correction

Dr. Nik Rumzi Nik IdrisDr. Nik Rumzi Nik Idris

SEE 1023 Circuit TheorySEE 1023 Circuit Theory

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Power FactorPower Factor

As we have seen before (for sinusoidal voltage and current),

p.f. = cos (vi)

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

P = Vrms Irms cos (vi)

Load+Vrms

Irms Vrms = Vrmsv

Irms = Irmsi

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Power FactorPower Factor

For a given Vrms and P, loads with high power factor draw LESS current compared with loads with low power factor

Pave

p.f. = cos 1

Vrms

I1, rms

Pave

p.f. = cos 2

Vrms

I2, rms

cos 1 > cos 2

1 < 2

PavePave

1 2

rmsrms,1 V

I 1Srms

rms,2 VI 2S

1SQ1

Q2

rms,2rms,1 II

2S

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Power FactorPower 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

Utility company (TNB) charge more to loads with LOW power factor

Therefore, it is desirable to increase the power factor

LoadSource+

I2R

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Power Factor CorrectionPower Factor Correction

Process of increasing the power factor without altering the voltage or current to the original load

PL

QL

1

2

QC

QT

(voltage and current to original load retained)

Before C added, S = PL + jQL p.f. = cos 1

After C added, S = PL + j(QL – QC) p.f. = cos 2 i.e. increased

+

VL

IL

+Vs (rms)

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Power Factor CorrectionPower Factor Correction

PL

1

QL

QC

2

QT

How do we calculate C?How do we calculate C?

Voltage across C = Vs (rms)

Impedance of C = C

1j

2

s

c

2

sc

QC

C1

QV

V