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Title: Power factor
correction
Aim:
1. To show the relationship between power, power
factor and volt-amps.
2. To
correct the power factor using power factor correction capacitors.
Apparatus: See your laboratory manual for the apparatus.
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THEORY:
A. Power Triangle
The apparent power is the
vector sum of real and reactive power. As shown in Figure 1, the power triangle
has the following components
1.
Real power or active power (P) watt [W]
2.
Reactive power (Q) volt-amperes reactive [Var]
3.
Complex power (S)
4.
Apparent Power (|S|) that is, the absolute value of complex power
S volt-ampere [VA]
5. Impedance angle (φ)
the angle of difference (in degrees) between voltage and current; Current
lagging Voltage (Quadrant I Vector), Current leading voltage (Quadrant IV
Vector)
Reactive power does not
transfer energy, so it is represented as the imaginary axis of the vector
diagram. Real power moves energy, so it is the real axis.
B. Power factor
The ratio between real power
and apparent power in a circuit is called the power factor. It's a practical
measure of the efficiency of a power distribution system. For two systems
transmitting the same amount of real power, the system with the lower power
factor will have higher circulating currents due to energy that returns to the source
from energy storage in the load. These higher currents produce higher losses
and reduce overall transmission efficiency. A lower power factor circuit will
have a higher apparent power and higher losses for the same amount of real
power.
The power factor is one when
the voltage and current are in phase. It is zero when the current leads or lags
the voltage by 90 degrees. Power factors are usually stated as
"leading" or "lagging" to show the sign of the phase angle,
where leading indicates a negative sign.
Purely capacitive circuits
cause reactive power with the current waveform leading the voltage wave by 90
degrees, while purely inductive circuits cause reactive power with the current
waveform lagging the voltage waveform by 90 degrees. The result of this is that
capacitive and inductive circuit elements tend to cancel each other out.
Where the waveforms are purely
sinusoidal, the power factor is the cosine of the phase angle (φ) between the
current and voltage sinusoid waveforms. Equipment data sheets and nameplates
often will abbreviate power factor as "cosφ" for this reason.
C. Power Factor correction
Most industrial loads (motors,
lights, etc.) are inductive in nature, so they have a lagging power factor.
Therefore, these loads draw more current (apparent power) than is required to
do the actual work (real power). This results in higher costs to the customer.
In the below example, a load is
simplified to its electrical equivalents. The load requires a current IL to run, so the
source must supply it.
Power Factor
Correction attempts to improve the power factor by the addition of capacitor(s)
in parallel with the load. These capacitors supply some or all of the reactive
power to the inductive load, which reduces the reactive power and therefore the
current that the power supply delivers. In this second circuit, the load still
requires a current IL to run, but some of the current is
coming from the capacitor, so the current the source must supply is less.
For Procedure and Circuit Diagram, See your Electrical
Laboratory Manual on page 3.
Fill Up Tables in Your Manual and Continue
To….
PRECAUTIONS:
For
Precautions, See General
Electrical Lab Precautions
Answers to Questions:
No Questions in this
experiment.
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