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Basics of Power Factor.... (Last update.... 17th
August 2013 ) |
Power Factor, Power Factor correction, value of capacitors
required etc….. |
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Power Factor:
In alternating current
circuits, alternating voltage and alternating current are not always
necessarily in phase with each other. i.e. to say, the current flowing in any
circuit component, which is a result of applied voltage, follows the voltage
wave form with a time gap. This time gap could be either ahead of the voltage
applied or it could be behind the voltage applied. In circuit with a lot of
inductance present, the current is behind the voltage applied. In such cases
the current is said to be “lagging”
the voltage, as if it is a bit reluctant to flow. The amount of “lag” depends
on the amont of inductance present. In circuits with a lot of capacitance
present, the voltage follows the current, as if the voltage is reluctant to
change. In such cases the current is said to be “leading” the voltage. Imagine a simple
circuit with a resistor connected to a dc voltage through a switch. The
moment the switch is closed, the dc voltage of the source appears across the resistor.
The resistor immediately starts carrying a current which is equal to Voltage
/ Resistance. ( V/R ). The current stops flowing immediately when the switch
is opened. The situation is some what different when the resistor is replaced
by an inductor. At the time of switching on the switch, there is a sudden
change in the level of voltage which is seen by the inductor, from zero volts
to V volts. As we know, the inductor always opposes any change in the state
of current which is carrying. It tries to hold on to the existing state of
current in it. If the voltage and current waveforms are sinusoidal, then
cosine of the angle between the voltage and current waveforms is called Power
Factor. Why cosine?
Why not sine or tan? Power is
Multiplication of Voltage and Current. But if the voltage and current are not
in phase with each other, simple multiplication does not result into power.
Power is therefore, multiplication of voltage and only that compoent of current which is in phase with voltage. i.e. P=
V x I x cos Ø. This explains why cosine and not sine or tangent of the angle! Imagine that you
want to go to a bank and deposit your money. When you reach the bank you find
that bank is closed. So you come back with the cash without delivering. By
the time you reach home, bank is again opened. You start again from home,
reach the bank but find that bank is again closed. That is to say your money
does not get transferred to the bank because opening of the bank and you
reaching the bank does not synchronize.
In other words, the two actions are not in phase with each other. Simiarly,
if the current is not phase with the voltage it does not create any transfer
of power from source to the load. The current simply travels without any
result. This is called wattless power. It does not result into “watts” or
power.
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In case of DC,
the voltage and the current are always in synchronism with each other and
therefore, there is no question of power factor. In a celing fan
the power factor is typically around 0.7 . This means that only 70 % of the
current drawn from the supply lines is utilised in generating mechanical
power by way of rotating the blades. It is inefficient use of current
provided by the Electric Supply Company. The energy meter fitted in the
incoming line, however, records actual energy consumed, taking into account
the low power factor of 0.7. Therefore, one does not pay more for making
poor use of current. However, the electricity company advises consumers
to improve power factor and make better use of the current supplied by them,
so that they need not lay thick copper cables for everyone. In industrial
environment, the situation is commercially different. Poor power factor by
industries is penalised by noting the power factor separately. Industrial
consumers of power therefore take measures to improve the power factor by
putting capacitors to compensate the inductance of motors in the factory. Click here
to see how to calculate the capacitorrs required. |
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