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Basics of Electricity
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Three-phase AC power is a series of
overlapping single-phase AC cycles as shown in
Figure 1.20. Each phase is offset by 1/3 of a cycle. Three-phase power is provided
as the main incoming supply in large commercial buildings and most industrial plants.
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Figure 1.20. Three-phase alternating current.
Work and Power For electricity to be useful, it must do work, such as produce light
or turn the shaft of a motor. There is useful work and wasted work.
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Light radiated by a lamp is
considered useful work since it provides a useful function.
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Heat produced by the same lamp is
considered wasted work since it performs no useful function in aiding vision.
The sum of the two types of work is called the total work. The term power is used to
describe the rate at which work is done by a load (lamp or motor) during some standard period of
time regardless of whether the work is useful or wasted. Practically all electricity used is
generated by companies who sell electricity. They must have some to measure how much electricity is
consumed by each of their customers. The simplest way is to measure the amount of electricity used
is to measure the work done (power) over a period of time. Power is measured in watts.
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To determine power (P) in watts, multiply voltage (V) of the circuit by the current
(IIII):P P = E x I.
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For example, if a lamp circuit has a voltage of 240 volts and a current of 0.25 amps, the
lamp is performing work at the rate of 240 x 0.25 = 60 watts.
If the 60W lamp burns for ten hours, total work done is 60 x 1 0 = 600 watt hours. Since the
watt hour is a relatively small unit compared to the amount of power consumed in a typical
application, the kilowatt hour (kWh) is used, being 1000 times greater.
If the 60W lamp burns for 1 000 hours, the work done is 60,000 watt hours, or 60 kWh ( which
is a much easier number to manage).
The electric supply company uses a kilowatt-hour meter to measure the amount of electricity
used by domestic and small commercial / industrial customers.
For larger users the voltage and current is measured separately so the customer is charged
for volt-amps, commonly quoted as kV (kilovolt amps). In this way the large users pay for all the
electricity they consume including the non-productive component (see
Power Factor).
Transformers
Power delivered from the electric supply company is at a very high voltage. Before it
reaches the consumer there is a transformer which reduces several thousand volts to a few hundred.
The reason for this is because it is much more efficient to transport electricity over long
distances at as high a voltage as possible. However, for the user a much lower voltage is safer and
more convenient to control.
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