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Current is classified into two types:
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| Direct Current (DC): DC is unidirectional
flow of electric charge that means its direction remains
constant.
Examples of DC power source are battery and solar cell,
etc.
Direct Current or DC is suitable for using with DC appliances
not to reverse polarity.
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Alternative Current (AC): AC is an electric current which
direction reverses cyclically. The usual waveform of AC
power is sine wave with frequency 50 or 60 Hz.
Examples of AC power sources are utility power supplies
to residences and businesses, audio and radio signals,
etc.
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Electrical power consumption
Electrical power consumed by DC electrical appliances can be
determined by voltage that they work on and current that they
consumed.
DC power consumption = Voltage (Volts) X
Load Current (Amps) = Watts
Electrical power consumed by AC electrical appliances is more
complicated than DC electrical appliances because the direction
of current is changed periodically then the value of voltage
and current should be measured in term of RMS (Root Mean Square)
to eliminate changing of current direction.
AC electrical appliances or AC loads are classified
into two types as the followings;
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| Linear load is electrical load consuming AC power
both real power and apparent power with a power factor of
1. Example of linear load is incandescent lamp. |
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| Non-linear load generates harmonic
currents in addition to the original AC current and its
power factor is less than 1. Examples of non-linear load
are fluorescent lamp, electronic ballast of fluorescent
lamp, PC and TV, etc. |
Each type of AC load has different consuming current characteristic
and the figures below show the current characteristic of linear
load and non-linear load with same input voltage at same power
rating of load.
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Current characteristic of
incandescent lamp,
which power factor = 1
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Current characteristic of
a set of PC,
which power factor = 0.52
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What are Watt and VA?
Watt is unit of power. Its measures a rate of energy
use or production and its symbol is W.
VA or volt-ampere is unit of electrical power consumed
by non-linear load. It measures apparent power.
Example
A Computer is consumed power as a non-linear load then
VA should be the measuring unit that indicates power consumed
by PC.
A UPS is used to supply power to PC once the utility
power is failed then a UPS should be declared its power in VA
as well.
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| AC power (Watt) measuring
To measure real AC power of electrical appliances, the measuring
equipment called "Power meter" is requested. This
equipment will measure voltage and current at the same time
and make calculation to get power in "Watt".
The following figures show measuring of a PC with 17"
monitor power consumption by using power meter. |
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| AC apparent power (VA) calculating
We can measure the power of the same set of a PC with 17"
monitor by measuring voltage (RMS) and current (RMS) after
calculation power by multiply working voltage with consumed
current. |
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AC apparent power
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= Voltage (RMS) X Load Current (RMS)
= VA
= 229.3 X 1.10 = 252.23 VA |
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| The relation of AC power and
AC apparent power
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AC power
(Watts)
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= AC apparent power X Power factor
= (VA) X Power factor |
Power factor (pf) of an AC electrical power system is
defined as ratio of real power to the apparent power and is
a number between 0 and 1.
Power factor of linear load equals one (=1) and power factor
of non-linear load is less than one (<1).
From a sample of a PC with 17" monitor, we can
calculate power factor by
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Watts
132
pf
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= VA X pf
= 252.23 X pf
= 132 / 252.23 = 0.523 |
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