The Familiar Forms of Electricity
Ever walked on a carpet and
been "zapped" when you touch a metal object? That is an example of
static electricity. Static electricity is used to a lesser extent than current
electricity in our every day lives.
A more useful form of
electricity is "current" electricity in which the electric current
"flows" in one direction only - Direct Current (DC). The batteries in
our torches, toys, portable radios and cars are the most common sources of DC
low voltage, low power electricity. Higher voltage, higher power DC systems are
used for particular applications, such as energy storage systems associated with
renewable energy systems that are not connected to an electricity supply
network. High voltage, high power DC power lines have been used successfully in
special applications such as interconnections between transmission systems and
undersea power cables. Large DC electric motors are common in certain
applications, such as electric locomotives where high starting torque and
variable speed are required.
The most useful type of
"current" electricity is the type in which the direction of flow of
the electrical current changes direction many times in each second - Alternating
Current (AC). AC electricity powers the appliances in our homes, turns the
electric motors of industry and energizes our electric lights.
The current in an AC system
does not instantaneously change direction. Rather, it gradually (in relative
terms) increases in magnitude until it reaches a maximum in one direction, then
gradually reduces to zero, gradually increases to a maximum in the other
direction, then reduces to zero - and the whole cycle starts all over again. The
number of complete cycles carried out in a second is called the frequency of the
AC electricity supply. In Australia, the AC frequency is 50 cycles per second,
with 60 cycles per second used in North America.
If DC and AC electricity can
both be used successfully, why is AC the dominant form of current electricity?
The answers lie in the
consideration of:
- economics - in general, AC
electrical equipment is smaller and cheaper to manufacture than DC equipment
of similar duty;
- voltage changes - changes in
voltages can be easily facilitated within an AC system, but voltage changes in
DC systems are complicated and require significant equipment. This ability
to change voltage is particularly important in transmission and distribution
systems where line losses are reduced if the voltage is increased. The
voltages used in a large electrical supply system and the importance of
having various voltages in the system are discussed in the Transmission and
Distribution section.
|