Phases
There is one further major
characteristic of an AC electricity supply that requires explanation - phases.
A DC circuit has two wires
through which the current in the circuit flows from a source of electricity
through a load and back to the source. A single-phase AC circuit also has two
wires connected to the source of electricity. However, unlike the DC circuit in
which the direction of the electric current does not change, the direction of
the current changes many times per second in the AC circuit. The 120 volt
electricity supplied to our homes is single phase AC electricity and has two
wires - an "active" and a "neutral".
The distribution line supplying
your home may be single phase and have only two wires strung between the poles
(we will use the overhead power lines as examples because they can be easily
seen). However, the distribution line may be made up of 4 lines. What are the
others? The other lines carry the currents from two other electrical circuits,
making a total of three circuits or phases. The reason why there are only 4
lines is because the 3 phases have a common neutral line (i.e. 3 active lines
and 1 common neutral line).
But why 3 phases? Why not 2 or
4? Because the magnitude and direction of the electricity flowing in each of the
phases is slightly displaced in time from the electricity flowing in the other
phases, the current flowing in the common neutral will be the sum of the neutral
currents from the 3 phases. The resultant current in the common neutral is
smaller in a 3 phase system than in systems with other numbers of phases. This
ability to use a common neutral of relatively small capacity has large economic
advantages and is the main reason why 3 phases are used.
3 phase electricity has another
advantage. We mentioned above that, in Canada, the voltage between the active
and neutral in the single phase, low voltage supply to our homes is 120 volts
and that this phase is only one of the phases in the 3 phase system. The voltage
between the phases of this 3 phase system is ??? volts (in Canada). A 120/208
volt, 3 phase supply is able to deliver more energy than a 120/240 volt, single
phase supply. 3 phase supplies are normally restricted to large electrical
loads, such as large electric motors.
As we travel back up the
electrical network, the voltage increases and the neutral disappears! Why? The
answer can be found in the consideration of why a neutral is used. A single
phase supply must have a neutral, whereas a 3 phase supply does not require a
neutral. More complicated reasons deal with fixing the voltage of the single
phase supply relative to the ground (because domestic appliances have their
metal enclosures connected to ground) and for fault protection purposes. 3
phase, medium voltage, distribution systems and high voltage transmission
systems therefore use one wire for each phase and no neutral.
The above discussions focused
on active and neutral conductors (wires) as being the means to convey the
electricity. One type of system uses the ground as the return path, with only
the active being conveyed by a wire conductor. This type of single-phase supply
system is called the Single Wire Ground Return system and is use to supply small
loads which are located far from the main distribution networks.
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