Environmental Engineering Reference
In-Depth Information
R
v
AB
A
B
i
AB
+-
v
Figure A.3
DC circuit example
Current
i
AB
through the battery is now negative,
v
AB
is still positive and therefore Equation
(A.1) gives a negative value to the power associated with the battery. If a line is drawn at
the interface AB, power is going into the interface if looked at towards the resistor, i.e. the
power is positive, but power is coming out of the interface if looked at towards the battery,
i.e. the power is negative. This, of course, satisfi es the conservation of energy principle at
the interface.
To sum up, the power associated with an element is taken as positive and therefore con-
sumed when the current is fl owing
through
the element from higher to lower potentials. If
the reverse were the case, the power would be negative and the element would be a generator.
All this is self-evident when dealing with DC circuits, but what would be the power associ-
ated with an element if
v
AB
and
i
AB
were continuously varying quantities such as sinusoids?
A.3 Why AC ?
In some cases AC holds no practical advantage over DC. In applications where electricity is
used to dissipate energy in the form of heat, the polarity or direction of current is irrelevant.
However, with AC it is possible to build electric generators, motors and power distribution
systems that are far more effi cient or fl exible than with DC, and so AC is used predominately
across the world in high power applications. Section 4.2 provides information on how AC is
generated through a rotating mechanical-electrical energy converter.
The relative simplicity of AC generators and motors translates into greater reliability and
lower cost of manufacture. There is, however, an additional very important advantage of AC
over DC. If there are two mutually inductive coils and one coil is energized with AC, an AC
voltage will be created in the adjacent coil. This device is known as a transformer and its
mode of operation is described in some detail in Section 4.3. The fundamental signifi cance
of a transformer is its ability to step voltage up or down from the powered coil to the unpow-
ered coil. As explained in Section 1.4 of Chapter 1, the transformer's ability gives AC an
advantage unmatched by DC in the realm of power transmission and distribution.
A.4 AC Waveforms
In practice AC generators are designed to produce a voltage that, to all intents and purposes,
is a pure
sine wave
. There is a good reason why pure sign waves are used in power networks.
