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Alternating Current
As mentioned above, this is
electricity that reverses its polarity
(and direction of flow) several
thousand times a second.
Again, as mentioned above, this is
electricity that reverses its polarity
(and direction of flow) several
thousand times a second.
Electricity & Radio 119
In an aircraft, it is typically used to
power flight instruments, or to
charge the battery in preference to a
generator, which does not work so
well at low engine RPM (exam
question). AC is more efficient at
transmitting energy over long
distances, and smaller conducting
elements are required. However,
alternators cannot self-excite – they
need battery power to function. The
higher alternating frequencies for
radio and radar (below) are produced
with oscillators.
If you can imagine changing the
connections to a battery very quickly
from one terminal to the other, you
would get the same effect, but the
results would be very jerky, and the
waves would be square. In contrast,
the transitions from an alternator are
very smooth and look like sine waves,
as in the diagram below. As it has
only one frequency, if you could
listen to it, it would sound like a
continuous tone.
The rate at which it does this is
called the frequency which, in a typical
North American home, is 60 cycles per
second, or 60 Hertz (1 Hertz is equal
to one cycle per second). In Europe,
it is 50 cycles per second, which
means that electric motors based on
60 cycles won’t work properly.
A cycle is a complete transition from
zero through a peak, down to a
trough and back up to zero, so the
more you can fit into a particular
time scale (the higher the frequency),
the shorter the wavelength is:
The rate at which it does this is
called the frequency which, in a typical
North American home, is 60 cycles per
second, or 60 Hertz (1 Hertz is equal
to one cycle per second). In Europe,
it is 50 cycles per second, which
means that electric motors based on
60 cycles won’t work properly.
A cycle is a complete transition from
zero through a peak, down to a
trough and back up to zero, so the
more you can fit into a particular
time scale (the higher the frequency),
the shorter the wavelength is:
The difference between the peak (or
crest) and the trough of a wave is the
amplitude (or, loosely, volume).
One complete transition of a wave,
no matter where it starts, is called a
cycle. The number of cycles per second is
known as the frequency. When two
waves transmitted at the same time
have their peaks and troughs
coinciding, they are said to be in
phase. When they don’t coincide, they
are out of phase by whatever angle is
created when the second wave start
its cycle:
120 Canadian Private Pilot Studies
This is the basis of finding direction
with the VOR, discussed later.
Because the AC waveform is not
square, its peak voltage must be
1.414 times that of DC to have the
same energy. The equivalent AC
voltage to match a DC one is called
the root mean square.
In the diagram above, the shaded
area inside the curve over the square
DC wave has the same energy as the
shaded parts of the DC wave outside
the curve. In your house, the peak
would actually be 170 volts, as
opposed to the "normal" 120.
The Alternator
AC is created in the first place with
an alternator (or an inverter, which
converts DC). On modern aircraft,
however, the term AC generator is
often used instead.
The other relevance of AC is that it
is the basis of radio waves ……..
Radio
We use radio to convey information.
For example, sound waves by
themselves don’t travel very far, so
what you do is create a carrier wave
at radio frequency and piggy-back a
sound wave on to it, decoding the
two at the receiving end–a radio is a
device for splitting up the two waves
and amplifying the result, since the
signal when it hits the aerial is quite
weak. Inside the box, therefore, are
several separate devices that work
together, needing different amounts
of electricity, including a resonant
circuit to ensure that you pick up only
one signal out of the many available.
The information to be sent modulates
(i.e. varies) the carrier wave through
the amplitude or the frequency. The
former is typically used in aviation,
and the latter by FM music stations
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