曝光台 注意防骗
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magnetised. Magnetic permeability is the
opposite, characterised by soft iron,
which is easily magnetised.
A magnetic field moving round a
conductor will induce an electrical
field in the conductor – the more the
flow, the bigger the field, which
follows the direction of flow. You
can get the same effect by moving
the conductor in the field, or by
moving them both together at the
same time.
This works in reverse as well – a
flow of electricity in any conductor
produces a magnetic field around it.
If you put a coil of wire round a soft
iron bar, and run electricity through
it, the iron will become a magnet for
as long as the current flows.
During engine operation, the battery
will be recharged with either a
generator or an alternator, based on DC
or AC, respectively, which will put
out more voltage than the battery
(typically 28v in a 24v system, or 42v
in a 36v system) to make sure the
battery doesn't drive the generator.
The alternator will charge at low
RPM, but some helicopters, notably
the Bell 206, use a starter/generator to
save space, despite this advantage
(the same unit is used to spin the
engine on startup, and switched over
when it's running to become a
generator). If an alternator were
used, you would need yet another
item attached to the engine.
Generators therefore have the
disadvantage of not producing lots
of electricity at low engine RPM
(exam question).
Direct Current
As previously mentioned, this
electricity that goes in one direction
only. It can be produced in many
ways, such as friction, heat, pressure and
photoelectricity, but we are concerned
with magnetism and chemical action (see
Batteries, above)
The Generator
These use magnetism to create DC.
A coil of wire round soft iron core
(an armature) is spun in a magnetic
field to cause a current to flow in
them - in fact, the current starts off
as AC and is converted with a
rectifier. The lines of magnetic force
either come from a permanent
magnet or an electromagnet formed
from the generator’s own current (in
which case it is called self-excited,
which can happen in the first place
through residual magnetism). When the
generator's field is provided through
the Master Switch, generator voltage
will drop to zero when the switch is
turned off (Master Switches are
mainly there for alternators, which
are not allowed to produce current
without a battery in the circuit).
Because the windings are in the same
direction, they are in series, and will
create a voltage equal to their sum.
Alternating Current
As mentioned above, this is
electricity that reverses its polarity
(and direction of flow) several
thousand times a second.
Electricity & Radio 119
Again, as mentioned above, this is
electricity that reverses its polarity
(and direction of flow) several
thousand times a second.
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
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