BASIC ELECTRICITY
Page 2
So where does electricity
come from, or rather how do we make electricity? What are the sources?
The first and most obvious
is the electrochemical process I described as a battery. Dissimilar metals
in an electrolyte form a cell from which electrons will flow. But
what about when the battery gets too low and we need to recharge it?
Where does that electricity come from? And where do we get this
Alternating Current?
That comes from a different
process call electromagnetic induction. Many hundreds of years ago
people discovered magnets. They also discovered that if you float a
magnet in a liquid it points to magnetic north, the north pole. This led
to the magnetic compass. But it wasn't until the last few hundred years
that scientists determined why. Our earth has a magnetic field. This
field radiates along lines of magnetic flux that flow north and south. Every
magnet also has a magnetic field around it with these same lines of
magnetic flux. See Wikipedia
on Magnetic Flux.
Then someone discovered that
if you move a wire (a complete circuit, not just a short piece of wire)
through a magnetic field it causes electrons to flow in the wire. You are
creating electricity. As the wire moves through the lines of
magnetic flux around the magnet, the electrons in the wire get all excited
and start dashing about.
So after much
experimentation it was discovered that if you rotate a magnet on a shaft
(a rotor), inside a coil of wire (the armature), electricity is
generated in the coil. This is a DC generator. If you are much over
40 you may remember that cars had generators, not alternators, up until
the 1970's. For the most part, the devices that make electricity are
still called generators, even though they are now mostly alternators. So
what's the difference? A generator puts out DC, and an alternator
puts out AC. See
Wikipedia on Electrical Generators
As I said on the previous
page, DC loses a lot of voltage when transmitted over long distances due
to resistance in the lines, and a thing called voltage drop. Someone
discovered that AC did not lose a lot of power this way. So until
recently power plants have all put out AC. In recent years methods
have been discovered to transmit DC over long distances as well, but power
plants are still mostly AC. In generators, if you wind half of the coil,
(the armature) one direction, and the other half the opposite direction,
as the rotor turns the electrons go one way in one half and the opposite
way in the other half. That gives you alternating current. If you spin
this rotor at 60 revolutions per second you get 60 cycle AC. See
the War of Currents on Wikipedia
So, power plants put out AC.
Car generators put out AC. Generators on your boat, whether on the
engine, or a separate generator, all put out AC. This is then
"rectified", that is, converted to DC and used to charge your
batteries and run equipment.
In the USA and around the
world, there are thousands of plants generating electricity by using steam
to turn the generators, water to turn the generators, wind turbines,
tides, and even nuclear power. The plants are all connected to a
power grid. In North America, the USA and Canada are both hooked up to the
same power grid. This grid is divided up into many little grids and
separate power sources, but by throwing switches the power you get in your
home can come from almost anywhere in North America. This power can then
be used by you to not only power all of your electric appliances, but also
to charge your batteries. See
Wikipedia on Electric Power Transmission.
Usually, the batteries in
your boat are recharged by the generator on your engine, but if hooked up
to shore power, you can use a charger to do this. You can also put
what is called a converter on your boat, convert the AC to 12 V DC and use
it directly to run all of your DC equipment, as well as charge the
batteries.
Also, there are alternative
methods to generate electricity. The best known is solar power. Certain
types of silicon, when exposed to sunlight, put out a small amount of
electric current. By connecting hundreds of these solar cells together,
large amounts of current can be generated. Many boats used for long
distance cruising use solar cells to recharge their batteries. See
Wikipedia on Solar Cells
Some cruisers also use wind powered generators to charge their batteries.
Ground
One of the most important
concepts when dealing with electricity is the concept of ground. It
literally means the earth. In order to measure the amount of voltage in a
circuit, you have to have a reference point that is considered zero
volts. In most cases this is ground, or earth. In the AC power
grid the ground point is literally the earth. A large rod is driven into
the earth and connected by cable to one side of the AC system. AC use
three wires. A black wire which is considered the hot side, a white wire
which is considered the neutral, or grounded side, and a third
green wire which is a safety wire, or grounding wire. If you go to
your local substation you will see such a ground. Even your house has a
ground. If you look at the wiring in your house you will see that it
is made up of three wires bundled in a cable. They are black, white, and
green. If you have 220 or 440 there will be a fourth wire which is red.
Both the white and the green are connected to ground. This is an important
point, because on shore side systems the white and green are connected.
But on a boat they are not. That is why the distinction between the grounded
wire and grounding wire is so important.
In DC
systems, ground is usually established by using the negative side as the
ground. However, some DC systems do use a third wire. The positive wire is
red, the negative wire is black or yellow, and the third wire is green.
All
wiring on boats is color coded. That is, for each type of equipment and
for main power circuits there is a specific color used for the positive
wire. The negative is always black or yellow. Go to Color
Codes table to see the colors used. The battery is connected to
the engine block and so all wires connected to the negative side of the
battery are grounded to the engine block. ISO Color Codes are
different. See Wikipedia
on Wiring Color Codes
Another
important point is that there should be only one ground point in any
system. This goes for both DC and AC. Both are grounded at the engine
block. This is the only point on the boat where the two systems are
connected.
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