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Synchronous Generators
3-Phase Generator (or Motor) Principles
All 3-phase generators (or motors) use a rotating magnetic field.
In the picture to the left we have installed three electromagnets around a
circle. Each of the three magnets is connected to its own phase in the
three phase
electrical grid.
As you can see, each of the three electromagnets
alternate between producing a South pole and a North pole towards the
centre. The letters are shown in black when the magnetism is strong, and
in light grey when the magnetism is weak. The fluctuation in magnetism
corresponds exactly to the fluctuation in voltage of each phase. When one
phase is at its peak, the other two have the current running in the
opposite direction, at half the voltage. Since the timing of current in
the three magnets is one third of a cycle apart, the magnetic field will
make one complete revolution per cycle.
Synchronous Motor Operation
The compass needle (with the North pole painted red) will follow the
magnetic field exactly, and make one revolution per cycle. With a 50 Hz
grid, the needle will make 50 revolutions per second, i.e. 50 times 60 =
3000 rpm (revolutions per minute).
In the picture above, we have in fact managed to build what is called a
2-pole permanent magnet synchronous motor. The reason why it is called a
synchronous motor, is that the magnet in the centre will rotate at a
constant speed which is synchronous with (running exactly like the cycle
in) the rotation of the magnetic field.
The reason why it is called a 2-pole motor is that it has one North and
one South pole. It may look like three poles to you, but in fact the
compass needle feels the pull from the sum of the magnetic fields around
its own magnetic field. So, if the magnet at the top is a strong South
pole, the two magnets at the bottom will add up to a strong North pole.
The reason why it is called a permanent magnet motor is that the compass
needle in the centre is a permanent magnet, not an electromagnet. (You
could make a real motor by replacing the compass needle by a powerful
permanent magnet, or an electromagnet which maintains its magnetism
through a coil (wound around an iron core) which is fed with direct
current).
The setup with the three electromagnets is called the stator in the motor,
because this part of the motor remains static (in the same place). The
compass needle in the centre is called the rotor, obviously because it
rotates.
Synchronous Generator Operation
If you start forcing the magnet around (instead of letting the current
from the grid move it), you will discover that it works like a generator,
sending alternating current back into the grid. (You should have a more
powerful magnet to produce much electricity). The more force (torque) you
apply, the more electricity you generate, but the generator will still run
at the same speed dictated by the frequency of the electrical grid.
You may disconnect the generator completely from the grid, and start your
own private 3-phase electricity grid, hooking your lamps up to the three
coils around the electromagnets. (Remember the principle of
magnetic /
electrical induction from the reference manual section of this web
site). If you disconnect the generator from the main grid, however, you
will have to crank it at a constant rotational speed in order to produce
alternating current with a constant frequency. Consequently, with this
type of generator you will normally want to use an indirect
grid connection of the generator. In practice, permanent magnet
synchronous generators are not used very much. There are several reasons
for this. One reason is that permanent magnets tend to become demagnetised
by working in the powerful magnetic fields inside a generator. Another
reason is that powerful magnets (made of rare earth metals, e.g. Neodynium)
are quite expensive, even if prices have dropped lately.
Wind Turbines With Synchronous Generators Wind turbines which use
synchronous generators normally use electromagnets in the rotor which are
fed by direct current from the electrical grid. Since the grid supplies
alternating current, they first have to convert alternating current to
direct current before sending it into the coil windings around the
electromagnets in the rotor.
The rotor electromagnets are connected to the current by using brushes and
slip rings on the axle (shaft) of the generator.
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Voltage Variation for Three Phase Alternating Current
It is indeed possible to obtain constant power from an AC system by having
three separate power lines with alternating current which run in parallel, and
where the current phase is shifted one third of the cycle, i.e. the red curve
above is running one third of a cycle behind the blue curve, and the yellow
curve is running two thirds of a cycle behind the blue curve.
With an alternating current in the electrical grid, the current changes
direction very rapidly, as illustrated on the graph above: Ordinary household
current in most of the world is 230 Volts alternating current with 50 cycles
per second = 50 Hz ("Hertz" named after the German Physicist H.R. Hertz
(1857-1894)). The number of cycles per second is also called the frequency of
the grid. In America household current is 130 volts with 60 cycles per second
(60 Hz).
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Energy
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Physicists define the word energy as the amount of work a physical system
is capable of performing. Energy, according to the definition of
physicists, can neither be created nor consumed or destroyed.
Energy, however may be converted or transferred to different forms: The
kinetic energy of moving air molecules may be converted to rotational
energy by the rotor of a wind turbine, which in turn may be converted to
electrical energy by the wind turbine generator. With each conversion of
energy, part of the energy from the source is converted into heat energy.
When we loosely use the expression energy loss (which is impossible by the
definition above), we mean that part of the energy from the source cannot
be used directly in the next link of the energy conversion system, because
it is converted into heat. E.g. rotors, gearboxes or generators are never
100 per cent efficient, because of heat losses due to friction in the
bearings, or friction between air molecules.
Most of us have the sensible notion, however, that as we e.g. burn fossil
fuels, somehow, loosely speaking, the global potential for future energy
conversion becomes smaller. That is absolutely true.
Physicists, however, use a different terminology: They say that the amount
of entropy in the universe has increased. By that they mean that our
ability to perform useful work converting energy decreases each time we
let energy end up as heat which is dissipiated into the universe. Useful
work is called exergy by physicists.
Since the vast majority of wind turbines produce electricity, we usually
measure their performance in terms of the amount of electrical energy they
are able to convert from the kinetic energy of the wind. We usually
measure that energy in terms of kilowatt hours (kWh) or megawatt hours MWh
during a certain period of time, e.g. an hour or a year.
People who want to show that they are very clever, and show that they
understand that energy cannot be created, but only converted into
different forms, call wind turbines Wind Energy Converters (WECs). The
rest of us may still call them wind turbines.
Note
Energy is not measured in kilowatts, but in kilowatt hours (kWh). Mixing
up the two units is a very common mistake, so you might want to read the
next section on power to understand the difference.
Energy Units
1 J (joule) = 1 Ws = 0.2388 cal
1 GJ (gigajoule) = 10 9 J 1 TJ (terajoule) = 10 12 J 1 PJ (petajoule) = 10 15 J 1 kWh (kilowatt hour) = 3,600,000 Joule 1 toe (tonne oil equivalent) = 7.4 barrels of crude oil in primary energy = 7.8 barrels in total final consumption = 1270 m 3 of natural gas = 2.3 metric tonnes of coal 1 Mtoe (million tonne oil equivalent) = 41.868 PJ
Power
Electrical power is usually measured in watt (W), kilowatt (kW), megawatt
(MW), etc. Power is energy transfer per unit of time.
Power may be measured at any point in time, whereas energy has to be
measured during a certain period, e.g. a second, an hour, or a year. (Read
the section on energy , if you have not done so yet).
If a wind turbine has a rated power or nameplate power of 1000 kW, that
tells you that the wind turbine will produce 1000 kilowatt hours (kWh) of
energy per hour of operation, when running at its maximum performance
(i.e. at high winds above, say, 15 metres per second (m/s)).
If a country like Denmark has, say 1000 MW of wind power installed, that
does not tell you how much energy the turbines produce. Wind turbines will
usually be running, say, 75 per cent of the hours of the year, but they
will only be running at rated power during a limited number of hours of
the year.
In order to find out how much energy the wind turbines produce you have to
know the distribution of wind speeds for each turbine. In Denmark's case,
the average wind turbines will return 2,300 hours of full load operation
per year. To get total energy production you multiply the 1000 MW of
installed power with 2,300 hours of operation = 2,300,000 MWh = 2.3 TWh of
energy. (Or 2,300,000,000 kWh).
In other areas, like Wales, Scotland, or Western Ireland you are likely to
have something like 3,000 hours of full load operation or more. In Germany
the figure is closer to 2,000 hours of full load operation.
The power of automobile engines are often rated in horsepower (HP) rather
than kilowatt (kW). The word "horsepower" may give you an intuitive idea
that power defines how much "muscle" a generator or motor has, whereas
energy tells you how much "work" a generator or motor performs during a
certain period of time.
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The
current magnetises the iron core and creates a pair of magnetic poles, one
North, and the other South. The two compass needles consequently point in
opposite directions. (You may repeat the experiment by clicking on the switch
again).
As you can see, the light bulb flashes the moment you connect the switch to
the battery.
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Changing Generator Rotational Speed
A Four Pole Generator
The speed of a generator (or motor) which is directly connected to a
three-phase grid is constant, and dictated by the frequency of the grid,
as we learned on the previous page.
If you double the number of magnets in the stator ,
however, you can ensure that the magnetic field rotates at half the speed.
In the picture to the left, you see how the magnetic field now moves
clockwise for half a revolution before it reaches the same magnetic pole
as before. We have simply connected the six magnets to the three phases in
a clockwise order.
This generator (or motor) has four poles at all times, two South and two
North. Since a four pole generator will only take half a revolution per
cycle, it will obviously make 25 revolutions per second on a 50
Hz grid, or
1500 revolutions per minute (rpm).
When we double the number of poles in the stator of a synchronous
generator we will have to double the number of magnets in the
rotor , as you see on the picture. Otherwise the poles will not match.
(We could use to two bent "horseshoe" magnets in this case).
Other Numbers of Poles
Obviously, we could repeat what we just did, and introduce another pair of
poles, by adding 3 more electromagnets to the stator. With 9 magnets we
get a 6 pole machine, which will run at 1000 rpm on a 50 Hz grid. The
general result is the following:
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Pole number
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50 Hz
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60 Hz
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2
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3000
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3600
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4
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1500
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1800
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6
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1000
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1200
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8
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750
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900
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10
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600
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720
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12
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500
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600
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Indirect Grid Connection of Wind Turbines
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Generating Alternating Current (AC) at Variable Frequency
