Asynchronous (Induction) Generators
The picture to the right illustrates the basic principles in the asynchronous generator, much as we saw it presented on the previous pages. In reality, only the rotor part looks different, as you will see on the this page.
Note: Asynchronous generator Before reading this page, you should have completed the previous three pages on Wind Turbine Generators. Most wind turbines in the world use a so-called three phase asynchronous (cage wound) generator, also called an induction generator to generate alternating current. This type of generator is not widely used outside the wind turbine industry, and in small hydropower units, but the world has a lot of experience in dealing with it anyway:
The curious thing about this type of generator is that it was really originally designed as an electric motor. In fact, one third of the world's electricity consumption is used for running induction motors driving machinery in factories, pumps, fans, compressors, elevators, and other applications where you need to convert electrical energy to mechanical energy.
One reason for choosing this type of generator is that it is very reliable, and tends to be comparatively inexpensive. The generator also has some mechanical properties which are useful for wind turbines. (Generator slip , and a certain overload capability).
The Cage Rotor
The key component of the asynchronous generator is the cage rotor. (It used to be called a squirrel cage rotor but after it became politically incorrect to exercise your domestic rodents in a treadmill, we only have this less captivating name).
It Cage rotor is the rotor that makes the asynchronous generator different from the synchronous generator. The rotor consists of a number of copper or aluminium bars which are connected electrically by aluminium end rings.
In the picture at the top of the page you see how the rotor is provided with an "iron" core, using a stack of thin insulated steel laminations, with holes punched for the conducting aluminium bars. The rotor is placed in the middle of the stator, which in this case, once again, is a 4-pole stator which is directly connected to the three phases of the electrical grid.
Motor Operation
When the current is connected, the machine will start turning like a motor at a speed which is just slightly below the synchronous speed of the rotating magnetic field from the stator. Now, what is happening? Rotor seen from top
If we look at the rotor bars from above (in the picture to the right) we have a magnetic field which moves relative to the rotor. This induces a very strong current in the rotor bars which offer very little resistance to the current, since they are short circuited by the end rings.
The rotor then develops its own magnetic poles, which in turn become dragged along by the electromagnetic force from the rotating magnetic field in the stator.
Generator Operation
Now, what happens if we manually crank this rotor around at exactly the synchronous speed of the generator, e.g. 1500 rpm (revolutions per minute), as we saw for the 4-pole synchronous generator on the previous page? The answer is: Nothing. Since the magnetic field rotates at exactly the same speed as the rotor, we see no induction phenomena in the rotor, and it will not interact with the stator.
But what if we increase speed above 1500 rpm? In that case the rotor moves faster than the rotating magnetic field from the stator, which means that once again the stator induces a strong current in the rotor. The harder you crank the rotor, the more power will be transferred as an electromagnetic force to the stator, and in turn converted to electricity which is fed into the electrical grid.
Generator Slip
The speed of the asynchronous generator will vary with the turning force (moment, or torque) applied to it. In practice, the difference between the rotational speed at peak power and at idle is very small, about 1 per cent. This difference in per cent of the synchronous speed , is called the generator's slip. Thus a 4-pole generator will run idle at 1500 rpm if it is attached to a grid with a 50 Hz current. If the generator is producing at its maximum power, it will be running at 1515 rpm.
It is a very useful mechanical property that the generator will increase or decrease its speed slightly if the torque varies. This means that there will be less tear and wear on the gearbox. (Lower peak torque). This is one of the most important reasons for using an asynchronous generator rather than a synchronous generator on a wind turbine which is directly connected to the electrical grid.
Automatic Pole Adjustment of the Rotor
Did you notice that we did not specify the number of poles in the stator when we described the rotor? The clever thing about the cage rotor is that it adapts itself to the number of poles in the stator automatically. The same rotor can therefore be used with a wide variety of pole numbers.
Grid Connection Required
On the page about the permanent magnet synchronous generator we showed that it could run as a generator without connection to the public grid.
An asynchronous generator is different, because it requires the stator to be magnetised from the grid before it works.
You can run an asynchronous generator in a stand alone system, however, if it is provided with capacitors which supply the necessary magnetisation current. It also requires that there be some remanence in the rotor iron, i.e. some leftover magnetism when you start the turbine. Otherwise you will need a battery and power electronics, or a small diesel generator to start the system).
© Copyright 1997-2003 Danish Wind Industry Association
Updated 19 September 2003
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