Introduction
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 picture illustrates the basic
principles in the asynchronous generator, much similar with the synchronous generator presented before. In reality, only the rotor part looks different.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, like the 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 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, as you see in the picture. In the picture is shown 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.If we look at the rotor bars from the
previous picture, there is 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
If we manually crank this rotor around at exactly the synchronous speed of the generator, e.g. 1500 rpm, as we saw for the 4-pole synchronous generator on the previous page nothing will happen. Since the magnetic field rotates at exactly the same speed as the rotor, there will be no induction phenomena in the rotor and it will not interact with the stator.
If speed is increased above 1500 rpm then 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 is cranked 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%. 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, because of 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
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 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.
However, an asynchronous generator in a stand alone system can be used 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 to start the turbine. Otherwise a battery and power electronics will be needed, or a small diesel generator to start the system.
