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The difference between asynchronous and synchronous generator


A synchronous generator is called “synchronous” because the waveform of the generated voltage is synchronized with the rotation of the generator. Each peak of the sinusoidal waveform corresponds to a physical position of the rotor. The frequency is exactly determined by the formula f = RPM x p / 120 where f is the frequency (Hz), RPM is the rotor speed (revolutions per minute) and p is the number of poles formed by the stator windings. A synchronous generator is essentially the same machine as a synchronous motor. The magnetic field of the rotor is supplied by direct current or permanent magnets.

The output frequency of an asynchronous generator is slightly (usually about 2 or 3%) lower than the frequency calculated from f = RPM x p / 120. If the RPM is held constant, the frequency varies depending on the power level. The peaks of the waveform have no fixed relationship with the rotor position. An asynchronous generator is essentially the same machine as an asynchronous or induction motor. The magnetic field of the rotor is supplied by the stator through electromagnetic induction.

The output frequency of a synchronous generator can be more easily regulated to remain at a constant value. Synchronous generators (large ones at least) are more efficient than asynchronous generators. Synchronous generators can more easily accommodate load power factor variations. Synchronous generators can be started by supplying the rotor field excitation from a battery. Permanent magnet synchronous generators require no rotor field excitation.

The construction of asynchronous generators is less complicated than the construction of synchronous generators. Asynchronous generators require no brushes and thus no brush maintenance. Asynchronous generators require relatively complicated electronic controllers. They are usually not started without an energized connection to an electric power grid, unless they are designed to work with a battery bank energy storage system. With an asynchronous generator and an electronic controller, the speed of the generator can be allowed to vary with the speed of the wind. The cost and performance of such a system is generally more attractive than the alternative systems using a synchronous generator.


Asynchronous (induction) 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 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.


A diagram of asychronous motor convert to generator

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