On the other hand if we use induction generators instead of synchronous generators, there will be two aspects:. Reactive power requirement of I. The I. G requires reactive power for its operation which needs to be provided either by capacitor banks or I. If we use I. If we have to install capacitor banks, there will be huge costing in installation of these equipment.
The output frequency of Induction Generator is variable which will create uncertainties in deciding ratings for electric equipment and load side equipment will have problems with varying frequency.
To deal with this we need to convert Variable AC power to DC power and back to Fixed frequency AC power using power electronic equipment which induces high amount of harmonic content in output spectrum which is undesirable and design of Harmonic filters and their implementation will increase cost.
Hence Synchronous Generator is used to supply power which are mainly large generation units like thermal, hydro, nuclear and Gas power plant etc. Induction Generators are installed mainly in wind power plants and small hydro power plants. Induction generator and synchronous generator both are used for large scale production of electricity while we use DC generators for few specific requirements.
Moreover, DC generators need carbon brushes for load current to flow and therefore we can't use them for large scale production also we may have to convert the same into AC which will be a more tedious task. So, DC is not at all feasible. Induction generator can be used and is used in wind turbines as mentioned appreciably well in one of the answers.
But we can't get reactive power from induction generator and therefore we need an alternate power source to run our system. In case of synchronous generator we can generate active as well as reactive power and therefore it meets our requirements completely and individually. But synchronous generator has issues as well. They have stability problem. 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. 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 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. This difference in per cent of the synchronous speed, is called the generator's slip. Thus a 4-pole generator will run idle at 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 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. Otherwise a battery and power electronics will be needed, or a small diesel generator to start the system.
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