Wind Turbine Generator
Types of Wind Turbine Generator
A wind turbine is made up of two major components and having looked at one of them, the rotor blade design in the previous tutorial, we can now look at the other, the Wind Turbine Generator or WTG’s which is the electrical machine used to generate the electricity. A low rpm electrical generator is used for converting the mechanical rotational power produced by the winds energy into usable electricity to supply our homes and is at the heart of any wind power system.
The conversion of the rotational mechanical power generated by the rotor blades (known as the prime mover) into useful electrical power for use in domestic power and lighting applications or to charge batteries can be accomplished by any one of the following major types of rotational electrical machines commonly used in a wind power generating systems:
- 1. The Direct Current ( DC ) machine, also known as a Dynamo
- 2. The Alternating Current ( AC ) Synchronous machine, also known as an AC Generator
- 3. The Alternating Current ( AC ) Induction machine, also known as an Alternator
All these electrical machines are electromechanical devices that work on Faraday’s law of electromagnetic induction. That is they operate through the interaction of a magnetic flux and an electric current, or flow of charge. As this process is reversible, the same machine can be used as a conventional electrical motor for converting the electrical power into mechanical power, or as a generator converting the mechanical power back into the electrical power.
The electrical machine most commonly used for wind turbines applications are those acting as generators, with the synchronous generator and the induction generator (as shown) being commonly used in larger wind turbine generator systems. Usually the smaller or home made wind turbines tend to use a low speed permanent magnet DC generator or Dynamo as they are small, cheap and a lot easier to connect up.
So does it make a difference what type of electrical generator we can use to produce wind power. Well, the simple answer is both Yes and No, as it all depends upon the type of system and application you want. The low voltage DC output from a generator or older style dynamo can be used to charge batteries while the higher AC sinusoidal output from an alternator can be connected directly to the local grid.
Also, the output voltage and power demand depends entirely upon the appliances you have and how you wish to use them. In addition, the location of the wind turbine generator, would the wind resource keep it constantly rotating for long periods of time or would the generator speed and therefore its output vary up and down with variations in the available wind.
A Wind Turbine Generator is what makes your electricity by converting mechanical energy into electrical energy. Lets be clear here, they do not create energy or produce more electrical energy than the amount of mechanical energy being used to spin the rotor blades. The greater the “load”, or electrical demand placed on the generator, the more mechanical force is required to turn the rotor. This is why generators come in different sizes and produce differing amounts of electricity.
In the case of a “wind turbine generator”, the wind pushes directly against the blades of the turbine, which converts the linear motion of the wind into the rotary motion necessary to spin the generators rotor and the harder the wind pushes, the more electrical energy can be generated. Then it is important to have a good wind turbine blade design to extract as much energy out of the wind as possible.
All electrical turbine generators work because of the effects of moving a magnetic field past an electrical coil. When electrons flow through an electrical coil, a magnetic field is created around it. Likewise, when a magnetic field moves past a coil of wire, a voltage is induced in the coil as defined by Faraday’s law of magnetic induction causing electrons to flow.
Simple Generator using Magnetic Induction
Then we can see that by moving a magnet past a single loop of wire, a voltage known as and emf (electro-motive force) is induced within the wire loop due to the magnetic field of the magnet.
As a voltage is induced across the wire loop, an electrical current in the form of an electron flow starts to flow around the loop generating electricity.
But what if instead of a single individual loop of wire as shown, we had many loops wound together on the same former to form a coil of wire, much more voltage and therefore current could be generated for the same amount of magnetic flux.
This is because the magnetic flux cuts across more wire producing a greater emf and this is the basic principal of Faraday’s law of electromagnetic induction and an AC generator uses this principal to convert a mechanical energy such as the rotation from a wind turbine or hydro turbine, into electrical energy producing a sinusoidal waveform.
So we can see that there are three main requirements for electrical generation and these are:
- A coil or set of conductors
- A magnetic field system
- Relative motion between the conductors and field
Then the faster the coil of wire rotates, the greater the rate of change by which the magnetic flux is cut by the coil and the greater is the induced emf within the coil. Similarly, if the magnetic field is made stronger, the induced emf will increase for the same rotational speed. Thus: Induced emf ∝ Φ*n. Where: “Φ” is the magnetic-field flux and “n” is the speed of rotation. Also, the polarity of the generated voltage depends on the direction of the magnetic lines of flux and the direction of movement of the conductor.
There are two basic types of electrical generator and alternator for that matter: the permanent-magnet generator and the wound-field generator with both types consisting of two main parts: the Stator and the Rotor.
The stator is the “stationary” (hence its name) part of the machine and can have either a set of electrical windings producing an electromagnet or a set of permanent magnets within its design. The rotor is the part of the machine that “rotates”. Again, the rotor can have output coils that rotate or permanent magnets. Generally, generators and alternators used for wind turbine generators are defined by how they make generate their magnetism, either electromagnets or permanent magnets.
There are no real advantages and disadvantages of both types. Most residential wind turbine generators on the market use permanent magnets within their turbine generator design, and which creates the required magnetic field with the rotation of the machine, although some do use electromagnetic coils.
These high strength magnets are usually made from rare earth materials such as neodymium iron (NdFe), or samarium cobalt (SmCo) eliminating the need for the field windings to provide a constant magnetic field, leading to a simpler, more rugged construction.
Wound field windings have the advantage of matching their magnetism (and therefore power) with the varying wind speed but require an external energy source to generate the required magnetic field.
We now know that the electrical generator provides a means of energy conversion between the mechanical torque generated by the rotor blades, called the prime mover, and some electrical load whether its charging batteries or dissipating power in a dump load.
The mechanical connection of the wind turbine generator to the rotor blades is made through a main shaft which can be either a simple direct drive, or by using a gearbox to increase or decrease the generator speed relative to the rotational speed of the blades.
The use of a gearbox allows for better matching of the generator speed to that of the turbine but the disadvantage of using a gearbox is that as a mechanical component it is subjected to wear and tear reducing the efficiency of the system. Direct drive however may be more simple and efficient, but the generators rotor shaft and bearings are subjected to the full weight and rotational force of the rotor blades.
Wind Turbine Generator Output Curve
So the type of wind turbine generator required for a particular location depends upon the energy contained in the wind and the characteristics of the electrical machine itself. All wind turbines have certain characteristics related to wind speed.
The generator (or alternator) will not produce output power until its rotational speed is above its cut-in wind speed where the force of the wind on the rotor blades is enough to overcome friction and the rotor blades accelerate enough for the generator to begin producing usable power.
Above this cut-in speed, the generator should generate power proportional to the wind speed cubed ( K.V3 ) until it reaches its maximum rated power output as shown.
Above this rated speed, the wind loads on the rotor blades will be approaching the maximum strength of the electrical machine, and the generator will be producing its maximum or rated power output as the rated wind speed window will have been reached.
If the wind speed continues to increase, the wind turbine generator would stop at its cut-out point to prevent mechanical and electrical damage, resulting in zero electrical generation. The application of a brake to stop the generator for damaging itself can be either a mechanical governor or electrical speed sensor.
Buying a wind turbine generator such as the ECO-WORTHY 400 Watt wind turbine generator for battery charging is not easy and there are a lot of factors to take into account. Price is only one of them. Be sure to choose an electrical machine that meets your needs. If you are installing a grid-connected system, choose an AC mains voltage generator.
If you intend to install a battery-based system, look for a battery-charging DC generator. Also consider the mechanical design of a generator such as size and weight, operating speed and protection from the environment as it will spend all of its life mounted at the top of a pole or tower.
In the next tutorial about Wind Turbine Generators we will look at DC machines and how we can use a PMDC generator to produce electricity from the power of the wind. To learn more about “Wind Turbine Generators”, or obtain more wind energy information about the various wind turbine generating systems available, or to explore the advantages and disadvantages of wind energy, Click Here to get your copy of one of the top “Wind Turbine Guides” today direct from Amazon.
14 Comments already about “Wind Turbine Generator”
What voltage level ie. 480v, 2400v is generator by the wind turbine and are voltage regulators incorporated, How is the wind turbine generator speed kept constant to provide a constant 60 HTZ so that it’s output can be in sync with the grid frequency when paralleled, and are there reversed power relays incorporated to prevent the wind turbine generator from appearing as a load ie. an online motor to the grid.
The type of electrical generator used to convert the rotational energy of the turbine blades into electrical energy depends on the electrical circuit it must supply. For small rated systems, Permanant Magnet type d.c. generators can be used. For larger single or 3-phase supplies, Induction or Synchronous generators are commonly used for power generation.
Constant speed drives are used for large generators that feed the generated power to the grid. The AC Energy produced by the Synchronous type Generator is converted into DC and then again converted to AC. Thus the power fed to the connected grid is automatically matched to the frequency of the grid and also at near unity power factor. This may also allow the generator to operate at variable wind speeds.
For inductuion machines, the stator is connected to the power grid with the rotor driven by the blades above Synchronous speed. For example, a 4-pole machine operating in a 60 Hz grid would have a synchronous speed of 1800 rpm. Again, if used with a suitable power converter, induction generators can run at variable speed. In either case, electronic feedback systems are used to monitor and control the generator speed to get the desired output.
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Thank you for your efforts in putting together these principles. Very helpful.
One question, however, is: since wind speed is varying voltage at which power is generated should also be variable. And / or current is variable. How is this taken care of? Voltage has to be the same as the grid. If suppose wind is low, will the turbine be driven by the Grid? In which case, it could be drawing power from the grid and work like a motor? Thank you for your reply.
Wind turbines have what are known as cut-in and cut-out speed limits. Cut-in refers to the minimum amount of wind speed required to enable the turbine to overcome friction and generate power. Cut-out refers to the maximum amount of wind speed that is safe for the turbine to operate in. Thus, below its cut-in speed the turbine would not produce any electrical power. Whereas above its cut-out speed the turbine is stopped to reduce the possibility of overspeed damage.
A wind turbine generators optimal rotational speed is generally just below its cut-out speed, so that it is spinning fast enough to generate a good amount of electrical power. The gearbox, speed governor or electrical controller ensures this optimal rotational speed. Thus as the wind speed and its kinetic energy varies, the turbines rotational velocity remains the same. Generally for large grid-tied installations and wind farms, doubly-fed induction generators are commonly used.
For small-scale wind power using DC generators charging batteries, a charge controller (or grid-tied inverter) is used to ensure a constant and steady output voltage and current.
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Hi, I’m using a lot of blower fan. is it possible the wind of blower fan use for wind turbin generator ?
You could do that but then you are using electricity to create electricity with losses, friction, windage, etc.
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