Additional topics (generators)
You may be thinking that a stator with twice as many magnets would be twice as expensive, but that is not really the case. Generators and motors are usually made with a very large number of stator magnets anyway, as can be sawn in the picture, where the stator coil windings have not still added on the iron.
The reason for this stator arrangement is that it is preferable to minimise the air gap between the rotor and the stator. At the the same time it is necessary to provide cooling of the magnets. The stator iron in reality consists of a large number of thin (0.5 mm) insulated steel sheets which are stacked to form the stator iron. This layering is done to prevent current eddies in the stator iron from decreasing the efficiency of the generator.
The problem of providing more generator poles on an asynchronous cage wound generator then really boils down to connecting the neighbouring magnets differently: either we take a bunch of magnets at a time, connecting them to the same phase as we move around the stator, or else we change to the next phase every time we get to the next magnet.
Two speed pole changing generators
Some manufacturers fit their turbines with two generators, a small one for periods of low winds and a large one for periods of high winds. A more common design on newer machines is pole changing generators, i.e. generators which, depending on how their stator magnets are connected, may run with a different number of poles and thus a different rotational speed.
Some electrical generators are custom built as two-in-one, i.e. they are able to run as e.g. either 150 kW or 600 kW generators and at two different speeds. This design has become ever more widespread throughout the industry.
Whether it is worthwhile to use a double generator or a higher number of poles for low winds depends on the local wind speed distribution and the extra cost of the pole changing generator compared to the price the turbine owner gets for the electricity. Keep in mind that the energy content of low winds is very small. A good reason for having a dual generator system, however, is that you may run your turbine at a lower rotational speed at low wind speeds. This is both more efficient aerodynamically and it means less noise from the rotor blades, which is usually only a problem at low wind speeds.
Incidentally, you may have a few pole changing motors in your house without even knowing it: washing machines which can also spin dry clothes usually have pole changing motors which are able to run at low speed for washing and at high speed for spinning. Similarly, exhaust fans in your kitchen may be built for two or three different speeds (in the latter case with a variable speed fan, you can use what you have learned about the energy in the wind: If you want to move twice as much air out of your house per minute using the same fan, it will cost you eight times as much electricity).
Variable speed generators for wind turbines
Manufacturers of electric motors have for many years been faced with the problem that their motors can only run at certain almost fixed speeds determined by the number of poles in the motor. As was shown on the previous page, the motor or generator slip in an asynchronous machine is usually very small for reasons of efficiency, so the rotational speed will vary with around 1% between idle and full load.
The slip, however is a function of the (DC) resistance (measured in ohms) in the rotor windings of the generator. The higher resistance, the higher the slip. So one way of varying the slip is to vary the resistance in the rotor. In this way one may increase generator slip to e.g. 10%.
On motors, this is usually done by having a wound rotor, i.e. a rotor with copper wire windings which are connected in a star, and connected with external variable resistors, plus an electronic control system to operate the resistors. The connection has usually been done with brushes and slip rings, which is a clear drawback over the elegantly simple technical design of an cage wound rotor machine. It also introduces parts which wear down in the generator, and thus the generator requires extra maintenance.
Evidently the use of resistors for controlling the slip of the generator introduces extra losses in the conversion process of mechanical power into electrical power, which also is a drawback.
An interesting variation of the variable slip induction generator avoids the problem of introducing slip rings, brushes, external resistors and maintenance altogether.
By mounting the external resistors on the rotor itself, and mounting the electronic control system on the rotor as well, you still have the problem of how to communicate the amount of slip you need to the rotor. This communication can be done very elegantly, however, using optical fibre communications and sending the signal across to the rotor electronics each time it passes a stationary optical fibre.
Running a pitch controlled turbine at variable speed
As will sawn in later pages, there are a number of advantages of being able to run a wind turbine at variable speed. One good reason for wanting to be able to run a turbine partially at variable speed is the fact that pitch control (controlling the torque in order not to overload the gearbox and generator by pitching the wind turbine blades) is a mechanical process. This means that the reaction time for the pitch mechanism becomes a critical factor in turbine design.
If you have a variable slip generator, however, you may start increasing its slip once you are close to the rated power of the turbine. The control strategy applied in a widely used Danish turbine design (600 kW and up) is to run the generator at half of its maximum slip when the turbine is operating near the rated power. When a wind gust occurs, the control mechanism signals to increase generator slip to allow the rotor to run a bit faster while the pitch mechanism begins to cope with the situation by pitching the blades more out of the wind. Once the pitch mechanism has done its work, the slip is decreased again. In case the wind suddenly drops, the process is applied in reverse.
Although these concepts may sound simple, it is quite a technical challenge to ensure that the two power control mechanisms co-operate efficiently.
Improving power quality
You may protest that running a generator at high slip releases more heat from the generator, which runs less efficiently. That is not a problem in itself, however, since the only alternative is to waste the excess wind energy by pitching the rotor blades out of the wind.
One of the real benefits of using the control strategy mentioned here is that you get a better power quality, since the fluctuations in power output are "eaten up" or "topped up" by varying the generator slip and storing or releasing part of the energy as rotational energy in the wind turbine rotor.
- Indicative comparison of generators (1 MW size): table comparing different generators