Very Like a Whale
In reality, the stator of a generator consists of a very large number of electromagnets.
Changing the Number of Generator Poles
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 you see in the picture. (We have not yet added the stator coil windings on the iron).
The reason for this stator arrangement is that we wish to minimise the air gap between the rotor and the stator. At the the same time we need 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 400 kW or 2000 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. (You should 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).
© Copyright 1997-2003 Danish Wind Industry Association
Updated 17 May 2003