Changing Generator Rotational Speed
A Four Pole Generator
The speed of a generator (or motor) which is directly connected to a three-phase grid is constant, and dictated by the frequency of the grid, as we learned on the previous page.
If you double the number of magnets in the stator , however, you can ensure that the magnetic field rotates at half the speed.
In the picture to the left, you see how the magnetic field now moves clockwise for half a revolution before it reaches the same magnetic pole as before. We have simply connected the six magnets to the three phases in a clockwise order.
This generator (or motor) has four poles at all times, two South and two North. Since a four pole generator will only take half a revolution per cycle, it will obviously make 25 revolutions per second on a 50 Hz grid, or 1500 revolutions per minute (rpm).
When we double the number of poles in the stator of a synchronous generator we will have to double the number of magnets in the rotor , as you see on the picture. Otherwise the poles will not match. (We could use to two bent "horseshoe" magnets in this case).
Other Numbers of Poles
Obviously, we could repeat what we just did, and introduce another pair of poles, by adding 3 more electromagnets to the stator. With 9 magnets we get a 6 pole machine, which will run at 1000 rpm on a 50 Hz grid. The general result is the following:

Synchronous Generator Speeds (rpm)
 Pole number 50 Hz 60 Hz 2 3000 3600 4 1500 1800 6 1000 1200 8 750 900 10 600 720 12 500 600
The term "synchronous generator speed" thus refers to the speed of the generator when it is running synchronously with the grid frequency. It applies to all sorts of generators, however: In the case of asynchronous (induction) generators it is equivalent to the idle speed of the generator.
High or Low Speed Generators?
Most wind turbines use generators with four or six poles. The reasons for using these relatively high-speed generators are savings on size and cost.
The maximum force (torque) a generator can handle depends on the rotor volume. For a given power output you then have the choice between a slow-moving, large (expensive) generator, or a high-speed (cheaper) smaller generator.