Are the basic speed the same as the synchronous speed?
Change of speed
A four-pole generator
The speed of a generator (or motor) that is connected directly to a 3-phase network is constant and determined by the network frequency, as we have seen on the previous page.
If we double the number of magnets in the stator, we can make the magnetic field rotate at half the speed.
In the picture on the left we see how the magnetic field rotates half a turn clockwise until it finds the same magnetic pole as before. We simply connected the six magnets to the three phases in a clockwise direction.
This machine has four poles, two north and two south poles at all times. Since a four-pole generator only needs half a revolution per period, it will consequently rotate in a 50 Hz network at 25 revolutions per second, or 1500 revolutions per minute (rpm).
If we double the number of poles in the stator in a synchronous machine, we also have to double the number of poles in the rotor, as the picture shows. Otherwise the poles don't match. (In this case we could use two horseshoe magnets).
Of course we can repeat the steps just set and insert another pair of poles by installing three more electromagnets in the stator. With 9 magnets we get a 6-pole machine that runs in a 50 Hz network at 1000 rpm. The following table shows further results:
Speed of synchronous generators (rpm)
The term "speed of synchronous generators" refers to the speed of the generator when it is running synchronously with the mains frequency. This applies to all types of generators; In the case of an asynchronous generator, this speed corresponds approximately to the idling speed.
Fast running or slow running generators?
Most wind turbines have generators with 4 or 6 poles. The reason for these relatively high-speed generators is savings in size and cost.
The maximum force (torque) that a generator can digest depends on the rotor volume. For a given nominal power, you have the choice between a slow-running, large (expensive) generator and a fast-running, smaller and cheaper one.
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