Electricity Production from Renewables Energies

3.7. “Fixed speed” wind turbines based on induction machines

3.7.1. Physicalprinciple

By again taking the example of Figure 3.28, and if we replace the magnet with a metal part, the latter will also revolve when stator windings are supplied by three— phase voltages. However, the rotational speed will be different from the synchronous speed, hence the designation of the term “induction machine” [LED 09]. The difference between the mechanical speed (Ω) and the synchronous speed ( Ω_s , imposed by the network frequency), is characterized by a slip:

[3.23]

A unit slip corresponds to the machine at standstill and a null slip corresponds to the synchronous speed.

3.7.2. Constitution of induction machines

The stator of an induction machine is made up of three windings, which are coupled in a star or triangle. Depending on the rotor design, two machine technologies can be distinguished.

For the first technology, industrial productions generally use a rotor. The latter is made up of copper or aluminum bars, which are conductive and short-circuited by a conductive ring at each end. This looks like a squirrel-cage, hence the name “squirrel-cage induction machines”. This operation enables the circulation of the induced currents (in the rotor), which, since they are revolving, they cause a second magnetic field. The second magnetic field even plays the same role as that of the magnet in Figure ...