AC electric machines

The working principle of the induction machines is based on the fact that rotor currents are induced by the stator currents, and this is the main cause of losses in the rotor windings, which makes these types of machines less efficient than other brushless machines [11]. However, asynchronous machines are widely used and their mass production makes them reliable and well developed, for that reason their cost is quite low. 

Synchronous AC machines have a stator similar to the asynchronous machines, which has usually a three-phase stator winding. Whereas there are two types of rotor structures round cylindrical rotor and salient pole rotor as illustrated schematically in Fig. 5.6. The rotor field is generated by permanent magnets or a DC current winding, as reported in the same figure. In the first case they are called permanent-magnet synchronous AC machines. 

The DC rotor winding is supplied with an external DC voltage by means of a brush-ring system, similar to the brush-collector system for the DC electric machine. The excitation of the rotor winding can also be obtained with a DC generator, known as exciter, located on the rotating shaft, or other excitation systems using AC exciter and solid state rectifiers. 

In synchronous AC machines the synchronization between the stator and rotor magnetic fields is achieved since the rotor magnetic field is fixed with respect to a rotor observer and thus it operates at the same speed as the rotor, which is the same speed of the stator magnetic field. 

Variable reluctance synchronous AC machines represent other types of synchronous machines which do not require any excitation system on the rotor [3]. 

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I. R. Smith, Response analysis of AC electrical machines. Research Studies Press Ltd. ISBN 0 863-80091-2 John Wiley Sons Inc. (1990). ISBN 0 471-92488-1... 

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