In permanent magnet synchronous machines it is common to use laminated rotor structures, i.e. disc rotors, wherein the actual body of the rotor around the shaft is made of a large number of identically shaped thin ferromagnetic metal discs which are stacked together as a tight assembly.
The ferromagnetic disc stack forms a good body for the rotor around the magnets, but particularly in large machines, heating of the rotor and, importantly, of the magnets may cause problems. Other problems besides heating include leakage fluxes of the magnets and the armature reaction, i.e. the magnetic flux tends to move crosswise at the rotor pole, which is not a desirable effect.
These problems have been overcome or at least reduced in the prior art laminated rotor structure of a permanent magnet machine, in which discs of a ferromagnetic material form the body of the rotor. The body is provided with bars of a damper winding that extend axially from one end of the body to the other in proximity to the surface and, on the inner side of the frame formed by the bars, with a circular arrangement of permanent magnets in V-formation. First ends of the permanent magnets are close to the outer perimeter of the rotor, while their second ends are closer to the central shaft of the rotor.
Two permanent magnets thus form a pair of permanent magnets in which the magnets are angled relative to each other, their first ends spaced apart from each other and their second ends in proximity to each other. Further, the structure comprises an air channel which extends in the axial direction through the laminar structure of the rotor and which is disposed in direct heat transfer contact with the second ends of the magnets in the pair of magnets. This way, the air flow in the air channel effectively cools the laminate structure and thereby the permanent magnets, and also directly the second end of the permanent magnets which is in direct heat transfer communication with the air flow.
Permanent magnet machines are basically fixedly magnetized, i.e. fixedly driven, which means that their magnetization cannot be adjusted as can be done with traditional synchronous machines. However, in certain applications such as in power generation there exists a need also to adjust, to some degree, the magnetization of a permanent magnet synchronous machine, and thereby the reactive power produced by the machine. This could be theoretically accomplished by adjusting the main magnetic flux of the permanent magnets. However, in practice this is not preferred because, in the case of a permanent magnet synchronous machine, an adjustment coupled in series with the main flux requires high magnetization power and thereby generates a great deal of heat, which is difficult to be conducted away from the proximity of the magnet. An adjustment implemented in this manner would weaken the performance, particularly efficiency, of the machine, as well as the magnetic properties of the magnet.