On the Possibility to Achieve a Pole Change in Synchronous Motors

The change of the number of poles in three-phase electrical machines has been proposed and used in induction motors, since the number of poles of the squirrel cage induction motor rotor automatically adapts to the stator number of poles. Consequently, only the stator winding polarity has to be changed. The main advantage of varying the number of poles in induction motors is the possibility to operate with two different synchronous speeds. This paper investigates the possibility to vary the number of poles in synchronous motors as well. Obviously, when the stator pole number of a synchronous machine is modified the rotor pole number has to be modified accordingly. Various possible solutions are illustrated, including a wound rotor motor, a line-start two-speed synchronous motor, a hybrid reluctance-permanent magnet motor, and a hybrid-excited synchronous motor. Some rotor configurations are designed and their performance is analysed by means of finite element analysis and experimental tests.