Alternative to field-weakening of surface-mounted permanent-magnet motors for variable-speed drives

Today, interest is increasing in the use of inverter-fed permanent-magnet (PM) synchronous motors in variable-speed drive applications. A problem is the difficulty in obtaining a sufficiently large field-weakening range. Especially for surface-mounted permanent-magnet (SMPM) motors, having the highest power density in relation to the required magnet material, this problem is very pronounced. An alternative method is studied of how to obtain a sufficiently wide field-weakening range. The basic idea is to connect different coil groups of the stator winding in different configurations. In this way, an extension in the speed range is obtained which is comparable to the field-weakening range of asynchronous motor drives. The influences on the motor design are studied and an example of a SMPM motor design is presented together with possible switching arrangements