Comparison of synchronous PM machine types for wide constant-power speed operation: converter performance

A thorough comparison of the converter performance characteristics for four permanent magnet (PM) synchronous machine configurations is presented. Two versions of an interior PM (IPM) machine with distributed windings are included. One version has a maximum back-EMF limit at the top speed while the second does not have any constraint on back-EMF amplitude. Two types of surface PM (SPM) machines are also considered, one with fractional-slot concentrated windings, and another with conventional distributed windings. The target application is an automotive direct-drive starter/alternator requiring a very wide 10 : 1 constant power speed ratio (CPSR). Detailed comparisons of the converter performance below and above the base speed are presented, evaluating significant issues, including the converter switching and conduction losses, output ripple current, and DC-link current ripple. Study results show that the higher excitation frequencies required by PM machines with high pole numbers have only a modest impact on converter efficiency for comparable output current waveform quality. In constrast, the imposition of maximum back-EMF amplitude constraints at top speed raise the machine rated current, resulting in elevated converter losses and larger DC-link capacitors.