A simple model for flux weakening in surface PM synchronous machines using back-to-back thyristors

Flux weakening in surface permanent magnet (PM) synchronous machines is revisited in this letter. The condition for achieving infinite constant power speed ratio (CPSR) is explained from the machine equivalent circuit and phasor diagram point of view. Back-to-back thyristors, or triac, switches feeding the three phases of a surface PM synchronous machine will be shown to be equivalent to a simple series reactance with respect to fundamental component behavior. Using such switches is equivalent to adding a series inductance to the machine. This additional inductance helps extend the CPSR of surface PM synchronous machines. This is significant because extending the CPSR of surface PM machines is usually a challenging task due to the presence of low-permeability surface magnets and the resulting low machine inductance.