Brushless Direct Current–Multiple Phase Reluctance Motor Modeling, Control, and Design

Quite a few electric machine technologies without PMs, such as reluctance synchronous and DC-excited synchronous machine drives, are currently used, both for line-start (constant-speed) and variable-speed applications, where, however, for the latter case, a PWM converter to produce variable voltage amplitude and frequency is required. The absence of PMs in RSM and DC-excited SMs leads to lower cost but a lower power factor (for RSMs) and, respectively, to larger-volume machines with DC joule losses (temperature-limited) rotors fed either through a brush slip-ring device or a contactless (rotary transformer) power transformer to the rotor (less than 3% power rating, though).