The electric and magnetic circuits of PM synchronous motors and PM d.c. brushless motors are similar, i.e., polyphase (usually three phase) armature windings are located in the stator and moving magnet rotor serves as the excitation system. PM synchronous motors are fed with three phase sinusoidal voltage waveforms and operate on the principle of magnetic rotating field. For constant voltage-to-frequency control technique no rotor position sensors are required. PM d.c. brushless motors operate from a d.c. voltage source and use direct feedback of the rotor angular position, so that the input armature current can be switched, among the motor phases, in exact synchronism with the rotor motion. This concept is known as self-controlled synchronization, or electronic commutation. The solid state inverter and position sensors are equivalent to the mechanical commutator in d.c. brush motors. Variable d.c. bus voltage can be obtained using

• a variable voltage transformer and diode rectifier • a controlled rectifier (thyristor, GTO or IGBT bridge) In the second case the d.c. bus voltage is a function of the firing angle of the controlled rectifier.