Brushless DC motor and sensorless drive arrangement therefor

A brushless motor and drive arrangement comprising in combination:

• a rotary output member rigidly coupled to a driven load, a magnetic rotor assembly including a plurality of permanent magnets mounted on the periphery of an annular magnetic ring, a stator assembly including at least two stator windings disposed relative to said magnetic rotor assembly such that (1) energization of an individual winding thereof with electric current develops torque for imparting rotary motion to the magnetic rotor assembly, and (2) movement of the rotor assembly relative to the stator assembly induces a voltage in the stator windings, a sensorless controller including means for initiating movement of the rotor assembly by energizing a given one of the stator windings and monitoring the voltage, whereby induced in at least one other stator winding to obtain rotor position information therefrom for synchronizing subsequenty energization of the stator windings with the rotor position, and resilient means for coupling the annular magnetic ring to the rotary output member so that when the controller energizes a given stator winding to initiate movement of the magnetic rotor assembly, the magnetic rotor assembly is able to rotate relative to the rotary output member and load at an effective velocity which induces a greater voltage in the other stator windings than would otherwise occur, thereby enabling the controller to obtain reliable rotor information for successfully starting the motor and load even though the load has relatively high inertia.