A rotor is constituted by a two-pole permanent magnet, and enabled to rotate by a predetermined angle in a direction corresponding to the direction electric current supplied to a stator coil. A data structure is generated that specifies a prioritized set of rules and storage locations within the CAM device for one or more match clauses of each of the rules. The controller transmits a common voltage measurement command to the respective voltage monitoring devices connected via a communication channel in a daisy-chain scheme.