A power tool includes a control for the motor of the power tool that senses an operating characteristic of the motor and controls the operation of the power tool based on the sensed characteristic. The sensed characteristic includes variations in reluctance of the motor. A controller may sense changes on load on the motor as a result of interaction between a working element such as a saw blade or drill bit and the work piece. The controlled operation may include variation in speed or torque or both, or may include stopping the motor. Emergency conditions may be sensed by changes in reluctance.