Eliminating sensors from closed-loopcontrol

Direct torque control regulates motor speed and torque without encoders or tachometers. Encoderless motor control proved to be a maintenance dream at a Georgia Pacific Gypsum Paper mill. Despite the harsh operating conditions, drives have operated for several years with no unscheduled downtime. Direct torque control is a motor-control method for ac drives that accurately regulates motor speed and torque without encoder feedback from the motor shaft. Eliminating encoders can simplify installation and system maintenance, and often results in less downtime, better control, and higher throughput. Here's how encoderless systems compare with more traditional motor-control methods. To operate efficiently, a conventional flux-vector PWM drive needs to know the rotor flux angle inside an ac-induction motor. A pulse encoder typically supplies information about rotor speed and angular position relative to the stator field. The controller uses this data to regulate voltage, current, and frequency through a pulse-width modulator to the motor. Torque, therefore, is controlled indirectly. This system provides good torque response, accurate speed control, and can produce full torque at zero speed, giving performance close to that of a dc drive. However, it requires rotor-position feedback. This adds cost