A Precise Digital Control System for a Commutatorless Electric Motor

A digital control system has been developed for the precise control of speed of a commutatorless, electronically switched, parallel-armature-connected, variable-speed synchronous motor. The control system uses a fixed-frequency 400-Hz supply and permits the control of the machine velocity in steps of 1 r/ min from slewing rates up to at least 4000 r/ min in either direction, each selected speed being controlled to 1 part in 105 using a crystal oscillator reference. Programmed changes between preselected velocity limits can also be obtained in which the maximum rates of acceleration or deceleration may be preset manually, or may be adjusted automatically, so that the motor is always operated under the condition of maximum allowable leading or lagging torque angle. The motor system has achieved efficiences of over 80 percent for a 2-hp output at 2000 r/min; the only foreseeable horsepower rating limitation to this type of drive will be governed by the rating of the thyristors available for use as the main current control elements. The system has potential applications in the general area of high-performance independent and coordinated variable-speed drive systems for process industries.