A New System of Speed Control For A-C Motors

The drive unit of this system consists of a constant speed a-c. motor supplemented by an adjustable speed d-c. machine of much smaller size. Both rotor and frame of the a-c. motor are mounted on bearings. The d-c. machine is mechanically connected to the frame of the a-c. motor so that the d-c. machine may drive or be driven by the frame. The d-c. machine is electrically connected through a motor-generator set of equivalent rating to the source of alternating current energy. The shaft speed of the a-c. motor is increased above the fixed speed by rotating the frame of the a-c. motor in the same direction as the rotor. The shaft speed is decreased by rotating the frame in the opposite direction. The direction of rotation and the speed are governed by adjusting the voltage impressed on the armature of the d-c. drive machine by the generator of the motor-generator set. When the unit is used to drive fans, the speed range is obtained by a combination of armature voltage control and field control of the d-c. drive machine. This permits a still further reduction in the rating of the d-c. drive machine so that for comparatively wide ranges of speed it forms but a small percentage of the total drive unit rating. Twenty-four units of this type, aggregating 7,020 hp., are now being built for Powerton Power Station for driving forced and induced draft fans.