Abridgment of 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 a-c. 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, and it is decreased by rotating the frame in the opposite direction. The direction of rotation and 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 7020 hp., are now being built for Powerton Power Station for driving forced and induced draft fans. This system of fan drive is being installed in preference to the two-speed squirrel-cage type induction motor system previously used, because it shows large savings in energy, costs little more, and provides a simple method of fan control which permits the adoption of a simplified system of automatic combustion control. The versatility of the system is further illustrated by a description of a 2500-hp. unit of this type designed to drive a high-pressure reciprocating boiler feed pump. During the first seven months of 1930, orders were placed for 35 of these units ranging in size from 166-hp. to 2500-hp., aggregating 16,000 hp. for installation in fou- different power stations.