Speed Control of Large Induction Motors by Thyristor Converters

An efficient method for achieving continuously variable speed control of slip ring induction motors with fan-type load is described in which the motor slip is controlled by a thyristor converter. By means of motor winding tap changes, the rating of the thyristor converter is reduced to a fraction, typically one third, of the motor. The reactive kVA associated with the converter is similarly reduced and the system full-load power factor is ideally 0.95. The motor starting current is controlled to be about one third of rated current. The scheme enables the initial cost of solid-state speed controls for large motors to be competitive with existing methods and additionally offers greater reliability, efficiency, and versatility, as well as reduced operating costs. The techniques discussed are especially suitable for motors ranging from tens to thousands of horsepower.