Thyristor Control of Resistive and Series DC Motor Loads Using Integral-Cycle Switching

The use of integral-cycle conduction of power appears to be useful for resistive load control, especially in heating applications. Uniform control sensitivity may be obtained over the whole power range if the controller has a fixed ON plus OFF time with variable ON/OFF ratio. The incremental load power controllable is proportional to the number of ON plus OFF supply cycles constituting a control period. Rectifier integral-cycle pulses of current were obtained from a single-phase supply using a pair of inverse-parallel connected thyristors and a diode bridge. This particular form of pulsewidth modulation was used for speed control of a fractional horsepower series dc motor on both open loop and closed loop. Steady-state characteristics in terms of mean current and mean speed were identical with those obtained from conventional dc supply. For high-speed low-torque operation there was little speed ripple and smooth, quiet motor performance was obtained. To obtain low speeds, the ON/OFF mark space ratio of the current was less than unity and the intermittent conduction caused speed ripple with motor noise and vibration.