Automatic speed control of one-quadrant DC drives

A DC machine with constant excitation is often used for speed-controlled electrical drives. The armature voltage is supplied by a chopper or phase-controlled rectifier. The speed control loop with a proportional integral controller has a nested current loop with the same type of controller. If the armature voltage source can supply positive and negative voltage and current, it is a four-quadrant drive. If the current can be only positive, it is called a one-quadrant drive. The performance of the speed control for a one-quadrant drive with controllers designed for a four-quadrant drive is analyzed. It is shown that this system operates in a linear and nonlinear mode. The performance in the nonlinear mode is poor, and even nonlinear oscillations are possible. A controller design is provided which improves the performance of the speed control in the nonlinear operation significantly without changing the linear design known from the four-quadrant drive. Therefore the performance in the linear range is not changed. The design is done by changing the nonlinearities of the controllers which are the negative output voltage limits. The speed control system is discussed taking into consideration the most common armature voltage supplies, such as two- and six-pulse single-bridge rectifiers and the one-quadrant chopper.