Closed-loop control of DC drives AND Microcomputer InterfaceA basic scheme of the Closed-loop speed control system employing current limit control ,also know as parallel current, is shown in fig 4-2A-1.Fig 4-2A-1 driver with current limit controlWn* sets the speed reference. A signal proportional to the motor speed is obtained from the speed sensor. The speed sensor output is filtered to remove the ac ripple and compared with the speed reference .the speed error is processed through a speed controller. The output of the speed controller Uc adjusts the rectifier firing angle α to make the actual speed close to reference speed. the speed controller is usually a PI(proportional and integral) controller and serves three purposes-----stabilizes the drive and adjusts the damping ratio at the desired value ,makes the steady-error close to zero by integral action, and filters out noise again due to the integral action. The drive employs current limit control, the purpose of which is to prevent the current from exceeding safe values .As long as IA
