A fast acting current limit for a PC-based DC drive

A PC based DC drive is implemented using an ultra fast current limit that intervenes only if the armature current exceeds a certain pre-defined threshold. This scheme normally operates in the speed control mode and the armature current is limited rather than controlled. If the armature current exceeds a certain pre-defined threshold, the drive switches to the current limiting mode by inhibiting the firing pulses delivered to the converter bridge. The scheme does not necessitate the smoothing or filtering of the armature current as it operates on its instantaneous value. This allowed the use of a personal computer as modest as a 12 MHz, XT equipped with a very low cost, low performance A/D card. The parallel port LPT1 was used to drive an all digital phase control firing circuit that delivers the appropriate gating signal to a B2C thyristor converter bridge. The system is controlled using a digital PI control algorithm. It was tested for starting, set point response as well as for load disturbances. The swift response of the current limit was demonstrated