A Fast-Acting Current Limit for a DC Motor Drive

The two basic methods of providing current-limit protection for a variable-speed dc motor drive are examined. The most common method, the current regulator, is a two-loop scheme consisting of a current-regulating loop enclosed within a speed-control loop. The alternate method is the interventionist system, which normally operates in the speed-control mode, the current limiter taking control only after the armature current has exceeded a threshold value. The paper reviews the existing literature on the topic and goes on to describe a new interventionist-type system. The speed of response is extremely high: with a typical, fully controlled three-phase bridge rectifier the current is limited with high accuracy within one-sixth of a cycle. Experimental results for a 50-hp machine are given.