Improving bridle low-speed regulation using cascaded current followers

The torque or current references to the current loops of motor drives providing power to helper-rolls in bridles are usually slaved directly from the current reference of the master speed regulated roll. At low line speeds, the current required to overcome losses in the master speed regulated roll will differ significantly from the current required to overcome losses in the helper-rolls, resulting in overall erratic speed regulation in the bridle. A scheme based on cascaded current major loops is described and shown to provide a substantial improvement in low-speed regulation. An additional benefit is that higher speed-loop bandwidths are made possible by the de-coupling effect provided by the cascaded current major loop.