A torque control strategy for motor starters

In heavy industries such as coal mining or quarry products, solid-state motor starters for induction motors have been used extensively to bring conveyor systems to full speed because of their softstart capability. A major problem for industrial motor starters using the standard voltage ramping technique is that they usually experience inconsistent overall motor run-up time and instability near full speed. Current industrial solutions for this problem require the use of current feedback or tacho feedback. In this paper, the authors show that the standard voltage ramp currently used in industrial motor starters does not correlate to the run-up time, nor guarantees substantial reduction of torque and current surges. Digital implementation of a voltage ramping power starter indeed confirms the validity of these results. A torque-control strategy is then proposed for a motor starter which would provide a substantial reduction of torque and current surges and at the same time would guarantee the required run-up time of the induction motor without the use of a tachogenerator. Experimental results show that an effective torque-controlled power starter can be implemented successfully