In recent years, the concept of using variable speed AC electric motors as mill drives has increased in popularity. One of the features of these motors is that motor torque is directly available as a signal output, eliminating the need for external torque measurement, or for inferring torque from other sources. In 2005, CSR Pioneer Mill replaced its steam turbines with electric motors. In addition to measuring torque from the electric motors, they contracted SRI to install strain gauges on the mill drive shafts to provide an independent measurement of torque, enabling a comparison of the two measurement systems to be made. The comparison of torque was conducted for the #1 mill and #1 mill pressure feeder tailbars. It showed that the motor torque measurement is of similar magnitude and variability to the shaft torque measurement. The results showed that the values differed more for the pressure feeder than for the mill, and it has been hypothesised that the greater variability comes from the variation in pressure feeder speed. The #1 mill speed, on the other hand, is generally constant at Pioneer Mill. The torque comparison has shown that, even for the more highly variable pressure feeder torque, the variability represents only 10% of the measured value. For the purpose of controlling torque, a measurement of this quality is sufficient. Consequently, motor torque is considered a satisfactory measure of torque for control purposes.
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