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.