Load Equalization between Two Synchronous Motors Driving a Grinding Mill

Abstract

Large Grinding Mills in the cement and mining industries are conventionally driven by two synchronous motors on opposite sides of a girth gear. Operational experience in these grinding mills shows that due to certain unavoidable inaccuracies in the manufacturing of the girth gear, there is a sustained load pulsation, 180o out of phase in the two motors in every mill revolution. This presents a double problem of wear on the gear teeth due to peak torque loading and motor over heating due to peak stator phase currents. To mitigate this problem, a drive system is presented in which the two synchronous motors are electrically connected through their stator and field windings, and mechanically coupled through the girth gear. Model equations for this system was developed and simulated on the MATLAB/SIMULINK environment. Results obtained shows that in the worst case event of such load pulsations, the torque developed by the two motors remained equal. Also, winding currents in the two motors were observed to be constant without any peaks and dips, thereby overcoming the motor overheating phenomenon. If these motors are connected to the same source as the original three-phase motor from which it was derived, the power capability is found to be the same for both the three- and the six-phase motors.