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.
