Abstract: A motor drive system for a mining shovel includes three a.c. motors which are driven by associated inverter circuits that receive power from a common d.c. bus. The d.c. bus connects to the a.c. power lines through a disconnect switch and a rectifier circuit and a chopper circuit is employed to dissipate power when the d.c. bus voltage rises due to motor regeneration. A power monitor and control circuit monitors the a.c. line voltage and the d.c. bus voltage and it controls the various system elements in such a manner as to maintain the d.c. bus voltage within an operating range. When the d.c. bus voltage cannot be maintained within the operating range, the disconnect switch is enabled to shut down the machine.