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Many of the orebodies presently being developed are in remote locations and are, or will be, served by electrical power systems of limited capacity. The largest single components of the electrical system of an underground mine are usually the production hoist drives; the power demands of these drives vary continuously throughout each hoisting cycle. Up to about 15 years ago, mine hoists were driven by either wound rotor induction motors (direction of motor rotation was controlled by stator reversing contactors, and speed and torque was controlled by rotor contactors and resistor grids) or shunt-wound separately excited dc motors (motor-generator sets supplied dc power to the hoist motor armature and fields). Static power conversion equipment now costs less to purchase and install and is claimed to be more efficient than motor generator sets. DC motors and variable-voltage power conversion equipment are, in turn, being superseded by synchronous motors and variable-frequency power supplies. On the debit side, however, the lagging kvars and harmonic currents produced by large static power conversion installations add to the power costs and may prove unacceptable to the power supply authorities.