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The application of static variable-frequency drives to provide power to deep-well submersible-pumps in oil-field applications is addressed. Introductory discussion of alternative variable-frequency drive circuits leads to a more detailed description of the complementary-commutated variable-voltage dc link arrangement chosen for this application. Physical considerations of enclosure and cooling for the Oil-field environment are reviewed. The expected benefits of basic variable-speed operation are set out, both in performance and economic terms. Further benefits become possible by extension of the use of variable-frequency drives. One example is the use of frequencies above 60 Hz to provide more pumping capacity from a pump of given diameter. Another is the use of down-hole sensors to enable closed loop control of pump suction pressure. The conclusion is that the use of variable-frequency drives in conjunction with existing submersible pumps provides immediate benefits. Increased flexibility in planning new installations is allowed by efficient matching to changing well conditions.