Microcomputer Control of Variable Frequency Drives Using the Submersible Motor as an Instrument

The production of a submersible pump can be optimized with the use of a microcomputer-controlled variable frequency drive. A new approach to monitoring the pump and motor performance is presented. Instead of downhole sensors, pressure bomb, or acoustical wave method, the three-phase induction motor is used as a sensing instrument to determine downhole pressure, both static and flowing. A new electronic device, installed at the surface, has been developed and tested to monitor the actual revolutions per minute (r/min) of the downhole pump and motor. Using the basic induction motor equivalent circuit, the motor terminal voltage can be determined and thus controlled to meet the motor manufacturer's design criteria. The detailed analysis then permits calculation of the liquid level which can be used as the governing parameter for oil production. Also a method is discussed to prevent cavitation, determine specific gravity, and calculate downhole temperature.