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.