In the oil and gas industry, an engine (e.g., a diesel, gasoline, natural gas, or propane engine) is sometimes used to power a three phase generator. The generator, in turn, supplies power to a three phase induction motor used to drive a mechanical pumping apparatus. If the induction motor in such a system is operated at variable speeds, the capability of the pumping system is greatly expanded. However, prior approaches to providing variable speed operation of the pump in such pumping systems have not been entirely satisfactory. In one previously used approach to operating a pump at variable speed in an engine driven pumping system, variable frequency and voltage power is delivered to a three phase motor driving the pump. Typically, in a system using a generator as the source of electrical power, an electronic variable speed drive (a.k.a., a variable frequency drive or VFD) is interposed between the generator and the induction motor to provide variable voltages and frequencies to the motor. There are many potential disadvantages to using a variable speed drive, including voltage harmonics generated on the motor leads that can damage the motor, current harmonics generated at the input of the VFD that can cause problems for the generator, the complexity in VFD design and/or the need for an expensive output transformer when higher voltages are used (most often when motor leads are very long).
To eliminate the need for a variable speed drive, some prior approaches employ a variable frequency power system that varies the speed and excitation of a three phase generator to produce a desired output frequency and voltage in an attempt to achieve the desired operation of the three phase motor in the pump. U.S. Pat. No. 7,170,262, to Pettigrew discloses a variable speed drive system of this type, and discusses the operation of such a system in comparison to earlier prior systems of the type using a variable speed drive as described herein above.
Unfortunately, variable frequency power systems, of the type exemplified by Pettigrew, have significant drawbacks. For example, simply attempting to control the output of the three phase generator may not produce the desired operation of the pump. The output speed of the induction motor driving the pump will vary based on factors such as pump loading. When that occurs, the user of such a variable frequency power system will not have accurate control of the pump speed and may encounter a variety of undesirable situations. For example, the pump may operate at a lower than expected speed, resulting in lower than optimal production or the pump may operate at a higher than desired speed and experience a “pump dry” or “pump off” condition causing the pump to run without fluid to cool and lubricate it or the pump will turn off to allow the fluid to refill the casing, causing sediment to settle in the pump. These conditions will result in damage to the pump, resulting in reduced life.
The present invention is directed to overcoming the drawbacks described and, in addition, improving the state of the art in the oil and gas industry.