Wells – Automatic
Reexamination Certificate
1999-05-22
2001-12-04
Bagnell, David (Department: 3672)
Wells
Automatic
C417S012000, C290S052000
Reexamination Certificate
active
06325142
ABSTRACT:
TECHNICAL FIELD
This invention relates to the general field of turbogenerator controls and more particularly to an improved high speed turbogenerator control system for providing electrical power to motors which have power requirements that normally vary in a repetitive manner over time.
BACKGROUND OF THE INVENTION
There are many industrial and commercial applications that utilize electrical motors to produce repetitive axial motions. The electrical motor's rotary motion can be converted into axial motion by any number of mechanisms such as cams, cranks, scotch yokes, or cable drums just to name a few. In any such application, the electrical power requirement of the motor is inherently variable and is cyclically locked to the repetitive axial motion. The motor power in these applications varies both due to inertial effects (the need to accelerate and decelerate the axially moving components of the system and the need to accelerate and decelerate the rotationally moving components of the system) and due to the work effects (changes in the work performed by the axially moving components as a function of their axial position and velocity). The magnitude of the motor power variation with time can be many times the average power requirement of the motor. Both the inertial effects and the work effects can cause the motor to function as a generator which produces electrical power at various times in the system's cyclical motion.
An elevator is one well-known example of an electrical motor producing axial motion wherein the motor's electrical power requirements vary with the passenger load, the axial velocity of the elevator and the axial acceleration/deceleration of the elevator. Deliberate deceleration or braking can be achieved by recovering the excess energy in the elevator's mechanical system (e.g. during the descent of a heavily loaded elevator) utilizing regeneration to convert that mechanical energy into electrical energy which can go back into an electrical distribution system.
Another example of a motor producing repetitive axial motion is a pump-jack type oil well. Also known as a walking beam (a large beam arranged in teeter totter fashion) or a walking-horse oil well, the pump-jack oil well generally including a walking beam suitably journaled and supported in an overhanging relationship to the oil well borehole so that a string of rods (as long as two miles) can be attached to the reciprocating end of the walking beam with the bottom end of the rods attached to a lift pump chamber at the bottom of the bore hole. A suitable driving means, such as an electrical motor or internal combustion engine, is connected to a speed reduction unit which drives a crank which in turn is interconnected to the other end of the walking beam by a pitman.
Conventionally, pump-jack oil wells utilize an induction motor powered by constant frequency, three-phase electrical power from a utility grid. The pump-jack pumping cycle varies the induction motor's speed only slightly as allowed by plus or minus a few percent of motor slip. However, the induction motor power typically varies over the pumping cycle by about four (4) times the average motor power level. At two (2) points in the pumping cycle, the motor power requirement peaks and at two (2) other points, the motor power requirements are at a minimum. Typically, at one of these minimum power requirement points in the pumping cycle, the induction motor extracts enough kinetic energy and/or work from the moving masses of the well to be able to function as a generator and produce electrical power which must be absorbed by the utility grid.
Whether the pump-jack oil well is driven by an induction motor or by an internal combustion engine, there is excess mechanical energy at some point(s) in the pumping cycle which must be absorbed to prevent excessive velocity induced stresses in the pump-jack oil well moving parts. When a pump-jack oil well is powered by an internal combustion engine, engine compression is the means by which this energy is dissipated (compression losses) while in the normal utility grid powered induction motor system, the induction motor is periodically driven at overspeed causing it to return power to the utility grid.
When a pump-jack type oil well is powered by constant frequency electrical power from a utility grid or a conventionally controlled turbogenerator, the oil extraction pumping rate may not be sufficient to keep up with the rate at which oil seeps into the well. In this case, potential oil production and revenues may be lost. Alternately, the oil extraction pumping rate may be greater than the rate at which oil seeps into the well. In this case, the oil well may waste power when no oil is being pumped or it may be necessary to shut down the oil well for a period of time to allow more oil to seep into the well.
For the reasons stated above, what is needed is an improved technique for providing power [generally] suitable for pump-jack oil well systems.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a turbogenerator with a low frequency inverter connected to an electric motor powering a cyclic motion machine, and a controller. The controller controls the turbogenerator and varies the frequency of the inverter to provide a generally constant power level to the electric motor.
An additional object of the present invention is to provide a turbogenerator with a low frequency inverter connected to an electric motor powering a pump-jack oil well, and a controller controlling the turbogenerator and inverter. The controller varies the frequency of the inverter to maintain a generally constant power output level for the turbogenerator.
A further object of the present invention is to provide a method to reduce variations in the power level provided to a cyclic motion machine having cyclically varying power requirements. The method includes connecting an induction motor to the cyclic motion machine to drive the machine, connecting a load inverter to the motor to provide power to the motor, and connecting a controller to the load inverter to control the power provided to the motor by varying the load inverter frequency. The controller monitors the inverter frequency certain machine positions during each machine cycle for a number of machine cycles to accumulate historical data. The controller further varies the load inverter frequency over each machine cycle in accordance with the accumulated historical data to reduce variations in the power level required by the motor.
The machine may be an oil well pump-jack. The controller may vary the voltage along with the frequency of the inverter. The controller and inverter may also be providing power to more than one motor, by varying the load inverter frequency in accordance with the average historical data of all motors at each machine position.
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Bosley Robert W.
Edelman Edward C.
Bagnell David
Capstone Turbine Corporation
Dougherty Jennifer
Irell & Manella LLP
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