Electricity: motive power systems – Constant motor current – load and/or torque control
Reexamination Certificate
2000-04-03
2002-07-02
Dang, Khanh (Department: 2837)
Electricity: motive power systems
Constant motor current, load and/or torque control
C318S433000, C388S904000, C417S018000, C417S022000
Reexamination Certificate
active
06414455
ABSTRACT:
BACKGROUND OF THE INVENTION
A variety of techniques are known for pump control and power management in petroleum well pumping systems. Typically, land-based pumping systems comprise a beam pump, an electric motor, a plunger and a barrel. Such beam pump units are known in the petroleum industry as “pump jacks” or “rod pumps.” Generally, these pumps are driven by AC electric motors.
A variety of techniques have been devised to regulate the speed of the beam pump unit to match the fluid accumulation in the well. For example, some pump speed control systems employ what is known as ‘damper regulation’ or ‘throttling.’ In damper regulation, the motor runs at a fixed speed, but works against a restriction. This results in relatively high energy consumption. In such constant speed systems, high torques are encountered on startup, the upstroke and the downstroke of a pump stroke cycle.
Alternatively, magnetic couplings, known as eddy current drives, are employed to vary the speed of the beam pump unit to match the oil well's production capabilities. Eddy currents “slip” a magnetic clutch to vary the speed of the pump. Clutch slip is inefficient, generates excessive heat in the clutch, and results in loss of energy. Further, eddy current drives can only slow, and not increase the speed of the beam pump.
Another issue in beam pump motor management is “pump-off.” While under extraction, the oil well can eventually pump off causing the plunger to “pound” the fluid in the barrel. To eliminate pounding, simple pump off controllers are installed to stop and restart the pump after a predetermined amount of time. In general, pump off controllers prevent damage to the system, but do not decrease mechanical loading. Consequently, they offer insignificant savings in energy usage.
Many attempts have been made to use variable speed drives to control the motor that drives the beam pump units. Such attempts have met with limited success. Variable drives, and the loads they are applied to, can generally be divided into two broad categories: “constant torque” and “variable torque.”
Even though called “constant torque,” the term “constant torque” is not literally accurate. The torque actually required can vary significantly. For example, “constant torque” applications frequently experience overload conditions with high torque demands. As a result, “constant torque” variable drives typically have a high overload rating (for example, 150% for 60 seconds) to handle the high peak torque demands. To handle these high torque excursions, excess horsepower is required. Consequently, additional energy is consumed during peak torque periods. This excess energy is wasted when torque demands are low. Thus, the beam pump unit is energy inefficient.
In another type of drive, known as a “constant speed” variable drive, the driving motor is driven at a desired constant speed. As with constant torque applications, constant speed drives have irregular energy usage characteristics. During a typical cycle of a cyclic load including startup, as well as parts of the upstroke and downstroke, constant speed drives require significant energy. They are particularly energy inefficient when in “regeneration” mode.
Regeneration is caused when the load “over speeds” the electric motor. This transforms the motor into a generator that generates excessive energy that returns to the variable drive. Generally, variable drives are not designed to absorb electrical energy from the motor. To handle the regenerated power, power dissipation devices such as resistors are employed. Use of resistors to consume regenerative power is very energy inefficient. There are, however, what are known as regenerative variable drives. Such drives are, however, generally expensive and do not reduce energy consumption.
Consequently, there in a need for a system and method for an energy efficient and cost effective way to control a motor driving a variable cyclic load.
SUMMARY OF THE INVENTION
A system and method for controlling the speed of an electric motor that drives a cyclic load is provided. In one embodiment, the system includes a signal conversion circuit, a signal measurement circuit, a control circuit, a signal inversion circuit and, optionally, a user interface circuit. The signal conversion circuit converts an AC electric signal to a DC electric signal. The signal measurement circuit periodically samples the DC electric signal to derive a DC electric characteristic signal from the DC electric signal. The control circuit receives the DC electric characteristic signal from the signal measurement circuit and derives and stores a DC current signal from the set of DC electric characteristic signals. The control circuit is responsive to the DC electric characteristic signal and a set of provided or derived operational parameters which may include, for example, a voltage-frequency profile and a set point parameter to generate a set of control signals. The signal inversion circuit, responsive to the set of control signals, inverts the DC electric signal and generates a set of drive signals to modify the speed of the motor through changes in supplied frequency or voltage. By modifying the motor speed, the system and method controls and regulates the motor attributes of speed and torque to selectively operate the motor either in a regulated “constant torque” mode or in a regulated “constant speed” mode.
In the regulated “constant torque” mode, in response to a set point parameter provided as a torque set point, the control circuit derives a set of torque reference signals. The control circuit compares the received set of DC electric characteristic signals and the stored DC current signal with the set of torque reference signals to generate a set of control signals. The control signals are representative of a determined selective adjustment to the frequency and voltage of the set of the drive signals. The signal inversion circuit generates a corresponding set of variable frequency and variable voltage drive signals and applies the set of variable frequency and variable voltage drive signals to selectively drive the motor at increased or decreased speeds by incrementing or decrementing the frequency of the set of drive signals. In alternative modes, the voltage of the drive signals may be modified.
In the regulated “constant speed” mode, in response to the set point parameter provided as a set point target torque range for a selected motor speed, the control circuit derives a set of speed reference signals. The control circuit compares the received set of DC electric characteristic signals and the stored DC current signal with the set of speed reference signals to generate the set of control signals. The control signals are representative of the selective adjustment to the frequency and voltage of the set of the drive signals. The signal inversion circuit generates a set of variable frequency and variable voltage drive signals and applies the set of variable frequency and variable voltage drive signals to selectively drive the motor at increased or decreased speeds by incrementing or decrementing the frequency of the set of drive signals according to the set point target torque range for the selected motor speed.
The optional user interface circuit is a supplied method to provide the control circuit with the set of operational parameters such as a voltage-frequency profile, and set point parameter appropriate for either the regulated “constant torque” or regulated “constant speed” mode. The set point parameter represents a motor control characteristic. The motor control characteristic may be provided to the system or derived from a motor load profile that typifies a set of characteristics of the motor under control. The motor load profile may be determined in the system by driving the motor through a complete cycle of the cyclic load.
The motor load profile may optionally be derived using a load sensor and a speed sensor. The motor may be operated at a selected motor torque to obtain a variable motor speed characteristic in the unregulated “constant torqu
Boyle Fredrickson Newholm Stein & Gratz S.C.
Dang Khanh
Leykin Rita
LandOfFree
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