Untitled record

Torque control of an induction motor in a pumping system

Induction Motor Torque Control in a Pumping System

Full description

Background of the invention

The present invention relates generally to controlling the performance of an induction motor and, more specifically, to methods for controlling the performance of an induction motor used in a pumping system.

In the oil and gas industry, an engine (for example, diesel, gasoline, natural gas, or propane engine) is sometimes used to power a three-phase generator. The engine, in turn, supplies power to a three-phase induction motor used to drive a mechanical pumping device. If the induction motor in the aforementioned system is operated at variable speeds, the capacity of the pumping system is greatly expanded. However, previous methods of supplying variable speed pump operation in these pumping systems were not entirely satisfactory.

In one method previously used to operate a variable speed pump in a motor-driven pumping system, the variable frequency and voltage are transmitted to a three-phase motor that drives the pump. Typically, in a system that uses a generator as a power source, an electronic variable speed drive (also known as a frequency variable drive) is interposed between the generator and the induction motor to provide variable voltages and frequencies to the motor. There are many potential drawbacks associated with using a variable speed rotating motor, including voltage harmonics generated on the motor leads that may damage the motor, current harmonics generated at the VFD input that may cause problems for the generator, complexity in VFD design, and /or the need for an expensive output transformer when higher voltages are used (often when the motor leads are very long).

To eliminate the need for a variable speed motor, some previous methods use a variable frequency power system that varies the speed and excitation of a three phase generator to produce the desired output frequency and voltage in an attempt to achieve the desired operation of a three phase motor in a pump. U.S. Patent No. 7,170,262, Pettigrew, discloses a variable speed drive system of this type, and discusses the operation of said system in comparison to previous systems of the type using a variable speed drive as described in the present application above.

Unfortunately, variable frequency power systems, of the type represented by Pettigrew, have major drawbacks. For example, simply trying to control the output of a three-phase generator may not produce the desired operation of the pump. The output speed of the induction motor that drives the pump varies based on factors such as pump load. When this occurs, the user of the variable frequency power system will not be able to accurately control the pump speed and may encounter a variety of undesirable situations. For example, the pump can operate at a lower speed than expected, resulting in less than ideal production, or the pump can operate at a higher than desired speed and experience a “pump dry” or “pump stall” condition that causes the pump to operate without Fluid to cool and lubricate it or the pump stops working to allow fluid to be refilled in the casing, causing deposits to settle in the pump. These conditions result in damage to the pump and a shortened service life.

The present invention is directed to overcoming the described defects and, in addition, improving the state of the art in the oil and gas industry.

General description of the invention

In one embodiment, a system and method are provided for controlling the rotational speed of a motor driving a pump. By controlling the speed of the rotor instead of the generator frequency, pump operation is more precise and precise in terms of control.

Other embodiments corresponding to the invention will be illustrated by the following detailed description accompanied by the accompanying figures.

Brief explanation of the drawings

The accompanying figures incorporated into and forming part of the specification illustrate several aspects of the present invention and, in addition to the description, represent an explanation of the principles corresponding to the invention. In shapes:

Figure 1 is a representative embodiment of an induction motor control system including an error control unit as directed in the present invention;

Figure 2 is a block diagram of a Simulink simulation run on a system similar to the system from Figure 1; And

Figures 3a-3d are a graphical representation of the results obtained from the simulation performed on the system from Figure 2.

While the invention is described within specific preferred embodiments, it is not intended to limit it to such embodiments. In contrast, the aim is to cover all alternatives, modifications and equivalents contained within the scope and scope of the invention as defined by the attached claims.

Detailed description

Referring now to Figure 1, a pump control system 10 is illustrated. As will be fully explained below, the error control system 20 of the pump control system 10 monitors one or more features of the electrical input to the cables. cables and the motor 16 and, based on what was observed, calculates the mechanical input values to the pump 18 and adjusts the operation of the motor 12 and the generator engine 14. By controlling the mechanical input values calculates the mechanical inputs to the pump 18, pump operation is more precise, accurate and reliable than by a system that only controls the electrical output values from the generator.

The motor 12 absorbs air and fuel and, by combustion of the mixture, generates rotary mechanical engine output 22. Rotary mechanical engine output 22 depends on the throttle position Txc, fuel used, air temperature and density, etc. . The motor 12 can be operated using diesel, gasoline, liquid propane, natural gas or other types of fuel. The motor 12 is operationally coupled and drives the generator, engine 14. In this regard, the rotary mechanical engine output 22 is typically directly coupled to the mechanical input from the engine 14. Even so, other types of connections can be used such as gearboxes, belts, hydraulic power coupling, etc. Similar based on the specific application. The generator 14 is a three phase synchronous generator, having a controllable field.

The engine 14 is electrically coupled by electrical cables to the motor, which is expressed entirely in 16. In the system shown, the pump is a centrifugal pump of a type known as an electrical submersible pump (ESP), such that Both the motor 16 and pump 18 are inside a well casing well below ground level and the cables 16 are of reasonable length. In a typical oil well, for example, cables can be 16,000 feet long. Through the cables 16, the generator 14 supplies the motor 16 with electrical energy. In the embodiment shown in Figure 1, motor 16 is a three-phase induction motor. The electrical input to the motor 16 can be characterized by voltages (Vm) and currents (Im). The motor generates 16 rotary mechanical outputs, which are characterized by pump speed (Up) and pump torque (Tp).

The motor 16 is mechanically coupled to, and drives, the pump 18, which is located at or near the bottom of the well casing and is submerged or partially submerged in the fluid (e.g., oil and/or water) being pumped. The pump generates 18 fluid output which can be characterized by, among other variables, pump head (Hp) and pump flow (Qp).

The error control unit 20 is, in general, in electrical communication with the motor 12 and the generator 14. The error control unit 20 is also configured to monitor the input voltage (Vm) and input current (Im) values to the cables. cables and motor 16. The error control unit 20 includes interface devices 24, a motor vector model 26, a rotary flux summation device 28, a rotary flux error controller 30, a motor speed summation device 32, and a motor speed error controller 34. The error control unit 20 optionally includes a torque linearizer 36.

The interface devices 24 are generally used to control or convert signals and data sent or received by the error control unit 20. In this regard, the interface devices 24 can include various electrical components such as, e.g., input devices / Parallel outputs, analog to digital (A/D) converters, digital to analog (D/A) converters, current and voltage sensors, flow estimators, filters, complements and the like.

In the embodiment shown, the interface devices 24 herein have two outputs. The first output is the throttle position command (Txc), which is converted from the torque error command (Tec) from the motor speed error control motor 34 or, in systems where the torque linearizer 36 is used, the torque linearizer command output (Tlc). From the motor torque evaluation method 36. The second output is the excitation current command (Eic) which is converted from the error estimate (λec) command generated by the rotor flux error control 30.

For input values, the interfaces 24 generate three motor voltage measurement signals (Vmm) to express the voltage values (Vm) on each phase of the cables and motor 16 and two motor current measurement signals (Imm) to express the currents (Im) on Two of the three phases. The third phase current can be easily calculated, since the three phase currents must sum to zero, but it can also be measured.

The motor vector model 26 is in electrical communication with interface devices 24, the rotor flux summation device 28, the supervisory controller 38, and the motor speed summation device 32, and when used, It is a means of evaluating the engine torque torque linearizer 36. The motor vector model 26 performs various mathematical functions using motor voltage measurement signals (Vmm) and motor current measurement signals (Imm) to provide estimates as described in US Patent No. 7,117,120 by Beck et al. (Patent No. 120'), which is incorporated into the present application by reference. For example, the motor vector model 26 calculates and outputs estimates of the performance of the induction motor 16 including the motor speed at the motor shaft (Ume), the rotor magnetic flux (λre), and the mechanical torque of the motor shaft ( tme ).

The rotor flux summation device 28 depicted in Figure 1 receives two input values, namely the rotor flux estimate (λre) from the motor vector model 26 and the issued rotor flux command (λrc). From supervisory controller 38. During normal operation, the rotor flow command (λrc) is generally maintained at a constant value to maintain proper torque (Tp) of the motor. However, during startup of motor 12, the rotor flux command (λrc) can be greatly reduced or even held at zero to reduce the load on motor 12. Depending on the received input values, the rotor flux summation means device 28 generates an output, namely an error estimate (λee).

The error estimate (λee) is received by the rotor flux estimate 30. In general, the rotor flux estimate 30 is a control circuit structure designed to adjust its output to drive the error duration to zero.

In the embodiment shown in Figure 1, the rotor flux error control 30 is a proportional-integral-derivative (PID) control device.

However, other types of control architectures can be used, such as proportional-integral (PI) controllers, stochastic controllers, and the like, depending on what is known in the art.

The rotor flux estimate 30 provides an output in the form of a flux excitation command (λec) to interface devices 24.

The motor speed summation device 32 shown in Figure 1 receives two input values, namely the motor speed estimate (Ume) from the motor vector model 26 and the motor speed command (Umc) from the supervisory controller 38. Based on the values The input received, the motor speed summation device 32 generates an output, namely a speed error estimate (Uee).

The speed error estimate (Uee) is received by the motor speed error control 34. Generally, the motor speed error control 34 is a control loop structure that is designed to adjust its output to drive the error duration to zero. In the model shown in Figure 1, the motor speed error control 34 is a proportional-integral-derivative (PID) control.

However, other types of control structures can be used, such as proportional-integral (PI) controls, approximate Boolean controls, and the like, depending on what is known in the art. As shown in Figure 1, the motor speed error control 34 generates and sends a torque linearizer (Tec) command to the optional motor torque evaluator 36. If the motor torque rectification facility 36 is canceled from the fault control unit 20, the motor torque command (Tec) is provided directly to the interface devices 24.

When used, the motor torque rectification device 36 is used to ensure that a given input is linked to an appropriate and required corrective action. For example, when a three percent (3%) change in engine torque is desired, a ten percent (10%) change in throttle speed may be necessary. Under such conditions, the motor torque rectification device 36 processes the motor torque command (Tec) received from the motor speed error control 34 such that a straight torque command (Tlc) is produced. Linear conversion of the torque command ensures that the system gain for throttle position control is close to steady state and, for this reason, it is easier to tune an optimal response. A straight torque command (Tlc) is output to interface devices 24 and fed to motor 12. Accordingly, the throttle position is changed as required to respond correctly to the torque linearizer (Tec) command. As shown in Figure 1, the motor torque evaluation device 36 receives two input values, namely the torque linearizer (Tec) command from the motor speed error control 34 and the motor speed estimate (Ume) from the motor vector model. vector model 26.

Referring also to Figure 1, most of the components of the fault control unit 20 are in electrical communication with the supervisory controller 38. In particular, the supervisory controller 38 transmits the motor speed command (Umc) to the speed collection device. motor speed summation device 32 and rotor flux command (λrc) to the rotor flux summation device rotor flux estimate 28. The values for these two commands may be derived by manual operator input, derived according to the methods described in Patent No. 120', or derived by other automated means. The supervisory controller 38 receives signals representing estimates of the motor speed at the motor shaft (Ume) and the mechanical torque of the motor shaft (Tme). The supervisory controller 38 also sets programmable operating variables (designated by the letter “P” in a circuit ), for interface devices 24, motor vector model 26, rotor flux estimate 30, motor speed error control 34 and, when To use it, the motor torque evaluation method is torque linearizer 36. Programmable operating variables include acquired values, filtering parameters, lookout table values, drive variables, and the like. The supervisory control device 38 may also receive a variety of other inputs from the error control unit 20 and the system 10 generates a variety of other outputs to the system 10.

During operation, the motor 12 is started within system 10 while the rotor flux command (λrc) is maintained at or near zero. Some flux is generally necessary to produce sufficient motor voltage (Im) for the motor vector model 26 to be able to estimate the motor speed (Ume). The stator speed of the motor 16 is determined by the motor vector unit 26 using motor voltage measurement signals (Vmm) and (Imm), which represent the voltages (Vm) and currents (Im) respectively of the cable phases and the motor 16. Initially, the supervisory controller 38 also limits the calculated value of the motor 16 speed reduction to zero (0) so that the calculated estimate of the motor speed (Ume) is the same as the electrical rotational speed of the stator of the motor 16 and, for this reason, is also proportional. With the rotational speed of the engine 12.

The motor vector model 26 provides the motor speed estimate (Ume) to the supervisory controller 38. Once the motor speed estimate (Ume) reaches a predetermined or desired level, the control escalates to the rotor flux command. (λrc) at a desired rate. When a predetermined or desired level is specified, the control method 38 may enter or use a constant variable or one or more lookup tables. When a lookup table is used, the supervisory controller 38 motor speed estimate (Ume) is used to locate the appropriate rotor flux estimate and corresponding command (λrc) on the lookup table. At this time, the supervisory controller 38, at a controlled rate, increases the limit on the calculated reduction value of the motor 16 such that the calculated estimate of the motor speed (Ume) becomes the actual rotational speed of the motor 16 and pump 18.

The rotor flux summation device 28 compares the rotor flux estimate (λrc) command with the rotor flux estimate (λre) provided by the motor vector model 26.

Based on this comparison, the rotor flux summation device 28 generates an error estimate (λee). The rotor flux estimate 30 is used where the flux excitation order (λec) is determined. The excitation current command (λec) then passes through the interface devices 24 and is received by the engine 14 as an excitation current command (Eic). An excitation current command (Eic) is received by the field current and is determined by the generator engine 14 and changes the voltages (Vm) and currents (Im) of the phases of the cables and motor 16. The process described above is repeated at a predetermined frequency. Accordingly, a feedback loop is triggered for the first control.

At all times during operation, the motor speed summation device 32 compares the motor speed command (Umc) with the motor speed estimate (Ume) received from the motor vector model 26 and generates a speed error estimate (Uee).

Next, the speed error estimate (Uee) is used by the motor speed error control 34 to generate the motor torque linearizer (Tec). The torque linearizer (Tec) command is converted to a linear command by the torque linearizer 36 (when using the calendar) to generate the torque linearizer (Tlc) command. The torque rectification (Tlc) command is transmitted through the interface devices 24 and the throttle position (Txc) command is produced and communicated to the motor 12. The process described above is repeated at a predetermined frequency. Thus, a second control feedback loop is triggered.

The motor 16 operating the pump 18 operates based on the throttle position command (Txc) and the excitation current command (Eic). Thus, pump speed (Up) and pump torque (Tp) are precisely controlled and pump head (Hp) and pump flow (Qp) are accurately controlled or stabilized according to the need of the specific application using first and second control feedback loops.

Those of skill in the art will recognize that the invention overcomes deficiencies in the prior art, such as Pettigrew's, to provide control of variable motor speed or significantly reduce the possibility of engine control not translating to desired motor and pump operation. Indeed, a representative embodiment of system 10 of Fig. 1, according to the invention, is based on motor torque control linearizer rather than the often flawed assumption that the desired frequency output from the generator results in the desired operation of the motor and pump as in prior systems.

As an additional advantage, the present invention, though the specific control and condensation of the rotating flux, reduces the peak starting current drawn by the motor and, as such, the current that must be supplied to the generator. In previous systems where the generator is operated at a frequency and voltage and then the motor is started "over the line," the motor current rises at a value as much as one thousand percent (1000%) of the motor's operating current. To handle the aforementioned large starting currents, previous systems using this approach needed many components in the system to be much larger and more accurate than they would otherwise be. The present invention reduces the peak current to less than one hundred and fifty percent (150%) of the normal operating current of the motor, thereby greatly reducing the weight, complexity and cost of the components required to apply the invention.

In order to test and evaluate System 10 (or a similar equivalent), a computer simulation was performed using the Simulink program developed by MathWorks. The simulation setup and structure are depicted schematically in the block diagram 40 of Figure 2.

The engine represented in the simulation was a four-cylinder engine from Simulink Demonstration Parts. In addition, motor speed error control 34 and rotor flux estimate 30 were proportional-integral (PI) control methods in the simulation. The simulation resulted in the results shown in Figures 3a to 3d. As can be seen in Figure 3B, the peak current in the rotating engine and the static engine during operation reached approximately 20 amps, while the peak currents in the steady state reached approximately 14 amps. In Figure 3c, the rotor flux estimate for the engine (solid line) smoothly overlies the rotor flux order (dashed line). Moreover, in Figure 3d, the motor speed (solid line) converged and continued at the commanded speed (dashed line).

All references, including publications, patent applications, and patents referenced herein are herein incorporated by reference as if they were individually cited by reference and are provided in their entirety herein.

[Definite and indefinite articles in the context of describing the invention (especially in the context of the following elements of protection) should be interpreted as including the singular and the plural, unless otherwise indicated in this application or the context expressly opposes this. The terms “including”, “thereby”, “includes”, and “containing” should be interpreted as having no fixed endings (i.e. meaning “including”, but not exclusively) unless otherwise indicated. The explanation of ranges of values in this application is intended only to provide a shorthand method of indicating individually each discrete value that falls within the range. Unless otherwise indicated herein, each discrete value is included in the specification as if it were mentioned individually in this application. the demand.

All methods described herein may be performed in any appropriate order unless otherwise indicated herein or the context expressly contravenes. The use of any and all examples, or representative language (e.g., “such as”) that are provided herein, is intended to further illustrate the invention and does not represent any limitation on the scope of the invention unless otherwise protected.

None of the language used in the specification should be construed as referring to any non-protected element that is essential to the implementation of the invention.

Preferred embodiments of the present invention are described herein, including the best known embodiment of the inventors for implementing the invention. Those of ordinary skill in the art may recognize alternative images of these preferred models upon reading the preceding description. The inventors expect that those of skill in the art will make appropriate use of such alternative forms, and it is the inventors' intent that the invention be implemented other than as specified herein. Therefore, the present invention incorporates all modifications and equivalents of the material described in the attached claims as permitted by applicable law. Furthermore, the invention includes any combination of the elements described above in all of their possible alternative forms unless otherwise indicated herein or the context expressly conflicts otherwise.