US20150211512A1

US20150211512A1 - System and method for driving multiple pumps electrically with a single prime mover

Info

Publication number: US20150211512A1
Application number: US14/166,991
Authority: US
Inventors: Herman Lucas Norbert Wiegman; Paul Robert Gemin; Benjamin Earl Ross; Stephen Duane Sanborn; Robert Earl McDowell
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2014-01-29
Filing date: 2014-01-29
Publication date: 2015-07-30

Abstract

A converterless motor-driven pump system includes an off-grid prime mover. The prime mover includes a rotational driveshaft that operates in response to a throttle or fuel input controller to control a rotation speed of the prime mover driveshaft. Operation of the throttle or fuel input controller is based on desired output characteristics of a pumping load. One or more electric power generators are driven by the off-grid prime mover to generate AC or DC power on an electrical bus shared by a plurality of variable speed electric motors. A plurality of pumps is connected to a common manifold shared by the plurality of pumps. The plurality of pumps is driven by the plurality of variable speed electric motors to generate a desired wellhead pressure or pumping load flow rate via the shared common manifold.

Description

BACKGROUND

The subject matter of this disclosure relates generally to motor-driven pumps, and more particularly, to a multiplicity of motor-driven pumps being fed from a singularity of generators tied to a single, throttle controlled prime mover.
A common way to drive multiple pumps uses multiple prime movers. The gas fracking industry, for example, generally employs a single diesel engine powering a single pump through a mechanical gear box. This technique employs many engines, gearboxes and pumps on many trailers, resulting in a very busy and very crowded pumping structure.
FIG. 1 illustrates a conventional system 1 that is known in the gas fracking industry for operating a pump. The system 1 uses a single prime mover including a diesel engine 2 that is fueled by either natural gas or diesel fuel to drive a fracking pump 3. Each fracking pump 3 is mechanically coupled via a transmission and corresponding gearbox 5 to a single reciprocating pump 3 to achieve optimization of the pumping system 1. System 1 is generally disposed on a single truck trailer. A plurality of such pump systems 1 together operate to provide a combined pressure in a common high pressure manifold 6 that supplies a desired pressure to a wellhead. A typical application may include about 16 such truck trailers, each including system 1, and that are backed up to one well head for fracking.
Another approach to driving pumps is through the use of electric motor driven pumps such as electric submersible pumps (ESPs). A conventional system in the oil and gas industry employs a variable speed drive (VSD) that is fed by a fixed frequency AC supply to drive a single ESP. The VSD synthesizes voltages and currents of such frequency as is necessary to operate the pump in the desired manner. In the oil and gas industry, the voltage output by the VSD is usually stepped up to a medium voltage using a transformer because high voltage motors are deployed in wells to reduce the size of the power cable needed to supply the motor. Similar to the gas fracking industry that generally requires each pump have a dedicated diesel engine, each ESP generally requires a dedicated VSD.
FIG. 2 illustrates a conventional system 10 that is known in the oil and gas industry for operating electric submersible pumps (ESPs) 12 in an off-grid application. One or more prime movers that are directly coupled to generators 14 produce an AC voltage having a fixed frequency and amplitude to supply one or more electrical loads 15. The prime mover(s) may comprise, for example, a reciprocating engine that is fueled by either natural gas or diesel fuel, or a turbine. The generated AC power is fed to a VSD 16 that is responsible for regulating the operation of the ESP 12 subsequent to stepping up the AC voltage to a medium voltage level that is supplied to ESP motor 18 via a suitable transformer 19.
In view of the foregoing, there is a need in the gas fracking industry to provide a pumping system that is less complex, less costly, and that has a smaller footprint. The pumping system should eliminate the necessity for using a plurality of diesel engines to power a plurality of pumps, and should also operate without the need to employ VSDs or other electronic controls to monitor and vary the voltage/frequency to the load.

BRIEF DESCRIPTION

According to one embodiment, a motor-driven pump system comprises:

- a single off-grid prime mover, the single off-grid prime mover comprising a rotational driveshaft and operating in response to a throttle or fuel input controller to control a rotation speed of the prime mover driveshaft;
- one or more electric power generators driven by the single off-grid prime mover rotational driveshaft to generate AC or DC power on a shared electrical bus;
- a plurality of variable speed motors configured to receive electric power via the shared electrical bus; and
- a plurality of pumps connected to a shared high pressure manifold and driven by the plurality of variable speed motors to generate a desired wellhead pressure via the shared high pressure manifold.

According to another embodiment, a method of operating a motor-driven pump system comprises:

- controlling a driveshaft rotation speed of an off-grid prime mover in response to a throttle or fuel input controller, wherein operation of the throttle or fuel input controller is based on known characteristics of a pumping load;
- controlling AC or DC power supplied to a shared electrical bus from at least one electric power generator in response to a driveshaft rotation speed of the off-grid prime mover;
- controlling a speed of a plurality of variable speed motors directly in response to the AC or DC power on the electrical bus shared by the plurality of variable speed motors; and
- generating a desired wellhead pressure or pumping load flow rate via a plurality of pumps driven by the plurality of variable speed motors and connected to a high pressure manifold shared by the plurality of pumps.

DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, wherein:

FIG. 1 illustrates a conventional engine-gearbox-pump system that is known in the gas fracking industry;

FIG. 2 illustrates a conventional electrical submersible pump (ESP) system that is known in the oil and gas industry;

FIG. 3 illustrates multiple motor-driven pumps powered from a shared electrical bus that is supplied by a single prime mover/generator according to one embodiment;

FIG. 4 illustrates multiple groups of motor-driven pumps, each group powered from a corresponding shared electrical bus that is supplied by a single prime mover/generator according to another embodiment; and

FIG. 5 is a graph illustrating control characteristics for one embodiment of a gas turbine engine/prime mover for both a conventional single speed (60 Hz) mode and a variable speed mode using direct electrical coupling to a plurality of motors.

While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.

DETAILED DESCRIPTION

The embodiments described herein are directed to control of motor-driven pumps in applications that are operating independently of a utility power grid, and combine the control of a single prime mover and one or more AC or DC generators to provide electric power to a plurality of motor-driven pumps in a manner that reduces system complexity, cost and footprint size. Such embodiments are particularly useful in the gas fracking industry where the usual control objective is to regulate wellhead pressures and/or flow rates.
FIG. 3 is a motor-driven pumping system 20 illustrating a plurality of motor-driven pumps 22, according to one embodiment. Each pump 22 is driven by a corresponding electric motor 24. The electric motors 24 are powered from a shared electrical bus 26 that is supplied by a single prime mover 28 mechanically coupled to one or more electric generators 30. Although shown as an AC powered system, the principles described herein are just as easily applied to a DC powered system in which motor winding field/ voltage regulator controls 32, 38 are not required.
The prime mover 28 may comprise a fuel based engine or other controllable source of rotational energy. In some applications, the prime mover 28 may comprise, without limitation, a gas turbine generator, a diesel engine, or a reciprocating engine that is fueled by either natural gas or diesel fuel. Although only a single prime mover 28 is depicted in FIG. 2, other applications may comprise multiple prime movers 28 so long as each prime mover 28 and associated generator(s) 30 supplies power to a corresponding plurality of motor-driven pumps on a corresponding shared electrical bus, such as the embodiment described herein with reference to FIG. 3.
According to one embodiment, the electric generator 30 is an AC generator comprising a field regulator 32. According to another aspect, the generator 22 may be a permanent magnet generator that does not require excitation, and therefore does not require a field/voltage regulator, as stated herein. It can be appreciated that use of a permanent magnet generator would further simplify the motor-driven pumping system 20 without sacrificing performance. The electric generator 30 then generates AC or DC power, depending on the particular application. The magnitude of the voltage output by the electric generator 30 is generally, but not necessarily, proportional to the rotation speed of the prime mover driveshaft 34.
Further, the electric generator 30 is electrically coupled to the plurality of respective AC or DC electric motors 24 via the shared electrical bus 26 that is common to the plurality of electric motors 24. The motor-driven pumping system 20 may comprise more than one electric generator 22, depending on the particular application. Regardless, any plurality of electric generators 22 that may be employed function together as a singularity of generators to supply electric power to the shared electric bus 26. In this way, each motor operates at the same frequency and/or voltage to supply power to the plurality of pumps 22 such that the plurality of pumps 22 together control a single wellhead pressure or pumping load flow rate.
According to one aspect, the need for variable speed and output pumping power is accomplished by control of the prime mover's throttle or fuel input controller/mechanism 36. The electric generator's voltage control element/regulator 32 is locally implemented and utilizes the rotation speed of the prime mover's driveshaft as a control input via a generator control element 38. Although described in terms of a single prime mover 28 and a single electric generator 30, the principles described herein apply as well to a plurality of prime movers, generators, load motors and rotational loads/pumps, such described herein with reference to FIG. 3. It can be appreciated that such elements may be mechanically connected in various fashions, but the common electrical connection/shared bus 26 is assumed; and pumping power/motors 24 are controlled through the prime mover(s) throttle control/mechanism 36 through the use of feedback and/or feedforward information such as, without limitation, wellhead pressure, and/or pumping load flow.
According to one aspect, the throttle control/mechanism 36 may be locally operated manually by an operator that has knowledge of pumping load characteristics, allowing the operator to manually control the electrical frequency or pumping speed in which the power to the load is approximated by a quadratic function of the electrical frequency or pumping speed. FIG. 5 is a graph 90 illustrating control characteristics for one embodiment of a gas turbine engine/prime mover for both a conventional single speed (60 Hz) mode and a variable speed mode using direct electrical coupling to a plurality of motors. The prime mover output power 92 and fuel efficiency 94 change in response to the speed of the prime mover. Both the output power 92 and fuel efficiency 94 are affected in different ways as depicted for variable speed operation 96 and constant speed operation 98. It can be appreciated that DC voltage effects DC motor speed and pump speed when using DC motors.
According to another aspect, the throttle control/mechanism 36 may be remotely operated from a programmable controller/computer operating center 31 where an operator may transmit a command to the throttle control/mechanism 36, allowing the operator to control the wellhead pressure and/or pumping load flow rate remotely. According to another aspect, the prime mover throttle control/mechanism 36 relies on feedback information from the electrical generator(s) 30, the variable speed motor(s) 24, and/or the pump(s) 22 in order to control the wellhead pressure and/or pumping load flow rate(s).
The motor-driven pumping system 20 thus uses and controls a plurality of pumps 22 to achieve a desired pressurization and hydraulic fracturing of a gas well. According to one aspect, the motor-driven pumping system 20 is mobile via a plurality of trailers 42, and advantageously provides a small footprint near the wellhead 40 by installing two or more pumps 22 and motors 24 per trailer 42, resulting in fewer prime movers of larger power rating either near the wellhead 40, or remote from the well head site. A corresponding manual disconnect mechanism 50 is employed to couple each electric motor 24 to the shared electrical bus 26. It can be appreciated that small prime movers are more efficient and can be logistically easier to employ and maintain, particularly when using a simple electrical connection to the loads.
Depending on the selection of the prime mover(s) 28 and the generator(s) 30, it may be desirable to use a gearbox to match the shaft speeds of the prime mover(s) 28 and generator(s) 30. It is preferable to use a fixed ratio gearbox to keep the system 20 as simple as possible. According to one aspect, the motor-driven pump(s) 22 are located on the same trailer 42 transporting the pump motors 24.
According to one embodiment, each motor 24 is mechanically coupled via a transmission 44 and corresponding gearbox 46 to a single reciprocating pump 22 to achieve optimization of the motor-driven pumping system 20. The plurality of reciprocating pumps 22 together operate to provide a combined pressure in a common high pressure manifold 48 that supplies a desired pressure to the wellhead 40. The wellhead pressure may be monitored via a pressure sensor 52 at or near the wellhead. The motor-driven pumping system 20 may further employ relay and protection equipment 50 that will shut down the system 20 due to predetermined overload/fault conditions that may occur during operation of the system 20.
The motor-driven pumping system 20 advantageously i) eliminates the need for a variable speed drive or a plurality of variable speed drives and transformer, simplifying the system, resulting in improved system reliability, ii) can optionally use pumped gas via the system pump(s) 22 as the fuel to run the prime mover(s) 28, resulting in very low fuel costs, iii) operates independently of a utility power grid; iv) eliminates the need for a multi-speed gear box between the engine prime mover(s) 28 and the pump(s) 22; v) eliminates a multitude of engine-gearbox-pump systems in favor of a single large prime mover 28 having a single control point; and vi) allows for a large prime mover 28 to be located some distance away from the pump(s) 22 and wellhead 40, thus giving more access to productive equipment e.g. slurry distribution, pumps, valves, safety equipment, etc.
It can be appreciated that there may be reasons to retain a transformer between the generator 28 and the motor driven pump(s) 22. Such reasons may include, without limitation, minimizing system cost and/or maximizing operational flexibility. According to one aspect, the decision to retain or remove the transformer(s) from the system 20 may be made on the basis of system optimization rather than conceptual operation of the system 20.
It can further be appreciated that the pump motor(s) 22 may be any electric motor that can be line started, including not only induction motors, but also a special class of permanent magnet motors known as line-start permanent magnet motors.
FIG. 3 is a simplified system diagram illustrating a motor-driven pumping system 60 using multiple groups 62, 64 of motor-driven pumps 22, each group 62, 64 of pumps 22 powered from a corresponding shared electrical bus 66, 68 that are each supplied by a corresponding single prime mover/ generator combination 70, 72 and 74, 76, according to another embodiment. A programmable controller 80 is programmed to monitor desired operating conditions such as, without limitation, well head pressure(s) and/or pumping load flow rates. The programmable controller 80 communicates control signals to the corresponding throttle or fuel input controls/ mechanisms 36, 37 that control the fuel flow to the respective prime movers 70, 74.
Each prime mover/ generator combination 70, 72 and 74, 76 may be controlled independently of each other based on control signals received from the programmable controller 80 that may be implemented as a single control unit or may be implemented in numerous ways as a distributed controller, based on the particular application. In this way, one group 62 of motor-driven pumps 22 can function to generate a desired wellhead pressure for a first gas well; while a second group 64 of motor-driven pumps 22 can function to generate a desired wellhead pressure for a second gas well independent of the first group 62 of motor-driven pumps 22. The number of motor-driven pump groups is limited only by the particular application, and is theoretically unlimited. According to one embodiment, the programmable controller 80 receives a single feedback signal from each group 62, 64 of motor-driven pumps 22 rather than a plurality of signals comprising a signal associated with each individual motor-driven pump 22. In this way, the programmable controller 80 controls each group 62, 64 of motor-driven pumps 22 based on a single feedback signal to achieve a desired pumping load characteristic such as, without limitation, wellhead pressure or pumping load flow rate associated with the corresponding high pressure manifold 47, 49. Thus, control of each group 62, 64 of motor-driven pumps 22 is based on a corresponding feedback signal that is common to all of the motors 24 and pumps 22 within the corresponding group.
The programmed controller 80 may include a processor and a memory device. The processor includes any suitable programmable circuit which may include one or more systems and microcontrollers, microprocessors, reduced instruction set circuits (RISC), digital signal processors (DSPs), application specific integrated circuits (ASIC), programmable logic circuits (PLC), field programmable gate arrays (FPGA), and any other circuit capable of executing the functions described herein. The above examples are exemplary only, and thus are not intended to limit in any way the definition and/or meaning of the term “processor.” The memory device includes a computer readable medium, such as, without limitation, random access memory (RAM), flash memory, a hard disk drive, a solid state drive, a diskette, a flash drive, a compact disc, a digital video disc, and/or any suitable device that enables the processor to store, retrieve, and/or execute instructions and/or data.
In the exemplary embodiment depicted in FIG. 3, programmed controller 80 includes a plurality of control interfaces 82, 84 that are coupled to prime mover throttle or fuel input controls/ mechanisms 36, 37 to control a fuel flow rate for each respective prime mover 70, 74. In addition, programmed controller 80 also includes a sensor interface 86 that is coupled to at least one sensor 52 such as shown and described with reference to FIG. 2. Each sensor 52 may transmit a signal continuously, periodically, or only once and/or any other signal timing that enables programmed controller 80 to function as described herein. Moreover, each sensor 52 may transmit a signal either in an analog form or in a digital form.
Programmed controller 80 may also include a display and a user interface. The display, according to one embodiment, includes a vacuum fluorescent display (VFD) and/or one or more light-emitting diodes (LED). Additionally or alternatively, the display may include, without limitation, a liquid crystal display (LCD), a cathode ray tube (CRT), a plasma display, and/or any suitable visual output device capable of displaying graphical data and/or text to a user.
Various connections are available between the programmed controller 80 and each throttle or fuel input control/ mechanism 36, 37. Such connections may include, without limitation, an electrical conductor, a low-level serial data connection, such as Recommended Standard (RS) 232 or RS-485, a high-level serial data connection, such as Universal Serial Bus (USB) or Institute of Electrical and Electronics Engineers (IEEE) 1394 (a/k/a FIRE WIRE), a parallel data connection, such as IEEE 1284 or IEEE 488, a short-range wireless communication channel such as BLUETOOTH, and/or a private network connection, whether wired or wireless.
In summary explanation, a motor-driven pumping system 20, 60 eliminates the need for a variable speed drive and, potentially, its associated transformer from a motor-driven pump system, resulting in a simpler system that reduces capital expense, weight and system footprint. The motor-driven pumping system 20, 60 employs a remotely located prime mover 28, improving access to the well head 40 region for fracking equipment and safety. The use of power generated on-site advantageously reduces the time it takes to put a well into production resulting from delays in getting the utility to install requisite power lines. Further, the use of natural gas produced by the well itself advantageously reduces the operating expense. The motor-driven pumping system 20, 60 is particularly advantageous in that a single prime mover controls a plurality of motor-driven pumps via a shared electrical bus and a shared high pressure manifold to manage a wellhead pressure and/or pumping load flow rate associated with a gas well that is common to all the motor-driven pumps.
Since the output of the generator(s) 30, 72, 76 is substantially sinusoidal when compared with the output of a variable speed drive, a filter is not required between the generator(s) and the motor(s) 24. The output of a variable VSD, for example, contains significant high frequency content, the result of chopping up DC voltage/current to produce AC voltage/current. This chopping action disadvantageously creates high frequency components called harmonics that are detrimental to the motor driving the pump. A filter is usually installed between the VSD and the motor; however, anecdotal data suggest that even such a filter may not always adequately filter out the harmonics, leading to accelerated aging of the insulation systems in the transformer, cable, and motor. This disadvantageously reduces the life of the system
A VSD also draws nonsinusoidal currents from its supply, unless an active front end is applied to the VSD. These resulting harmonics are detrimental to the generator supplying the VSD. Many system designs oversize the generator so that it can better tolerate the harmonic currents drawn by the VSD. Other system designs will use an active power filter to source the harmonic currents drawn by the VSD, thereby alleviating the generator from having to supply them. Either of such approaches adds to the cost and complexity of the system. Further, VFD's occupy space near the wellhead 40, and can be a reliability problem.
The principles described herein with reference to the various embodiments include reduced capital expense and more timely well production. The off-grid motor-driven system embodiments described herein advantageously allow putting a well into production sooner since there is frequently a substantial waiting period for the utility to install supply lines to the well site, as stated herein. At such time as utility power is available, the well operator can remove the prime mover and generator, replacing them with a variable speed drive and transformers if desired.
However, this is typically not done in the gas fracking industry since fracking is done by bringing in tractor trailer trucks with a diesel-transmission-pump system on each bed 42. Such tractor trailer trucks consume substantial amounts of fuel and emit substantial amounts of smoke and noise. It is easily appreciated that a wellhead 40 may get very crowded with, for example, 10-12 fracking trucks in close proximity to the wellhead 40.
Looking again at FIG. 2, the prime mover driveshaft 34 is coupled directly or indirectly to the generator 30; while the generator 30 is electrically coupled to a plurality of motors 24 that may be line start motors such as induction motors, permanent magnet motors, or DC motors used in association with a DC generator system, via a shared electrical bus 26; and each motor driveshaft is directly or indirectly coupled to a corresponding well pump 22, as depicted in FIG. 2. The prime mover 28 is turned-on to rotate its driveshaft 34, causing the generator 30 to produce AC or DC power sufficient to power the plurality of motors 24, that subsequently drive the plurality of well pumps 22. It can be appreciated that if the generator derives variable DC electricity, the motor(s) must be a DC motor(s); and that if the generator produces variable frequency AC, the motor(s) can be either an induction, permanent magnet or wound field synchronous motor(s).
The present inventors realized that the engine throttle control element 36 is the only control element necessary to control the plurality of well pumps 22, recognizing the nature of the pumping load, where power to the load is typically a quadratic function of speed/frequency. Therefore, the engine throttle control element 36 may be controlled in an open loop mode, a closed loop mode, or a combination thereof, either manually or automatically, in which the engine throttle setting is based on a desire to do more or less pumping at the load. The desire to do more or less work is based on the nature of the pumping load, as stated herein, and can be compared with driving a motor vehicle where in order to get home faster, one need only press down harder on the accelerator (throttle). It is not necessary to measure the output power and precisely set the accelerator (throttle). The speed of the motor vehicle is regulated by approximately pressing on the accelerator.
The engine throttle control element 36, when operated in a closed loop mode, may operate in response to one or more desired operational characteristics, including without limitation, electric motor voltage, electric motor frequency, pump speed/rotational frequency, wellhead pressure(s), pumping load flow rates, and pump operating point(s), among others.
Since some applications may employ a permanent magnet generator that does not require excitation, it can be appreciated that a generator exciter will not be required in such applications. The use of a permanent magnet generator further simplifies the motor-driven pumping system 20 without sacrificing performance, as stated herein.
Although particular embodiments have been described herein with application to electric motor-driven gas well fracking pumps, the principles described herein can just as easily be applied to other applications including without limitation, geothermal applications. In such applications, gas turbines, reciprocating engines, or other rotational energy sources can be employed to rotate the generator.
The programmed controller 80 may further be configured with synchronization logic and programmed according to yet another embodiment to generate a control signal that activates an auxiliary/spare generator to provide a parallel operation capability.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.

Claims

1. A motor-driven pump system, comprising:

a single off-grid prime mover, the single off-grid prime mover comprising a rotational driveshaft and operating in response to a throttle or fuel input controller to control a rotation speed of the prime mover driveshaft;

one or more electric power generators driven by the single off-grid prime mover rotational driveshaft to generate AC or DC power on a shared electrical bus;

a plurality of variable speed motors configured to receive electric power via the shared electrical bus; and

a plurality of pumps connected to a shared high pressure manifold and driven by the plurality of variable speed motors to generate a desired wellhead pressure via the shared high pressure manifold.

2. The motor-driven pump system according to claim 1, wherein the throttle or fuel input controller is integrated with the motor-driven pump system.

3. The motor-driven pump system according to claim 1, wherein the throttle or fuel input controller is remote from the motor-driven pump system.

4. The motor-driven pump system according to claim 1, wherein the throttle or fuel input controller is a manually operated throttle or fuel input controller.

5. The motor-driven pump system according to claim 1, wherein the throttle or fuel input controller is an automated throttle or fuel input controller.

6. The motor-driven pump system according to claim 1, wherein the single off-grid prime mover comprises at least one of a reciprocating engine, a turbine, or a rotational energy source.

7. The motor-driven pump system according to claim 1, wherein the one or more electric power generators comprise at least one of a permanent magnet generator, a wound-field synchronous generator, a DC generator, an induction generator, a synchronous reluctance generator, a homopolar induction alternator, or an AC generator.

8. The motor-driven pump system according to claim 7, further comprising a generator exciter for providing excitation to the generator.

9. The motor-driven pump system according to claim 8, further comprising a flow sensor configured to monitor a flow rate associated with the plurality of pumps and to generate a sensor signal in response thereto.

10. The motor-driven pump system according to claim 9, wherein the throttle or fuel input controller is programmed to control the excitation of the generator exciter in response to the flow sensor signal.

11. The motor-driven pump system according to claim 1, further comprising at least one pressure sensor configured to monitor the wellhead pressure.

12. A method of operating a motor-driven pump system, the method comprising:

controlling a driveshaft rotation speed of an off-grid prime mover in response to a throttle or fuel input controller, wherein operation of the throttle or fuel input controller is based on known characteristics of a pumping load;

controlling AC or DC power supplied to a shared electrical bus from at least one electric power generator in response to a driveshaft rotation speed of the off-grid prime mover;

controlling a speed of a plurality of variable speed motors directly in response to the AC or DC power on the electrical bus shared by the plurality of variable speed motors; and

generating a desired wellhead pressure or pumping load flow rate via a plurality of pumps driven by the plurality of variable speed motors and connected to a high pressure manifold shared by the plurality of pumps.

13. The method of operating a motor-driven pump system according to claim 12, wherein controlling a driveshaft rotation speed of an off-grid prime mover comprises controlling a driveshaft rotation speed of at least one of a reciprocating engine, a turbine engine, or a rotational energy source.

14. The method of operating a motor-driven pump system according to claim 12, wherein controlling the AC or DC power supplied to the shared electrical bus comprises controlling an AC power output of at least one of a wound-field synchronous generator, a permanent magnet generator, a DC generator, an induction generator, a synchronous reluctance generator, a homopolar induction alternator, or an AC generator.

15. The method of operating a motor-driven pump system according to claim 14, wherein controlling an AC power output of the wound-field synchronous generator comprises controlling an AC power output of a wound-field exciter.

16. The method of operating a motor-driven pump system according to claim 15, further comprising controlling the AC power output of the wound-field exciter in response to the driveshaft rotation speed of the off-grid prime mover.

17. The method of operating a motor-driven pump system according to claim 14, further comprising controlling the AC power output of the permanent magnet generator in response to the driveshaft rotation speed of the off-grid prime mover.

18. The method of operating a motor-driven pump system according to claim 12, further comprising linking the generated AC or DC power to the variable speed motor via a transformer.

19. The method of operating a motor-driven pump system according to claim 12, further comprising operating the throttle or fuel input controller in response to known characteristics of a pumping load, wherein the known characteristics of the pumping load are a quadratic function of motor speed.