LINEAR PUMPING SYSTEM AND METHODS FOR CONTROLLING THE SAME

Abstract

An example linear pumping system suitable for small size and low flow rate operation. The system includes a pump casing that encloses a linear motor and one or two (an upper and lower) pumps. The pump casing may include an upper fluid inlet port, a lower fluid inlet port, and a fluid-outlet port. The linear motor includes a stator and a mover, where the mover includes an upper plunger portion and a lower plunger portion and is configured to move alternately up and down relative to the stator. The upper and lower pumps positioned on both sides of the mover may each include a pump chamber, a fluid inlet valve, and a fluid outlet valve. In operation, hydraulic pressure from the upper and lower pump outlet valves causes fluid within an interior cavity of the pump casing to be expelled through the fluid outlet port. The motion of the mover can be controlled using pressure and/or plunger position measurements.

Claims

1. A linear pumping system, comprising: (a) a pump casing having an interior cavity and at least one fluid inlet port receiving fluid from outside the pump casing, and at least one fluid outlet port expelling fluid from the interior cavity to the outside of the pump casing; (b) a linear motor enclosed within the pump casing and including a stator and a mover, the mover including a plunger and being configured to move alternately up and down with respect to the stator; and (c) a first pump enclosed within the pump casing and including a first pump chamber, a first pump fluid inlet valve configured to receive fluid into the first pump chamber from outside the pump casing through the at least one fluid inlet port of the pump casing, and a first pump fluid outlet valve configured to expel fluid from the first pump chamber into the interior cavity of the pump casing, wherein the first pump chamber is configured to receive a top portion of the plunger of the linear motor mover, the received top plunger portion causing fluid to be drawn into the first pump chamber through the first pump inlet valve on a downstroke of the linear motor mover, and causing fluid to be expelled from the first pump chamber into the interior cavity of the pump casing through the first pump outlet valve on an upstroke of the linear motor mover; and wherein hydraulic pressure from the first pump causes fluid within the interior cavity of the pump casing to be expelled through the fluid outlet port of the pump casing.

2. The linear pumping system of claim 1, further comprising (d) a second pump enclosed within the pump casing and including a second pump chamber, a second pump fluid inlet valve configured to receive fluid into the second pump chamber from outside the pump casing through the at least one fluid inlet port of the pump casing, and a second pump fluid outlet valve configured to expel fluid from the second pump chamber into the interior cavity of the pump casing, wherein the second pump chamber is configured to receive a bottom portion of the plunger of the linear motor mover, the received bottom plunger portion causing fluid to be drawn into the second pump chamber through the second pump inlet valve on an upstroke of the linear motor mover, and causing fluid to be expelled from the second pump chamber into the interior cavity of the pump casing through the second pump outlet valve on a downstroke of the linear motor mover; and wherein hydraulic pressure from the first pump and the second pump causes fluid within the interior cavity of the pump casing to be expelled through the fluid outlet port of the pump casing.

3. The linear pumping system of claim 1, wherein the linear motor further includes an expandable diaphragm that seals oil within the linear motor and is configured to allow the oil within the linear motor to expand or contract with temperature.

4. The linear pumping system of claim 2, wherein the first and second pump fluid inlet valves and first and second pump fluid outlet valves are one-way valves that are configured to operate in any orientation.

5. The linear pumping system of claim 4, wherein at least one of the first and second pump fluid inlet valves and first and second pump fluid outlet valves comprises a ball valve having a buoyant ball.

6. The linear pumping system of claim 4, wherein at least one of the first and second pump fluid inlet valves and the first and second pump outlet valves comprises a ball valve having a metallic ball and a magnetic valve seat.

7. The linear pumping system of claim 2, wherein the first and second pump fluid inlet ports are configured to receive fluid in a direction that is substantially perpendicular to a lengthwise direction of the first and second pump chambers.

8. The linear pumping system of claim 2, further comprising one or more pressure sensors configured to measure pressure within at least one of the first and second pump chambers.

9. The linear pumping system of claim 8, further comprising a motor controller that is configured to control movement of the linear motor based at least in part on pressure measured by the one or more pressure sensors.

10. The linear pumping system of claim 9, wherein the motor controller is configured to control one or more motion parameters of the linear motor dependent on the pressure measured within at least one of the first or second pump chambers.

11. The linear pumping system of claim 2, further comprising one or more sensors configured to determine a location of the mover in relation to at least one of the first and second pump chambers.

12. The linear pumping system of claim 11, further comprising a motor controller configured to control a stroke length of the linear motor based at least in part on the position of the mover.

13. The linear pumping system of claim 1, further comprising one or more temperature sensors configured to measure an operating temperature of the linear motor.

14. The linear pumping system of claim 13, further comprising a motor controller that is configured to control one or more motion parameters of the linear motor based at least in part on the measured operating temperature of the linear motor.

15. The linear pumping system of claim 14, wherein the motor controller is configured to stop movement of the linear motor if the measured operating temperature of the linear motor reaches a predetermined threshold.

16. A method of controlling operation of a linear pumping system for use in an oil or gas well, the linear pumping system including a linear motor and one or more pump chambers, comprising: measuring pressure within the one or more pump chambers of the linear pumping system; measuring a position of a plunger rod of the linear motor in relation to the one or more pump chambers; and controlling movement of the plunger rod within the one or more pump chambers based at least in part on the measured pressure and position.

17. The method of claim 16, wherein a stroke length of the linear motor is controlled based at least in part on the measured position to cause a maximum penetration of the plunger rod within the one or more pump chambers.

18. The method of claim 16, wherein movement of the linear motor is stopped if the measured pressure exceeds a predetermined threshold.

19. The method of claim 16, further comprising: measuring an operational temperature of the linear motor; and controlling movement of the linear motor based at least in part on the measured operational temperature.

20. The method of claim 19, wherein movement of the linear motor is stopped if the measured operational temperature exceeds a predetermined threshold.

21. The method of claim 16, further comprising the step of using at least one of (a) pressure measurements within the one or more pump chambers and (b) linear motor temperature to provide an estimate of the fluid/gas mixture in the portion of the oil or gas well where the linear pumping system is located.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 depicts an example linear pumping system for pumping fluids from an oil or gas well in accordance with this disclosure.

[0010] FIGS. 2A, 2B, 2C, and 2D depict an example of the fluid pumping action of the linear pumping system shown in FIG. 1.

[0011] FIG. 3 is a stroke-pressure diagram illustrating an example method of operating a linear pumping system to regulate pressure buildup within the pump chambers.

[0012] FIGS. 4A, 4B, and 4C are diagrams illustrating different types of linear motors that can be used in different embodiments.

[0013] FIGS. 5A and 5B illustrate an embodiment of a linear motor with a mover and stator, and the mechanism to control the motion of a mover relative to the stator.

[0014] FIGS. 6A, 6B, 6C, and 6D are a series of diagrams illustrating how the pump behavior can be determined from the piston pressure.

DETAILED DESCRIPTION

[0015] FIG. 1 depicts an example linear pumping system for pumping fluids from an oil or gas well. The left-hand portion 14 of FIG. 1 shows separate components of a partially disassembled linear pumping system, and the right-hand portion 16 of FIG. 1 depicts a fully assembled pumping system.

[0016] The example linear pumping system depicted in FIG. 1 includes a pump casing 11 that encloses a linear motor comprising stator 5 and mover 15, an upper pump 12 and a lower pump 6, such that the linear motor (including stator 5 and mover 15) is integrated into the pump assembly. The pump casing 11 includes an upper fluid inlet port 22, a lower fluid inlet port 23, and a fluid outlet port 1.

[0017] As noted, the linear motor includes a stator 5 and a mover 15. The linear motor mover 15 includes a plunger rod made of mechanically strong material that is surrounded by permanent magnets 9 at a center portion of the rod, defining an upper plunger portion 8 extending above the permanent magnets 9 and a lower plunger portion 19 extending below the permanent magnets 9. In embodiments, the plunger rod of the linear motor may have a small diameter (e.g., less than one inch) to provide a pump system that is suitable for use in deep wells at low flow rates and also pumping fluid from a small diameter oil or gas well. A person of skill in the art will appreciate that the primary advantage of a small pump is that it reduces the force the linear motor has to generate to move the pump plunger, which in turn makes the pump small and cheaper to build. In operation, the linear motor mover 15 is caused to move alternately up and down within the motor assembly when the stator 5 is coupled to a power source. The up and down movement of the linear motor mover 15 within the linear motor assembly is respectively referred to herein as the upstroke and downstroke of the linear motor.

[0018] In an embodiment, the linear motor may be an oil-filled motor assembly to improve longevity and provide a highly efficient motor with close tolerances between the mover 15 and stator 5. Oil may be contained in the motor assembly using pressure balancing diaphragms made from a soft material, such as silicone or an elastomer, to allow the oil to expand and contract with temperature. In the example illustrated in FIG. 1, oil is sealed within the motor assembly by moving seals 3, 17 that provide seals between the mover 15 and the upper and lower pump chambers 2, 13. The illustrated example further includes bellows 4, 18 that allow the oil to expand and contract with temperature within the motor chamber.

[0019] The upper and lower pumps (12 and 6, respectively) each include a fluid inlet valve 7, 10 and a fluid outlet valve 21, 20 respectively, and each define a pump chamber 2 (upper) and 13 (lower) for receiving the upper and lower plunger rod portions 8, 19 of the linear motor mover 15. Specifically, the upper pump chamber 2 is configured to receive the upper plunger portion 8 and the lower pump chamber 13 is configured to receive the lower plunger portion 19, as illustrated by the fully assembled pump shown on the right-hand side 16 of FIG. 1. In this way, a duplex pump is formed with minimal complexity and size from two simplex pumps mounted on either end of the linear motor (stator 5 and mover 15), allowing fluid to be pumped in both stroking directions.

[0020] The fluid inlet (7, 10) and outlet (21, 20) valves to the upper and lower pump chambers 2, 13 are one-way valves (i.e., non-return valves), and are preferably configured to allow the linear pumping system to operate in any orientation without being dependent on gravity. Specifically, the fluid inlet valves 7, 10 are one-way valves that are respectively coupled between the upper and lower pump chambers 2, 13 and the upper and lower fluid inlet ports at 22, 23, and are configured to receive fluid into the pump chambers 2, 13 from outside of the pump casing 11. The fluid inlet valves 7, 10 may preferably be coupled to fluid inlet ports 22, 23 that are oriented such that the fluid inlets are not in line with the length of the pumping chamber, as shown in the illustrated embodiment. The fluid outlet valves 21, 20 are one-way valves that are respectively coupled between the upper and lower pump chambers 2, 13 and an interior cavity 24 of the pump casing 11, and are configured to expel fluid from the pump chambers 2, 13 into the interior cavity 24 of the pump casing. In an embodiment, the one-way inlet (7, 10) and outlet (21, 20) valves may comprise buoyant balls in ball valves that are configured to move with the fluid without being dependent on gravity. The buoyant ball valves may, for example, utilize a retaining spring force provided by a spring return mechanism. In another example, the buoyant ball valves may include buoyant metallic balls (e.g., hollow metal balls) and magnets in the ball seats to attract the balls back into the valve seat. Other one-way valves known in the art may also be used, as appropriate.

[0021] In operation, on an upstroke of the linear motor mover 15, fluid is drawn into the lower pump chamber 13 and expelled from the upper pump chamber 2, and on a downstroke of the linear motor mover 15, fluid is drawn into the upper pump chamber 2 and expelled from the lower pump chamber 13. Fluid from both pump chambers 2, 13 is expelled into the interior cavity 24 of the pump casing 11, and the resulting hydraulic pressure causes the fluid to be expelled through the fluid outlet port 1 of the casing, for example into a fluid pipeline. The linear pump system may preferably be configured to operate in an oil or gas well at a minimum of 20 strokes per minute and a maximum of 1200 strokes per minute, such that the pump system may achieve reasonable flow rates with small diameter pumping chambers. An example of the fluid pumping action of the linear pumping system is shown in the diagrams of FIG. 2.

[0022] The example illustrated in FIGS. 2A-2D depicts the operation of a linear pumping system in four stages, where certain reference numerals used in FIG. 1 have been omitted for clarity. The linear pumping system in the illustrated example is submersed within a fluid 25, such as oil. In the first stage (FIGS. 2A to 2B) of operation, a down stroke of the linear motor causes a suction within the upper pump chamber 2 that draws fluid into the upper pump chamber 2 through the upper fluid inlet port 22. In the second stage (FIGS. 2B to 2C) of operation, an upstroke of the linear motor causes the fluid to be expelled from the upper pump chamber 2 through the one-way fluid outlet valve 21 into an interior cavity 24 of the pump casing. On the same upstroke of the linear motor in the second stage (FIGS. 2B to 2C) of operation, a suction is created in the lower pump chamber 13, drawing fluid into the lower pump chamber 13 through the lower fluid inlet port 23.

[0023] In the third stage of operation (FIGS. 2C to 2D), the next down stroke of the linear motor causes fluid to refill the upper pump chamber 2 and fluid to be expelled from the lower pump chamber 13 through the one-way fluid outlet valve 20 and into the interior cavity 24 of the pump casing. At the third stage of operation, fluid expelled from both the upper and lower pump chambers 2, 13 has intermixed within the interior cavity 24 of the pump casing, and the resulting hydraulic pressure causes fluid to exit the pump casing through the fluid outlet port 1. Similarly, in the fourth stage of operation (FIGS. 2C to 2D), the next upstroke of the linear motor causes fluid to refill the lower pump chamber 13, fluid to be expelled from the upper pump chamber 2 into the interior cavity of the pump casing 24, and fluid to exit the pump casing through the fluid outlet port 1. The linear pumping system then continues to cycle between the third and fourth operational stages (FIGS. 2C, 2D), pumping fluid out of the pump casing from the fluid outlet port 1 on each upstroke and down stroke of the linear motor.

[0024] In embodiments of the disclosure, the linear pumping system includes one or more sensors (not shown here) that are configured to measure the location of the motor mover 15 within the motor assembly. In embodiments, the linear pumping system may further include a motor controller (not illustrated) that is configured to regulate the movement and stroke of the pumping system based at least in part on the measured location of the motor mover 15. In this way, the upper and lower plunger rod portions 8, 19 may be cycled to the end of their respective pump chambers 2, 13, providing a maximum compression ratio for the pump.

[0025] In specific embodiments of the disclosure, the linear pumping system may include one or more sensors (not shown) that are configured to measure the pressure and/or temperature within the pump chambers 2, 13. In embodiments, the linear pumping system may further include a motor controller (not shown) that is configured to regulate the movement and stroke of the pumping system based at least in part on the measured pressure within the pump chambers 2, 13 in order to regulate pressure, e.g., to prevent the pump from becoming hydraulically locked. In this way, the linear pumping system may provide full compression on gassy fluids and reduced compression on fluids.

[0026] FIG. 3 is a stroke-pressure diagram illustrating an example of how a linear pumping system may be controlled to regulate pressure buildup within the pump chambers. The diagram in FIG. 3 includes an example plot of measured pressure within a pump chamber during a cycle (i.e., upstroke and downstroke) of the linear motor. The plotted solid line is an example of a normal pump cycle that includes a ramping portion 31 where pressure is building within the chamber, followed by a flowing portion 33 (i.e., the horizontal flat portion) where fluid is pumped out of the chamber. If a pump outlet becomes blocked, however, then pressure will continue to build within the pump chamber, as illustrated in FIG. 3 by the continuation of the ramping pressure shown by the dotted line 32. By monitoring the pressure within the pump chamber, the pump movement may be regulated to stop movement of the pump when the pressure reaches a predetermined threshold. In the illustrated example, pump movement is stopped at position 34 to prevent any further increase in pressure. The pump may, for example, be regulated to stroke only to position 34 to maintain the maximum possible pressure, or to stop under control of the motor controller. Details of the implementation control following this processing operation are familiar to those of skill in the art and need not be described in detail.

[0027] In another example, a feedback system similar to that shown in FIG. 3 may be used to monitor and control the motor stator temperature. The linear motor may, for example, be stopped if the stator temperature reaches a predetermined threshold, or may be regulated to slow the motor or cause the motor to operate with a reduced stroke to maintain the stator temperature below the predetermined threshold (i.e., the maximum operating temperature.) Again, details of the practical implementation of such control need not be described in detail.

[0028] In yet additional embodiments, the linear pumping system may be further configured to measure or compute the amount of gas in the pump based on the motor current and chamber pressures. In this way, the stroke length of the linear motor may be altered to better suit the fluid being pumped, allowing the linear pumping system to operate effectively with fluids of different viscosities and with varying amounts of intermixed gas.

[0029] Linear motors can be implemented with a simple coils and metal structure where the induction of large currents in the mover creates movement, a linear induction motor. A linear motor can also be implemented with a reluctance linear motor with several windings and an alternating magnetic mover so that moving the field along the length of the motor provides movement of the mover. This implementation is a synchronous linear reluctance motor. The third possibility is to incorporate permanent magnets into the mover and/or stator increasing the flux density and energy density of the motor, which is a synchronous permanent magnet linear motor. FIGS. 4A-4C illustrate this principle.

[0030] In FIG. 4A, a linear induction motor is shown with active windings in the stator 42 and a cylindrical solid metal mover 41. The motor is an induction motor inducing high currents in the solid mover. In FIG. 4B the motor consists of a stator 44 with alternating magnetic properties and a stator 43 with alternating field coils. The field in the stator coils is alternated to create a moving magnetic force which can be alternated to create linear motion, as illustrated in more detail in the following FIG. 5B. The control and switching of the stator windings has to be synchronized to the position of the mover, hence the term synchronous motor can be used.

[0031] In FIG. 4C, the motor consists of a stator 46 with alternating magnetic properties (and in one embodiment permanent magnets) and a stator 45 with alternating field coils, and permanent magnets. The field in the stator coils is alternated to create a moving magnetic force which can be alternated to create linear motion. The control and switching of the stator windings has to be synchronized to the position of the mover, hence the term synchronous permanent magnet motor can be used.

[0032] The issue of the manner in which motion can be controlled in a linear oscillating motor as shown above has several important aspects. The main areas of concern in controlling the motor motion is starting and controlling the stops and starts at the ends of the motion range.

[0033] FIG. 5A illustrates again a mover 15 and stator 5 in a specific embodiment. FIG. 5B provides a closer look at the magnetic system. In particular, as shown, the mover has several pairs of alternating magnetic elements 52 and 53 that make up the complete mover, while the stator has several alternating windings 50 and 51. It will be appreciated that in the above arrangement reversing the electrical field in windings 50 and 51 (and all the other stator windings) in anti-phase results in moving the mover in one direction or the other. To create continuous movement, the fields in windings 50 and 51 must remain in one polarity until a pair of magnetic elements in the mover moves over the windings pair. Once the pair of magnetic elements in the mover moves past the pair of windings, the fields in the stator windings must be reversed to then continue the movement to the next pair of stator field coils. Reversing the fields again continues the movement of the mover 15.

[0034] This principle of operation requires the stator fields to be reversed at a frequency dependent on the distance between the magnetic components and the linear speed of the motor. Accordingly, for this motor to operate, the stator field coils must be switched at the correct point to create a moving field which is always creating motion in one direction. When the mover approaches the end of its stroke, the rate of field switching should preferably be slowed to slow down the motion of the mover, and also finally be applied out of phase to stop the motion. Preferably, the system will ramp down the linear mover velocity toward the end of the stroke, bringing the mover to a halt a very small distance away from the end of the mechanical travel, without making contact with the end of the pump chamber. Such operation requires sensing of the position of the mover and in particular determining how close the mover is to the end of the stroke.

[0035] Sensing of the mover position is therefore important in the control of a linear synchronous motor. There are several methods of control that can be used in different embodiments. One simple control mechanism in applications where the motor will operate a long way from the power supply in a deep well, is to sense the current drawn by the stator coils. This current is proportional to the relative position of the mover magnetic circuit (whether passive, as in a reluctance motor or active with a permanent magnet motor). Thus, by sensing the current drawn by the stator it is possible to establish where the mover is relative to the stator. It is, however, problematic to determine how close the mover is to the end of travel. One possible method of establishing this end of travel position is to switch fields slowly at start up until the mover comes in contact with the end of the pump chamber, at which point the stator coil current will increase rapidly, and the mover will not move. Once the end position is known, the controller can simply count the stator current changes as the mover moves through the stator windings. Since the size of the motor is known and the number of coils is known, counting the current changes can be used as a position sensor.

[0036] In alternative embodiments, electronic position sensors may be fitted to the motor so that its absolute position and velocity can be sensed real time. This approach provides more accurate and direct feedback for controlling the motion of the mover. A person of skill in the art will appreciate details of the implementation of such control mechanisms, which are thus not discussed in further detail.

[0037] Controlling the motion of the mover in the manner described above is another aspect of this invention. In particular, in this aspect the general control mechanism which in its simplest form is used to create linear oscillating motion of the mover relative to the stator, is also further controlled to adapt the pump behavior to respond more appropriately to changing fluid conditions in the pump. The main additional aspects of control include prevention of over pressure, adapting the motor speed to react to motor winding over temperature, and dealing with gassy fluids and gases.

[0038] Over pressure can happen because of stuck valves, or heavy fluids, deep wells, etc. To address such conditions, in a specific embodiment a pressure sensor can be fitted to one or both of the pumping chambers. Data from this pressure sensor(s) can be used to regulate the motor velocity and stop the motor short of the end position to prevent over pressure. Mechanisms to take into account pressure sensor data to control the mover are described in more detail below.

[0039] In alternative embodiments, if the motor winding temperature is measured or computed, this temperature can be regulated by simply shutting the motor off if it gets too hot, or slowed down to reduce the power consumption to a level where the temperature rise becomes stable.

[0040] Additionally, it will be appreciated that gassy fluids will cause reduced pressure build up as the gas compresses with the stroking of the pump. This means that the pump and motor can move a lot faster during the compression stroke and only slow down again as the gas reaches the opening pressure of the non-return valve. With this understanding, in accordance with another embodiment the velocity profile of the pump can be changed to make it pump gassy fluids more effectively. Such a change can be implemented in practice by measuring the pressure in the pumping chamber or, where no pressure sensor is available, the current drawn by the motor. At any given depth this current will be proportional to the force developed, and so the force required to stroke the pump can be calculated from the motor current. It will also be appreciated that the motor current is also proportional to the depth of the pump and the friction in the seals, which can also be taken into account for more accurate control.

[0041] Implementation details of these and other control mechanisms will be appreciated by those of skill in the art, and employed in practical implementations without departing from the principles of this invention. The following description is intended to provide additional detail and illustration.

[0042] FIG. 6A shows one of the two identical pumping sections in FIG. 1, illustrating that the pressure at the fluid inlet port 22 may be measured using a sensor 70. Pressure in the piston chamber 2 may also be measured using a second sensor. In different embodiments, the position of the mover 8 can be measured using an additional down hole sensor 63, or computed at the surface.

[0043] FIG. 6B shows the pressure build up in the chamber with the piston moving during one stroke of the piston. In an upstroke of the piston, the fluid in the chamber is compressed, with the pressure building as the velocity of the motor increases and the pressure rises to overcome the tubing pressure, opening the upper valve. This portion of the diagram is illustrated by line 64. Then, as the valve opens, the pressure becomes steady at constant velocity, as illustrated by line 65. The pressure is directly proportional to force and so the motor current will also provide a measure of cylinder pressure, albeit also related to several other factors like friction and pump mover inertia. Accordingly, one can measure the activity in the pump chamber either directly with pressure sensors (such as 70), or indirectly via the motor current.

[0044] FIG. 6C illustrates a pump chamber with some fluid and some gas in it. This combination leads to a complex pressure stroke diagram with different segments, such as 66, 67 and 68 illustrated in FIG. 6C, corresponding to different stages in the mover stroke and fluid-gas combinations. In particular, the initial pressure rise will be slow and the piston will initially compress the gas, leading to a stroking pressure with the gas in compression and the incompressible fluid flowing. In the case of a complex fluid-gas mixture, the pressure stroke overall diagram is likely to be complex as gas under pressure will likely release rapidly through the exit valve.

[0045] FIG. 6D illustrates a chamber filled with gas, which will restrict the pressure achievable as the pump compresses the gas it may never generate enough pressure as shown by line 69 in FIG. 6D to open the valve, or it may in the final portion of the pump stroke compress the gas sufficiently to open the exit valve.

[0046] It is an intention of this invention that either using pressure sensors and/or current measurement one can measure the pressure behavior in the pump chambers and determine the fluid and gas mix the pump is working with. Importantly, in accordance with an aspect of the invention, information about the fluid mix in the chamber can be used to adjust the pump control, so the piston movement suits the property of the mix.

[0047] Note that in the event the pump is immersed in gas only, this will alter the pressure and current behavior as indicated above, but also reduce the cooling available to the motor, and accordingly would also result in increased motor winding temperatures. The measurement of motor temperature is therefore a useful measurement, as well as the pressure to determine what fluid or gas mixture the pump is working in.

[0048] This application uses examples to illustrate the invention, the scope of which is determined by the attached claims. Other examples falling within the scope of the invention may be apparent to those of skill in the art. It is noted that the figures described herein are not necessarily to scale. Certain features of the instant disclosure may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. It is to be recognized that the different teachings of the embodiments discussed herein may be employed separately or in any suitable combination to produce the desired results.