Multi-speed drive unit on the compression shaft line

Abstract

A drive unit for driving a load, such as a centrifugal compressor, a pump, etc., having a driving shaft is connected to the load to be driven. The drive unit includes a number of electric motors connected to the driving shaft and a number of variable frequency drives electrically connected to a power grid (G) used to power each of the electric motors.

Description

[Technical Field]

[0001] The present disclosure relates to a multi-speed drive unit that operates on a compressor driving shaft or shaft line to drive a load such as a compressor, pump, or the like. [Background technology]

[0002] In some technical fields, such as the oil and gas sector, it is usually required to drive loads such as compressors or pumps, and nowadays the application of so-called "all-electric" compression shaftline solutions is steadily increasing, since no exhaust gases originating from the combustion of chemical fuels such as gas or diesel are released into the environment.

[0003] However, in the applications described above, electric motors typically must generate a significant amount of electrical power. Therefore, electric motors are equipped with variable frequency drives (VFDs) (or variable speed drive systems, VSDSs) to control the electrical power supply to the electric motor and, therefore, the torque produced thereby. More specifically, a variable frequency drive (VFD) is a power electronics device for powering an electric motor that can control the speed and torque of driven equipment such as pumps, compressors, fans, etc., as described above.

[0004] For example, in the oil and gas industry, particularly in liquefied natural gas (LNG) applications, the compression shaft line requires a variable frequency drive with a high rated power output in the range of 50 MW to 100 MW, and LNG systems driven solely by VFDs are commonly referred to as "e-LNG." In this power range, there are very few VFDs referenced. Therefore, to drive a load at the required high power output, two or more electric motors, each powered by a respective variable frequency drive of a smaller power output, are operably connected to the shaft line.

[0005] However, while supplying the necessary power to the associated electric motor, VFDs also generate disturbing AC torque components, which can excite natural frequencies in the shaft line that can cause excessive shaft vibration with possible mechanical failure.

[0006] Also, the VFD of each electric motor is connected to the power grid, and even in this case, the disturbance current harmonics injected into the power grid by the VFD can cause distortion and heating problems on the network. Summary of the Invention [Problem to be solved by the invention]

[0007] An improved multi-speed drive unit with two or more electric motors acting on the same shaft line, capable of driving significant loads and at the same time preventing the generation of torsional vibrations on the shaft line as well as avoiding the generation of electrical harmonics on the grid, would be welcome in the art.

[0008] In one aspect, the subject matter disclosed herein is directed to a drive unit for driving a load, such as a centrifugal compressor or pump. The drive unit includes a driving shaft coupled to the load to drive the load. The drive unit also includes a plurality of electric motors, each mechanically coupled to the driving shaft in series with one another. A plurality of variable frequency drives are also envisioned. Each variable frequency drive is connected or coupled to the electric motor to regulate the associated torque and angular velocity. During operation, the variable frequency drives typically cause the generation of torque harmonics other than the average torque value and current harmonic components on the power grid other than the fundamental current component, which can cause malfunctions of the entire system. The drive unit also includes a plurality of isolation transformers, each connected or coupled to the variable frequency drive and the power grid. The electric motors and isolation transformers are configured to reduce torque harmonics other than the average torque value and current harmonic components other than the fundamental current component. Such torque harmonics are vibration torque harmonics.

[0009] In another aspect, disclosed herein is an electric motor including a stator having a plurality of windings, the windings of each stator being offset from one another by a predetermined angle to reduce the overall AC torque component acting on the driving shaft.

[0010] In another aspect, disclosed herein, each electric motor is three-phase and the stator has three windings.

[0011] In another aspect, disclosed herein is an electric motor having a rotor mechanically coupled to a driving shaft and having a predetermined physical angular displacement relative to the rotors of the other electric motors.

[0012] In another aspect, the subject matter disclosed herein relates to each isolation transformer having a primary winding connected to a common point on the power grid and a secondary winding connected to an associated variable frequency drive. The primary windings of each isolation transformer are connected to the power grid at the same common point of coupling. The primary or secondary windings of the isolation transformers are arranged in different vector groups to reduce current harmonics injected into the power grid.

[0013] In another aspect, disclosed herein is a driving unit including a control logic unit connected to at least one of the variable frequency drives. The control logic unit is configured to control power generated by the electric motor and delivered to the load. The control logic unit provides an angular velocity reference to a first variable frequency drive, which is capable of providing a torque reference to the multiple variable frequency drives to maintain a required angular velocity of the driving shaft. [Brief explanation of the drawings]

[0014] A complete understanding of the disclosed embodiments of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. [Figure 1] FIG. 1 is a schematic diagram of a drive unit for driving a centrifugal compressor according to a first embodiment. [Figure 2] FIG. 2 is a schematic diagram of a drive unit according to a second embodiment. [Figure 3] FIG. 3 is a schematic diagram of a drive system according to a third embodiment. [Figure 4] FIG. 4 is a schematic diagram of the operation of the drive system of FIG. [Figure 5] FIG. 5 is a diagram showing the delta-delta connection of windings of an isolation transformer. [Figure 6] FIG. 6 is a diagram showing a delta-wye connection of the windings of an isolation transformer. [Figure 7] FIG. 7 is a schematic diagram of a drive unit according to the fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] In the oil and gas sector, it is common to need to drive loads such as centrifugal compressors or pumps that require high driving power. Today, electric motors are the preferred device for driving these loads, as they do not spread pollution into the environment. To reach the power required to drive the aforementioned loads, several electric motors are needed, coupled in series with each other, and the power they generate is added together.

[0016] Electric motors are powered by variable frequency drives, which introduce disturbing AC torque components into the shaft lines and / or inject disturbing current harmonics into the grid. According to the present invention, it is possible to reduce such disturbing AC components by placing an electric motor between them and / or by placing a voltage isolation transformer so that the disturbing AC components cancel each other out. This has several advantages, including but not limited to reducing, minimizing, and / or even eliminating undesirable torsional vibrations on the drive shaft, thus reducing or avoiding mechanical problems for the operation of the drive unit. This can also have the advantage of extending the operating life of other components, such as the drive shaft and / or coupling.

[0017] As used herein, a "voltage isolation transformer" is an electric machine that can transfer power from an alternating current (AC) source to some equipment or device while isolating the powered device from the source for safety reasons. An isolation transformer changes the amplitude of the AC voltage, blocking the transmission of the DC component of the signal from one circuit to another and allowing the AC component to pass.

[0018] Isolation transformers can be electrically connected in several different ways, depending on how the primary and secondary windings of the transformer are electrically connected. As will be explained in more detail below, among the available connections are the so-called "delta-wye connection" and "delta-delta connection," which allow for different voltage ratios and phase shifts between the primary and secondary windings of the isolation transformer.

[0019] Referring now to the drawings, Figure 1 shows a first embodiment of a drive unit 1 for driving a load L. The drive unit 1 comprises a driving shaft 2 mechanically coupled to the load L to be driven, a power unit 3 comprising two electric motors 31, 32 each mechanically coupled to the driving shaft 2 for driving the load L, and two variable frequency drives (VFDs) 41, 42 electrically connected to and supplying power to each of the electric motors 31, 32 to regulate the torque generated by the electric motors 31, 32. The drive unit 1 also comprises a control logic unit 5 operably connected to the first VFD 41 in the embodiment shown in Figure 1.

[0020] According to the present disclosure, the load L is a centrifugal compressor. A centrifugal compressor is a rotary machine that achieves pressure increase by adding kinetic energy / velocity to a fluid through an impeller. However, in other embodiments, the type and number of loads L may be different. More specifically, to remain in an oil and gas field, the load L may be, for example, a pump for pumping oil through a pipeline.

[0021] According to the present disclosure, as can be understood with reference to FIG. 1 , electric motors 31 and 32 are connected in series. Also, as described above, each electric motor 31 and 32 operates on driving shaft 2. In particular, the rotor of each electric motor 31 and 32 is mechanically coupled to driving shaft 2. In some embodiments, the types and numbers of electric motors 31 and 32 may be different. In particular, some embodiments may include more than two electric motors, such as three or four electric motors, operating on the same driving shaft 2 connected in series.

[0022] In this embodiment, each electric motor 31 and 32 is of the three-phase type that is common in this application, however, different types of electric motors may be considered.

[0023] Each of the VFDs 41 and 42 supplies power to the respective electric motors 31 and 32 to regulate the torque T applied by the electric motors 31 and 32 to the driving shaft 2. More specifically, given the power absorbed by the load L and the angular velocity ω of the driving shaft 2, a certain torque is required by the load L, so that each of the electric motors 31 and 32 must generate a specific torque. The VFDs 41 and 42 then control the power supply of the associated electric motor 31 and 32 to generate the appropriate torque to drive the load L according to the required rotational speed ω.

[0024] In the first embodiment of the drive unit 1 shown in FIG. 1 , the control logic unit 5 controls a first VFD 41 connected to a first electric motor 31. In particular, the control logic unit 5 can control the first electric motor 31 in a speed regulation mode via the first VFD 41. The second electric motor 32 is controlled in a torque regulation mode via the second VFD 42. Thus, the control logic unit 5 is configured to operate the first VFD 41 to set the angular speed ω of the first electric motor 31, and thus of the driving shaft 2, and the second variable frequency drive VFD 42 can regulate the torque T of the second electric motor 32, as will be explained in more detail below.

[0025] The control logic unit 5 may be embodied as a programmable microcontroller, a PLC, etc. The control logic unit 5 may be manually programmed using any suitable programming technique and / or programming language, such as C++, to include computer readable instructions that, when executed by a computer processor, cause the computer processor to read, for example, the operating state of each VFD 41 and 42 and each electric motor 31 and 32, i.e., the absorbed power, the generated torque, the angular velocity of the drive shaft 2.

[0026] Control unit 5 is then programmed, in one embodiment, to generate a first output signal for controlling VFD 41 and a second output signal for controlling VFD 42, and to adjust the angular velocity of each electric motor 31 or 32 to balance the torque reference value T generated by each electric motor. In this way, cancellation of torque harmonic (excluding average torque value) and current harmonic components (excluding fundamental current component) is enabled through coupling of electric motors 31, 32 and isolation transformer arrangements, as will be better explained below.

[0027] Typically, control logic unit 5 is embodied on a motherboard electrically connected to the VFD of the electric motor it controls. Control logic unit 5 may, in some cases, be located remotely relative to the VFD equipment to which it is connected.

[0028] The control logic unit 5, as described above, is operatively connected to the first VFD 41. In particular, the control logic unit 5 can determine an angular velocity reference or setpoint ω and can transmit the angular velocity reference ω to the first VFD 41. The two VFDs 41 and 42 exchange torque and speed data to maintain the angular velocity reference ω required by the control logic unit 5 and appropriately distribute the torque T produced by both the electric motor 31 and the electric motor 32 according to the torque demands of the load L.

[0029] 1, the two VFDs 41 and 42 are connected to each other and work together. Specifically, the two VFDs 41 and 42 are electronic devices that can control the power supplies of the respective electric motors connected thereto to control their speed and torque by adjusting the frequency and voltage of the motor power supplies, as described above.

[0030] Also, the angular speed ω of the driving shaft 2 is typically determined by a load L, such as a compressor, where different angular speeds ω are required depending on the operating regime. Therefore, the total torque required is also set based on the power required by the load L, and the control logic unit 5 indirectly adjusts the operation of the first electric motor 31, and therefore the second electric motor 32, to distribute the torque generated to drive the load L as proportionally as possible. If the torque generated by the first electric motor 31 and the torque generated by the second electric motor 32 are the same, the AC torque components will have the same amplitude and, if appropriately shifted, can cancel each other out, as will be explained in more detail below.

[0031] Continuing to refer to FIG. 1, each electric motor 31 and 32 mechanically coupled to driving shaft 2 has an associated stator winding 311 and 321, respectively.

[0032] When each electric motor 31 and 32 is powered by its associated VFD 41 and 42, disturbance torque harmonics, i.e., harmonics different from the average torque value, may be introduced, which may generate torsional vibrations in the drive shaft 2 and cause mechanical problems in the operation of the drive unit 1. As mentioned above, if the torque generated by the first electric motor 31 and the torque generated by the second electric motor are the same, the amplitudes of the AC torque components generated by each electric motor 31 and 32 will have the same amplitude.

[0033] As described above, the stators 311 and 321 of the first electric motor 31 and the second electric motor 32 have windings designed to be phase-shifted by the aforementioned angle θ. In particular, the stator windings 311a, 311b, and 311c of the first electric motor 31 are physically shifted by the aforementioned predetermined displacement angle θ relative to the associated stator windings 321a, 321b, and 321c of the second electric motor 32. More specifically, the stator windings 321a, 321b, and 321c of the second electric motor 32 are physically shifted by the aforementioned displacement angle θ in a radial direction (i.e., perpendicular to the length of the driving shaft 2 to which the electric motor 2 is coupled) relative to the stator windings 311a, 311b, and 311c of the first electric motor 31.

[0034] First electric motor 31 is three-phase and therefore includes three-phase windings 311a, 311b, and 311c that are arranged at a predetermined physical angular displacement θ relative to three-phase windings 321a, 321b, and 321c of second electric motor 32. In accordance with the present disclosure, the angular displacement θ is 30°, as in a six-phase winding electric motor. However, in some embodiments, a different physical angular displacement θ may be used based on the particular application and motor construction technique.

[0035] If the drive unit 1 comprises three or more motors, there will be an angular displacement between each of the subsequent electric motors coupled in series to the driving shaft 2 that is appropriately calculated to reduce the AC torque component.

[0036] Still referring to FIG. 1 , according to the present disclosure, each electric motor 31 and 32 mechanically coupled to the driving shaft 2 is a three-phase winding motor with a predetermined angular displacement θ between two sets of three-phase windings 311a, 311b, and 311c and 321a, 321b, and 321c of the stators of the two associated electric motors 31 and 32. Therefore, some torque harmonics different from the average torque value are reduced from the equivalent resultant air-gap torque acting on the two associated motor rotors. In other words, and more specifically, each electric motor 31 and 32 has torque harmonic components. However, because the stator windings of each motor 31 and 32 are shifted by the aforementioned displacement angle θ and such electric motors 31 and 32 are mechanically coupled to the same driving shaft 2, the resultant torque generated by the same electric motors 31 and 32, which is the sum of the associated torques, reduces undesirable torque harmonics.

[0037] As mentioned above, torque harmonics are generated by the use of VFDs 41 and 42. In particular, such harmonics are superimposed on the average torque T as an oscillatory torque which may be, for example, the excitation of torsional resonance modes of the train for LNG applications, leading to vibration problems in the shaft line, i.e., driving shaft 2.

[0038] Considering that the electric motors 31 and 32 driven by the two VFDs 41 and 42 are coupled to the same driving shaft 2, the same two VFDs 41 and 42 preferably operate at the same power level (i.e., the same torque distribution, which, as described above, is an optimized solution for better distributing the power supplied by the electric motors 31 and 32), and the configuration of the electric motor stators 311 and 321 shown in Figure 1 maximizes the reduction of undesired torque harmonics (i.e., harmonics that differ from the average torque value, as described above). In practice, such torque harmonics for each VFD 41 and 42 will have the same amplitude for a given rotational speed ω if the average torque generated by each electric motor is the same.

[0039] Specifically, in some embodiments, the arrangement of the two sets of three-phase windings 311a, 311b, and 311c and 321a, 321b, and 321c of the stators 311 and 321 of the two associated electric motors 31 and 32 allows for cancellation of AC torque components, which are generated by the VFDs 41 and 42, as described above. More specifically, the phase shift of the windings of the stator 321 of the second electric motor 32 is designed so that the phasors of undesired harmonics are phase shifted by 180° relative to the phasors of the same undesired harmonics generated by the VFD 41 of the first electric motor 32, so that they can cancel each other out.

[0040] Additionally, through the operation of the control logic unit 5, optimization of the operation of the drive unit 1 is achieved, since by having two motors, they can generate the same torque so that the cancellation of the alternating torques is maximized. The same applies to the cancellation of current harmonic components injected into the power grid via phase shifting of the isolation transformer windings, as will be explained in more detail below, which current harmonic components are other than the fundamental current component set at 50 Hz or 60 Hz, depending on the grid G ​​and the electrical network.

[0041] 2, there is shown a second embodiment of the drive unit 1. In particular, the control logic unit 5 is now connected to both a first VFD 41 and a second VFD 42, each of which is also connected in this case to a first electric motor 31 and a second electric motor 32, respectively.

[0042] Also in this embodiment, the first electric motor 31 and the second electric motor 32 are mechanically coupled to a driving shaft 2 which is mechanically coupled to a load L.

[0043] In this second embodiment, control of torque T or power and angular velocity ω is performed directly by control logic unit 5 which is coupled to both VFD 41 and VFD 42, as described above.

[0044] In this embodiment, VFD 41 and VFD 42 are not in direct communication with each other. In particular, control logic unit 5 is configured to determine an angular velocity reference ω and a torque reference or torque setpoint T to maintain an angular velocity on driving shaft 2, and to distribute the generated torque T between electric motor 31 and electric motor 32.

[0045] Furthermore, the control logic unit 5 is configured and programmed to send an angular speed reference value ω to the first VFD 41 and a torque reference value T to the second VFD 42 in order to control the two VFDs 41 and 42 in speed and / or torque, respectively, as described above. In this way, once the desired angular speed ω on the driving shaft 2 is set, the control logic unit 5 enables the control of the two electric motors 31 and 32 by the first VFD 41 and the second VFD 42 to deliver the total torque T needed to generate the required power sent to the load L.

[0046] The operation of the drive unit 1 of the second embodiment of Figure 2 is the same as that of the first embodiment, however, in this case the control logic unit 5 is directly connected to the second VFD 42, and therefore more specific control of the operation of the second electric motor 32 can be performed directly by the control logic unit 5.

[0047] The drive unit 1 of the second embodiment shown in FIG. 2 can also cancel the AC torque components generated by the VFDs 41 and 42 through the arrangement of two sets of three-phase windings 311a, 311b, and 311c, and 321a, 321b, and 321c of the two associated electric motors 31 and 32 of the stator, where again the two associated electric motors 31 and 32 are phase shifted by a displacement angle θ to cancel the undesired AC torque component of the first VFD 41 with that of the second VFD 42, shifting the phase of the latter by 180°.

[0048] Thus, in both the first embodiment shown in FIG. 1 and the second embodiment of FIG. 2, the drive unit 1 can suppress or cancel one or more AC torque components, two reducing possible mechanical resonances on the driving shaft 2.

[0049] In some embodiments, the control logic unit 5 is also configured to control the power generated by the electric motors 31, 32 and delivered to the load L. The control logic unit 5 is then configured to provide a torque reference T to the first variable frequency drive 41 and a torque reference T to the second variable frequency drive 42 in order to maintain a required angular velocity of the driving shaft 2.

[0050] 3 and 4, a third embodiment of the drive unit 1 is shown, again comprising two electric motors 31 and 32 mechanically coupled to the driving shaft 2, which in turn is mechanically coupled to the load L. As with the previous embodiment, the electric motors 31 and 32 are driven and powered by associated VFDs, again indicated by reference numerals 41 and 42, respectively.

[0051] The drive unit 1 also comprises two isolation transformers, one for each VFD 41 and 42, in particular a first isolation transformer 61 and a second isolation transformer 62. Specifically, the first isolation transformer 61 is connected between the first VFD 41 and the power grid G, and the second isolation transformer 62 is connected between the second VFD 42 and the power grid G. Each of the isolation transformers 61 and 62 also comprises a primary winding, denoted by reference numerals 611 and 621, respectively, and a secondary winding, denoted by reference numerals 612 and 622, respectively.

[0052] Two isolation transformers 61 and 62 are capable of transferring power from the power grid G ​​to the VFDs 41 and 42 while isolating the same VFDs 41 and 42 from the power grid G.

[0053] The primary windings 611 and 621 of the first isolation transformer 61 and second isolation transformer 62 of each VFD 41 and 42 are connected to grid G ​​at the same point of common coupling, as shown in Figures 3 and 4. The secondary windings 612 and 622 of the first isolation transformer 61 and second isolation transformer 62 of each VFD 41 and 42 are arranged to suppress current harmonic components generated on grid G ​​by drive unit 1. The principle of operation is to take the current harmonics generated from each transmission system 41 and / or 42 and combine them together with one source of the harmonics shifted 180° relative to the other, thus resulting in cancellation of these current harmonics injected on grid G.

[0054] For example, in a three-phase power distribution system, the fifth and seventh harmonics are the dominant harmonics and typically cause distortion and heating problems. Cancellation of these current harmonic components generated by each VFD 41 and 42 can be achieved by arranging the vector groups of the isolation transformers 61 and 62 in a first, or "delta-delta," configuration and a second, or "delta-wye," configuration, respectively, as better shown in FIGS. 5 and 6, which illustrate the vector group arrangements of the primary and secondary windings of the "delta-delta" and delta-wye transformers. More specifically, in a delta-delta connected configuration, the primary and secondary windings of the three-phase transformer are electrically connected as a delta (i.e., "Δ"). Alternatively, in a delta-wye connected configuration, the primary winding of the three-phase transformer is still connected as a delta, and the secondary winding is still electrically connected as a "wye."

[0055] In particular, the delta-delta configuration of the first isolation transformer 61 causes a 0° phase shift in the current, and the delta-wye configuration of the second isolation transformer 62 causes a 30° current phase shift in the current supplied to the second VFD 42.

[0056] The fifth harmonic in the delta-wye transformer 62 is phase shifted by five times 30°, resulting in a phase shift of 150°. Additionally, the fifth harmonic is a negative-sequence harmonic; therefore, it is in the opposite direction of the fundamental, which is phase shifted 30° in the opposite direction, resulting in a total phase shift of 180°. In this way, the fifth harmonics generated by the two VSD systems have a phase shift of 180° relative to each other, causing cancellation of the harmonic components.

[0057] Similarly, the seventh harmonic in the second delta-wye transformer 62 is phase shifted by seven times 30°, resulting in a phase shift of 210°. Because the seventh harmonic is a positive sequence harmonic, its relative shift with respect to the fundamental is again 180°.

[0058] The harmonic cancellation mentioned above can be visualized in FIG. 4, where it can be seen that the fifth and seventh AC harmonic components coming from the primary windings 611 and 621 of the first isolation transformer 61 and the second isolation transformer 62 are cancelled out in view of the connection to grid G.

[0059] In this way, the drive unit 1 according to the third embodiment shown in Figures 3 and 4 can cancel both AC torque components by the angular displacement θ between the two sets of three-phase windings 311a, 311b and 311c and 321a, 321b and 321c of the two associated electric motors 31 and 32 of the stators, and the interference current harmonic components other than the fundamental current component of the grid G, via the isolation transformers 61 and 62.

[0060] Other connections of the primary windings 611 and 621 and secondary windings 612 and 622 of the first isolation transformer 61 and the second isolation transformer 62 can also be foreseen in order to cancel out undesired current harmonics.

[0061] As described above, the electric motors 31 and 32 driven by the two VFDs 41 and 42 are connected to the same driving shaft 2, and the two VFDs 41 and 42 preferably operate at the same power level, thus maximizing harmonic cancellation at the point of common coupling because the harmonic components have the same amplitude. In principle, the same effect of harmonic cancellation could be achieved with two independent VFDs 41 and 42 connected to the same common coupling, but the size and operating conditions of the independent VFDs 41 and 42 are typically dictated by the process requirements of the driven machine (e.g., a centrifugal compressor), and it is unlikely that these VFDs 41 and 42 can operate continuously at the same power level that maximizes harmonic cancellation.

[0062] 7, a fourth embodiment of the drive unit 1 is shown in which the arrangement of the three-phase stator windings 311a, 311b and 311c of the first electric motor 31 is the same as the associated stator windings 321a, 321b and 321c of the second electric motor 32. However, the rotor 312 of the first electric motor 31 has a predetermined physical angular displacement θ relative to the rotor 322 of the second electric motor 32.

[0063] Therefore, the effect of torque harmonic reduction (i.e., harmonics different from the average torque value) is achieved in this embodiment by physically shifting the rotor 312 of the first electric motor 31 relative to the rotor 322 of the second electric motor 32 by a predetermined displacement angle θ while maintaining the same arrangement of the electric motor stator windings, as shown schematically in FIG. 7.

[0064] The operation of the fourth embodiment of the drive unit 1 is the same as the third embodiment of Figure 3 or 4. In this case, the drive unit 1 also comprises a first isolation transformer 61 connected between the VFD 41 and the grid G ​​and a second isolation transformer 62 connected between the VFD 42 and the grid G. In this way, the drive unit 1 can theoretically cancel out disturbance current harmonics when the power absorbed by the electric motor is the same.

[0065] 4, 5, and 6, one operating cycle of an embodiment of the present invention will now be described for illustrative purposes. Specifically, during operation, electric motors 31 and 32 are powered by associated VFDs 41 and 42 as they operate to drive load 2. Shifting motor stator windings 311 and 321 by a displacement angle θ allows for cancellation of undesired torque harmonics, i.e., harmonics that differ from the average torque value. In this manner, electric motors 31 and 32 transmit an average torque value to load L without or with reduced undesired mechanical torque oscillations on the shafts.

[0066] At the same time, VFDs 41 and 42 are powered by transformers 61 and 62, respectively. In the illustrated embodiment, the primary and secondary windings of three-phase transformer 61 are connected in a delta-delta connection with a phase shift equal to 0° of the current supply of VFD 41 (which supplies first electric motor 31), while the primary and secondary windings of three-phase transformer 62 are connected in a delta-wye connection with a phase shift equal to 30° of the current supply of the other VFD 42 (which supplies second electric motor 32), so that the fifth and seventh current harmonics cancel each other out in grid G.

[0067] While aspects of the present invention have been described in terms of various specific embodiments, it will be apparent to those skilled in the art that many modifications, changes, and omissions are possible without departing from the spirit and scope of the claims. Additionally, unless otherwise specified herein, the order or sequence of any process or method steps may be varied or rearranged according to alternative embodiments.

[0068] Reference will now be made in detail to the embodiments of the present disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the disclosure, not as a limitation of the disclosure. Indeed, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. References throughout this specification to "one embodiment" or "an embodiment" or "some embodiments" mean that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the disclosed subject matter. Thus, the appearances of the phrases "in one embodiment," "in an embodiment," or "in some embodiments" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0069] When presenting elements of various embodiments, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements. The terms "comprising," "including," and "having" are intended to be non-exclusive and mean that there may be additional elements other than the listed elements.

Claims

1. A drive unit (1) for driving a load (L) such as a centrifugal compressor, a pump, etc. a driving shaft (2) connectable to the load (L) to be driven; a plurality of electric motors (31, 32) each mechanically coupled to the driving shaft (2) in series with one another to drive the load (L); a plurality of variable frequency drives (41, 42) electrically coupled to associated electric motors (31, 32), each capable of adjusting the torque and / or angular speed of the connected electric motors (31, 32), the operation of which causes the generation of torque harmonics other than an average torque value and current harmonic components on the power grid (G) other than a fundamental current component; a plurality of isolation transformers (61, 62) each coupled to a variable frequency drive (41, 42) and the power grid (G); a first electric motor (31) of the plurality of electric motors (31, 32) mechanically coupled to the driving shaft (2) for driving the load (L); a first variable frequency drive (41) coupled to the first electric motor (31) and capable of adjusting the torque and / or angular speed of the first electric motor (31) on the driving shaft (2); a first isolation transformer (61) of the plurality of isolation transformers (61, 62) coupled to the first variable frequency drive (41) and the power grid (G); a second electric motor (32) of the plurality of electric motors (31, 32) mechanically coupled to the driving shaft (2) for driving the load (L); a second variable frequency drive (42) coupled to the second electric motor (32) and capable of adjusting the torque and / or angular speed of the second electric motor (32) on the driving shaft (2); a second isolation transformer (62) coupled to the second variable frequency drive (42) and the power grid (G); The second electric motor (32) is disposed on the driving shaft (2) between the first electric motor (31) and the load (L); the electric motors (31, 32) and the isolation transformers (61, 62) of the drive unit (1) are configured to reduce torque harmonics other than the average torque value and / or current harmonic components other than the fundamental current component; Each isolation transformer (61, 62) primary windings (611, 621) connected to a common point of the power grid (G); a secondary winding (612, 622) connected to the associated variable frequency drive (41, 42); the primary windings (611, 621) of the isolation transformers (61, 62) are connectable to the power grid (G) at the same point of common coupling; the primary windings (611, 621) or the secondary windings (612, 622) of the isolation transformers are arranged with different vector groups to reduce the current harmonic components injected into the power grid (G); the first isolation transformer (61) having a primary winding (611) connected to a common point of the power grid (G) and a secondary winding (612) connected to the first variable frequency drive (41); the primary winding (611) and the secondary winding (612) of the first isolation transformer (61) are connected in a "delta-delta" configuration; the second isolation transformer (62) having a primary winding (621) connected to a common point of the power grid (G) and a secondary winding (622) connected to the second variable frequency drive (42); the primary winding (621) and the secondary winding (622) of the second isolation transformer (62) are connected in a "delta-wye" configuration; Drive unit (1).

2. 2. The drive unit (1) of claim 1, wherein one or more torque harmonics of the second electric motor (32) are phase shifted relative to respective torque harmonics of the first electric motor (31) to reduce overall torque harmonics acting on the driving shaft (2).

3. Among the plurality of electric motors (31, 32), the first electric motor (31) is a reference electric motor; Each electric motor (31, 32) comprises a stator (311, 321); Each stator (311, 321) has a plurality of windings (311a, 311b, 311c; 321a, 321b, 321c), 3. The drive unit (1) of claim 1 or 2, wherein the stator windings (321a, 321b, 321c) of each electric motor (31, 32) are physically shifted radially by a predetermined displacement angle (θ) relative to the stator windings (311a, 311b, 311c) of the reference electric motor (31) to reduce the overall torque harmonics acting on the driving shaft (2).

4. The first electric motor (31) comprises a stator (311); The stator (311) of the first electric motor (31) has a plurality of windings (311a, 311b, 311c); The second electric motor (32) comprises a stator (321); The stator (321) has a plurality of windings (321a, 321b, 321c), 4. The drive unit (1) of claim 3, wherein the windings (321a, 321b, 321c) of the stator (321) of the second electric motor (32) are shifted by a predetermined displacement angle (θ) relative to the windings (311a, 311b, 311c) of the stator (311) of the first electric motor (32) to reduce the overall torque harmonics acting on the driving shaft (2).

5. The first electric motor (31) is a three-phase motor, and the stator (311) has three windings (311a, 311b, 311c); 5. A driving unit (1) according to claim 4, wherein the second electric motor (32) is three-phase and the stator (321) has three windings (321a, 321b, 321c).

6. A driving unit (1) as described in claim 3, wherein the predetermined shifted displacement angle (θ) of each stator (311, 321) relative to the stator (311) of the reference electric motor (31) is set to suppress or reduce one or more torque harmonics in order to reduce possible mechanical excitation on the driving shaft (2).

7. Each electric motor (31, 32) comprises a rotor (312, 322); 2. The drive unit (1) of claim 1, wherein each rotor (312, 322) of each electric motor (31, 32) is mechanically connected to the driving shaft (2) and has a predetermined physical angular displacement (θ) relative to the rotor (322) of the other electric motor (32).

8. the first electric motor (31) comprises a rotor (312) mechanically connected to the driving shaft (2); the second electric motor (32) comprises a rotor (322) mechanically connected to the driving shaft (2); 8. The drive unit (1) of claim 7, wherein the rotor (322) of the second electric motor (32) has a predetermined physical angular displacement (θ) relative to the rotor (312) of the first electric motor (32).

9. 9. The drive unit (1) of claim 8, wherein the predetermined displacement angle (θ) is set to suppress or reduce one or more torque harmonics, two reducing possible mechanical excitations on the driving shaft (2).

10. 2. The driving unit (1) according to claim 1, wherein each of the electric motors (31, 32) generates an equal torque.

11. a control logic unit (5) connected to at least one of the variable frequency drives (41, 42); the control logic unit (5) is configured to control the power generated by the plurality of electric motors (31, 32) and transmitted to the load (L); the control logic unit (5) is configured to provide the angular velocity reference value (ω) to the first variable frequency drive (41); 2. The driving unit (1) of claim 1, wherein the first variable frequency drive (41) is capable of providing the torque reference value (T) to the plurality of variable frequency drives (42) to maintain the required angular velocity of the driving shaft (2).

12. 12. The driving unit (1) according to claim 11, wherein the control logic unit (5) is connected to the plurality of variable frequency drives (41, 42).

13. the control logic unit (5) is configured to control the power generated by the plurality of electric motors (31, 32) and transmitted to the load (L); 13. The driving unit (1) of claim 11 or 12, wherein the control logic unit (5) is configured to provide a torque reference value (T) to the first variable frequency drive (41), and the control logic unit (5) is configured to provide torque reference values ​​(T) to the plurality of variable frequency drives (42) to maintain the required angular velocity of the driving shaft (2).

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