METHODS AND SYSTEMS FOR SYNCHRONIZING SYNCHRONOUS MOTORS WITH AN ELECTRICAL NETWORK BASED ON THE DETECTED POSITION OF THE ROD

Abstract

The system (100) is used to start and synchronize synchronous motors (10) with electrical power from a power grid (20). The system (100) includes a variable frequency drive (30) for starting the synchronous motors (10), and an electrical diverter (61, 63) for each synchronous motor (10). The electrical diverter (61, 63) has a control input electrically coupled to a control output of the variable frequency drive (30) to selectively couple the stator windings (12) of the synchronous motors (10) to a power output (33) of the variable frequency drive (30) or to the power grid (20) based on signals received at a control input of the variable frequency drive (30) that represent a position of the stator shafts of the synchronous motors (10). Therefore, the interruption occurs at the desired shaft positions.

Description

METHODS AND SYSTEMS FOR SYNCHRONIZING SYNCHRONOUS MOTORS WITH THE GRID ELECTRICAL BASED ON THE DETECTED POSITION OF THE STEM FIELD OF INVENTION The subject matter described herein relates to methods for synchronizing synchronous motors with electrical power from an electrical network and to systems for starting and synchronizing synchronous motors with electrical power from an electrical network. BACKGROUND OF THE INVENTION A synchronous motor has a plurality of poles, for example, N poles associated with the stator and N poles associated with the rotor. When the motor rotates, the rotor poles rotate synchronously with the stator poles. To start a synchronous motor, that is, to change its rotor speed from zero to the motor's rated speed, a variable frequency drive (VFD) is used. A VFD is an electrical / electronic unit that drives the motor windings (specifically, and generally, both the stator and excitation windings) until the drive voltage amplitude and drive voltage frequency are close to the motor's rated voltage amplitude and rated voltage frequency; more precisely, the VFD brings the motor to a state where Ref. 346100 states that the drive voltage amplitude is equal to the motor's nominal voltage amplitude and the drive voltage frequency is very close to the motor's nominal voltage frequency. Once this condition is reached, the motor can be energized directly from the mains (and no longer from the VFD), provided that the mains voltage frequency and amplitude match the motor's nominal voltage amplitude and voltage frequency. However, to prevent malfunction and / or damage, a power switch from the VFD to the mains requires at least one other condition to be met. Therefore, such a power switch should only be activated when there is a phase difference between the power from the variable frequency drive and the power from the electrical network that is zero (or practically zero). Existing VFDs activate the aforementioned power switch as soon as any voltage alignment condition (i.e., amplitude, frequency, and phase) is reached. As is known, if a synchronous motor has N poles, there are N / 2 distinct possible pole alignment conditions within a 360° rotation of the rotor, and therefore N / 2 voltage alignment conditions. This means that once motor startup is complete and a power interruption occurs, it is impossible to determine the rotor position of the rotating motor. SUMMARY OF THE INVENTION The shaft of a synchronous motor is frequently mechanically coupled to the shaft of a machine, for example, a reciprocating compressor. In this case, the lack of knowledge of the rotor's position corresponds to the lack of knowledge of the machine's position at any time since the motor's starting phase began; the machine's operating position might be, for example, the exact position of the piston within the cylinder of a reciprocating compressor. Therefore, it would be desirable to start a synchronous motor in such a way that its rotor position is known, preferably known precisely, just after the starting phase, i.e., when it begins to be driven by electrical energy from an electrical network. According to the first aspect, the subject matter described herein relates to a method for synchronizing a synchronous motor with electrical power from a power grid. A switch from the variable frequency drive to the power grid is activated at a moment when one of the motor's rotors is in a predetermined angular position. In particular, the method comprises the steps of: A) starting the synchronous motor by supplying power to the motor through a variable frequency drive, B) repeatedly comparing the voltage amplitude and frequency supplied to the synchronous motor with the voltage amplitude and frequency of the power grid, and C) if the comparison is positive and the phase difference is zero, activating the switch at the aforementioned moment. According to a second aspect, the subject matter described herein relates to a method for starting and synchronizing synchronous motors with electrical power from a power grid. The system includes an improved variable frequency drive for starting each motor by appropriately energizing the stator and excitation windings. The system also includes an electrical diverter for each motor, driven by the improved variable frequency drive, to selectively couple the motor's stator windings to either a power output from the variable frequency drive or the power grid, based on signals received at a control input of the improved variable frequency drive.The system also includes an interrupt enable unit having a control input and a control output; this control input is arranged to be electrically coupled to angular position detectors of the stem of synchronous motors, while this control output is electrically coupled to the control input of the variable frequency drive; the interrupt enable unit is configured to allow the interruption of power from the variable frequency drive to the electrical network. According to a third aspect, the subject matter described herein relates to a fluid compression plant that includes synchronous motors and a compressor mechanically coupled to each motor. The plant also includes an improved system for starting and synchronizing each of the motors. The improved system is designed to start the motors so that when all motors are energized by the same electrical network, the driven compressors are in predetermined relative operating positions. BRIEF DESCRIPTION OF THE FIGURES A more complete appreciation of the described embodiments of the invention and many of its consequent advantages will be readily obtained, as it will be better understood with reference to the following detailed description when considered in relation to the accompanying figures, where: Figure 1 shows a block diagram of one modality of an improved system for starting and synchronizing synchronous motors electrically connected to an electrical network and a plurality of synchronous motors; Figure 2 shows a block diagram of an improved variable frequency drive electrically connected to some components of the system in Figure 1 used to interrupt power to a synchronous motor; Figure 3 shows a detailed block diagram of the improved variable frequency drive of Figure 2; Figure 4 shows timing diagrams of electrical signals in the system of Figure 1 related to a synchronous motor power switch; Figure 5 shows a flow diagram of one modality of a method for synchronizing synchronous motors with electrical power from an electrical network; and Figure 6 shows a general block diagram of one modality of a fluid compression plant. DETAILED DESCRIPTION OF THE INVENTION A variable frequency drive is capable of starting a synchronous motor. It supplies electricity to the motor's stator and excitation windings. The voltage amplitude and frequency of the electricity are increased until they approach the motor's rated voltage amplitude and frequency. Once this condition is met, the variable frequency drive is ready to interrupt power, that is, to connect the motor to the electrical grid so that the motor receives electricity from the grid and no longer from the drive, when the phase difference is zero.According to the improvement of the present invention, such a power switch is not activated at any time, for example, immediately after reaching the nominal voltage amplitude and voltage frequency values, but rather when the motor shaft is in a predetermined position as indicated by, e.g., a suitable sensor or detector. In this way, any machine connected to the motor shaft and operated by the motor is synchronized with the electrical network as desired. We will now turn to different modes of description, an example of which is illustrated in the figures. This example is provided to explain the description, not to limit it. In fact, it will be evident to those skilled in the art that various modifications and variations can be made to this description without departing from its scope or spirit. Figure 1 shows one embodiment of a starting and synchronizing system 100. The system 100 is electrically coupled to an electrical network 20 and a plurality of synchronous motors 10; partial detailed views of this embodiment are shown in Figures 2 and 3. According to this embodiment, four identical motors 10 are provided; however, the system 100 can be used to start and synchronize any number of synchronous motors, whether identical or similar to each other (e.g., having the same number of pole pairs). The electrical network 20 is a source of AC electrical power and is used to energize the motors 10 during both starting and normal operation; the electrical network energizes a motor directly during normal operation and indirectly, i.e., through an electric actuator, during starting. In Figure 1, some components of motor 10 are highlighted and associated with reference numbers, as they are helpful for understanding the following description, while other components are omitted. Specifically, a motor 10 includes stator windings 12 (shown schematically as black circles), excitation windings 14, and an electrical excitation unit 45 that is electrically connected to the excitation windings 14 and is arranged to generate electrical power voltages and currents suitable for the excitation windings of the synchronous motor. It should be noted that the electrical excitation unit is frequently considered part of the synchronous motor; however, it can also be a separate component. Essentially, system 100 includes an improved variable frequency drive 30 and one or more electrical diverters (a combination of elements 61 and 63). There must be one electrical diverter for each synchronous motor, as shown in Figure 1, operating in the same manner and performing the same functions. An electrical diverter is an electrical component designed to divert electricity between two electrical paths. According to the embodiment in Figure 1, the electrical diverter includes two interconnected electrical switches, namely a first electrical switch 61 and a second electrical switch 63. The electrical diverter is arranged to selectively connect the stator windings of a synchronous motor to a power output from the variable frequency drive or to the mains electricity supply. Specifically, the first electrical switch 61 is arranged to selectively connect the stator windings 12 to the mains electricity supply 20, and the second electrical switch 63 is arranged to selectively connect the stator windings 12 to a power output 33 from the variable frequency drive 30.Switches 61 and 63 can be arranged so that the stator windings 12 of a synchronous motor 10 are electrically coupled to a single source of electricity at the same time; according to a first alternative, switches 61 and 63 are arranged so that the stator windings 12 are initially electrically coupled to the actuator 30, finally to the network 20, and during a short intermediate time interval to both sources of electricity; according to a first alternative, switches 61 and 63 are arranged so that the stator windings 12 are initially electrically coupled to the actuator 30, finally to the network 20, and during a short intermediate time interval to no source of electricity. The improved variable frequency drive 30 is electrically coupled to the mains power supply 20 to receive electrical power from the mains and drive both the stator windings and the excitation windings of a synchronous motor at a time; electrical power is supplied to terminal 31; drive signals for the excitation windings are produced from terminal 32 and drive signals for the stator windings are produced from terminal 33. According to the configuration in Figure 1, terminal 33 corresponds to a power output of an improved variable frequency drive 30; in fact, for example, the improved variable frequency drive 30 is capable of directly generating electrical power voltages and electrical power currents suitable for the stator windings of a synchronous motor. According to the configuration shown in Figure 1, terminal 32 corresponds to an excitation output of an enhanced variable frequency drive 30. In fact, for example, the enhanced variable frequency drive 30 is not capable of directly generating electrical power voltages and currents suitable for the excitation windings of a synchronous motor, but it is electrically coupled to the electrical excitation unit 45 of a synchronous motor, which is used to directly generate electrical power voltages and currents suitable for the excitation windings of a synchronous motor. Such a solution can be used if the synchronous motor has its own integrated or associated electrical excitation unit. Alternatively, a variable frequency drive may be capable of directly generating electrical power voltages and currents suitable for the excitation windings of a synchronous motor. According to the embodiment in Figure 1, the electrical excitation unit 45 of a synchronous motor is arranged to receive excitation signals from an enhanced variable-frequency drive 30 during startup and from an external control unit 40 (see Figure 2) during normal operation; in fact, it has two excitation inputs. The drive 30 and the unit 40 can be coordinated to avoid sending excitation signals to the unit 45 at the same time; alternatively, an electrical diverter can be used (not shown in Figure 1). The control unit 40 can be part of a machine control unit, i.e., a control unit arranged to control not only synchronous motors 10. The improved variable frequency drive 30 operates only during the starting of a synchronous motor 10. In particular, according to the modality of Figure 1, the variable frequency drive 30 can start a first motor 10, then once the first motor 10 is energized directly by the electrical network 20, it can start a second motor 10, then once the second motor 10 is energized directly by the electrical network 20, it can start a third motor 10, then once the third motor 10 is energized directly by the electrical network 20, it can start a fourth motor 10, and then once the fourth motor 10 is energized directly by the electrical network 20 it can remain idle; as already clarified, the number of motors to be started can be one or two or three or four or any higher number. It should be noted that in Figure 1, for the sake of simplicity and clarity, only the electrical (power) connection between the improved variable frequency drive 30 and one motor 10 (i.e., the one on the left in the figure) is shown; similar connections exist with the other motors 10. It should also be noted that Figure 1 shows some electrical switches without reference numbers and some electrical transformers without reference numbers that are not strictly necessary for the modality shown and are in accordance with usual practice in the art. As shown in Figure 3, the variable frequency drive 30 has other terminals, in particular a terminal 35 for a control input and terminals 36 for the control outputs for the synchronous motors 10; these terminals and their electrical connections are not shown in Figures 1 and 2 for reasons of simplicity and clarity; they are explained in what follows with the help of Figure 3. A control output provides interruption signals to an electrical diverter associated with a motor 10; in other words, the enhanced variable frequency drive 30 is arranged to determine whether the stator windings 12 of a synchronous motor 10 are powered by the power output 33 of the variable frequency drive 30 or by the electrical grid 20, and to interrupt the power between them, specifically interrupting the power from the variable frequency drive to the electrical grid. It should be noted that the same control output can be used for several motors 10; for example, during a first time period, it can be used for a first motor, during a second time period after the first time period, it can be used for a second motor, and so on.In Figure 3, two terminals 36 are shown electrically coupled respectively to a control input 62 of the first electrical switch 61 and a control input 64 of the second electrical switch 63; according to alternative embodiments, a single terminal 36 may be sufficient if the control signal for switch 61 is exactly opposite to the control signal for switch 63. Control input 35 receives interrupt enable signals from an interrupt enable unit; in other words, the enhanced variable frequency drive 30 is configured to perform a power interruption only if enabled (or authorized) from an external unit. It should be noted that, according to some embodiments, the interrupt enable unit may be integrated into a variable frequency drive of the invention. As shown in Figure 3, the system 100 includes an interrupt enable unit 50 having a control input 51 and a control output 52. The control output 52 is electrically coupled to the control input 35 of the enhanced variable frequency drive 30. The control input 51 is electrically coupled to a detector 17; this detector is arranged to detect an angular position of a synchronous motor 10, preferably a single predetermined angular position. As will be explained further below, the unit 50 generates interrupt enable signals based on the angular position of the shaft of the motor driven by an enhanced variable frequency drive 30; in other words, the power interruption occurs only when the motor shaft is in a predetermined position.Detector 17 can be part of a measuring station arranged to detect not only a specific angular position, but also, for example, the rotational speed of the motor. As already explained, the variable frequency drives of the prior art perform power interruption as soon as any voltage alignment condition of the synchronous motor being started is reached. In contrast, the variable frequency drive of the invention, such as the variable frequency drive 30, performs the power interruption taking into account an additional condition based on the angular position of the motor shaft. The internal control logic for the improved variable frequency drive 30 can be likened to a logic AND port having four logic inputs, one for each condition that must be met: zero or very small phase difference (condition determined by the internal circuitry of the controller), zero or very small voltage amplitude difference (condition determined by the internal circuitry of the controller), voltage frequency difference less than a predetermined threshold (condition determined by the internal circuitry of the controller), and the interrupt being enabled (controller input condition). These conditions will be explained in more detail later. The variable frequency drive 30 of the invention can be implemented, for example, essentially through a combination of a static frequency converter model, e.g., ABB's LCI Megadrive type, and an excitation system model, e.g., ABB's UNITROL 1020 type. Such an excitation system is capable, among other things, of driving such a static frequency converter to perform voltage adaptation prior to synchronization. The drive signal of such an excitation system can be combined with another signal from a control output (e.g., terminal 52 in Figure 3) of an interrupt enable unit (e.g., unit 50 in Figure 3) so that such a static frequency converter is driven taking into account not only the voltage adaptation but also the rotor position. Considering, for example, Figure 4, it is assumed that at time t0 (and at any subsequent time) a sinusoidal signal driving the first motor 10 is such that the drive voltage amplitude at the power output 33 of the variable frequency drive 30 is equal to the voltage amplitude of a network sinusoidal signal 20 and that the drive voltage frequency at the power output 33 of the variable frequency drive 30 is very close to the voltage frequency value of the network sinusoidal signal 20 (e.g., the frequency difference may be 0.1–0.2 Hz); such a condition is achieved by the operation of the variable frequency drive 30. At time ti, the electrical phase difference between the drive sinusoidal signal and the network sinusoidal signal is zero; in a short time interval stl around time ti, the phase difference is approximately zero and synchronization is possible.In this condition A1, one can assume, for example, a rotor position of approximately 0° (i.e., a first rotor condition with respect to an arbitrary reference). At time t2, the electrical phase difference between the drive sinusoidal signal and the mains sinusoidal signal is again zero; in a short time interval st2 around time t2, the phase difference is approximately zero, and synchronization is again possible. In this condition A2, one can assume, for example, a rotor position of approximately PHI (corresponding to 360° divided by the number of pole pairs of the motor) (i.e., a second rotor condition with respect to an arbitrary reference).At time t3, the electrical phase difference between the drive sinusoidal signal and the mains sinusoidal signal is again zero; in a short time interval st3 around time t3, the phase difference is approximately zero, and synchronization is again possible. In this condition A3, a rotor position of approximately 2*PHI can be assumed, for example (i.e., a third rotor condition with respect to an arbitrary reference). At time t4, the electrical phase difference between the drive sinusoidal signal and the mains sinusoidal signal is again zero; in a short time interval st4 around time t4, the phase difference is approximately zero, and synchronization is again possible. In this condition A4, a rotor position of approximately 3*PHI can be assumed, for example (i.e., a fourth rotor condition with respect to an arbitrary reference).At time t5, the electrical phase difference between the drive sinusoidal signal and the mains sinusoidal signal is again zero; in a short time interval st5 around time t5, the phase difference is approximately zero, and synchronization is again possible. In this condition A5, a rotor position of approximately 4*PHI (i.e., a fifth rotor position with respect to an arbitrary reference) can be assumed, for example. The short time intervals st1, st2, st3, st4, st5 mentioned above can be equal to, for example, one, two, or three periods of the mains sinusoidal signal, or in the range of, e.g., 50 ms to 300 ms. Within the long time intervals It1, lt2, lt3, lt4, respectively, between t1 and t2, t2 and t3, t3 and t4, t4 and t5, the phase difference is not zero, and synchronization is not possible. the length of such long time intervals Itl, lt2, lt3, lt4 could be in the range of, e.g.2 s, e.g., 10 s. The interrupt enable unit 50 chooses the desired rotor position for the first motor 10 (according to the example in Figure 4, this is equal to a mechanical phase shift of 2*PHI), and generates an interrupt enable signal that is transmitted at the control output 52 of unit 50 (see the corresponding timing diagram in Figure 4) and received at the control input 35 of the actuator 30. As can be seen in Figure 4, the interrupt enable signal has a short pulse each time the rotor position corresponds exactly to a mechanical phase shift of, e.g., 2*PHI; the interrupt must occur on one of these pulses, but only if synchronization is possible (during the time interval st3 in this case). The interrupt enable unit 50 determines the rotor position based on a signal received at its control input 51 from detector 17.Detector 17 can generate a short pulse each time the rotor position corresponds exactly to the arbitrary reference. Figure 5 shows a flowchart 500 of a specific method for synchronizing a synchronous motor with electrical power from a power grid, such as one of the motors 10 in Figure 1. Specifically, this flowchart relates to a starting phase; block 510 corresponds to the beginning of the phase, and block 580 corresponds to the end of the phase. In block 520, a synchronous motor is energized by a variable-frequency drive to start the motor rotating. In block 530, the drive voltage frequency and / or the drive voltage amplitude is slightly increased by the variable-frequency drive.In block 540, the drive voltage frequency and drive voltage amplitude are compared to the motor's rated voltage frequency and rated voltage amplitude. If the difference is zero or small, the control proceeds to block 550; otherwise, the control returns to block 540. In block 550, a voltage alignment condition (amplitude, frequency, and phase) is tested. If this condition exists, the control proceeds to block 560; otherwise, the control loops back, and after some time, the test in block 550 is repeated. In block 560, a stem angular position (derived from, for example, an angular position detector) is tested. If the stem angular position is a desired position, the control proceeds to block 570; otherwise, the control loops back, and after some time, the test in block 550 is repeated.In block 570, a power switch is activated so that the motor is powered by the mains electricity and no longer by the variable frequency drive. The above procedure can be repeated, for example, for all 10 motors in Figure 1. In this case, the desired angular positions of the motor shafts can be different. Alternatively, the desired angular positions of the motor shafts can be the same. Generally, a method for synchronizing a synchronous motor with electrical power from an electrical network comprises the following steps: A) Starting the synchronous motor by supplying power to the synchronous motor through a variable frequency drive, so that the voltage amplitude supplied to the synchronous motor is raised to a nominal voltage amplitude of the synchronous motor and the voltage frequency supplied to the synchronous motor is raised to a nominal voltage frequency of the synchronous motor, B) repeatedly perform a comparison of the voltage amplitude supplied to the synchronous motor with the voltage amplitude of the electrical power, and a comparison of the voltage frequency supplied to the synchronous motor with the voltage frequency of the electrical power, and C) If the comparison is positive, activate a power switch, stopping the power supply to the synchronous motor through the variable frequency drive, and starting the power supply to the synchronous motor from the electrical network. In stage C, the comparison is positive if the supplied voltage amplitude is equal to or approximately equal to (e.g., less than approximately 3%) the nominal voltage amplitude or the network voltage amplitude, and the supplied voltage frequency is close to (e.g., less than approximately 0.5% or approximately 0.1-0.2 Hz) the nominal voltage frequency or the network voltage frequency; it is assumed that the nominal voltage amplitude corresponds to the network voltage amplitude and that the nominal voltage frequency corresponds to the network voltage frequency.Thanks to the small difference between the supplied voltage frequency and the mains voltage frequency, the motor slowly changes its synchronization state with respect to the mains, and it is possible to select exactly when to synchronize (consider, for example, Figure 4 and its explanation), i.e., when the phase difference between the variable frequency drive power and the mains power is zero or practically zero (e.g., less than approximately 2°). Furthermore, according to this method, the power switch is activated when a synchronous motor rotor is in a predetermined angular position. This predetermined angular position can be considered to correspond to a predetermined synchronization state of the motor with respect to the electrical grid. The default angular position for when to activate the power switch can be selected from a set of preset angular positions. The preset angular position can be selected by an operator, for example, when the motor is installed or during maintenance. In this case, with reference to Figures 1 and 2, it can be assumed that the improved variable frequency drive 30 is set to receive (directly or indirectly) input from an operator. In this case, the preset angular position for a motor remains constant for an extended period (for example, a year or even longer). Alternatively, the default angular position can be selected based on input from an electronic control unit. In this case, with reference to Figures 1 and 2, it can be assumed that the improved variable frequency drive 30 is configured to receive input from an electronic control unit. In this case, the default angular position for a motor can change quite frequently (e.g., weekly, daily, or even less often) and / or at any time. It should be noted that a change to the aforementioned predetermined angular position has no immediate effect on the motor's operation and its synchronization with the power grid. This position is used only when a power interruption occurs. After the interruption, that is, once the motor is directly energized by the power grid (i.e., with reference to the example in Figure 1, switch 63 is opened and switch 61 is closed), any change to this position will only affect the motor's subsequent synchronization with the power grid. As explained earlier, according to the present description, the power interruption in stage C can only occur under certain conditions (consider, for example, blocks 550 and 560 in Figure 5 and their explanation); one such condition is the angular position of the shaft. To verify this condition (consider, for example, block 560 in Figure 5 and its explanation), a stage D can be implemented, which consists of detecting the angular position of the synchronous motor during its rotation. Based on the detected angular position, a power interruption can be triggered. The power interruption can also be triggered based on a time interval starting from the detected angular position.It should be noted that although the letter D follows the letters A, B, and C, the detection of the angular position of the motor stem can be carried out not only at any time during voltage alignment or after the motor is fully rotating, but can be initiated when the motor begins to rotate and continued thereafter. At least stages A, B, and C, and possibly D, can be performed for at least one other synchronous motor after completing stage C for a first synchronous motor. This means synchronizing a set of synchronous motors to the same electrical network. An advantageous application of the system described above and / or the method described above is in fluid compression plants, for example, plant 1000 in Figure 6. Figure 6 shows, as an example, two synchronous motors 10 whose shafts are mechanically coupled to two compressors 600. However, any number of motors and / or any number of compressors is possible. Furthermore, the motors and compressors do not necessarily have to be identical. This solution is particularly effective if these compressors are reciprocating compressors, since in a reciprocating compressor the angular position of its shaft corresponds to a longitudinal position of its piston and a pressure in its compression chamber. In the configuration of Figure 6, there is a rod angular position detector 17 for each synchronous motor 10; each angular position detector 17 is electrically coupled to the same interrupt enable unit (see, for example, block 50 in Figure 2) of the synchronization system 100, which may be called the main synchronization system. System 100 allows for the proper synchronization of the compressors to the electrical grid, as well as with each other. For example, considering the case of four synchronous motors and four corresponding reciprocating compressors (see, e.g., Figure 1), during normal operation, at a given moment the first compressor may be at approximately 0% compression, the second compressor may be at approximately 33% of its maximum compression, the third compressor may be at approximately 66% of its maximum compression, and the fourth compressor may be at maximum compression; at a subsequent moment, the first compressor may be at approximately 33% of its maximum compression, the second compressor may be at approximately 66% of its maximum compression, the third compressor may be at maximum compression, and the fourth compressor may be at approximately 33% of its maximum compression; and so on. In Figure 6, a compressor shaft can be mechanically coupled to a synchronous motor shaft via a mechanical coupler (not shown in the figure). Specifically, the mechanical coupler can be arranged to couple the shafts in a plurality of different positions, each position corresponding to a different rotation angle between the shafts. In this way, the synchronization of the compressors to the electrical grid and to each other can be determined not only through the synchronization system. 100 but also manually. It should be noted that the improved variable frequency drive of the synchronization system can be configured to selectively disregard interrupt enable signals from its control input. This feature can be useful, for example, during maintenance, or if the synchronization system is not operating correctly, or if, under certain operating conditions, synchronization is achieved in a different way, for example, solely through mechanical means. In the configuration shown in Figure 6, in addition to synchronization system 100, there is a second synchronization system 200 that is identical or nearly identical to system 100. System 200 is provided to act as a backup synchronization system in the event that system 100 is faulty or inactive. It is hereby stated that, as of this date, the best method known to the applicant for putting the aforementioned invention into practice is the one that is clear from the present description of the invention.

Claims

1. A method for synchronizing a synchronous motor with electrical power from an electrical network, characterized in that it comprises the steps of: A) starting the synchronous motor by supplying power to the synchronous motor through a variable frequency drive, such that the voltage amplitude supplied to the synchronous motor rises to a nominal voltage amplitude of the synchronous motor and the voltage frequency supplied to the synchronous motor rises to a nominal voltage frequency of the synchronous motor, B) repeatedly comparing the voltage amplitude supplied to the synchronous motor with the voltage amplitude of the electrical power, and comparing the voltage frequency supplied to the synchronous motor with the voltage frequency of the electrical power, C) if the comparison is positive, interrupting the power supply by stopping the power supply to the synchronous motor through the variable frequency drive,and initiate the supply of power to the synchronous motor from the electrical network; wherein the power interruption occurs if both a first and a second condition are met, the first condition being met when a phase difference between the power of the variable frequency drive and the power of the electrical network is zero, and the second condition being met when a rotor of the synchronous motor is in a predetermined angular position.

2. The method according to claim 1, characterized in that the predetermined angular position is selected from a set of predetermined angular positions.

3. The method according to claim 2, characterized in that the set of predetermined angular positions comprises an angular position for each pair of poles of the synchronous motor.

4. The method according to claim 2, characterized in that the predetermined angular position is selected from a set of predetermined angular positions based on input from an operator and / or an electronic control unit.

5. The method according to claim 1, characterized in that it further comprises the steps of: D) detecting an angular position of the synchronous motor during the rotation of the synchronous motor; wherein the power interruption is carried out based on the detected angular position.

6. The method according to claim 5, characterized in that the power interruption is also based on a time interval starting from the detected angular position.

7. The method according to claim 1, characterized in that at least stage A, stage B and stage C are carried out for at least one other synchronous motor after completing stage C for the synchronous motor.

8. A system arranged for starting and synchronizing synchronous motors with electrical power from an electrical network, characterized in that it comprises a variable frequency drive having: a power input arranged to be electrically coupled to the electrical network, an excitation output arranged to be electrically coupled to excitation windings of the synchronous motors, a power output arranged to be electrically coupled to stator windings of the synchronous motors, a control input arranged to receive interrupt enable signals, and a control output arranged to emit interrupt signals, wherein the variable frequency drive is arranged to start the synchronous motors;The system further comprises an electric diverter for each synchronous motor, the electric diverter being arranged to selectively couple the stator windings of the synchronous motors to the power output of the variable frequency drive or to the electrical network, wherein the electric diverter has a control input coupled to the control output of the variable frequency drive;The system further comprises an interrupt enable unit having a control input and a control output, wherein the control input of the interrupt enable unit is arranged to be electrically coupled to rod angular position detectors of synchronous motors, wherein the control output of the interrupt enable unit is electrically coupled to the control input of the variable frequency drive, wherein the interrupt enable unit is configured to permit the interruption of power from the variable frequency drive to the electrical network.

9. The system according to claim 8, characterized in that the electric diverter comprises: a first electric switch arranged to be electrically coupled to the electric network, and a second electric switch electrically coupled to the power output of the variable frequency drive.

10. The system according to claim 8, characterized in that it further comprises an electrical excitation unit electrically coupled between the excitation output of the variable frequency drive and the excitation windings of the synchronous motors.

11. The system according to claim 10, characterized in that the electrical excitation unit is arranged to be electrically coupled to a control unit, the control unit being arranged to control at least synchronous motors.

12. The system according to claim 8, characterized in that the variable frequency drive is arranged to selectively dismiss the control input interrupt enable signals.

13. A fluid compression plant, characterized in that it comprises a first synchronization system in accordance with any of the preceding claims 8 to 12.

14. The plant according to claim 13, characterized in that each of the motors comprises or is associated with a rod angular position detector, wherein each angular position detector is electrically coupled to a first synchronization system interruption enable unit.

15. The plant according to claim 13, characterized in that it further comprises a plurality of synchronous motors.

16. The plant according to claim 15, characterized in that it further comprises a plurality of reciprocating compressors mechanically coupled respectively to the plurality of synchronous motors through a corresponding mechanical coupler, wherein the mechanical coupler is arranged to couple a rotating rod of a motor with a rotating rod of a compressor in a plurality of different positions, each different position corresponding to a different angle of rotation between rods.

17. The plant according to claim 13, characterized in that it further comprises a second synchronization system according to any of the preceding claims 8 to 12, the second synchronization system being arranged to act as a backup synchronization system.

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