Variable frequency driving system

Abstract

A variable frequency drive system is provided. In accordance with an embodiment of the present disclosure, a variable frequency drive system includes a transformer having a set of input terminals, a first set of output terminals, and a second set of output terminals. A first rectifier is coupled to the first set of output terminals and a second rectifier is coupled to the second set of output terminals. The transformer is configured to convert a three-phase alternating current (AC) provided to the set of input terminals into a first three-phase AC output at the first set of output terminals and a second three-phase AC output at the second set of output terminals. The first three-phase AC output is phase shifted from the second three-phase AC output.

Description

Technical Field

[0001] The present disclosure generally relates to an improved variable frequency drive system for controlling an electric motor that allows for reduction of time harmonics. Background Art

[0002] An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and the current in the wire windings to produce a force in the form of torque applied to the motor shaft. Electric motors can be classified based on the type of power supply, application, etc. Industrial applications include alternating current (AC) motors used in HVAC, pumps, fans, etc.

[0003] Some AC motors run at fixed speeds, while others run at variable speeds. A variable frequency drive (VFD) is a device that can control the rotational speed of a variable speed AC motor. A VFD can control the speed and torque of an AC motor by varying the frequency of the wire winding current and by controlling the magnitude of the current or associated voltage depending on the topology.

[0004] A VFD typically includes three different subsystems: a rectifier, a direct current (DC) link, and an inverter. The rectifier converts the AC input power into DC power. The most basic rectifier for a VFD used in industrial applications is configured as a three-phase full-wave diode bridge. The DC link includes capacitors that smooth the DC power output to the inverter. The inverter converts the DC input power into three-phase AC power that drives the wire windings of an AC motor (e.g., a three-phase induction motor).

[0005] Harmonic distortion is a measure of the deviation from a pure sinusoidal waveform that can be caused by a non-linear load (VFDs are considered non-linear loads because the rectifier portion draws current in a non-sinusoidal pattern). Utility Model Content

[0006] An overview is presented below to provide a basic understanding of one or more embodiments of the present disclosure. This overview is not intended to identify key or important elements, or to limit any scope of the embodiments or any scope of the claims. Its sole purpose is to present the concepts in a simplified form as a prelude to a more detailed description that will be presented later.

[0007] According to an embodiment of the present disclosure, a variable frequency drive system includes: a transformer, the transformer including a group of input terminals, a first group of output terminals, and a second group of output terminals; a first rectifier and a second rectifier, the first rectifier being coupled to the first group of output terminals and the second rectifier being coupled to the second group of output terminals; wherein the transformer is configured to transform three-phase alternating current provided to the group of input terminals into a first three-phase alternating current output at the first group of output terminals and a second three-phase alternating current output at the second group of output terminals, respectively; wherein the first three-phase alternating current output is phase-shifted from the second three-phase alternating current output.

[0008] Preferably, the variable frequency drive system further comprises: a third rectifier; wherein the transformer comprises a third set of output terminals coupled to the third rectifier; wherein the transformer is configured to transform the three-phase AC power provided to the set of input terminals into the first three-phase AC power output at the first set of output terminals, the second three-phase AC power output at the second set of output terminals, and the third three-phase AC power output at the third set of output terminals; wherein the third three-phase AC power output is phase-shifted from the first three-phase AC power output and the second three-phase AC power output.

[0009] Preferably, the first three-phase AC power output leads the second three-phase AC power output, and the third three-phase AC power output lags behind the second three-phase AC power output.

[0010] Preferably, the first three-phase AC power output leads the second three-phase AC power output by 20 degrees, and the third three-phase AC power output lags the second three-phase AC power output by 20 degrees.

[0011] Preferably, the amount by which the first three-phase AC power output leads the second three-phase AC power output is within a range of plus or minus 10% of the amount by which the third three-phase AC power output lags behind the second three-phase AC power output.

[0012] Preferably, the variable frequency drive system further comprises a first inverter coupled to the first rectifier, a second inverter coupled to the second rectifier, and a third inverter coupled to the third rectifier.

[0013] Preferably, the variable frequency drive system further includes a low voltage motor coupled to the first inverter, the second inverter and the third inverter.

[0014] Preferably, the variable frequency drive system further comprises a low voltage motor coupled to the first rectifier, the second rectifier and the third rectifier.

[0015] Preferably, the transformer comprises: a group of primary windings; a first group of secondary windings, a second group of secondary windings and a third group of secondary windings; wherein the first group of secondary windings and the third group of secondary windings are arranged in a zigzag configuration, while the second group of secondary windings is not arranged in a zigzag configuration.

[0016] Preferably, each of the first set of secondary windings and the third set of secondary windings is arranged in a triangular zigzag configuration.

[0017] According to another embodiment of the present disclosure, a variable frequency drive system includes: a first rectifier, a second rectifier and a third rectifier, the first rectifier includes a first group of input terminals and a first DC output terminal, the second rectifier includes a second group of input terminals and a second DC output terminal, and the third rectifier includes a third group of input terminals and a third DC output terminal; a first inverter, a second inverter and a third inverter, the first inverter includes a first group of output terminals and a first DC input terminal electrically connected to the first DC output terminal, the second inverter includes a second group of output terminals and a second DC input terminal electrically connected to the second DC output terminal, and the third inverter includes a third group of output terminals and a third DC input terminal electrically connected to the third DC output terminal; a motor, the motor is electrically connected to the first group of output terminals of the first inverter, the second group of output terminals of the second inverter, and the third group of output terminals of the third inverter.

[0018] Preferably, each of the first rectifier, the second rectifier and the third rectifier is a passive rectifier.

[0019] Preferably, the variable frequency drive system also includes: a transformer, the transformer including a group of input terminals, a first group of output terminals, a second group of output terminals and a third group of output terminals; wherein the first group of output terminals of the transformer is electrically connected to the first group of input terminals of the first rectifier; wherein the second group of output terminals of the transformer is electrically connected to the second group of input terminals of the second rectifier; wherein the third group of output terminals of the transformer is electrically connected to the third group of input terminals of the third rectifier.

[0020] Preferably, the transformer is configured to transform the three-phase AC power provided to the set of input terminals of the transformer into a first three-phase AC power output at the first set of output terminals of the transformer, a second three-phase AC power output at the second set of output terminals, and a third three-phase AC power output at the third set of output terminals.

[0021] Preferably, the first three-phase AC power output is phase-shifted from the second three-phase AC power output, and wherein the third three-phase AC power output is phase-shifted from the second three-phase AC power output.

[0022] Preferably, the first three-phase AC power output leads the second three-phase AC power output, and the third three-phase AC power output lags behind the second three-phase AC power output.

[0023] Preferably, the first three-phase AC power output leads the second three-phase AC power output by 20 degrees, and the third three-phase AC power output lags the second three-phase AC power output by 20 degrees.

[0024] Preferably, the amount by which the first three-phase AC output leads the second three-phase AC output is within a range equal to plus or minus 10% of the amount by which the third three-phase AC output lags the second three-phase AC output.

[0025] Preferably, the transformer comprises: a group of primary windings; a first group of secondary windings, a second group of secondary windings and a third group of secondary windings; wherein the first group of secondary windings and the third group of secondary windings are arranged in a zigzag configuration, while the second group of secondary windings is not arranged in a zigzag configuration.

[0026] Preferably, the variable frequency drive system further comprises a three-phase motor having three-phase input terminals connected to corresponding three-phase output terminals of the first inverter, the second inverter and the third inverter through electrical conductors of equal length.

[0027] By reading the following detailed description and the accompanying drawings (which will be briefly described later), these and other features, aspects and advantages of the present disclosure will be apparent. The present disclosure includes any combination of two, three, four or more of the above-described embodiments and any combination of two, three, four or more features or elements set forth in the present disclosure, regardless of whether these features or elements are explicitly stated to be combined in the specific embodiment description herein. The present disclosure should be read as a whole so that any separable features or elements of the disclosure disclosed in any of the various aspects and embodiments of the present disclosure should be considered combinable, unless otherwise clearly indicated herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] To describe in detail various exemplary embodiments, reference will now be made to the following accompanying drawings, in which:

[0029] Figure 1 A medium voltage electric motor is shown being controlled by a medium voltage variable frequency drive.

[0030] Figure 2 A low voltage electric motor controlled by a low voltage variable frequency drive is shown.

[0031] Figure 3 is a block diagram of a variable frequency drive system with medium voltage input and low voltage output.

[0032] Figure 4 is configured for 12-pulse input rectifier operation Figure 3 An example of a variable frequency drive system is shown in .

[0033] Figure 5 is configured for 18-pulse input rectifier operation Figure 3 Another example of a variable frequency drive system is shown in .

[0034] Figure 6 is configured for 24-pulse input rectifier operation Figure 3 Another example of a variable frequency drive system is shown in .

[0035] Figure 7 is designed with a delta-connected primary winding and a delta-connected secondary winding Figure 5 Schematic diagram of an example of a step-down transformer.

[0036] Figure 8 is designed with a star-connected primary winding and a star-connected secondary winding Figure 5 Schematic diagram of another example of a step-down transformer.

[0037] Fig. 9 is designed with a delta-connected primary winding and a star-connected secondary winding Figure 5 Schematic diagram of yet another example of a step-down transformer.

[0038] Fig. 10A yes Figure 7 Schematic diagram of an example of a step-down transformer.

[0039] Fig. 10B It is shown Fig. 10A Vector diagram of the relative phase angle between the secondary windings of the step-down transformer.

[0040] Fig.11 yes Fig. 10B A partial schematic diagram of an example of a step-down transformer.

[0041] Fig.12 is a schematic diagram of example variable frequency drives connected in parallel.

[0042] The use of the same reference numbers in different drawings indicates similar or identical items. DETAILED DESCRIPTION

[0043] The following discussion is directed to various exemplary embodiments. However, those skilled in the art will appreciate that the examples disclosed herein have broad applications, and the discussion of any embodiment is only meant as an example of that embodiment, and does not imply that the scope of the present disclosure (including claims) is limited to that embodiment.

[0044] The drawings are not necessarily to scale. For the sake of clarity and conciseness, certain features and components herein may be exaggerated in scale or shown in somewhat schematic form, and some details of conventional elements may be omitted.

[0045] In the subsequent description and claims, the terms "including" and "comprising" are used in an open manner and should be interpreted as "including but not limited to...". Similarly, the term "coupled" is intended to mean an indirect or direct connection. Thus, if a first device is coupled to a second device, the connection may be through a direct connection of the two devices, or through an indirect connection established via other devices, components, nodes, and connections. In addition, when used herein (including in the claims), the words "about", "usually", "substantially", "approximately", etc. mean within a range of plus or minus 10%, unless otherwise stated herein.

[0046] Figure 1 A medium voltage VFD 102 is shown coupled between an industrial HVAC system 104 and a three-phase medium voltage feed 106. The HVAC system 104 includes a three-phase medium voltage AC motor 110 electrically connected to and controlled by the three-phase medium voltage VFD 106.

[0047] Three-phase medium voltage VFDs are expensive to manufacture. In addition, three-phase medium voltage VFDs are bulky, which makes them difficult to build and transport to customers. The market for three-phase medium voltage VFDs is diverse and includes many (e.g., 14+) unique voltage ratings, and many (e.g., 11+) unique power ratings. Therefore, VFD suppliers are often required to offer three-phase medium voltage VFDs in multiple configurations to meet various market needs, which can be challenging from a manufacturing standpoint.

[0048] If the VFD manufacturer's customers use standard low voltage AC motors instead of medium voltage AC motors, some of the problems associated with three-phase medium voltage VFDs can be alleviated. However, using three-phase low voltage (e.g., 480V) AC motors is not directly compatible with the full range of customer feeder voltages. Low voltage can be considered less than 690V.

[0049] Figure 2 A three-phase low voltage VFD 202 is shown coupled between an HVAC system 204 and a harmonic filter 206. The HVAC system 204 includes a three-phase low voltage AC motor 212. Figure 2Also included is a three-phase step-down transformer 210 that transforms the three-phase medium voltage input provided by the three-phase medium voltage feed 106 into a three-phase low voltage output required by the low voltage VFD 202 and / or the low voltage AC motor 212 .

[0050] Even at low voltage, AC motor 212 may require more Figure 1 The medium voltage AC motor 110 shown requires a higher current to provide the same power to the compressor of the HVAC system 204. VFDs produce harmonic distortion. High current, low voltage VFDs produce substantial harmonic distortion. Harmonic filter 206 is added to reduce the harmonic distortion of VFD 202. However, harmonic filter 206 increases Figure 2 Complexity and cost of the system shown.

[0051] Figure 3 is a block diagram illustrating a VFD system 300 configured to address the above-mentioned problems and other problems. The VFD system 300 has a three-phase medium voltage input and a three-phase low voltage output, which is used to drive a low voltage AC motor 212 of the HVAC system 204. Although the present disclosure is made with reference to three-phase low voltage AC motors employed in industrial HVAC systems, it should be understood that the present disclosure should not be limited thereto.

[0052] As shown, the system 300 includes a plurality of three-phase low voltage VFDs 302 connected in parallel and a step-down transformer, which are connected in combination to the AC motor 212 of the HVAC system 204, and the step-down transformer includes a plurality of three-phase outputs. The step-down transformer is configured to transform the three-phase medium voltage provided by the feeder 106 into a plurality of three-phase symmetrical phase-shifted low voltage outputs. Each of the plurality of outputs of the step-down transformer provides a three-phase low voltage (e.g., 460V) voltage to a corresponding VFD 302. The VFD 302 can provide the AC motor 212 with the same voltage as the VFD 302. Figure 2 However, power is distributed in parallel to the AC motors 212 by the VFDs 302. All factors being equal, the power delivered to the AC motors 212 by each VFD 302 is substantially less than the power delivered by the VFDs 202.

[0053] The step-down transformer includes a set of primary windings and multiple sets of secondary windings. The multiple sets of secondary windings are configured to symmetrically phase shift the three-phase low voltage output provided to the corresponding VFD 302. The three-phase output of the secondary windings is phase shifted to produce harmonic cancellation and thus reduce the total harmonic distortion of the VFD system 300.

[0054] The amount of power required by the AC motor 212 may vary from user to user. The number of secondary windings and corresponding VFD 302 may vary based on the customer's power requirements. Figure 4-Figure 6An example of a VFD system 300 providing different levels of power to the AC motor 212 is shown. Figure 4 A VFD system 300 is shown having a pair of secondary windings 402 that provide two phase-shifted outputs to the respective VFDs 302. More specifically, the three-phase output of the secondary winding 402-1 is phase-shifted by +30° from the phase of the three-phase voltage input provided to the primary winding, and the three-phase output of the secondary winding 402-2 is phase-shifted by -30° from the phase of the three-phase voltage input provided to the primary winding. The 30-degree symmetrical phase shift results in 12 conduction pulses per 360-degree electrical cycle. This is commonly referred to as 12-pulse operation. Figure 5 A VFD system 300 is shown having three secondary windings 502 that provide three phase-shifted outputs to respective VFDs 302. More specifically, the three-phase output of secondary winding 502-1 is phase-shifted by +20° from the phase of the three-phase voltage input provided to the primary winding, the three-phase output of secondary winding 502-2 is in phase with the phase of the three-phase voltage input provided to the primary winding (phase shifted by 0°), and the three-phase output of secondary winding 502-3 is phase-shifted by -20° from the phase of the three-phase voltage input provided to the primary winding. The symmetrical phase shift of 20 degrees results in 18 conduction pulses per 360 degree electrical cycle. This is commonly referred to as 18-pulse operation. Figure 6 A VFD system is shown with four secondary windings 602 that provide three-phase shifted outputs to respective VFDs 302. More specifically, the three-phase output of secondary winding 602-1 is phase-shifted by +30° from the phase of the three-phase voltage input provided to the primary winding, the three-phase output of secondary winding 602-2 is phase-shifted by +15° from the phase of the three-phase voltage input provided to the primary winding, the three-phase output of secondary winding 602-3 is phase-shifted by -15° from the phase of the three-phase voltage input provided to the primary winding, and the three-phase output provided by secondary winding 602-4 is phase-shifted by -30° from the phase of the three-phase voltage input provided to the primary winding. The symmetrical phase shift of 15 degrees results in 24 conduction pulses per 360 degree electrical cycle. This is commonly referred to as 24-pulse operation.

[0055] As will be described more fully, the secondary windings may be connected in a "zig-zag" configuration to introduce phase shift. Zig-zag transformers use additional winding interconnections to produce a phase shift characteristic. The term "zig-zag" is most commonly used for star connections, but it can also be used to describe a delta phase shift configuration. Delta-zig-zag is also known as a delta-polygon connection.

[0056] Each VFD 302 provides three-phase voltages of equal magnitude (eg, 460V) to the low voltage motor 212. In other words, Figure 4-Figure 6Each VFD system 300 shown in FIG. 1 provides the same low voltage input to the low voltage AC motor 212. However, Figure 6 The VFD system 300 shown may be compared to Figure 5 The VFD system 300 shown provides more power while Figure 5 The VFD system 300 shown can be compared to Figure 4 The illustrated VFD system 300 provides more power to the low voltage AC motor 212 while maintaining low harmonic distortion due to symmetrical phase shifting.

[0057] Figure 4-Figure 6 The step-down transformer shown in can be used in many different configurations. Figure 7-Figure 9 It shows that it can be Figure 5 Three examples of step-down transformers used in the system shown. Figure 7-Figure 9 The page also shows Figure 5 The VFD system is 300. Figure 7 and Fig. 9 In the case of a rectifier, the primary winding AC is connected in a "delta" configuration. Figure 8 In the MOSFET, the primary winding AC is connected in a "star" configuration. Figure 8 and Fig. 9 The secondary windings a1-c1, a2-c2 and a3-c3 in are connected in a star configuration, and Figure 7 The secondary windings a1-c1, a2-c2 and a3-c3 in are connected in a delta configuration. Other configurations for both the primary and secondary windings are contemplated. Figure 7-Figure 9 Not clearly shown in the drawings, the secondary windings a1-c1 and a3-c3 are connected in a zigzag configuration to introduce a phase shift relative to each other and the windings a2-c2. Thus, the secondary windings a1-c1 are connected to introduce a +20° phase shift relative to the three-phase output provided by the secondary windings a2-c2, and the secondary windings a3-c3 are connected to introduce a -20° phase shift relative to the phase shift provided by the secondary windings a2-c2.

[0058] As mentioned above, a symmetrical phase shift is introduced into the secondary winding of the step-down transformer to reduce the total harmonic distortion. Figure 7-Figure 9 The example shown in introduces positive and negative 20° phase shifts relative to the output of secondary windings a2-c2. Fig. 10A Show Figure 7 One embodiment of the connections in the secondary windings a1-c1, a2-c2 and a3-c3 is shown. For ease of explanation and illustration, the terminals of the secondary windings a1-c1, a2-c2 and a3-c3 are also indicated as a1-c1, a2-c2 and a3-c3.

[0059] like Fig. 10BAs shown, windings a1-c1, a2-c2 and a3-c3 are connected in a delta configuration. Windings a1-c1 and a3-c3 (but not winding a2-c2) are connected in a "zigzag" configuration. Fig.11 An exemplary step-down transformer is shown, which includes a primary winding AC arranged in a delta configuration, and one secondary winding (ie, a1-c1) arranged in a delta-zigzag configuration. To simplify the illustration, Fig.11 The secondary windings a2-c2 and a3-c3 are not shown.

[0060] Fig. 10A and Fig.11 The winding a1-c1 is shown subdivided into two parts. The winding a1 is subdivided into sub-windings wa1 and wa2, each of which is magnetically connected with the primary winding A. For the purpose of explanation only, magnetically connected windings refer to windings wound around the same core. Fig. 10A and Fig.11 It is also shown that winding b1 is divided into sub-windings wb1 and wb2, each of which is in magnetic communication with primary winding B. Winding c1 is subdivided into sub-windings wc1 and wc2, each of which is in magnetic communication with primary winding C.

[0061] Fig. 10A It is shown that the secondary windings a2-c2 are respectively connected to the primary winding AC magnetically. Each of the windings a3-c3 is subdivided into two parts. Winding a3 is divided into sub-windings wa4 and wa5, each of which is connected to the primary winding A magnetically. Winding b3 is divided into sub-windings wb4 and wb5, each of which is connected to the primary winding B magnetically. Winding c3 is divided into sub-windings wc4 and wc5, each of which is connected to the primary winding C magnetically. Windings a1-c1 are respectively connected to windings a2-c2 magnetically, and windings a2-c2 are respectively connected to windings a3-c3 magnetically.

[0062] As described above, windings a1-c1 and a3-c3 are connected in a zigzag delta configuration to introduce a +20° and -20° relative phase shift. Fig. 10B It is shown Fig. 10A Vector diagram of the relative phase angles between the secondary windings of the step-down transformer in FIG. Fig. 10A and 10B , terminal a1 is fed by windings wa1 and wc2. The two windings are spaced 120 degrees apart. The voltage values ​​in each winding are chosen so that the vector combination of wa1 and wc2 adds up to a +20 degree phase shift compared to wa3. Terminal a3 is similar except that phase b instead of phase c is used to produce a negative phase shift. The phase shift is related to the turns ratio in the sub-windings. Fig. 10A and Fig.11The exemplary step-down phase-shifting transformer of demonstrates a 4160V primary winding and a 460V secondary winding. The non-phase-shifted winding (wa3, wb3, wc3) uses a turns ratio of 0.110 compared to the primary winding. The phase-shifted winding includes two sections. The longer section (wa1, wb1, wc1, wa4, wb4, wc4) uses a turns ratio of 0.082 relative to the primary winding. The shorter section (wa2, wb2, wc2, wa5, wb5, wc5) uses a turns ratio of 0.0437 relative to the primary winding.

[0063] Figure 4-Figure 6 The output of the secondary winding of the example VFD system 300 shown in is provided to the input of a corresponding VFD 302. Fig. 10A The secondary windings a1-c1, a2-c2, and a3-c3 are shown coupled to the corresponding rectifiers of the VFD 302. Figure 5 An example VFD system 300 is shown. More specifically, the output terminals of the secondary windings a1-c1, also indicated as a1-c1, are connected to the respective inputs of the rectifier R1, the output terminals of the secondary windings a2-c2, also indicated as a2-c2, are connected to the respective inputs of the rectifier R2, and the output terminals of the secondary windings a3-c3, also indicated as a3-c3, are connected to the respective inputs of R3. The rectifiers R1-R3 are electrically connected to the inverters I1-I3, respectively, and provide DC power to the inverters I1-I3, respectively. As shown, the DC link capacitors C1-C3 are electrically connected between the rectifiers and the inverters. The outputs of the inverters are connected in parallel. The parallel connection points of multiple VFDs can be formed at the motor terminals. The wire lengths from the VFDs to the motors should be equal. This wire length introduces a small amount of inductance to help balance the current sharing of the parallel VFDs.

[0064] Fig.12 An example multiple VFDs 302 connected in parallel are shown, and the combination of multiple VFDs 302 is connected in series between the corresponding secondary windings a1-c1, a2-c2, and a3-c3 (not shown) and the low voltage motor 212. Each VFD 302 includes a DC link capacitor connected between a rectifier R and an inverter I. Each phase of the rectifier R includes a high-side diode DH connected to a low-side diode DL. Each phase of the inverter I includes a high-side switch SH connected to a low-side switch SL. In the example shown, each high-side switch SH includes an insulated gate bipolar transistor (IGBT) connected in parallel with a diode DHx, and each low-side switch includes an IGBT connected in parallel with a diode. Alternative switches such as MOSFETs may also be considered.

[0065] The high-side switches SH are respectively connected in series with the low-side switches SL, which are in turn connected to the corresponding terminals of the motor 212. The collectors of the high-side switches SH are connected to the V- input terminal of the rectifier R. A DC voltage Vdc is received from the rectifier R between the V+ and V- input terminals.

[0066] The high-side switch SH and the low-side switch SL are controlled by a microcontroller (or other similar data processing device) through corresponding gate drivers (not shown). The gate driver is a circuit that receives a low-power input signal from a device (such as a microcontroller) and generates a corresponding high-current output signal required to control the gate of the power switch.

[0067] The control of the inverter is relatively simple. The high-side switches SH1-SH3 of each inverter 1 receive corresponding control signals (e.g., pulse width modulation signals) H1-H3 from a microcontroller or similar device, and the low-side switches SL1-SL3 of each inverter 1 receive corresponding control signals (e.g., pulse width modulation signals) L1-L3 from a microcontroller or similar device. The microcontroller activates the high-side switches SH1-SH3 by respectively validating the control signals H1-H3, and the microcontroller activates the low-side switches SL1-SL3 by respectively validating the control signals L1-L3. When activated, each of the switches SH1-SH3 and SL1-SL3 conducts current to or from the motor 212. As described above and reiterated below, the present disclosure includes, but is not limited to, the following exemplary embodiments.

[0068] Item 1. A device comprising a transformer, the transformer comprising a set of input terminals, a first set of output terminals, and a second set of output terminals; a first rectifier and a second rectifier, the first rectifier being coupled to the first set of output terminals and the second rectifier being coupled to the second set of output terminals, respectively; wherein the transformer is configured to transform a three-phase alternating current (AC) provided to the set of input terminals into a first three-phase AC output at the first set of output terminals and a second three-phase AC output at the second set of output terminals, respectively; wherein the first three-phase AC output is phase-shifted from the second three-phase AC output.

[0069] Item 2. The device as described in the preceding manner further includes a third rectifier; wherein the transformer includes a third set of output terminals coupled to the third rectifier; wherein the transformer is configured to transform the three-phase alternating current AC provided to the set of input terminals into the first three-phase AC output at the first set of output terminals, the second three-phase AC output at the second set of output terminals, and the third three-phase AC output at the third set of output terminals; wherein the third three-phase AC output is phase-shifted from the first three-phase AC output and the second three-phase AC output.

[0070] Clause 3. An apparatus as described in any preceding manner, wherein the first three-phase AC output leads the second three-phase AC output, and the third three-phase AC output lags the second three-phase AC output.

[0071] Clause 4. The apparatus of any preceding means, wherein the first three-phase AC output leads the second three-phase AC output by 20 degrees, and the third three-phase AC output lags the second three-phase AC output by 20 degrees.

[0072] Clause 5. The apparatus of any preceding means, wherein the amount by which the first three-phase AC output leads the second three-phase AC output is substantially equal to the amount by which the third AC output lags the second three-phase AC output.

[0073] Clause 6. The apparatus of method 2 further comprising a first inverter coupled to the first rectifier, a second inverter coupled to the second rectifier, and a third inverter coupled to the third rectifier.

[0074] Clause 7. The apparatus of any preceding manner, further comprising a low voltage motor coupled to the first inverter, the second inverter, and the third inverter.

[0075] Clause 8. The apparatus of any preceding manner, further comprising a low voltage motor coupled to the first rectifier, the second rectifier, and the third rectifier.

[0076] Clause 9. An apparatus as described in any of the preceding methods, wherein the transformer comprises: a group of primary windings; a first group of secondary windings, a second group of secondary windings, and a third group of secondary windings; wherein the first group of secondary windings and the third group of secondary windings are arranged in a zigzag configuration, while the second group of secondary windings is not arranged in a zigzag configuration.

[0077] Clause 10. The apparatus of any preceding manner, wherein each of the first and third sets of secondary windings are arranged in a triangular zigzag configuration.

[0078] Clause 11. Another device includes a first rectifier, a second rectifier and a third rectifier, the first rectifier including a first set of input terminals and a first direct current (DC) output terminal, the second rectifier including a second set of input terminals and a second direct current (DC) output terminal and the third rectifier including a third set of input terminals and a third direct current (DC) output terminal; a first inverter, a second inverter and a third inverter, the first inverter including a first set of output terminals and a first DC input terminal electrically connected to the first DC output terminal, the second inverter including a second set of output terminals and a second DC input terminal electrically connected to the second DC output terminal and the third inverter including a third set of output terminals and a third DC input terminal electrically connected to the third DC output terminal; and a motor electrically connected to the first set of output terminals of the first inverter, the second set of output terminals of the second inverter and the third set of output terminals of the third inverter.

[0079] Clause 12. The apparatus of the preceding manner, wherein each of the first rectifier, the second rectifier, and the third rectifier is a passive rectifier.

[0080] Clause 13. The device as described in any of the preceding methods further includes: a transformer, which includes a group of input terminals, a first group of output terminals, a second group of output terminals and a third group of output terminals; wherein the first group of output terminals of the transformer is electrically connected to the first group of input terminals of the first rectifier; wherein the second group of output terminals of the transformer is electrically connected to the second group of input terminals of the second rectifier; wherein the third group of output terminals of the transformer is electrically connected to the third group of input terminals of the third rectifier.

[0081] Clause 14. An apparatus as described in any of the preceding methods, wherein the transformer is configured to transform three-phase alternating current (AC) provided to the set of input terminals of the transformer into a first three-phase AC output at the first set of output terminals of the transformer, a second three-phase AC output at the second set of output terminals, and a third three-phase AC output at the third set of output terminals.

[0082] Clause 15. The apparatus of any preceding manner, wherein the first three-phase AC output is phase shifted from the second three-phase AC output, and wherein the third three-phase AC output is phase shifted from the second three-phase AC output.

[0083] Clause 16. The apparatus of any preceding means, wherein the first three-phase AC output leads the second three-phase AC output, and the third three-phase AC output lags the second three-phase AC output.

[0084] Mode 17. The apparatus of any preceding mode, wherein the first three-phase AC output leads the second three-phase AC output by 20 degrees, and the third three-phase AC output lags the second three-phase AC output by 20 degrees.

[0085] Clause 18. The apparatus of any preceding means, wherein the amount by which the first three-phase AC output leads the second three-phase AC output is substantially equal to the amount by which the third three-phase AC output lags the second three-phase AC output.

[0086] Clause 19. An apparatus as described in any of the preceding methods, wherein the transformer comprises: a set of primary windings; first, second and third sets of secondary windings; wherein the first set of secondary windings and the third set of secondary windings are arranged in a zigzag configuration, and the second set of secondary windings is not arranged in a zigzag configuration.

[0087] Clause 20. An apparatus as described in any preceding manner, comprising a three-phase motor having three-phase input terminals connected to corresponding three-phase output terminals of the first inverter, the second inverter and the third inverter through electrical conductors of equal length.

[0088] Although exemplary embodiments have been shown and described, modifications may be made thereto by those skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not restrictive. Many variations and modifications of the systems, devices, and processes described herein are possible and are within the scope of the present disclosure. Therefore, the scope of protection is not limited to the embodiments described herein, but is limited only by the appended claims, the scope of which shall include all equivalents of the subject matter of the claims. Unless otherwise expressly stated, the steps in the method claims may be performed in any order. Identifiers such as (a), (b), (c) or (1), (2), (3) listed before the steps in the method claims are not intended to specify or do not specify a specific order of the steps, but are used to simplify subsequent references to these steps.

Claims

1. A variable frequency drive system, characterized in that: The variable frequency drive system comprises: a transformer, the transformer comprising a set of input terminals, a first set of output terminals, and a second set of output terminals; a first rectifier and a second rectifier, the first rectifier coupled to the first set of output terminals and the second rectifier coupled to the second set of output terminals; wherein the transformer is configured to transform the three-phase alternating current supplied to the set of input terminals into a first three-phase alternating current output at the first set of output terminals and a second three-phase alternating current output at the second set of output terminals, respectively; Wherein the first three-phase AC power output is phase shifted from the second three-phase AC power output.

2. The variable frequency drive system according to claim 1, characterized in that: The variable frequency drive system further comprises: The third rectifier; wherein the transformer includes a third set of output terminals coupled to the third rectifier; wherein the transformer is configured to transform the three-phase AC power provided to the set of input terminals into the first three-phase AC power output at the first set of output terminals, the second three-phase AC power output at the second set of output terminals, and the third three-phase AC power output at the third set of output terminals; Wherein the third three-phase AC power output is phase-shifted from the first three-phase AC power output and the second three-phase AC power output.

3. The variable frequency drive system according to claim 2, characterized in that: The first three-phase AC power output leads the second three-phase AC power output, and the third three-phase AC power output lags the second three-phase AC power output.

4. The variable frequency drive system according to claim 3, characterized in that: The first three-phase AC power output leads the second three-phase AC power output by 20 degrees, and the third three-phase AC power output lags the second three-phase AC power output by 20 degrees.

5. The variable frequency drive system according to claim 4, characterized in that: The amount by which the first three-phase AC power output leads the second three-phase AC power output is within a range of plus or minus 10% of the amount by which the third three-phase AC power output lags the second three-phase AC power output.

6. The variable frequency drive system according to claim 2, characterized in that: The variable frequency drive system further includes a first inverter coupled to the first rectifier, a second inverter coupled to the second rectifier, and a third inverter coupled to the third rectifier.

7. The variable frequency drive system according to claim 6, characterized in that: The variable frequency drive system further includes a low voltage motor coupled to the first inverter, the second inverter, and the third inverter.

8. The variable frequency drive system according to claim 2, characterized in that: The variable frequency drive system further includes a low voltage motor coupled to the first rectifier, the second rectifier, and the third rectifier.

9. The variable frequency drive system according to claim 2, characterized in that: The transformer comprises: A set of primary windings; a first set of secondary windings, a second set of secondary windings, and a third set of secondary windings; Wherein, the first set of secondary windings and the third set of secondary windings are arranged in a zigzag configuration, while the second set of secondary windings is not arranged in a zigzag configuration.

10. The variable frequency drive system according to claim 9, characterized in that: Each of the first and third sets of secondary windings are arranged in a triangular zigzag configuration.

11. A variable frequency drive system, characterized in that: The variable frequency drive system comprises: a first rectifier, a second rectifier and a third rectifier, the first rectifier comprising a first set of input terminals and a first DC output terminal, the second rectifier comprising a second set of input terminals and a second DC output terminal and the third rectifier comprising a third set of input terminals and a third DC output terminal; a first inverter, a second inverter and a third inverter, the first inverter comprising a first set of output terminals and a first DC input terminal electrically connected to the first DC output terminal, the second inverter comprising a second set of output terminals and a second DC input terminal electrically connected to the second DC output terminal and the third inverter comprising a third set of output terminals and a third DC input terminal electrically connected to the third DC output terminal; A motor is electrically connected to the first set of output terminals of the first inverter, the second set of output terminals of the second inverter, and the third set of output terminals of the third inverter.

12. The variable frequency drive system according to claim 11, characterized in that: Each of the first rectifier, the second rectifier, and the third rectifier is a passive rectifier.

13. The variable frequency drive system according to claim 11, characterized in that: Also includes: a transformer, the transformer comprising a set of input terminals, a first set of output terminals, a second set of output terminals, and a third set of output terminals; wherein the first group of output terminals of the transformer is electrically connected to the first group of input terminals of the first rectifier; wherein the second group of output terminals of the transformer is electrically connected to the second group of input terminals of the second rectifier; Wherein, the third group of output terminals of the transformer is electrically connected to the third group of input terminals of the third rectifier.

14. The variable frequency drive system according to claim 13, characterized in that: The transformer is configured to transform three-phase alternating current provided to the set of input terminals of the transformer into a first three-phase alternating current output at the first set of output terminals of the transformer, a second three-phase alternating current output at the second set of output terminals, and a third three-phase alternating current output at the third set of output terminals.

15. The variable frequency drive system according to claim 14, characterized in that: The first three-phase AC output is phase shifted from the second three-phase AC output, and wherein the third three-phase AC output is phase shifted from the second three-phase AC output.

16. The variable frequency drive system according to claim 14, characterized in that: The first three-phase AC power output leads the second three-phase AC power output, and the third three-phase AC power output lags the second three-phase AC power output.

17. The variable frequency drive system according to claim 16, characterized in that: The first three-phase AC power output leads the second three-phase AC power output by 20 degrees, and the third three-phase AC power output lags the second three-phase AC power output by 20 degrees.

18. The variable frequency drive system according to claim 17, characterized in that: The amount by which the first three-phase AC power output leads the second three-phase AC power output is within a range equal to plus or minus 10% of the amount by which the third three-phase AC power output lags the second three-phase AC power output.

19. The variable frequency drive system according to claim 13, characterized in that: The transformer comprises: A set of primary windings; a first set of secondary windings, a second set of secondary windings, and a third set of secondary windings; Wherein, the first set of secondary windings and the third set of secondary windings are arranged in a zigzag configuration, while the second set of secondary windings is not arranged in a zigzag configuration.

20. The variable frequency drive system according to claim 19, characterized in that: The variable frequency drive system further includes a three-phase motor having three-phase input terminals connected to corresponding three-phase output terminals of the first inverter, the second inverter, and the third inverter through electrical conductors of equal length.

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