Title of Invention	A CIRCUIT BRAKER
Abstract	The invention relates to a braking circuit device, comprising an array of power cells (251-253, 261-263, 271-273,281-283) electrically connected to receive power from a source (210) and deliver power to a load (230), the array comprising a first rank (250,270) of regenerative power cells (251-253,271- 273); and a second rank (260,280) of non-regenerative power cells (261- 263,281-283) connected to the first rank (250,270) of regenerative power cells. A control circuit (292,295) connected to the array of power cells, wherein the control circuit is configured to generate PWM voltage commands for controlling the regenerative power cells (251-253,271-273) and the non-regenerative power cells (261-263,281-283).

Title of Invention

A CIRCUIT BRAKER

Abstract

The invention relates to a braking circuit device, comprising an array of power cells (251-253, 261-263, 271-273,281-283) electrically connected to receive power from a source (210) and deliver power to a load (230), the array comprising a first rank (250,270) of regenerative power cells (251-253,271- 273); and a second rank (260,280) of non-regenerative power cells (261- 263,281-283) connected to the first rank (250,270) of regenerative power cells. A control circuit (292,295) connected to the array of power cells, wherein the control circuit is configured to generate PWM voltage commands for controlling the regenerative power cells (251-253,271-273) and the non-regenerative power cells (261-263,281-283).

Full Text	FIELD OF THE INVENTION The present invention relates to variable-frequency drive with regeneration capacity. More particularly, the invention relates to a circuit braker controlling non-generative power cells when the load is in regenerative operation. BACKGROUND OF THE INVENTION [0002] In recent years, circuits for medium- voltage variable frequency drive (VFD) applications have received attention. Several novel methods have been introduced in the past decade. For example, in a circuit comprising series-connected inverters as described in U.S. Patent No. 5,625,545 to Hammond, the disclosure of which is incorporated herein by reference in its entirety, an inverter or power cell 110 includes a three-phase diode-bridge rectifier 112, one or more direct current (DC) capacitors 114, and an H-bridge inverter 116. The rectifier 112 converts the input 118 alternating current (AC) voltage to a substantially constant DC voltage that is supported by the capacitors 114 that are connected across the rectifier 112 output. The output stage of the inverter 110 includes an H-bridge inverter 116 includes two poles, a left pole and a right pole, each with two devices. The inverter 110 transforms the DC voltage across the DC capacitors 114 to an AC output 120 using pulse-width modulation (PWM) of the semiconductor devices in the H-bridge inverter 116. [0003] A circuit including power cells such as 110 in FIG. 1 , when connected to a load, such as a motor, can provide power from an input source to the motor when operating in the motoring mode. However, when the motor speed needs to be reduced, power from the motor needs to be absorbed by the inverter. This mode of operation, when power must be absorbed by the inventor, is referred to as the regeneration mode. The diode-bridge rectifiers 112 in each power cell do not allow power to be transferred back to the source. Hence, the power absorbed by the circuit is strictly limited by the losses in the inverter and the capacitors within each power cell and is usually in the range of about 0.2% to about 0.5% of rated power. [0004] The disclosure contained herein describes attempts to solve one or more of the problems described above. SUMMARY [0005] In an embodiment, a braking circuit includes an arrangement of power cells electrically connected to receive power from a source and deliver power to a load. The circuit includes a first rank of regenerative power cells, a second rank of non-regenerative power cells, and a control circuit. The output voltages of the regenerative power cells and non-regenerative power cells may be maintained at least substantially at their rated values when the load is operated at less than rated flux and less than rated current. The control circuit may at least substantially use the current capability of the regenerative power cells when the load is operated at rated flux and less than rated current. The control circuit may at least substantially use the current capability of the regenerative power cells and non-regenerative power cells when the load is operated at rated flux and full current. A voltage drop may occur across all of the cells during braking. [ 0 0 0 6 ] In some embodiments, each regenerative power cell may include an inverter bridge, a capacitor set electrically connected across terminals of the inverter bridge, and an active front end comprising a plurality of transistors electrically connected as a three-phase bridge. In an alternate embodiment, each regenerative power cell may include an inverter bridge, a capacitor set electrically connected across terminals of the inverter bridge, a three-phase diode bridge rectifier electrically connected across the terminals, and a series-connected transistor and resistor combination that is electrically connected across the terminals. In either embodiment, the inverter bridge may comprise, for example, a four-transistor H-bridge inverter or an eight-transistor H-bridge inverter based on a neutral-point-clamped connection. [0007] In some embodiments, each non-regenerative power cell may include an inverter bridge, a capacitor set electrically connected across terminals of the inverter bridge, and a three-phase bridge rectifier electrically connected across the terminals. This inverter bridge also may comprise, for example, a four-transistor H- bridge inverter or an eight-transistor H-bridge inverter based on a neutral-point- clamped connection. In some embodiments, the regenerative power cells and non- regenerative power cells are removably and interchangeably installed in a housing. [0008] In an alternate embodiment, an electrical device includes a plurality of single-phase power cells electrically connected to receive power from a source and deliver power to a load. The single-phase power cells include a first rank of regenerative power cells and a second rank of non-regenerative power cells. Each non-regenerative power cell may include an inverter bridge, a capacitor set electrically connected across terminals of the inverter bridge, and a three-phase bridge ' rectifier electrically connected across the terminals. The non-regenerative power cells may provide reactive power when the plurality of cells are used for braking of a motor. [0009] In an alternate embodiment, an electrical device, includes a first rank that includes at least three single-phase non-regenerative power cells, a second rank that includes at least three single-phase regenerative power cells, and a control circuit. Each non-regenerative power cell includes an inverter bridge, a capacitor set electrically connected across terminals of the inverter bridge, and a three-phase bridge rectifier electrically connected across the terminals. When the device is used for braking of a motor, a three-phase power cell is not required, and the non-regenerative power cells provide reactive power. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS [0010] Aspects, features, benefits and advantages of the present invention will be apparent with regard to the following description and accompanying drawings, of which: [0011] FIG. 1 depicts a prior art power cell. [0012] FIG. 2 depicts a circuit comprising a plurality of power cells connected to a load. [0013] FIGs. 3A and 3B are a block diagram of exemplary regenerative power cells. [0014] FIG. 4 is a diagram of an arrangement of regenerative power cells and non-regenerative power cells. [0015] FIGs. 5A and 5B illustrate exemplary voltage-current relationships for motoring, regeneration, and maximum braking in various embodiments. [0016] FIG. 6 illustrates an exemplary voltage-current relationship of a circuit in a motor speed range that is greater than the speed at which rated flux can be applied to the motor. [0017] FIG. 7 illustrates an exemplary voltage-current relationship of a circuit in a motor speed range that is less than the speed at which rated flux can be applied to the motor but greater than the speed at which rated current can be applied to the motor. [0018] FIG. 8 is a block diagram of an exemplary control circuit. [0019] FIGs. 9A - 9D illustrate simulation results of a variable frequency drive at various motor speeds. DETAILED DESCRIPTION [0020] Before the present methods, systems and materials are described, it is to be understood that this disclosure is not limited to the particular methodologies, systems and materials described, as these may vary. It is also to be understood that the terminology used in the description is for the purpose of describing the particular versions or embodiments only, and is not intended to limit the scope. For example, as used herein and in the appended claims, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. In addition, the following terms are intended to have the following definitions herein: [0021] capacitor set - one or more capacitors. [0022] comprising - including but not limited to. [0023] control circuit - a first electrical device that signals a second electrical device to change a state of the second electrical device. [0024] electrically connected or electrically coupled — connected in a manner adapted to transfer electrical energy. [0025] H-bridge inverter - a circuit for controlled power flow between AC and DC circuits having four transistors and four diodes. Referring to FIG. 1, an H- bridge inverter 116 generally includes a first phase leg and a second phase leg electrically connected in parallel. Each leg includes two transistor/diode combinations. In each combination, the diode is electrically coupled across the base and emitter of the transistor. [0026] inverter - a device that converts DC power to AC power or AC power to DC power. [0027] medium voltage - a rated voltage greater than 690 volts (V) and less than 69 kilovolts (kV). In some embodiments, medium voltage may be a voltage between about 1000 V and about 69 kV. [0028] neutral-point clamped connection - in the context of an eight- transistor H-bridge inverter, an arrangement of eight transistors to include a first phase leg and a second phase leg electrically connected in parallel at the DC terminals. Each leg includes four transistors. The mid-point between the upper pair of transistors and the mid-point between the lower pair of transistors of each phase leg is electrically connected through diodes to form a neutral point. [0029] non-regenerative power cell - a power cell that does not have the capability of absorbing regenerative power. [0030] power cell - an electrical device that has a three-phase alternating current input and a single-phase alternating current output. [0031] rank - an arrangement of power cells established across each phase of a three-phase power delivery system. [0032] rated speed - the number of times that the shaft of a motor may turn in a time period, such as revolutions per minute (RPM), when it is operating at its rated torque. [0033] regenerative power cell - a power cell that has the capability of absorbing regenerative power. [0034] substantially - to a great extent or degree. [0035] three-phase bridge rectifier - a device including an arrangement of semiconductive devices such diodes that converts three-phase alternating current to direct current. [0036] In various embodiments, a multi-level power circuit uses single-phase series-connected regenerative cell and non-regenerative cell inverters to provide limited braking capability. FIG. 2 illustrates an exemplary embodiment of a circuit having such inverters. In FIG. 2, a transformer 210 delivers three-phase, medium- voltage power to a load 230 such as a three-phase induction motor via an array of single-phase inverters (also referred to as power cells). A three-phase inverter is not required in the array. The transformer 210 includes primary windings 212 that excite a number of secondary windings 214 - 225. Although primary winding 212 is illustrated as having a star configuration, a mesh configuration is also possible. Further, although secondary windings 214 - 225 are illustrated as having a mesh configuration, star-configured secondary windings are possible, or a combination of star and mesh windings may be used. Further, the number of secondary windings illustrated in FIG. 2 is merely exemplary, and other numbers of secondary windings are possible. The circuit may be used for medium voltage applications or, in some embodiments, other applications. [0037] Any number of ranks of power cells are connected between the transformer 210 and the load 230. A "rank" is considered to be a three-phase set, or a group of power cells established across each of the three phases of the power delivery system. Referring to FIG. 2, rank 250 includes power cells 251-253, rank 260 includes power cells 261-263, rank 270 includes power cells 271-273, and rank 280 includes power cells 281-283. Fewer than four ranks, or more than four ranks, are possible. A central control system 295 sends command signals to local controls in each cell over fiber optics or another wired or wireless communications medium 290. [0038] As mentioned above, the prior art power cells such as those illustrated in FIG. 1 do not allow any significant amount of regeneration. In order to achieve the desired braking capability in the embodiments described herein, alternate power cells are used. FIGs. 3A and 3B show two embodiments of power cells that do permit regeneration. Referring to FIG. 3A, a power cell 300 includes an active front end 310 that serves as a three-phase bridge as it receives power from dedicated three- phase secondary windings of the transformer via an input 342. The cell 300 also includes a plurality of current-controlling devices such as transistors or thyristors 312 - 317, for example insulated gate bipolar transistors (IGBTs), integrated gate commuted thyristors or other devices, generally referred to herein as the front-end transistors. Although six transistors in a bridge format - in this example, three pairs of two transistors each connected in parallel across the DC terminals - are illustrated in FIG. 3A, other numbers of transistors may be used. These transistors can be controlled by a local and remote control system (292 and 295, respectively, in FIG. 2) to transfer energy in either direction thus allowing motoring or regenerating to full capacity (i.e., approximately or fully 100%). Any suitable method to achieve such operation with front-end transistors may be used. The remainder of the power cell 300 includes one or more capacitors 320 and an H-bridge inverter 330, each connected across the output or DC terminals of the active front end 310, to deliver AC power to the output 344. Other inverter bridges may be used as substitutes for the four-transistor H-bridge 330 illustrated in FIG. 3A. For example, an H-bridge comprising eight transistors based on the neutral-point clamped connection may be used. [0039] In an alternate embodiment, FIG. 3B illustrates a power cell 350 which includes the elements of a rectifier 360, capacitors 375, and an inverter bridge such as an H-bridge inverter 380 connected in parallel between an input 392 and output 394. A three-phase diode bridge rectifier 360 receives power from dedicated three-phase secondary windings of the transformer via the input 392. In addition, a brake circuit 370 includes a transistor 374 (referred to herein as a brake transistor) and a resistor 372 electrically connected in series with respect to each other and in parallel across the DC capacitors 375 and the DC output of the rectifier 360. The brake transistor 374 is controlled by a local controller, and during motoring the brake transistor 374 is controlled to be "off and does not participate in the energy transfer from the AC input 392 to the AC output 394. However, during regenerating the brake transistor 375 may be controlled to turn on and off in order to dissipate the energy from the motor in the resistor 372 and hence maintain the DC voltage across the capacitors 375 at a pre-determined value. [0040] A power cell that allows power to be absorbed from the motor is referred to herein as a regenerative cell (RC), and a power cell that does not allow any significant amount of regeneration (such as the one shown in FIG. 1 and is described above) is referred to as a non-regenerative cell (NRC). Although two examples of RCs are shown in FIGs. 3A and 3B, other RC cells may be used in the embodiments described herein. [0041] Thus, referring back to FIG. 2, ranks 250 and 270 may comprise regenerative cells, and ranks 260 and 280 may comprise non-regenerative cells, or vice-versa. It should be noted that the number of cells per phase depicted in FIG. 2 is exemplary, and more than or less than four ranks may be possible in various embodiments. For example, two ranks, four ranks, eight ranks, or other numbers of ranks are possible. In fact, the number of regenerative power cells used can be selected based on the desired degree of braking provided. In addition, the power cells may be removably installed in a housing so that a single housing unit may be used for various applications, with regenerative cells being exchanged for non-regenerative cells, or vice versa, depending on the desired application such as braking or regeneration desired. [0042] To obtain full regeneration capability for a circuit such as that shown in FIG. 2, RCs may be used in the entire circuit of power cells. However, in many applications, the braking capability that is needed is a much smaller fraction of the rated capacity of the load, such as approximately 10 percent to approximately 20% for ship propulsion and large fan applications. The use of RCs in the entire circuit makes the solution more expensive than it needs to be. However, we have discovered an exemplary series-connected arrangement of inverters that provides limited, but not full, braking capability. [0043] Referring to FIG. 4, a series-connected arrangement of inverters can be used to connect both RCs and NRCs within the same circuit. A first rank 410 includes three single-phase NRCs 411, 412 and 413 (one cell for each phase in a three-phase circuit), while a second rank 420 includes three single-phase RCs 421, 422 and 423 (also one for each phase). In this embodiment, three-phase power cells are not required. Each cell receives power from dedicated secondary windings of an input transformer (210 in FIG. 2). For each phase, a first output terminal of each RC cell is electrically connected to the output terminal of opposite polarity for the phase's corresponding NRC cell. The second output terminal of each RC cell is electrically connected to an output line. The remaining output terminals of all three RC cells in the rank are electrically connected to each other to form a star point 440. As shown in FIG. 2, additional ranks, such as four ranks total, six ranks total, eight ranks total, or more, may be present in the circuit. The number of ranks of RCs selected may be determined based on the desired level of regeneration required. [0 044] In some embodiments, the circuit may include a bypass feature that allows continued operation of the circuit if one or more of the power cells should fail. For example, as illustrated in U.S. Patent No. 5,986,909, and in particular FIG. 1B and the accompanying text, which are incorporated herein by reference, a bypass may create a shunt path between the output lines of a power cell if the cell fails so that current can then flow through the bypass instead of the power cell. [0045] During motoring, the RC ranks and NRC ranks may provide a substantially equal amount of power to the load. However, during regeneration, the diode-bridge rectifiers in the NRC prevent transfer of power to the utility, but front- end transistors (AFE) in the RCs are controlled to absorb the braking energy by transferring power from the load (such as a motor) to the input source. The control of the front-end transistors may be the same as or similar to known control methods, such as the regulation of the DC-voltage within each RC. However, control of the output H-bridges may require different methods. For example, FIG. 4A illustrates exemplary voltage and current vectors during motoring, while FIG. 4B illustrates exemplary voltage and current vectors during regeneration. When motoring, as illustrated in FIG. 4A, the output voltage vectors of cell 1 (an RC) and cell 2 (an NRC) are along the motor voltage. Each power cell provides an equal or substantially equal amount of power to the load. Referring to FIG. 4B, during regeneration the voltage of cell 2 (NRC) is controlled to be in quadrature with respect to the drive current. This prevents cell 2 from absorbing any real (or active) power. However, the output of cell 1 (RC) must make up the difference between the motor voltage and the output of cell 2. This restricts the amount of available voltage that the drive can produce and will require reduced flux operation at high speeds during regeneration. When a mixture of NRC and RC are used, as shown in FIG. 4, maximum braking torque is obtained when the NRC and RC voltages are also in quadrature to each other as shown in FIG. 5C. Although it is noted that a quadrature relationship (of 90 degrees) is desired to maintain zero power flow into the NRC cells, for practical implementation this angle may be reduced below 90 degrees to ensure that either no power flows into the NRC cells or some power flows out of the NRC cells. [0046] The approach of having both sets of cells (NRC and RC) generate voltages during the entire braking process allows for continuous braking during the entire speed range. Unlike prior art methods, the methods described herein can produce regenerative torque before the motor speed has dropped below the voltage capability of the RC. The cells' output voltages are substantially used (with normal system losses) to provide power to the motor when the motor is running at or near its rated speed. When the motor voltage is reduced (as for braking), the NRCs and RCs also participate so that the NRCs provide reactive power during braking, and current is reversed through the RCs. This, unlike the prior art, all cells can participate during braking. [0047] For the purpose of the following discussion, the following symbols are defined on a per-unit basis as follows: Voltage capability of the RC, VRC = x per-unit Voltage capability of the NRC, VNRC = (1-x) per-unit Total voltage capability of drive =1.0 per-unit No-load current of motor = INL Speed below which rated flux can be applied on the motor = Wv Speed below which rated current can be applied to the motor = WI Braking torque capability = TB Torque current = Iqs Magnetizing current = IdS Motor speed = w Motor voltage = Vmotor [ 0048 ] To understand operation of an exemplary drive circuit with limited braking capability, the entire operating speed range may be considered to include three speed ranges as described below. During each speed range, there may be a separate limit on the achievable braking torque. This limit depends on the relative voltage capabilities of the RC and the NRC and the magnetizing current of the motor. Although the description below is in the context of an induction motor, similar results may be achieved with a synchronous motor by operating at reduced voltage on stator side. Although the total voltage capability of the drive is described below as having a unit value of 1.0, this value can be different from 1.0 as long as x is less than that value. [0049] Speed range # 1: wv [ 0050 ] In this speed range, the drive cannot operate the motor at rated flux, because of the requirement to operate the NRC cell at quadrature with the motor current. Hence, the motor is operated at reduced flux and reduced current. Referring to FIG. 6, the maximum voltage output is given as: [0051]Vmax = √(1-2x + 2x2) (1) [0052] To increase or obtain maximum possible torque within this speed range, the motor may be operated at or near this maximum voltage. Therefore, the voltages for the cells are fixed at VRC = x, and VNRC = (1 - x). As speed decreases from rated speed, motor flux gradually increases until it equals its rated value at w = wv. Hence, in per-unit terms, Wv = Vmax. FReferring again to FIG. 6, torque current is obtained as: [0053]Iqs=x Ids/(1-x) (2) [0054] At rated speed the (minimum) braking torque capability then may be given by the following equation: [0055] TB = Vmax Iqs =INL (1 -2x + 2x2) x / (1 - x) (3) [0056] where, it is assumed that at light loads and reduced flux Id = INL Vmax [0057] Speed range # 2: wI [0058] In this speed range, the motor is operated at rated flux but reduced current, and the RC provides rated output. Hence, referring to FIG. 7: [0059] VRC = x, and VNRC = √(w2 -x2), where w represents the speed and the per-unit motor voltage, (4) [ 0060] and Iqs = x Ids/√(w2 - x2) (5) [0061] As speed decreases, Iq becomes larger and approaches rated torque current. When w = WI (where w1 = x/PF_rated), rated current is applied to the motor and rated braking torque can be obtained. [0062] Speed range #3: 0 [0063] In this speed range, the motor is operated at rated flux and rated current. The voltages from the NRC and the RC are reduced linearly with speed as shown below, so that motor current is maintained at rated: [0064] VRC = x w/wI (6) [0065] VNRC = w √(wI2 - x2) / wI. (7) [0066] The above equations show exemplary methods for controlled operation of the RCs and NRCs during regeneration. A block diagram showing an exemplary control system 800 to implement such equations in a typical motor drive controller is given in FIG. 8. In this figure, the "Flux Reference Generator" 805, "Limited Regen Voltage Allocation" 810 and "Regen Limit" 815 help to provide limited regeneration control. As shown in the figure, the exemplary circuit includes at least two inputs, the flux demand λDMD 801 and the speed reference wref 802. The Flux Reference Generator 805 calculates the flux reference using the maximum voltage given by equation (1) above and the stator frequency, ws 804, and provides the flux reference,λref 803, as an output. The Flux Reference Generator 805 also ensures that the flux reference is always less than or equal to the flux demand, ΛDMD 801. The Flux Regulator 820 compensates for the difference between the flux reference, λref 803, and the flux feedback, ΛDS 809, where ΛDS 809 is the estimated actual flux value given by the measured motor voltage and the stator speed. The output of Flux Regulator 820 is the motor magnetizing current reference, Ids 821. [0067] The speed reference, wref 802, is compared with the motor speed, w 850, in the Speed Regulator 855, which provides the motor torque current reference, IqS ref 858 as the output. When the motor is commanded to slow down, the Regen Limit block 815 calculates equations (2) and (5) above and provides a limit on the torque current reference. [0068] The circuit shown in FIG. 8 includes two Current Regulators 860 and 862 that control the magnetizing current and the torque current. Their outputs are voltage references, Vds ref 863 and VqS.rcf 861. The Limited Regen Voltage Allocation 810 block splits the motor voltage references, Vds ref 863 and Vqs.ref 861, into voltage references for the RCs and NRCs. In the Limited Regen Voltage Allocation 810 block, the magnitude of the voltages for RC and NRC may be first found using x, Vmotor and ws from equations (4), (6) and (7) above. Then, the d- and q-axis components of the voltage commands for the NRCs and the RCs (VqdsNRc 868 and VqdsRC 869) are identified using Ids ref, and IqS ref. Finally, d- and q-axis components of VRC and VNRC are converted to 3-phase voltages in the stationary frame, and are used as references to generate PWM voltage commands for controlling the inverters. [0069] FIGs. 9A - 9D provide exemplary simulation results to show the operation in regeneration for a drive configuration such as that shown in FIG. 4 with x = 0.5 (i.e. an equal number of RCs and NRC's). It should be noted that having an equal number of RCs and NRCs is not a requirement of the embodiments described herein. The drive may be commanded to go into regeneration at t = 5 seconds by decreasing the speed reference wref. [0070] FIGs. 9A and 9B illustrate exemplary motor speed (FIG. 9A) and the output voltage (FIG. 9B) of a drive having a combination of RC and NRC ranks. In each Figure, time=0 to time=5 seconds represents motoring, while the remaining time periods represent different speed ranges of braking. Unlike the prior art, as illustrated in FTGs. 9A-9B, a voltage drop is present across the non-regenerative power cells during braking. In fact, such a voltage is present during the entire braking period. [0071] Referring to FIG. 9B, during Speed Range #1, the drive output voltage is maintained at the value specified by the equation (1) while the output voltage of RC and NRC are held at their respected rated values (i.e. 100%). In the Speed Range #2, the voltage of NRC is reduced whereas the voltage of RC is maintained at its rated voltage as given by the equation (4). As soon as the motor speed enters Speed Range #3, the voltage of both RC and NRC decreases linearly as the motor slows down as specified by equations (6) and (7). [0072] FIG. 9C shows exemplary motor current components during regeneration. In Speed Range #1, the magnetizing current (IDS) is lowered to satisfy the maximum motor voltage specified by the equation (1). Subsequently, the torque current (IQS) is also reduced so that the motor current remains in quadrature with respect to the output voltage of NRC as shown in FIG. 6. Throughout Speed Range #1, both the magnetizing current and the torque current increase equally in ratio so as to bring up the motor flux towards its rated value (i.e., approximately 100%) while maintaining the power factor and keeping the quadrature relation between the motor current and the output voltage of the NRC cells. During Speed Range #2, there is more room to accommodate the larger torque current as the voltage of NRC decreases, as evident from FIG. 7. As was mentioned earlier, Speed Range #3 starts at the instant when the total motor current reaches its rated value. [0073] FIG. 9D shows active power delivered from the drive, the RC cells and the NRC cells. The active power from the NRC remains substantially zero throughout the duration of motor deceleration. In this example, all of the active power (or total power) from the motor is absorbed by the RC, thereby illustrating that the methods described herein may successfully divert all the power absorbed from the motor into the RC during regeneration. In addition, the generated power in the motor is controlled to be less than the power rating of RC. [0074] As noted above, there may be a different number of RCs and NRCS than the examples expressly described above. When the combination of RCs to NRCs is changed, the value of braking torque at full speed also may change according to equation (3). This is tabulated in the following table of braking torque for different cell combinations, assuming motor no-load current, INL, equals 25%: [0 07 5] As noted above, when the bypass feature is provided with each of the power cells, then it is possible to operate the circuit if one or more of the power cells fail. Under such a condition, the equations provided above can still be applied, but with slight adjustments as detailed here. When one or more power cells are bypassed, the total voltage capability of the RC and the NRC may change to y and z, respectively, such that t = y + z. The same procedure as detailed in equations (1) - (7) above may be followed using t, y, z instead of 1.0, x, and 1-x to determine the operation of the control with a smaller number of power cells in the circuit. [0076] Still other embodiments will become readily apparent to those skilled in this art from reading the above-recited detailed description and drawings of certain exemplary embodiments. It should be understood that numerous variations, modifications, and additional embodiments are possible, and accordingly, all such variations, modifications, and embodiments are to be regarded as being within the spirit and scope of this application. For example, regardless of the content of any portion (e.g., title, field, background' summary, abstract, drawing figure, etc.) of I this application, unless clearly specified to the contrary, such as via an explicit definition, there is no requirement for the inclusion in any claim herein (or of any claim of any application claiming priority hereto) of any particular described or illustrated characteristic, function, activity, or element, any particular sequence of activities, or any particular interrelationship of elements. Moreover, any activity can be repeated, any activity can be performed by multiple entities, and/or any element can be duplicated. Further, any activity or element can be excluded, the sequence of activities can vary, and/or the interrelationship of elements can vary. Accordingly, the descriptions and drawings are to be regarded as illustrative in nature, and not as restrictive. Moreover, when any number or range is described herein, unless clearly stated otherwise, that number or range is approximate. When any range is described herein, unless clearly stated otherwise, that range includes all values therein and all subranges therein. Any information in any material (e.g., a United States patent, United States patent application, book, article, etc.) that has been incorporated by reference herein, is only incorporated by reference to the extent that no conflict exists between such information and the other statements and drawings set; forth herein. In the event of such conflict, including a conflict that would render invalid any claim herein or seeking priority hereto, then any such conflicting information in such incorporated by reference material is specifically not incorporated by reference herein. We Claim: 1. A braking circuit, comprising: an arrangement of power cells electrically connected to receive power from a source and deliver power to a load (230), the arrangement comprising: a first rank (250,270) of regenerative power cells (251-253, 271-273); and a second rank (260,280) of non-regenerative power cells (261-263, 281- 283); wherein the second rank of non-regenerative power cells is connected to the first rank of regenerative power cells; and wherein the braking circuit further comprises a control circuit (800) connected to the arrangement of power cells, wherein the control circuit is configured to generate pulse width modulation voltage commands for controlling the regenerative power cells and the non- regenerative power cells. 2. The circuit as claimed in claim 1, wherein a voltage drop is present across the non-regenerative cells (261-263, 281-283) during braking of the load (230). 3. The circuit as claimed in claim 1, wherein the load (230) is a motor. 4. The circuit as claimed in claim 3, wherein the output voltages of the regenerative power cells (251-253,271-273) and non-regenerative power cells (261-263,281-283) are maintained at least substantially at their rated values when the load (230) is operated at less than rated flux and less than rated current of the motor. 5. The circuit as claimed in claim 4, wherein the control circuit (800) at least substantially uses the current capability of the regenerative power cells (251- 253,271-273) when the load (230) is operated at rated flux and less than rated current of the motor. 6. The circuit as claimed in claim 5, wherein the control circuit (800) at least substantially uses the current capability of the regenerative power cells (251- 253,271-273) and non-regenerative power cells (261-263,281-283) when the load (230) is operated at rated flux and full current of the motor. 7. The circuit as claimed in claim 1, wherein each regenerative power cell (300) comprises: an inverter bridge a capacitor set (320) electrically connected across terminals of the inverter bridge (330); and an active front end (310) comprising a plurality of transistors (312-317) electrically connected as a three-phase bridge. 8. The circuit as claimed in claim 1, wherein each regenerative power cell (350) comprises: an inverter bridge; a capacitor set (375) electrically connected across terminals of the inverter bridge (380); a three-phase diode bridge rectifier (360) electrically connected across the terminals; and a series-connected transistor (374) and resistor (372) combination that is electrically connected across the terminals. 9. The circuit as claimed in claim 1, wherein each non-regenerative power cell comprises: an inverter bridge; a capacitor set electrically connected across terminals of the inverter bridge ;and a three-phase bridge rectifier electrically connected across the terminals. 10. The circuit as claimed in claim 8 or claim 9, wherein the inverter bridge comprises a four-transistor H-bridge inverter (330) or an eight-transistor H- bridge inverter based on a neutral-point-clamped connection. 11.The circuit as claimed in claim 1, wherein the regenerative power cells (251- 253, 271-273) and non-regenerative power cells (261-263,281-283) are removably and interchangeably installed in a housing. 12. An electrical device, comprising a braking circuit as claimed in claim 9, wherein the power cells comprise single phase power cells; and wherein the non- regenerative power cells provide reactive power when the plurality of cells are used for braking of a motor. 13. An electrical device comprising a braking circuit as claimed in claim 9; wherein the ranks comprise at least three single-phase power cells; and wherein when the device is used for braking of a motor, a three-phase power cell is not required, and the non-regenerative power cells provide reactive power. 14. An electrical device as clamed in claim 13, wherein the regenerative power cells output substantially rated current when the load is operated at rated flux and less than rated current; and the regenerative power cells and non- regenerative power cells output substantially rated current when the load is operated at rated flux and full current. ABSTRACT TITLE: A CIRCUIT BRAKER The invention relates to a braking circuit device, comprising an array of power cells (251-253, 261-263, 271-273,281-283) electrically connected to receive power from a source (210) and deliver power to a load (230), the array comprising a first rank (250,270) of regenerative power cells (251-253,271- 273); and a second rank (260,280) of non-regenerative power cells (261- 263,281-283) connected to the first rank (250,270) of regenerative power cells. A control circuit (292,295) connected to the array of power cells, wherein the control circuit is configured to generate PWM voltage commands for controlling the regenerative power cells (251-253,271-273) and the non-regenerative power cells (261-263,281-283).

Full Text

FIELD OF THE INVENTION
The present invention relates to variable-frequency drive with regeneration capacity. More
particularly, the invention relates to a circuit braker controlling non-generative power cells when
the load is in regenerative operation.
BACKGROUND OF THE INVENTION
[0002] In recent years, circuits for medium- voltage variable frequency drive (VFD) applications
have received attention. Several novel methods have been introduced in the past decade. For
example, in a circuit comprising series-connected inverters as described in U.S. Patent No.
5,625,545 to Hammond, the disclosure of which is incorporated herein by reference in its entirety,
an inverter or power cell 110 includes a three-phase diode-bridge rectifier 112, one or more direct
current (DC) capacitors 114, and an H-bridge inverter 116. The rectifier 112 converts the input
118 alternating current (AC) voltage to a substantially constant DC voltage that is supported by
the capacitors 114 that are connected across the rectifier 112 output. The output stage of the
inverter 110 includes an H-bridge inverter 116 includes two poles, a left pole and a right pole,
each with two devices. The inverter 110 transforms the DC voltage across the DC capacitors 114
to an AC output 120 using pulse-width modulation (PWM) of the semiconductor devices in the
H-bridge inverter 116.
[0003] A circuit including power cells such as 110 in FIG. 1 , when connected to a load, such as
a motor, can provide power from an input source to the motor when operating in the motoring
mode. However, when the motor speed needs to be reduced, power from the motor needs to be
absorbed by the inverter. This mode of operation, when power must be absorbed by the inventor,
is referred to as the regeneration mode. The diode-bridge rectifiers 112 in each power cell do not
allow power to be transferred back to the source. Hence, the power absorbed by the circuit is
strictly limited by the losses in the inverter and the capacitors within each power cell and is
usually in the range of about 0.2% to about 0.5% of rated power.
[0004] The disclosure contained herein describes attempts to solve one or more of the problems
described above.

SUMMARY
[0005] In an embodiment, a braking circuit includes an arrangement of
power cells electrically connected to receive power from a source and deliver power
to a load. The circuit includes a first rank of regenerative power cells, a second rank
of non-regenerative power cells, and a control circuit. The output voltages of the
regenerative power cells and non-regenerative power cells may be maintained at least
substantially at their rated values when the load is operated at less than rated flux and
less than rated current. The control circuit may at least substantially use the current
capability of the regenerative power cells when the load is operated at rated flux and
less than rated current. The control circuit may at least substantially use the current
capability of the regenerative power cells and non-regenerative power cells when the
load is operated at rated flux and full current. A voltage drop may occur across all of
the cells during braking.
[ 0 0 0 6 ] In some embodiments, each regenerative power cell may include an
inverter bridge, a capacitor set electrically connected across terminals of the inverter
bridge, and an active front end comprising a plurality of transistors electrically
connected as a three-phase bridge. In an alternate embodiment, each regenerative
power cell may include an inverter bridge, a capacitor set electrically connected
across terminals of the inverter bridge, a three-phase diode bridge rectifier electrically
connected across the terminals, and a series-connected transistor and resistor
combination that is electrically connected across the terminals. In either embodiment,
the inverter bridge may comprise, for example, a four-transistor H-bridge inverter or
an eight-transistor H-bridge inverter based on a neutral-point-clamped connection.
[0007] In some embodiments, each non-regenerative power cell may include
an inverter bridge, a capacitor set electrically connected across terminals of the
inverter bridge, and a three-phase bridge rectifier electrically connected across the
terminals. This inverter bridge also may comprise, for example, a four-transistor H-
bridge inverter or an eight-transistor H-bridge inverter based on a neutral-point-
clamped connection. In some embodiments, the regenerative power cells and non-
regenerative power cells are removably and interchangeably installed in a housing.
[0008] In an alternate embodiment, an electrical device includes a plurality
of single-phase power cells electrically connected to receive power from a source and
deliver power to a load. The single-phase power cells include a first rank of

regenerative power cells and a second rank of non-regenerative power cells. Each
non-regenerative power cell may include an inverter bridge, a capacitor set
electrically connected across terminals of the inverter bridge, and a three-phase bridge
' rectifier electrically connected across the terminals. The non-regenerative power cells
may provide reactive power when the plurality of cells are used for braking of a
motor.
[0009] In an alternate embodiment, an electrical device, includes a first rank
that includes at least three single-phase non-regenerative power cells, a second rank
that includes at least three single-phase regenerative power cells, and a control circuit.
Each non-regenerative power cell includes an inverter bridge, a capacitor set
electrically connected across terminals of the inverter bridge, and a three-phase bridge
rectifier electrically connected across the terminals. When the device is used for
braking of a motor, a three-phase power cell is not required, and the non-regenerative
power cells provide reactive power.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0010] Aspects, features, benefits and advantages of the present invention
will be apparent with regard to the following description and accompanying drawings,
of which:
[0011] FIG. 1 depicts a prior art power cell.
[0012] FIG. 2 depicts a circuit comprising a plurality of power cells
connected to a load.
[0013] FIGs. 3A and 3B are a block diagram of exemplary regenerative
power cells.
[0014] FIG. 4 is a diagram of an arrangement of regenerative power cells
and non-regenerative power cells.
[0015] FIGs. 5A and 5B illustrate exemplary voltage-current relationships
for motoring, regeneration, and maximum braking in various embodiments.
[0016] FIG. 6 illustrates an exemplary voltage-current relationship of a
circuit in a motor speed range that is greater than the speed at which rated flux can be
applied to the motor.
[0017] FIG. 7 illustrates an exemplary voltage-current relationship of a
circuit in a motor speed range that is less than the speed at which rated flux can be

applied to the motor but greater than the speed at which rated current can be applied
to the motor.
[0018] FIG. 8 is a block diagram of an exemplary control circuit.
[0019] FIGs. 9A - 9D illustrate simulation results of a variable frequency
drive at various motor speeds.
DETAILED DESCRIPTION
[0020] Before the present methods, systems and materials are described, it is
to be understood that this disclosure is not limited to the particular methodologies,
systems and materials described, as these may vary. It is also to be understood that
the terminology used in the description is for the purpose of describing the particular
versions or embodiments only, and is not intended to limit the scope. For example, as
used herein and in the appended claims, the singular forms "a," "an," and "the"
include plural references unless the context clearly dictates otherwise. Unless defined
otherwise, all technical and scientific terms used herein have the same meanings as
commonly understood by one of ordinary skill in the art. In addition, the following
terms are intended to have the following definitions herein:
[0021] capacitor set - one or more capacitors.
[0022] comprising - including but not limited to.
[0023] control circuit - a first electrical device that signals a second
electrical device to change a state of the second electrical device.
[0024] electrically connected or electrically coupled — connected in a
manner adapted to transfer electrical energy.
[0025] H-bridge inverter - a circuit for controlled power flow between AC
and DC circuits having four transistors and four diodes. Referring to FIG. 1, an H-
bridge inverter 116 generally includes a first phase leg and a second phase leg
electrically connected in parallel. Each leg includes two transistor/diode
combinations. In each combination, the diode is electrically coupled across the base
and emitter of the transistor.
[0026] inverter - a device that converts DC power to AC power or AC
power to DC power.

[0027] medium voltage - a rated voltage greater than 690 volts (V) and less
than 69 kilovolts (kV). In some embodiments, medium voltage may be a voltage
between about 1000 V and about 69 kV.
[0028] neutral-point clamped connection - in the context of an eight-
transistor H-bridge inverter, an arrangement of eight transistors to include a first
phase leg and a second phase leg electrically connected in parallel at the DC
terminals. Each leg includes four transistors. The mid-point between the upper pair
of transistors and the mid-point between the lower pair of transistors of each phase leg
is electrically connected through diodes to form a neutral point.
[0029] non-regenerative power cell - a power cell that does not have the
capability of absorbing regenerative power.
[0030] power cell - an electrical device that has a three-phase alternating
current input and a single-phase alternating current output.
[0031] rank - an arrangement of power cells established across each phase
of a three-phase power delivery system.
[0032] rated speed - the number of times that the shaft of a motor may turn
in a time period, such as revolutions per minute (RPM), when it is operating at its
rated torque.
[0033] regenerative power cell - a power cell that has the capability of
absorbing regenerative power.
[0034] substantially - to a great extent or degree.
[0035] three-phase bridge rectifier - a device including an arrangement of
semiconductive devices such diodes that converts three-phase alternating current to
direct current.
[0036] In various embodiments, a multi-level power circuit uses single-phase
series-connected regenerative cell and non-regenerative cell inverters to provide
limited braking capability. FIG. 2 illustrates an exemplary embodiment of a circuit
having such inverters. In FIG. 2, a transformer 210 delivers three-phase, medium-
voltage power to a load 230 such as a three-phase induction motor via an array of
single-phase inverters (also referred to as power cells). A three-phase inverter is not
required in the array. The transformer 210 includes primary windings 212 that excite
a number of secondary windings 214 - 225. Although primary winding 212 is
illustrated as having a star configuration, a mesh configuration is also possible.

Further, although secondary windings 214 - 225 are illustrated as having a mesh
configuration, star-configured secondary windings are possible, or a combination of
star and mesh windings may be used. Further, the number of secondary windings
illustrated in FIG. 2 is merely exemplary, and other numbers of secondary windings
are possible. The circuit may be used for medium voltage applications or, in some
embodiments, other applications.
[0037] Any number of ranks of power cells are connected between the
transformer 210 and the load 230. A "rank" is considered to be a three-phase set, or a
group of power cells established across each of the three phases of the power delivery
system. Referring to FIG. 2, rank 250 includes power cells 251-253, rank 260
includes power cells 261-263, rank 270 includes power cells 271-273, and rank 280
includes power cells 281-283. Fewer than four ranks, or more than four ranks, are
possible. A central control system 295 sends command signals to local controls in
each cell over fiber optics or another wired or wireless communications medium 290.
[0038] As mentioned above, the prior art power cells such as those
illustrated in FIG. 1 do not allow any significant amount of regeneration. In order to
achieve the desired braking capability in the embodiments described herein, alternate
power cells are used. FIGs. 3A and 3B show two embodiments of power cells that do
permit regeneration. Referring to FIG. 3A, a power cell 300 includes an active front
end 310 that serves as a three-phase bridge as it receives power from dedicated three-
phase secondary windings of the transformer via an input 342. The cell 300 also
includes a plurality of current-controlling devices such as transistors or thyristors 312
- 317, for example insulated gate bipolar transistors (IGBTs), integrated gate
commuted thyristors or other devices, generally referred to herein as the front-end
transistors. Although six transistors in a bridge format - in this example, three pairs
of two transistors each connected in parallel across the DC terminals - are illustrated
in FIG. 3A, other numbers of transistors may be used. These transistors can be
controlled by a local and remote control system (292 and 295, respectively, in FIG. 2)
to transfer energy in either direction thus allowing motoring or regenerating to full
capacity (i.e., approximately or fully 100%). Any suitable method to achieve such
operation with front-end transistors may be used. The remainder of the power cell
300 includes one or more capacitors 320 and an H-bridge inverter 330, each
connected across the output or DC terminals of the active front end 310, to deliver AC

power to the output 344. Other inverter bridges may be used as substitutes for the
four-transistor H-bridge 330 illustrated in FIG. 3A. For example, an H-bridge
comprising eight transistors based on the neutral-point clamped connection may be
used.
[0039] In an alternate embodiment, FIG. 3B illustrates a power cell 350
which includes the elements of a rectifier 360, capacitors 375, and an inverter bridge
such as an H-bridge inverter 380 connected in parallel between an input 392 and
output 394. A three-phase diode bridge rectifier 360 receives power from dedicated
three-phase secondary windings of the transformer via the input 392. In addition, a
brake circuit 370 includes a transistor 374 (referred to herein as a brake transistor) and
a resistor 372 electrically connected in series with respect to each other and in parallel
across the DC capacitors 375 and the DC output of the rectifier 360. The brake
transistor 374 is controlled by a local controller, and during motoring the brake
transistor 374 is controlled to be "off and does not participate in the energy transfer
from the AC input 392 to the AC output 394. However, during regenerating the brake
transistor 375 may be controlled to turn on and off in order to dissipate the energy
from the motor in the resistor 372 and hence maintain the DC voltage across the
capacitors 375 at a pre-determined value.
[0040] A power cell that allows power to be absorbed from the motor is
referred to herein as a regenerative cell (RC), and a power cell that does not allow any
significant amount of regeneration (such as the one shown in FIG. 1 and is described
above) is referred to as a non-regenerative cell (NRC). Although two examples of
RCs are shown in FIGs. 3A and 3B, other RC cells may be used in the embodiments
described herein.
[0041] Thus, referring back to FIG. 2, ranks 250 and 270 may comprise
regenerative cells, and ranks 260 and 280 may comprise non-regenerative cells, or
vice-versa. It should be noted that the number of cells per phase depicted in FIG. 2 is
exemplary, and more than or less than four ranks may be possible in various
embodiments. For example, two ranks, four ranks, eight ranks, or other numbers of
ranks are possible. In fact, the number of regenerative power cells used can be
selected based on the desired degree of braking provided. In addition, the power cells
may be removably installed in a housing so that a single housing unit may be used for
various applications, with regenerative cells being exchanged for non-regenerative

cells, or vice versa, depending on the desired application such as braking or
regeneration desired.
[0042] To obtain full regeneration capability for a circuit such as that shown
in FIG. 2, RCs may be used in the entire circuit of power cells. However, in many
applications, the braking capability that is needed is a much smaller fraction of the
rated capacity of the load, such as approximately 10 percent to approximately 20% for
ship propulsion and large fan applications. The use of RCs in the entire circuit makes
the solution more expensive than it needs to be. However, we have discovered an
exemplary series-connected arrangement of inverters that provides limited, but not
full, braking capability.
[0043] Referring to FIG. 4, a series-connected arrangement of inverters can
be used to connect both RCs and NRCs within the same circuit. A first rank 410
includes three single-phase NRCs 411, 412 and 413 (one cell for each phase in a
three-phase circuit), while a second rank 420 includes three single-phase RCs 421,
422 and 423 (also one for each phase). In this embodiment, three-phase power cells
are not required. Each cell receives power from dedicated secondary windings of an
input transformer (210 in FIG. 2). For each phase, a first output terminal of each RC
cell is electrically connected to the output terminal of opposite polarity for the phase's
corresponding NRC cell. The second output terminal of each RC cell is electrically
connected to an output line. The remaining output terminals of all three RC cells in
the rank are electrically connected to each other to form a star point 440. As shown in
FIG. 2, additional ranks, such as four ranks total, six ranks total, eight ranks total, or
more, may be present in the circuit. The number of ranks of RCs selected may be
determined based on the desired level of regeneration required.
[0 044] In some embodiments, the circuit may include a bypass feature that
allows continued operation of the circuit if one or more of the power cells should fail.
For example, as illustrated in U.S. Patent No. 5,986,909, and in particular FIG. 1B
and the accompanying text, which are incorporated herein by reference, a bypass may
create a shunt path between the output lines of a power cell if the cell fails so that
current can then flow through the bypass instead of the power cell.
[0045] During motoring, the RC ranks and NRC ranks may provide a
substantially equal amount of power to the load. However, during regeneration, the
diode-bridge rectifiers in the NRC prevent transfer of power to the utility, but front-

end transistors (AFE) in the RCs are controlled to absorb the braking energy by
transferring power from the load (such as a motor) to the input source. The control of
the front-end transistors may be the same as or similar to known control methods,
such as the regulation of the DC-voltage within each RC. However, control of the
output H-bridges may require different methods. For example, FIG. 4A illustrates
exemplary voltage and current vectors during motoring, while FIG. 4B illustrates
exemplary voltage and current vectors during regeneration. When motoring, as
illustrated in FIG. 4A, the output voltage vectors of cell 1 (an RC) and cell 2 (an
NRC) are along the motor voltage. Each power cell provides an equal or substantially
equal amount of power to the load. Referring to FIG. 4B, during regeneration the
voltage of cell 2 (NRC) is controlled to be in quadrature with respect to the drive
current. This prevents cell 2 from absorbing any real (or active) power. However, the
output of cell 1 (RC) must make up the difference between the motor voltage and the
output of cell 2. This restricts the amount of available voltage that the drive can
produce and will require reduced flux operation at high speeds during regeneration.
When a mixture of NRC and RC are used, as shown in FIG. 4, maximum braking
torque is obtained when the NRC and RC voltages are also in quadrature to each other
as shown in FIG. 5C. Although it is noted that a quadrature relationship (of 90
degrees) is desired to maintain zero power flow into the NRC cells, for practical
implementation this angle may be reduced below 90 degrees to ensure that either no
power flows into the NRC cells or some power flows out of the NRC cells.
[0046] The approach of having both sets of cells (NRC and RC) generate
voltages during the entire braking process allows for continuous braking during the
entire speed range. Unlike prior art methods, the methods described herein can
produce regenerative torque before the motor speed has dropped below the voltage
capability of the RC. The cells' output voltages are substantially used (with normal
system losses) to provide power to the motor when the motor is running at or near its
rated speed. When the motor voltage is reduced (as for braking), the NRCs and RCs
also participate so that the NRCs provide reactive power during braking, and current
is reversed through the RCs. This, unlike the prior art, all cells can participate during
braking.
[0047] For the purpose of the following discussion, the following symbols
are defined on a per-unit basis as follows:

Voltage capability of the RC, VRC = x per-unit
Voltage capability of the NRC, VNRC = (1-x) per-unit
Total voltage capability of drive =1.0 per-unit
No-load current of motor = INL
Speed below which rated flux can be applied on the motor = Wv
Speed below which rated current can be applied to the motor = WI
Braking torque capability = TB
Torque current = Iqs
Magnetizing current = IdS
Motor speed = w
Motor voltage = Vmotor
[ 0048 ] To understand operation of an exemplary drive circuit with limited
braking capability, the entire operating speed range may be considered to include
three speed ranges as described below. During each speed range, there may be a
separate limit on the achievable braking torque. This limit depends on the relative
voltage capabilities of the RC and the NRC and the magnetizing current of the motor.
Although the description below is in the context of an induction motor, similar results
may be achieved with a synchronous motor by operating at reduced voltage on stator
side. Although the total voltage capability of the drive is described below as having a
unit value of 1.0, this value can be different from 1.0 as long as x is less than that
value.
[0049] Speed range # 1: wv [ 0050 ] In this speed range, the drive cannot operate the motor at rated flux,
because of the requirement to operate the NRC cell at quadrature with the motor
current. Hence, the motor is operated at reduced flux and reduced current. Referring
to FIG. 6, the maximum voltage output is given as:
[0051]Vmax = √(1-2x + 2x2) (1)

[0052] To increase or obtain maximum possible torque within this speed
range, the motor may be operated at or near this maximum voltage. Therefore, the
voltages for the cells are fixed at VRC = x, and VNRC = (1 - x). As speed decreases
from rated speed, motor flux gradually increases until it equals its rated value at w =
wv. Hence, in per-unit terms, Wv = Vmax. FReferring again to FIG. 6, torque current is
obtained as:
[0053]Iqs=x Ids/(1-x) (2)
[0054] At rated speed the (minimum) braking torque capability then may be
given by the following equation:
[0055] TB = Vmax Iqs =INL (1 -2x + 2x2) x / (1 - x) (3)
[0056] where, it is assumed that at light loads and reduced flux Id = INL Vmax
[0057] Speed range # 2: wI [0058] In this speed range, the motor is operated at rated flux but reduced
current, and the RC provides rated output. Hence, referring to FIG. 7:
[0059] VRC = x, and VNRC = √(w2 -x2), where w represents the speed and the
per-unit motor voltage,
(4)
[ 0060] and Iqs = x Ids/√(w2 - x2) (5)
[0061] As speed decreases, Iq becomes larger and approaches rated torque
current. When w = WI (where w1 = x/PF_rated), rated current is applied to the motor
and rated braking torque can be obtained.
[0062] Speed range #3: 0 [0063] In this speed range, the motor is operated at rated flux and rated
current. The voltages from the NRC and the RC are reduced linearly with speed as
shown below, so that motor current is maintained at rated:
[0064] VRC = x w/wI (6)
[0065] VNRC = w √(wI2 - x2) / wI. (7)
[0066] The above equations show exemplary methods for controlled
operation of the RCs and NRCs during regeneration. A block diagram showing an
exemplary control system 800 to implement such equations in a typical motor drive
controller is given in FIG. 8. In this figure, the "Flux Reference Generator" 805,
"Limited Regen Voltage Allocation" 810 and "Regen Limit" 815 help to provide
limited regeneration control. As shown in the figure, the exemplary circuit includes at

least two inputs, the flux demand λDMD 801 and the speed reference wref 802. The Flux
Reference Generator 805 calculates the flux reference using the maximum voltage
given by equation (1) above and the stator frequency, ws 804, and provides the flux
reference,λref 803, as an output. The Flux Reference Generator 805 also ensures that
the flux reference is always less than or equal to the flux demand, ΛDMD 801. The Flux Regulator 820 compensates for the difference between the flux reference, λref 803, and
the flux feedback, ΛDS 809, where ΛDS 809 is the estimated actual flux value given by
the measured motor voltage and the stator speed. The output of Flux Regulator 820 is
the motor magnetizing current reference, Ids 821.
[0067] The speed reference, wref 802, is compared with the motor speed, w
850, in the Speed Regulator 855, which provides the motor torque current reference,
IqS ref 858 as the output. When the motor is commanded to slow down, the Regen
Limit block 815 calculates equations (2) and (5) above and provides a limit on the
torque current reference.
[0068] The circuit shown in FIG. 8 includes two Current Regulators 860 and
862 that control the magnetizing current and the torque current. Their outputs are
voltage references, Vds ref 863 and VqS.rcf 861. The Limited Regen Voltage Allocation
810 block splits the motor voltage references, Vds ref 863 and Vqs.ref 861, into voltage
references for the RCs and NRCs. In the Limited Regen Voltage Allocation 810
block, the magnitude of the voltages for RC and NRC may be first found using x,
Vmotor and ws from equations (4), (6) and (7) above. Then, the d- and q-axis
components of the voltage commands for the NRCs and the RCs (VqdsNRc 868 and
VqdsRC 869) are identified using Ids ref, and IqS ref. Finally, d- and q-axis components of
VRC and VNRC are converted to 3-phase voltages in the stationary frame, and are
used as references to generate PWM voltage commands for controlling the inverters.
[0069] FIGs. 9A - 9D provide exemplary simulation results to show the
operation in regeneration for a drive configuration such as that shown in FIG. 4 with x
= 0.5 (i.e. an equal number of RCs and NRC's). It should be noted that having an
equal number of RCs and NRCs is not a requirement of the embodiments described
herein. The drive may be commanded to go into regeneration at t = 5 seconds by
decreasing the speed reference wref.
[0070] FIGs. 9A and 9B illustrate exemplary motor speed (FIG. 9A) and
the output voltage (FIG. 9B) of a drive having a combination of RC and NRC ranks.

In each Figure, time=0 to time=5 seconds represents motoring, while the remaining
time periods represent different speed ranges of braking. Unlike the prior art, as
illustrated in FTGs. 9A-9B, a voltage drop is present across the non-regenerative
power cells during braking. In fact, such a voltage is present during the entire braking
period.
[0071] Referring to FIG. 9B, during Speed Range #1, the drive output
voltage is maintained at the value specified by the equation (1) while the output
voltage of RC and NRC are held at their respected rated values (i.e. 100%). In the
Speed Range #2, the voltage of NRC is reduced whereas the voltage of RC is
maintained at its rated voltage as given by the equation (4). As soon as the motor
speed enters Speed Range #3, the voltage of both RC and NRC decreases linearly as
the motor slows down as specified by equations (6) and (7).
[0072] FIG. 9C shows exemplary motor current components during
regeneration. In Speed Range #1, the magnetizing current (IDS) is lowered to satisfy
the maximum motor voltage specified by the equation (1). Subsequently, the torque
current (IQS) is also reduced so that the motor current remains in quadrature with
respect to the output voltage of NRC as shown in FIG. 6. Throughout Speed Range
#1, both the magnetizing current and the torque current increase equally in ratio so as
to bring up the motor flux towards its rated value (i.e., approximately 100%) while
maintaining the power factor and keeping the quadrature relation between the motor
current and the output voltage of the NRC cells. During Speed Range #2, there is
more room to accommodate the larger torque current as the voltage of NRC
decreases, as evident from FIG. 7. As was mentioned earlier, Speed Range #3 starts at
the instant when the total motor current reaches its rated value.
[0073] FIG. 9D shows active power delivered from the drive, the RC cells
and the NRC cells. The active power from the NRC remains substantially zero
throughout the duration of motor deceleration. In this example, all of the active
power (or total power) from the motor is absorbed by the RC, thereby illustrating that
the methods described herein may successfully divert all the power absorbed from the
motor into the RC during regeneration. In addition, the generated power in the motor
is controlled to be less than the power rating of RC.
[0074] As noted above, there may be a different number of RCs and NRCS
than the examples expressly described above. When the combination of RCs to NRCs

is changed, the value of braking torque at full speed also may change according to
equation (3). This is tabulated in the following table of braking torque for different
cell combinations, assuming motor no-load current, INL, equals 25%:

[0 07 5] As noted above, when the bypass feature is provided with each of the
power cells, then it is possible to operate the circuit if one or more of the power cells
fail. Under such a condition, the equations provided above can still be applied, but
with slight adjustments as detailed here. When one or more power cells are bypassed,
the total voltage capability of the RC and the NRC may change to y and z,
respectively, such that t = y + z. The same procedure as detailed in equations (1) - (7)
above may be followed using t, y, z instead of 1.0, x, and 1-x to determine the
operation of the control with a smaller number of power cells in the circuit.
[0076] Still other embodiments will become readily apparent to those skilled
in this art from reading the above-recited detailed description and drawings of certain
exemplary embodiments. It should be understood that numerous variations,
modifications, and additional embodiments are possible, and accordingly, all such
variations, modifications, and embodiments are to be regarded as being within the
spirit and scope of this application. For example, regardless of the content of any
portion (e.g., title, field, background' summary, abstract, drawing figure, etc.) of I this
application, unless clearly specified to the contrary, such as via an explicit definition,
there is no requirement for the inclusion in any claim herein (or of any claim of any

application claiming priority hereto) of any particular described or illustrated
characteristic, function, activity, or element, any particular sequence of activities, or
any particular interrelationship of elements. Moreover, any activity can be repeated,
any activity can be performed by multiple entities, and/or any element can be
duplicated. Further, any activity or element can be excluded, the sequence of activities
can vary, and/or the interrelationship of elements can vary. Accordingly, the
descriptions and drawings are to be regarded as illustrative in nature, and not as
restrictive. Moreover, when any number or range is described herein, unless clearly
stated otherwise, that number or range is approximate. When any range is described
herein, unless clearly stated otherwise, that range includes all values therein and all
subranges therein. Any information in any material (e.g., a United States patent,
United States patent application, book, article, etc.) that has been incorporated by
reference herein, is only incorporated by reference to the extent that no conflict exists
between such information and the other statements and drawings set; forth herein. In
the event of such conflict, including a conflict that would render invalid any claim
herein or seeking priority hereto, then any such conflicting information in such
incorporated by reference material is specifically not incorporated by reference
herein.

We Claim:
1. A braking circuit, comprising:
an arrangement of power cells electrically connected to receive power
from a source and deliver power to a load (230), the arrangement comprising:
a first rank (250,270) of regenerative power cells (251-253, 271-273); and
a second rank (260,280) of non-regenerative power cells (261-263, 281-
283);
wherein the second rank of non-regenerative power cells is connected to the
first rank of regenerative power cells; and wherein the braking circuit further
comprises a control circuit (800) connected to the arrangement of power cells,
wherein the control circuit is configured to generate pulse width modulation
voltage commands for controlling the regenerative power cells and the non-
regenerative power cells.
2. The circuit as claimed in claim 1, wherein a voltage drop is present across the
non-regenerative cells (261-263, 281-283) during braking of the load (230).
3. The circuit as claimed in claim 1, wherein the load (230) is a motor.

4. The circuit as claimed in claim 3, wherein the output voltages of the
regenerative power cells (251-253,271-273) and non-regenerative power cells
(261-263,281-283) are maintained at least substantially at their rated values
when the load (230) is operated at less than rated flux and less than rated
current of the motor.
5. The circuit as claimed in claim 4, wherein the control circuit (800) at least
substantially uses the current capability of the regenerative power cells (251-
253,271-273) when the load (230) is operated at rated flux and less than rated
current of the motor.
6. The circuit as claimed in claim 5, wherein the control circuit (800) at least
substantially uses the current capability of the regenerative power cells (251-
253,271-273) and non-regenerative power cells (261-263,281-283) when the
load (230) is operated at rated flux and full current of the motor.

7. The circuit as claimed in claim 1, wherein each regenerative power cell (300)
comprises:
an inverter bridge
a capacitor set (320) electrically connected across terminals of the inverter
bridge (330); and
an active front end (310) comprising a plurality of transistors (312-317)
electrically connected as a three-phase bridge.
8. The circuit as claimed in claim 1, wherein each regenerative power cell
(350) comprises:
an inverter bridge;
a capacitor set (375) electrically connected across terminals of the inverter
bridge (380);
a three-phase diode bridge rectifier (360) electrically connected across the
terminals; and
a series-connected transistor (374) and resistor (372) combination that is
electrically connected across the terminals.

9. The circuit as claimed in claim 1, wherein each non-regenerative power
cell comprises:
an inverter bridge;
a capacitor set electrically connected across terminals of the
inverter bridge ;and
a three-phase bridge rectifier electrically connected across the
terminals.
10. The circuit as claimed in claim 8 or claim 9, wherein the inverter bridge
comprises a four-transistor H-bridge inverter (330) or an eight-transistor H-
bridge inverter based on a neutral-point-clamped connection.
11.The circuit as claimed in claim 1, wherein the regenerative power cells (251-
253, 271-273) and non-regenerative power cells (261-263,281-283) are
removably and interchangeably installed in a housing.
12. An electrical device, comprising a braking circuit as claimed in claim 9,
wherein the power cells comprise single phase power cells; and wherein the non-
regenerative power cells provide reactive power when the plurality of cells are
used for braking of a motor.

13. An electrical device comprising a braking circuit as claimed in claim 9;
wherein the ranks comprise at least three single-phase power cells; and
wherein when the device is used for braking of a motor, a three-phase power
cell is not required, and the non-regenerative power cells provide reactive
power.
14. An electrical device as clamed in claim 13, wherein the regenerative power
cells output substantially rated current when the load is operated at rated flux
and less than rated current; and the regenerative power cells and non-
regenerative power cells output substantially rated current when the load is
operated at rated flux and full current.

ABSTRACT

TITLE: A CIRCUIT BRAKER
The invention relates to a braking circuit device, comprising an array of power
cells (251-253, 261-263, 271-273,281-283) electrically connected to receive
power from a source (210) and deliver power to a load (230), the array
comprising a first rank (250,270) of regenerative power cells (251-253,271-
273); and a second rank (260,280) of non-regenerative power cells (261-
263,281-283) connected to the first rank (250,270) of regenerative power cells.
A control circuit (292,295) connected to the array of power cells, wherein the
control circuit is configured to generate PWM voltage commands for controlling
the regenerative power cells (251-253,271-273) and the non-regenerative power
cells (261-263,281-283).

A CIRCUIT BRAKER

Documents:

Inventors:

PCT Conventions: