SE2051108A1 - Inductive machine with an actively rectified exciter winding - Google Patents

Abstract

An exciterless inductive machine (too) comprises a stator (102) and a rotor (104), which is separated by an air gap (112) from the stator and includes an exciter winding (110) disposed adjacent the air gap, an actively switched field winding (108) and a rotating DC voltage supply (114), which is coupled to both windings. During normal operation, the DC voltage supply is configured to perform harmonics harvesting in that it rectifies an exciter winding current and supplies the rectified current to the field winding. The DC voltage supply includes an actively switched rectifier (118) configured to apply a magnetizing voltage to the exciter winding when the inductive machine is in a short-circuit condition. The rectifier (118) may be further operable to apply a demagnetizing voltage to the field winding.

Description

INDUCTIVE MACHINE WITH ANACTIVELY RECTIFIED EXCITER WINDING TECHNICAL FIELD

[0001] The present disclosure concerns synchronous machines having fieldwindings requiring excitation. In particular, the present teachings concernsynchronous motors and generators, including brushless machines requiring excitation of field windings.

BACKGROUND

[0002] The applicant”s prior disclosure US20160211787A1 relates to an exciterlesssynchronous machine concept based on the principle that airgap flux densityharmonics that do not contribute to torque production can be harvested with a set ofcoils properly placed on the rotor pole surface. Such harmonics, called “slotharmonics”, are harvested as they have a strong flux density variation and highfrequency, which result in an amount of power enough to power the field of the maingenerator (or machine). One or more phases of so-called Excitation Coils (EC) may beconnected to diode rectifier and charge a DC link capacitor. Then, a set of activeswitches (half- or full-bridge) can be used to regulate the field current, as shown in figure 7.

[0003] This harvesting system works perfectly in normal conditions, it is simpleand relatively robust, but is not capable to deal with short circuit conditions. During ashort circuit, however, the field generated by the stator currents exactly opposes theone generated by the rotor on the d-axis. As a result, the fundamental flux dropssignificantly. One can conceptually imagine the stator as having a “mirroring effect”on all the flux generated by the rotor. It is then easy to understand that during short-circuit (SC) conditions there is not much power available for harvesting in the airgap.Additionally, generators of this type are usually required to have a stator currentoutput of 3 pu (per-unit) during SC, which is generally achieved with a field current ofaround 1.8 pu with respect to nominal condition. This means that the field neces-sitates approximately four times the amount of power required in nominal conditions for 10 seconds.

[0004] One can think the exciterless concept as a (inside-out) reluctance-based machine, where the stator slots are shaping the reluctance path. The “reluctance exciter” works as a generator, i.e. generating power to feed the main machine field.The fundamental flux (by large amount) of the main generator is magnetizing the“reluctance exciter”. However, when a SC happens, the fundamental flux in the airgapdrops, and all the magnetizing flux of our reluctance exciter disappears. Theexcitations coils, because they are naturally connected to a diode rectifier, are not able to provide a magnetizing current needed to extract torque/ power from it.

SUMMARY

[0005] One objective is to make available an inductive machine that can cope with the lack of power to harvest in the airgap during SC conditions.[0006] This and other objectives are achieved by the invention defined in claim 1.

[0007] Generally, all terms used in the claims are to be interpreted according totheir ordinary meaning in the technical field, unless explicitly defined otherwiseherein. All references to “a/ an/ the element, apparatus, component, means, step, etc.”are to be interpreted openly as referring to at least one instance of the element,apparatus, component, means, step, etc., unless explicitly stated otherwise. The stepsof any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Aspects and embodiments are now described, by way of example, with reference to the accompanying drawings, on which:figure 1 is a schematic view of an exciterless synchronous machine; figure 2 illustrates a portion of a rotating DC voltage supply with an actively switched harvesting block (rectifier) and a field winding block with a full-bridge topology;figure 3 illustrates a partial sectional view of a synchronous machine having exciterwindings; figure 4 discloses alternative layouts of the active rectifier; figure 5 is a conceptual power balance diagram for SC conditions; figure 6 is a plot of a simulation of torque as a function of time in an inductivemachine without active rectification (earlier portion) and with active rectification (later portion); and 3 figure 7 illustrates, similar to figure 2, a portion of a rotating DC voltage supplyaccording to prior art, where a harvesting block has a passive diode bridge and the field winding block has a half-bridge topology.

DETAILED DESCRIPTION

[0009] The aspects of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, on which certainembodiments of the invention are shown. These aspects may, however, be embodiedin many different forms and should not be construed as limiting; rather, theseembodiments are provided by way of example so that this disclosure will be thoroughand complete, and to fully convey the scope of all aspects of invention to those skilled in the art. Like numbers refer to like elements throughout the description.

[0010] With reference to figure 1, an exciterless synchronous machine 100according to the present teachings, exemplified as a generator, includes a stator 102and a rotor 104. The rotor 104 includes poles 106, only one of which is shown forconvenience of illustration. Pole 106 includes field windings 108, and exciterwindings 110. The exciter windings 110 are disposed adjacent the air gap 112 of thesynchronous machine 100. While the illustrated rotor 104 is an internal rotor 104, the present teachings can also be applied to external rotor 104 geometries.

[0011] The air gap 112 of a synchronous machine 100 contains significant fluxcontent. The flux distribution in the air gap 112 of synchronous machine 100 candepend on several factors, including but not limited to space harmonics due todistribution of the windings, slotting, salient poles, time harmonics due to non-idealcurrents in the stator windings and switching of power electronics devices when thestator windings are connected to a power electronic converter. The harmonicdistribution present in the air gap 112 can also vary at different operation modes,including no, full or partial load conditions. Harmonics in an air gap 112 of an open-slot type line-fed synchronous machine mainly contain slotting harmonics due to thelarge slot openings in the stator 102. A Fourier expansion of the air gap magnetic fluxdensity waveform shows that the slotting harmonics, which are the 24fh-orderharmonics in the case of the machine 100 of figure 1, are very significant compared tothe other harmonics present in the air gap 112 flux distribution during nominal operation. Similar results can be seen in the no load and short circuit cases. Further, 4 a converter-fed synchronous machine can contain low order harmonics in addition to slotting harmonics.

[0012] With continued reference to figure 1, the exciter windings 110 are disposedadjacent the air gap 112, where damper bars can typically be placed in brushed orbrushless synchronous machines with separate exciter machines. However, theexciter winding 110 pitch can be optimized for maximum energy harvesting from theslotting harmonics. According to one aspect of the present teachings, damper bars arereplaced by the exciter windings 110 in order to capture the electromotive forcepresent due at least in part to the slotting harmonics in the air gap 112. One of theprimary functions of traditional damper bars is to damp transients in synchronousmotors. Such transients can cause “hunting” behavior by the synchronous machineand are undesirable. The exciter windings 110 can perform this and other functionsperformed by traditional damper bars, including mitigating against transients, as-sisting in motor starting, protection of the field winding, and reduction of transient orsub-transient reactances. Unlike damper bars, which can be shorted at the axial endsof the rotor 104, the exciter windings 110 are connected to additional componentsdescribed herein that capture the current generated in the exciter windings 110 andprovide that current to the field windings 108 of the rotor 104. It should be noted thattraditional damper bars can be implemented in addition to the exciter windings 110,for example to maintain reactances at desired levels. Further, from one to all of theexciter windings 110 can be configured to selectively operate as traditional damperbars, for example by shorting one or more the exciter windings 100 and bypassing thevoltage supply 114. For example, in an arrangement where the exciter windings 110are arranged in a multi-pole configuration, one or more of the multiple poles of the exciter windings 110 can be shorted in such a way to behave as damper bars.

[0013] With continued reference to figure 1, the voltage in the exciter windings110 can be fed to rotating voltage supply 114. Rotating voltage supply 114 can includean active damper 116, a rectifier 118, and a DC-to-DC converter 120. The activedamper 116 receives ACIN through leads 124. Active damper 116 can include a triac150 and a resistor 152 configured to disperse over-voltage conditions, for exampleduring direct on-line starting of the machine. The example active damper 116 isillustrated without limitation, and active damper 116 can also include a variety of over voltage protection circuits and components. The rectifier 118 can be a multiphase passive rectifier including active switching circuitry 119 to be described below.Rectifier 118 converts AC1N to DC1N. The slot harmonics have a higher frequency thanthe fundamental frequency of the machine, and so fast switching diodes are preferredover rectifiers based on conventional thyristors, which thyristors may not be ideal forsuch high frequencies. However, new high frequency switching thyristors could beimplemented in voltage supply 114. The depicted DC-to-DC converter 120 includesinductor L, switch T, diode D, and capacitor C arranged in a boost converterconfiguration. According to other aspects of the present teachings, the DC-to-DCconverter 120 can be a buck converter, flyback converter or other form of DC-to-DCconverter including DC-to-DC converters implementing active or passivecomponents. The converter 120 receives DC1N from rectifier 118 and produces, byaction of the switch T, the variable DC output DCVAR. The voltage supply 114 suppliesDCvAR to the field windings 108 through leads 121. According to another aspect of thepresent teachings, voltage supply 114 can implement a thyristor rectifier to perform control of current in the field windings 108.

[0014] Stator leads (not shown) can couple the stator 102 of the synchronousmachine 100 to three phase AC. Through detection of the conditions at the statorleads, the current required in the rotor windings 108 to generate the desired field inair gap 112 can be determined. The current of the rotor windings 108 can be indirectlycontrolled by commanding the rotating voltage supply 114. The commands may besent to the voltage supply 114 on a wired or wireless communication link. Somesuitable wireless forms of communication can include radio modulation techniques,optical communication, or through use capacitive or inductive communicationtechniques. According to one aspect of the present teachings, the rotating voltagesupply 114 can be disposed close to the center of the rotor 104 to reduce rotationalaccelerations experienced by the supply 114, which can reach as high as 20g undershort circuit conditions. According to another aspect of the present teachings, a singlesupply 114 can be implemented, or multiple supplies 114 can be implemented, including but not limited to redundant backup supplies 114.

[0015] Figure 3 shows a single-phase harmonic exciter winding 300. According toone aspect of the present teachings, the number of winding slots 302 is about twicethe number of stator slots 304 across the air gap 305 for a single-phase energy harvesting winding 300. The number of exciter windings can vary with the number of phases of exciter windings, the width and number of rotors, the size of the machine, and number of winding slots in the stator.

[0016] The number of windings 300 will vary for multiphase operation. Thenumber of windings that can be placed will be limited by the saturation of the energyharvesting winding region, which has a consequence of significantly reducing themaximum energy harvested. The configuration for attaining the maximum amount ofinduced current in the exciter windings 300 from the slot harmonics can vary basedon the distance between the windings 300, slot opening width, the air gap of themachine, the speed of the machine and type of power electronics converterimplemented, and such aspects can be varied to generate the required amount of induced current in the exciter windings 300.

[0017] The plurality of exciter windings 300 are disposed within each of the poles306 of the rotor 308 at the radial edge 310 of the pole 306. According to still anotheraspect of the present teachings, the rotor 308 is a salient pole rotor, having fieldwindings 312 disposed inwardly relative to pole transverse portion or pole “shoe” 314,and around the pole core 316. According to a further aspect of the present teachings,the rotor 308 and stator 320 depicted in figure 3 are for a 20 MVA class generator.The exciter windings 300 can be disposed in slots 320 that are open. Slots 320 can besized and shaped so to include only exciter windings to the exclusion of otherwindings, including but not limited to the field windings 312, damper bars, or otherauxiliary windings. Additional slots can be included on the rotor edge 310 that contain exciter windings 300 in addition to damper bars or other windings.

[0018] With continued reference to figure 3, the individual exciter windings 300can be connected using a wave winding configuration, which in a single phase exciterwinding 300 configuration can result in adjacent windings differing by 180 electricaldegrees, such that the adjacent windings 300 carry current in antiparallel directions.It should be noted that other winding configurations can also be implementedaccording to the present teachings. For example, the number of turns can be varied inorder to bring the voltage level near to the desired voltage. According to one aspect ofthe present teachings, the windings 300 are stranded, which can be desirable due toeddy current considerations. In one alternative example, traditional damper bars canbe disposed in one or more of the slots 320 in combination with exciter windings 300. 7

[0019] Figure 2 shows details of the active switching circuitry 119 connected to theexciter winding 110. Instead of full-bridge diode rectifiers like in the prior art, theinvention uses full-bridge or half-bridge voltage source converters. While figure 2shows switches implemented as insulated-gate bipolar transistors (IGBT), a viablealternative may be to use metal-oxide semiconductor field-effect transistors(MOSFET). The voltage source converters function as a drive, providing the neededmagnetizing current to the reluctance exciter. The inductive machine 100 will stillwork as a generator, i.e., the power is coming from the rotating mass of the main machine pushed by the prime mover.

[0020] The configuration according to figure 2 has the capability of magnetizingthe (inside-out) exciterless concept under short-circuit conditions of the mainmachine. In addition, it provides the capability of fast de-magnetization, that is achieved by applying the negative DC voltage to the field with field winding.

[0021] In figure 2, two blocks of power electronics elements are used: a harvestingblock and a field winding block. For this latter one, a full-bridge is recommendable asa half-bridge would not provide the de-magnetization capability. On the harvestingside, both half-bridge (as in figure 2) and full-bridge configurations can be used, de-pending on the number of phases. Half-bridge configuration minimizes the numberof switches. In the case of a three-phase harvesting system, it has the advantage that avery standard control can be applied. However, a full bridge configuration could bebuilt, from a control perspective, as a standalone single-phase unit, and deployed onseveral machines of the portfolio irrespective of the number of harvesting phasesoptimized for a specific machine. As the number of minimum independent phases isrelated to the machine design, and in particular with the slot-per-phase-per-pole (q)number, having a single-phase block with an independent control could be advantageous.

[0022] Turning to the control aspects, an aim of the active rectifiers will be tokeep the DC voltage up to a required level. In order to control the power factor of the“reluctance exciter” it is important to calculate the dq flux component of thisreluctance machines. This is no challenge (under rotating conditions), but requiresknowing the current in the Excitation Coils EC. This can be either measured directlywith a current sensor on the EC coils, or by reconstructing the current by knowing the DC current and the switching state of each drives, which are all known variables. 8 State-of-the-art control schemes can be adopted as field-oriented control; further-more, the speed is known and constant, which make the control easier as speedestimation is not necessary. This is required to be sure that there is a constantmagnetizing current provided by the EC. Because of the fast de-excitation capability,the energy stored in the field winding of the generator can be efficiently transferred tothe shaft, controlling the bridges in as a motor supplied by the field winding, and thisapplies torque to the shaft.

[0023] The configuration shown in figure 2 is operable to provide fastdemagnetization of the field winding. This helps meeting regulatory grid requirements.

[0024] The described configuration is also suitable for motors. Whenever the synchronous machine work with flux values under the rated ones, it becomes harderto harvest. Provided the DC link is charged, the slot reluctance can be magnetized bythe active rectifier, taking power from the shaft. One way of charging the DC link in a motor application could be by injecting a small rotating flux.

[0025] Figure 4 shows alternative layouts suitable for the active rectifier.Figure 4A shows a multiphase half-bridge arrangement, and figure 4B shows a full- bridge multiphase arrangement.

[0026] Figure 5 is a schematic power balance diagram of a generator in SCconditions, where Pin is the power supplied by the input shaft, Pbfaking is the powergenerated from the rotor-stator action tending to slow down the shaft, and Pfieid denotes power dissipated in the field windings 108.

[0027] In figure 6, the torque developed by an example generator is shown. Themodel simulates a SC condition, with a 3 pu output stator current. At time 0.08 s, thecurrent is injected in the EC, with proper phase shift angle, mimicking the controlthat the drive elements would do. The machine is running at 600 rpm. A brakingtorque is present because of the high current loading dissipating mainly on the statorresistance. When the “drives” are switched on, a just of 10.8 kN is observed in theaverage torque of the main AMG. A mechanical power of about 650 kW is being deducted from the excitation system and can thus be used to feed the field winding. 9

[0028] This invention proposes a concept for active rectification in an exciterlessmachine. In particular, this invention helps solving the short circuit condition, which is a very stringent and challenging requirement for the exciterless concept.

[0029] The aspects of the present disclosure have mainly been described abovewith reference to a few embodiments. However, as is readily appreciated by a personskilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims (11)

1. An exciterless inductive machine (100) comprising:a stator (102; 320); and a rotor (104; 308) separated by an air gap (112) from the stator and including anexciter winding (110; 300) disposed adjacent the air gap, an actively switched fieldwinding (108; 312) and a rotating DC voltage supply (114), which is coupled to bothwindings and configured to, during normal operation, rectify an exciter winding current and supply the rectified current to the field winding, characterized in that the DC voltage supply includes an actively switched rectifier (118).

2. The inductive machine of claim 1, wherein the rectifier is configured to apply amagnetizing voltage to the exciter winding during a detected short-circuit condition of the inductive machine.

3. The inductive machine of claim 2, wherein the magnetizing voltage is controlled on the basis of a sensed or estimated exciter winding current.

4. The inductive machine of claim 2 or 3, wherein the magnetizing voltage is controlled to stabilize a power factor of the exciter winding current.

5. The inductive machine of any of the preceding claims, wherein the rectifier is a full- or half-bridge voltage source converter.

6. The inductive machine of any of the preceding claims, wherein the rectifier is operable to apply a demagnetizing voltage to the field winding.

7. The inductive machine of claim 6, wherein the actively switched field winding has a full-bridge topology (210).

8. The inductive machine of any of the preceding claims, wherein the statorcomprises a plurality of slots (304) open towards the air gap, and the exciter winding is arranged to capture at least slotting harmonics.

9. The inductive machine of any of the preceding claims, which is a generator.

10. The inductive machine of claims 1 to 8, which is a motor. 11

11. The inductive machine of any of the preceding claims, which is a synchronous inductive machine.