Method and apparatus to neutralize bearing currents in an electrical machine

Abstract

An electrical machine includes a rotor with a shaft, and a stator, the rotor supported for rotation relative to the stator by bearings. A voltage attenuator circuit applies a compensating voltage signal to the shaft to offset a voltage induced on the bearings from stray current flow in the rotor. The attenuator circuit includes an input that receives a common mode voltage signal applied to stator or rotor windings. A compensation circuit generates the compensating voltage signal from the common mode voltage signal, and a capacitive coupler applies the compensating voltage signal to the shaft. A feedback circuit detects a voltage on the shaft and adjusts the compensation circuit.

Description

METHOD AND APPARATUS TO NEUTRALIZEBEARING CURRENTS IN AN ELECTRICAL MACHINE

[0001] The present disclosure relates generally to protecting bearings in electrical machines, and more particularly to reducing bearing damage caused by currents induced in the bearings.BACKGROUND

[0002] Electrical machines (i.e., electrical generators and motors) utilize bearings to support rotation of the rotor relative to the stator. These bearings are prone to damage caused by stray current flow through the bearings. Specifically, in operation of the electrical machine, voltages are induced across parasitic capacitances in the machine, the voltages resulting in a stray current flow in the rotor. This current flow induces a voltage on the bearing component connected to the rotor. The bearings are typically constructed as an inner race, an outer race, and roller bodies between the inner and outer races. One of the races is mounted on the rotor shaft and the other race is stationarily fixed, e.g., to the stator or machine frame. A lubricating film is provided between the bearing components. When the voltage induced on the rotor bearing component exceeds the breakdown strength of the lubricating film, current is discharged through the bearing to the stator or machine housing, resulting in damage to the bearing.

[0003] The stray current problem noted above is particularly relevant when the machine windings (stator or rotor) are supplied with current from electronic switching elements in a converter (i.e. a frequency converter). As the switching elements turn on and off, the voltages across the parasitic capacitances continuously change, which leads to the stray current flow in the rotor discussed above. Electrical generators in a wind turbine that are controlled by a PWM (Pulse Width Modulation) main converter are prone to this issue and have experienced early generator bearing failures therefrom.

[0004] It is known to provide the bearings of electrical machines with insulation (typically a ceramic layer on the inner or outer race, or ceramic roller bodies) to minimize the bearing currents. It is also known to ground the rotor via groundingbrushes. These solutions, however, are expensive, not always effective, and require maintenance and continuous component replacement.

[0005] The art would benefit from an improved, reliable, and cost-effective system and method to reduce the damage caused to bearings in electrical machines from stray current flow.BRIEF DESCRIPTION

[0006] Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.

[0007] Aspects of the invention encompass an electrical machine (e.g., a generator or a motor) that includes a rotor with a shaft, and a stator. The rotor is supported for rotation relative to the stator by a bearing, wherein a pair of bearings may be configured at opposite ends of the rotor shaft. A voltage attenuator circuit is configured to apply a compensating voltage to the rotor to offset a voltage induced on the bearing from stray current flow in the rotor. The voltage attenuator circuit may include an input that receives a signal corresponding to a common mode voltage applied to stator or rotor windings in the machine (depending on how the machine is configured). A compensation circuit generates an output signal from the common mode voltage signal, the output signal corresponding to the compensating voltage. A capacitive coupler is configured to supply the compensating voltage signal to the rotor shaft. A dynamic feedback circuit is configured to continuously or periodically detect a voltage on the rotor shaft and adjust the compensation circuit such that the compensating voltage signal offsets the detected voltage on the shaft thereby minimizing damage to the bearing from stray current flow through the bearing.

[0008] In a particular embodiment, the electrical machine is configured as an electrical generator and the rotor includes a plurality of windings fed by a pulse width modulation (PWM) converter. The electrical generator may be configured in an offshore or onshore wind turbine installation.

[0009] The compensation circuit may include various functionalities to generate the desired compensating voltage. For example, the circuit may include a dynamic inverting amp stage that inverts the input voltage and sets the gain and amplitude ofthe output voltage signal, wherein the gain and amplitude values are adjustable by a controller in response to the feedback circuit. The compensation circuit may include a dynamic transport delay stage that controls the offset time between the input and output signals of the voltage attenuator circuit.

[0010] In a particular embodiment, the compensation circuit may include a step- up transformer downstream of the inverting amp stage to achieve a sufficient voltage attenuation while minimizing the size of the downstream capacitive coupler.

[0011] In one embodiment, the capacitive coupler is a capacitor defined by a cylinder disposed around the shaft with an air gap between the cylinder and the shaft. The cylinder may be formed from a different metal than the shaft. The cylinder may be open-ended and define a sleeve on the shaft, or may be closed-ended and define a cap on the end of the rotor shaft. Other types of capacitive couplings could also be used, such as a parallel plate mechanism.

[0012] The electrical machine (either a generator or a motor) may include a machine frame, wherein the stator and the rotor are housed within the machine frame. The rotor shaft may include an end portion that extends through an end wall of the machine frame, wherein the capacitive coupler is configured on the end portion of the shaft outside of the machine frame. For example, the capacitive coupler may include a cylinder disposed around the end portion of the shaft extending through the machine frame.

[0013] The invention also encompasses various method embodiments for reducing damage to a bearing in an electrical machine from stray current flow through the bearing, wherein the electrical machine includes a rotor having a shaft, and a stator, the rotor supported for rotation relative to the stator by the bearing. The method includes providing an input to a voltage attenuator circuit, the input corresponding to a common mode voltage applied to stator or rotor windings in the electrical machine. The method includes modifying the input signal to generate an output voltage signal. The output voltage is applied as a compensating voltage to the rotor shaft via a capacitive coupling to offset a voltage induced on the rotor shaft (and thus the bearing) from the stray current flow in the rotor. The method includes providing a feedback loop that detects a voltage on the rotor shaft and adjusts thecompensating voltage to offset the rotor shaft voltage, thereby minimizing the stray current flow through the bearing.

[0014] The method may be particularly beneficial for an electrical generator having a plurality of windings fed by a pulse width modulation (PWM) converter, such as a conventional wind turbine generator.

[0015] The method includes compensation functionalities to generate the compensating voltage signal from the input signal, which may include inverting the input signal, and setting a gain, amplitude, and delay of the output voltage signal.

[0016] The method may include increasing the compensating voltage applied to the rotor shaft with a step-up transformer downstream thereby minimizing the size of the coupling capacitor needed to transfer the compensating voltage to the rotor shaft.

[0017] Various other method embodiments may include any combination of the functions and aspects of the electrical machine discussed above.

[0018] These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0019] A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:

[0020] Fig. l is a perspective view of an embodiment of a wind turbine according to the present disclosure;

[0021] Fig. 2 is a simplified, internal view of an embodiment of a nacelle according to the present disclosure;

[0022] Fig. 3 is a schematic view of an embodiment of a wind turbine electrical power system suitable for use with the wind turbine shown in Fig. 1;

[0023] Fig. 4 is a diagram view of an embodiment of an electrical machine in accordance with aspects of the invention;

[0024] Fig. 5 is a single-phase equivalent circuit of a generator in accordance with aspects of the invention;

[0025] Fig. 6 is a single-phase equivalent circuit of another embodiment of a generator in accordance with aspects of the invention; and

[0026] Fig. 7 depicts a method in accordance with aspects of the invention.

[0027] Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements in the invention.DETAILED DESCRIPTION

[0028] Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

[0029] The terms “coupled,” “fixed,” “attached to,” and the like refer to both direct coupling, fixing, or attaching, as well as indirect coupling, fixing, or attaching through one or more intermediate components or features, unless otherwise specified herein.

[0030] Approximating language, as used herein throughout the specification and claims, is applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, “approximately”, and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value, or the precision of the methods or machines for constructing or manufacturing the components and / or systems. For example, the approximating language may refer to being within a 10 percent margin.

[0031] As discussed above, the present invention relates to electrical machines in general, including conventional electrical motors and generators, that utilize bearings susceptible to damage from stray current flow. Although not limited to such, the invention has particular usefulness with respect to wind turbine generators. For nonlimiting purposes of illustration and ease of explanation only, the invention will be explained relative to implementation in a wind turbine generator.

[0032] Referring now to the drawings, Fig. 1 illustrates a perspective view of an embodiment of a wind turbine 10 according to the present disclosure. The wind turbine 10 described herein may be an onshore wind turbine, as shown in Fig. 1, or an offshore wind turbine. Further, as shown in Fig. 1, the wind turbine 10 includes a tower 12 extending from a support surface 14, a nacelle 16 mounted on the tower 12, and a rotor 18 coupled to the nacelle 16. The rotor 18 includes a rotatable hub 20 and at least one rotor blade 22 (e.g., three blades 22) coupled to and extending outwardly from the hub 20. Each rotor blade 22 is spaced about the hub 20 to facilitate rotating the rotor 18 to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. For instance, the hub 20 may be rotatably coupled to an electric generator 24 (Fig. 2) positioned within the nacelle 16 to permit electrical energy to be produced.

[0033] The wind turbine 10 may also include a wind turbine controller 26 centralized within the nacelle 16. However, in other embodiments, the controller 26 may be located within any other component of the wind turbine 10 or at a location outside the wind turbine 10. Further, the controller 26 may be communicatively coupled to any number of the components of the wind turbine 10 in order to control the operation of such components and / or implement a corrective or control action. As such, the controller 26 may include a computer or other suitable processing unit. Thus, in several embodiments, the controller 26 may include suitable computer- readable instructions that, when implemented, configure the controller 26 to perform various different functions, such as receiving, transmitting and / or executing wind turbine control signals. Accordingly, the controller 26 may generally be configured to control the various operating modes (e.g., start-up or shut-down sequences), de-rating or up-rating the wind turbine, and / or individual components of the wind turbine 10.

[0034] Referring now to Fig. 2 , a simplified, internal view of an embodiment of the nacelle 16 of the wind turbine 10 shown in Fig. 1 is illustrated. As shown, a generator 24 may be disposed within the nacelle 16 and supported atop a bedplate 46. In general, the generator 24 may be coupled to the rotor 18 for producing electrical power from the rotational energy generated by the rotor 18. For example, as shown in the illustrated embodiment, the rotor 18 may include a rotor shaft 34 coupled to the hub 20 for rotation therewith. The rotor shaft 34 may, in turn, be rotatably coupled to a generator shaft 36 of the generator 24 through a gearbox 38. As is generally understood, the rotor shaft 34 may provide a low speed, high torque input to the gearbox 38 in response to rotation of the rotor blades 22 and the hub 20. The gearbox 38 may then be configured to convert the low speed, high torque input to a high speed, low torque output to drive the generator shaft 36 and, thus, the generator 24.

[0035] The wind turbine 10 may also include one or more pitch drive mechanisms 32 communicatively coupled to the wind turbine controller 26, with each pitch adjustment mechanism(s) 32 being configured to rotate a pitch bearing 40 and thus the individual rotor blade(s) 22 about its respective pitch axis 28. In addition, as shown, the wind turbine 10 may include one or more yaw drive mechanisms 42 configured to change the angle of the nacelle 16 relative to the wind (e.g., by engaging a yaw bearing 44 of the wind turbine 10 that is arranged between the nacelle 16 and the tower 12 of the wind turbine 10).

[0036] In addition, the wind turbine 10 may also include one or more sensors 66, 68 for monitoring various wind conditions of the wind turbine 10. For example, the incoming wind direction 52, wind speed, or any other suitable wind condition near of the wind turbine 10 may be measured, such as through use of a suitable weather sensor 66. Suitable weather sensors may include, for example, light detection and ranging devices, sonic detection and ranging devices, anemometers, wind vanes, barometers, radio detection and ranging devices or any other sensing device which can provide wind directional information now known or later developed in the art. Further sensors 68 may be utilized to measure additional operating parameters of the wind turbine 10, such as voltage, current, vibration, etc. as described herein.

[0037] Referring now to Fig. 3, a schematic diagram of an embodiment of a wind turbine power system 100 is illustrated in accordance with aspects of the presentdisclosure. As mentioned, the rotor 18 of the wind turbine 10 may, optionally, be coupled to the gearbox 38, which is, in turn, coupled to a generator 102, which may be a doubly fed induction generator (DFIG). As shown, the generator 102 may be connected to a stator bus 104. Further, a power conversion assembly 106 may be connected to the generator 102 via a rotor bus 108, and to the stator bus 104 via a line side bus 110. As such, the stator bus 104 may provide an output multiphase power (e.g., three-phase power) from a stator of the generator 102, and the rotor bus 108 may provide an output multiphase power (e.g., three-phase power) from a rotor of the generator 102. The power conversion assembly 106 may also include a rotor side converter (RSC) 112 and a line side converter (LSC) 114. The generator 102 is coupled via the rotor bus 108 to the rotor side converter 112. Additionally, the RSC 112 is coupled to the LSC 114 via a DC link 116 across which is a DC link capacitor 118. The LSC 114 is, in turn, coupled to the line side bus 110.

[0038] The RSC 112 and the LSC 114 may be configured for normal operating mode in a three-phase, pulse width modulation (PWM) arrangement using one or more switching devices, such as insulated gate bipolar transistor (IGBT) switching elements. In addition, the power conversion assembly 106 may be coupled to a converter controller 120 in order to control the operation of the rotor side converter 112 and / or the line side converter 114 as described herein. It should be noted that the converter controller 120 may be configured as an interface between the power conversion assembly 106 and the turbine controller 26 and may include any number of control devices.

[0039] In typical configurations, various line contactors and circuit breakers including, for example, a grid breaker 122 may also be included for isolating the various components as necessary for normal operation of the generator 102 during connection to and disconnection from a load, such as the electrical grid 124. For example, a system circuit breaker 126 may couple a system bus 128 to a transformer 130, which may be coupled to the electrical grid 124 via the grid breaker 122. In alternative embodiments, fuses may replace some or all of the circuit breakers.

[0040] In operation, alternating current power generated at the generator 102 by rotating the rotor 18 is provided to the electrical grid 124 via dual paths defined by the stator bus 104 and the rotor bus 108. On the rotor bus side 108, sinusoidal multi-phase (e.g., three-phase) alternating current (AC) power is provided to the power conversion assembly 106. The rotor side converter 112 converts the AC power provided from the rotor bus 108 into direct current (DC) power and provides the DC power to the DC link 116. As is generally understood, switching elements (e.g., IGBTs) used in the bridge circuits of the rotor side converter 112 may be modulated to convert the AC power provided from the rotor bus 108 into DC power suitable for the DC link 116.

[0041] In addition, the line side converter 114 converts the DC power on the DC link 116 into AC output power suitable for the electrical grid 124. In particular, switching elements (e.g., IGBTs) used in bridge circuits of the line side converter 114 can be modulated to convert the DC power on the DC link 116 into AC power on the line side bus 110. The AC power from the power conversion assembly 106 can be combined with the power from the stator of the generator 102 to provide multi-phase power (e.g., three-phase power) having a frequency maintained substantially at the frequency of the electrical grid 124 (e.g., 50 Hz or 60 Hz).

[0042] Additionally, various circuit breakers and switches, such as grid breaker 122, system circuit breaker 126, stator sync switch 132, converter breaker 134, and line contactor 136 may be included in the wind turbine power system 100 to connect or disconnect corresponding buses, for example, when current flow is excessive and may damage components of the wind turbine power system 100 or for other operational considerations. Additional protection components may also be included in the wind turbine power system 100.

[0043] Moreover, the power conversion assembly 106 may receive control signals from, for instance, the local control system 176 via the converter controller 120. The control signals may be based, among other things, on sensed states or operating characteristics of the wind turbine power system 100. Typically, the control signals provide control of the operation of the power conversion assembly 106. For example, feedback in the form of a sensed speed of the generator 102 may be used to control the conversion of the output power from the rotor bus 108 to maintain a proper and balanced multi-phase (e.g., three-phase) power supply. Other feedback from other sensors may also be used by the controller(s) 120, 26 to control the power conversion assembly 106, including, for example, stator and rotor bus voltages and currentfeedbacks. Using the various forms of feedback information, switching control signals (e.g., gate timing commands for IGBTs), stator synchronizing control signals, and circuit breaker signals may be generated.

[0044] The power conversion assembly 106 also compensates or adjusts the frequency of the three-phase power from the rotor for changes, for example, in the wind speed at the hub 20 and the rotor blades 22. Therefore, mechanical and electrical rotor frequencies are decoupled, and the electrical stator and rotor frequency matching is facilitated substantially independently of the mechanical rotor speed.

[0045] Under some states, the bi-directional characteristics of the power conversion assembly 106, and specifically, the bi-directional characteristics of the LSC 114 and RSC 112, facilitate feeding back at least some of the generated electrical power into the generator rotor. More specifically, electrical power may be transmitted from the stator bus 104 to the line side bus 110 and subsequently through the line contactor 136 and into the power conversion assembly 106, specifically the LSC 114 which acts as a rectifier and rectifies the sinusoidal, three-phase AC power to DC power. The DC power is transmitted into the DC link 116. The capacitor 118 facilitates mitigating DC link voltage amplitude variations by facilitating mitigation of a DC ripple sometimes associated with three-phase AC rectification.

[0046] The DC power is subsequently transmitted to the RSC 112 that converts the DC electrical power to a three-phase, sinusoidal AC electrical power by adjusting voltages, currents, and frequencies. This conversion is monitored and controlled via the converter controller 120. The converted AC power is transmitted from the RSC 112 via the rotor bus 108 to the generator rotor. In this manner, generator reactive power control is facilitated by controlling rotor current and voltage.

[0047] The RSC 112 also functions in the voltage compensation circuit in accordance with aspects of the invention described below.

[0048] Fig. 4 depicts an electrical machine 200 that may incorporate aspects of the present invention. The machine 200 is configured as an electrical generator (e.g., the wind turbine generator 102 depicted in Fig. 3) and includes a stator 206 mounted within and to a housing or frame 242 having opposite end walls 242, 243. The stator 206 includes stator windings 207 that supply power to a load (e.g., the grid).

[0049] The electrical machine 200 includes a rotor 202 configured on a rotor shaft 204. As discussed above with respect to Figs. 2 and 3, in a wind turbine embodiment, the rotor shaft 204 is rotationally driven by the wind turbine main shaft 34 via a gearbox 38. The rotor 202 includes a plurality of rotor windings 203.

[0050] The rotor shaft also includes an end portion 240 that extends through the opposite end wall 242 of the machine frame 238.

[0051] The rotor shaft 204 is rotationally supported by bearings 208 that may be mounted in the machine frame end walls 242, 243. An electrical insulation material 244 may be provided between the bearings 208 and the machine frame 238 to aid in electrically isolating the bearings 208 from the machine frame 238.

[0052] Conventional bearings may be utilized that include an outer race 212, and inner race 210, and rolling bodies (e.g., ball bearings or roller bearings) between the races 210, 212.

[0053] The rotor shaft extends through the end wall 243 and is electrically coupled to a frequency converter, such as the RSC 112 discussed above with respect to Fig. 3, which facilitates feeding back at least some of the generated electrical power via the rotor bus 108 into the generator rotor 202, specifically to the rotor windings 203. In particular, the frequency converter 112 provides a phase voltage to each of the rotor windings 203 at a star connection. A common mode voltage Vcm is applied to the rotor via the star connection and can be measured directly between the star connection and the machine housing 238, the Vcm having a basic sinusoidal voltage profile.

[0054] The electrical machine 200 is configured with a voltage attenuator circuit 216 that is configured to apply a compensating voltage signal to the rotor 202 (e.g., to the rotor shaft 204) to offset a voltage induced on the rotor 202 from stray current flow in the rotor 202. By reducing the rotor voltage, the voltage on the bearings 208 is also reduced, which reduces the voltage potential across the bearings 208 and minimizes stray current flow through the bearings to the grounded machine frame 238. The voltage attenuator circuit 216 is explained in greater detail below.

[0055] It is inherent in the construction of the electrical machine 200 that various stray capacitances are generated, as depicted in the phase diagram of Fig. 5. The construction leads to a capacitance Cws between the rotor winding 203 and the machine housing 238. Another capacitance Cwr is defined between the rotor winding203 and the rotor shaft 204. A capacitance Cg is defined between the rotor shaft 204 and the machine housing 238. An insulation capacitance Cil and Ci2 is defined across the bearing insulation material 244 (Fig. 4) between the rotor shaft 204 and the machine housing 238.

[0056] A capacitance Cbl and Cb2 is also defined at each of the bearings 208, respectively, which is active between the outer ring of the bearing and the rotor shaft. 204.

[0057] Due to the combination of various stray capacitances described above and the common mode voltage Vcm (at the star connection), a stray current flow is developed between the rotor windings 203 and the rotor 202 / rotor shaft 204. Thus, current flows between the rotor shaft 204 and the grounded machine frame 238. This stray current results in a voltage forming across the bearings 208. When this voltage exceeds the breakdown strength of the lubricating film in the bearings 208, current flows through the bearings 208 to the machine frame 238 causing damage to the bearings 208.

[0058] Referring to Figs. 5 and 6, the voltage attenuator circuit 216 includes an input 218 that receives a signal corresponding to the common mode voltage Vcm applied to rotor windings 203 or the stator windings 207 depending on how the electrical machine 200 is configured. In the embodiment depicted in the figures, the electrical machine 200 is an electrical generator and the common mode voltage Vcm is applied to the rotor windings 203, as discussed above.

[0059] The voltage attenuator circuit 216 includes a compensation circuit 220 having functionalities to generate the compensating voltage signal from the input signal, as described in greater detail below.

[0060] A capacitive coupler 232 applies the compensating voltage signal to rotor shaft 204. This voltage offsets the induced voltage on the bearings 208 at least to an extent to substantially prevent the damaging current flow through the bearings 208 discussed above.

[0061] As depicted in Fig. 6, a dynamic feedback circuit 246 is provided and is configured to continuously or periodically detect a voltage on the rotor shaft 204 and adjust the compensation circuit such that the compensating voltage signal is sufficient to prevent the stray current flow through the bearings 208.

[0062] Fig. 6 depicts an embodiment of the components and functionalities of the compensation circuit 220. An inverting amp stage is provided by an adjustable gain amp 226 configured to receive and invert the input signal. The gain constant is established by a controller configured with the compensation circuit 220 and may be continuously or periodically adjusted by the feedback loop 246.

[0063] An adjustable transport delay stage is provided a transport delay circuit 228 downstream of the amp 226 and is used to set a delay between receipt of the input signal (corresponding to the common mode voltage Vcm at the rotor winding star connection) and application of the compensating voltage to the rotor shaft 204. This delay accounts for the time it takes for the voltage increases at the various parasitic capacitances described above, generation of the stray rotor current, and voltage increase at the bearings 208.

[0064] The compensation circuit 220 may also include a step-up transformer 230 to increase (e.g., xlO increase) the compensating voltage signal applied to the rotor shaft 204 through the capacitive coupler 232. By increasing the signal strength, the size of the capacitive coupler 232 can be reduced while still applying sufficient compensating voltage to attenuate the voltage peaks at the bearings 208. To further increase current on the primary side of the transform 230, multiple amplifiers 226 may be configured in parallel upstream of the transformer 230.

[0065] The function of the capacitive coupler 232 may be performed by any suitable capacitor configuration. In a unique embodiment of the invention depicted in Fig. 4, the capacitive coupler 232 includes a cylinder 234 disposed around the rotor shaft 204 with an air gap between the cylinder 234 and the shaft 204. The cylinder 234 is formed from a different metal than the shaft 204, whereas air in the gap between the metals functions as a dielectric. The cylinder 234 may be configured around an end portion 240 of the shaft 204 that extends through the end wall 242 of the machine frame 238. Thus, the capacitive coupler 232 is outside of the machine frame. The cylinder 234 may be open-ended and configured as a sleeve around the end portion 240 of the shaft 204.

[0066] In the alternate embodiment also depicted in Fig. 4, the cylinder 234 is closed-ended and is configured as a cap 236 onto the end portion of the shaft 204.

[0067] The present invention also encompasses various embodiments of a method for reducing damage to a bearing in an electrical machine from stray current flow through the bearing, wherein the electrical machine includes a rotor having a shaft, and a stator, the rotor supported for rotation relative to the stator by the bearings. Steps of the method are depicted in the diagram of Fig. 7. Aspects of the method are also discussed above with reference to Figs. 4-6.

[0068] Referring to Fig. 7, an embodiment of the method 300 includes the step 302 of providing an input to a voltage attenuator circuit, the input corresponding to a common mode Vcm voltage applied to stator or rotor windings in the electrical machine.

[0069] At step 304, the input signal is modified to generate an output voltage signal.

[0070] At step 306, the output voltage signal is applied as a compensating voltage signal to the rotor shaft via a capacitive coupling to offset a voltage induced on the rotor shaft and bearing from the stray current flow in the rotor.

[0071] At step 308, in a dynamic feedback loop, a voltage on the rotor shaft is continuously or periodically detected and the compensating voltage is adjusted accordingly at step 310 to offset the rotor shaft voltage and bearing voltage, thereby minimizing the stray current flow through the bearing.

[0072] As discussed above, the input signal may be inverted an amplified to generate the compensating voltage signal.

[0073] The method 300 may include modifying the timing between the input signal and the output voltage signal in the generation of the compensating voltage signal.

[0074] Additionally, the method 300 may include increasing the compensating voltage signal applied to the rotor shaft with a step-up transformer downstream, as discussed abo e.

[0075] Further aspects of the invention are provided by the subject matter of the following clauses:Clause 1 : An electrical machine, comprising: a rotor, the rotor comprising a shaft; a stator, the rotor supported for rotation relative to the stator by a bearing; a voltage attenuator circuit configured to apply a compensating voltage signal to the shaft to offset a voltage induced on the bearing from stray current flow in the rotor,the voltage attenuator circuit comprising: an input that receives a signal corresponding to a common mode voltage applied to stator or rotor windings; a compensation circuit that generates the compensating voltage signal from the common mode voltage signal; a capacitive coupler configured to supply the compensating voltage signal to the shaft; and a dynamic feedback circuit configured to continuously or periodically detect a voltage on the shaft and adjust the compensation circuit such that the compensating voltage signal offsets the detected voltage on the shaft thereby minimizing damage to the bearing from stray current flow through the bearing.Clause 2: The electrical machine as in clause 1, wherein the electrical machine comprises an electrical generator, the rotor comprising a plurality of windings fed by a pulse width modulation (PWM) converter.Clause 3 : The electrical machine as in clause 1 or 2, wherein the electrical generator is a wind turbine electrical generator.Clause 4: The electrical machine as in any preceding clause, wherein the compensation circuit comprises an adjustable inverting amp stage and an adjustable transport delay stage.Clause 5: The electrical machine as in any preceding clause, wherein the compensation circuit further comprises a step-up transformer downstream of the inverting amp stage.Clause 6: The electrical machine as in any preceding clause, wherein the capacitive coupler comprises a cylinder disposed around the shaft with an air gap between the cylinder and the shaft.Clause 7: The electrical machine as in any preceding clause, wherein the cylinder is closed-ended and defines a cap on an end of the shaft.Clause 8: The electrical machine as in any preceding clause, wherein the cylinder is open-ended and defines a sleeve around the shaft.Clause 9: The electrical machine as in any preceding clause, further comprising a machine frame, the stator and the rotor housed within the machine frame, the shaft comprising an end portion extending through an end wall of the machine frame, the capacitive coupler configured with the end portion of the shaft outside of the machine frame.Clause 10: The electrical machine as in any preceding clause, wherein the capacitive coupler comprises a cylinder disposed around the end portion of the shaft.Clause 11 : A method for reducing damage to a bearing in an electrical machine from stray current flow through the bearing, wherein the electrical machine includes a rotor having a shaft, and a stator, the rotor supported for rotation relative to the stator by the bearing, the method comprising: providing an input to a voltage attenuator circuit, the input corresponding to a common mode voltage applied to stator or rotor windings in the electrical machine; modifying the input signal to generate an output voltage signal; applying the output voltage signal as a compensating voltage signal to the rotor shaft via a capacitive coupling to offset a voltage induced on the rotor shaft and bearing from the stray current flow in the rotor; and in a dynamic feedback loop, detecting a voltage on the rotor shaft and adjusting the compensating voltage to offset the rotor shaft voltage and bearing voltage thereby minimizing the stray current flow through the bearing.Clause 12: The method as in clause 11, wherein the electrical machine is an electrical generator, the rotor having a plurality of windings fed by a pulse width modulation (PWM) converter.Clause 13: The method as in clause 11 or 12, wherein the electrical generator is a wind turbine electrical generator.Clause 14: The method as in one of clauses 11-13, wherein the input signal is inverted and amplified to generate the compensating voltage signal.Clause 15: The method as in one of clauses 11-14, wherein timing between the input signal and the output voltage signal is modified to generate the compensating voltage signal.Clause 16: The method as in one of clauses 11-15, further comprising increasing the compensating voltage signal applied to the rotor shaft with a step-up transformer downstream.Clause 17: The method as in one of clauses 11-16, wherein the capacitive coupling is defined by a cylinder around the rotor shaft with an air gap defined between the cylinder and the rotor shaft.Clause 18: The method as in one of clauses 11-17, wherein the cylinder is open-ended and defines a sleeve around the shaft.Clause 19: The method as in one of clauses 11-18, wherein the electrical machine includes a machine frame, the stator and the rotor housed within the machine frame, the rotor shaft having an end portion extending through an end wall of the machine frame, the method further comprising configuring the capacitive coupler with the end portion of the rotor shaft outside of the machine frame.Clause 20: A wind turbine, comprising the electrical machine of any one of clauses 1-10 configured as an electrical generator.

Claims

WHAT IS CLAIMED IS:

1. An electrical machine, comprising: a rotor, the rotor comprising a shaft; a stator, the rotor supported for rotation relative to the stator by a bearing; a voltage attenuator circuit configured to apply a compensating voltage signal to the shaft to offset a voltage induced on the bearing from stray current flow in the rotor, the voltage attenuator circuit comprising: an input that receives a signal corresponding to a common mode voltage applied to stator or rotor windings; a compensation circuit that generates the compensating voltage signal from the common mode voltage signal; a capacitive coupler configured to supply the compensating voltage signal to the shaft; and a dynamic feedback circuit configured to continuously or periodically detect a voltage on the shaft and adjust the compensation circuit such that the compensating voltage signal offsets the detected voltage on the shaft thereby minimizing damage to the bearing from stray current flow through the bearing.

2. The electrical machine as in claim 1, wherein the electrical machine comprises an electrical generator, the rotor comprising a plurality of windings fed by a pulse width modulation (PWM) converter.

3. The electrical machine as in claim 2, wherein the electrical generator is a wind turbine electrical generator.

4. The electrical machine as in claim 1, wherein the compensation circuit comprises an adjustable inverting amp stage and an adjustable transport delay stage.

5. The electrical machine as in claim 4, wherein the compensation circuit further comprises a step-up transformer downstream of the inverting amp stage.

6. The electrical machine as in claim 1, wherein the capacitive coupler comprises a cylinder disposed around the shaft with an air gap between the cylinder and the shaft.

7. The electrical machine as in claim 6, wherein the cylinder is closed-ended and defines a cap on an end of the shaft.

8. The electrical machine as in claim 6, wherein the cylinder is open-ended and defines a sleeve around the shaft.

9. The electrical machine as in claim 1, further comprising a machine frame, the stator and the rotor housed within the machine frame, the shaft comprising an end portion extending through an end wall of the machine frame, the capacitive coupler configured with the end portion of the shaft outside of the machine frame.

10. The electrical machine as in claim 9, wherein the capacitive coupler comprises a cylinder disposed around the end portion of the shaft.

11. A method for reducing damage to a bearing in an electrical machine from stray current flow through the bearing, wherein the electrical machine includes a rotor having a shaft, and a stator, the rotor supported for rotation relative to the stator by the bearing, the method comprising: providing an input to a voltage attenuator circuit, the input corresponding to a common mode voltage applied to stator or rotor windings in the electrical machine; modifying the input signal to generate an output voltage signal; applying the output voltage signal as a compensating voltage signal to the rotor shaft via a capacitive coupling to offset a voltage induced on the rotor shaft and bearing from the stray current flow in the rotor; and in a dynamic feedback loop, detecting a voltage on the rotor shaft and adjusting the compensating voltage to offset the rotor shaft voltage and bearing voltage thereby minimizing the stray current flow through the bearing.

12. The method as in claim 1, wherein the electrical machine is an electrical generator, the rotor having a plurality of windings fed by a pulse width modulation (PWM) converter.

13. The method as in claim 12, wherein the electrical generator is a wind turbine electrical generator.

14. The method as in claim 11, wherein the input signal is inverted and amplified to generate the compensating voltage signal.

15. The method as in claim 14, wherein timing between the input signal and the output voltage signal is modified to generate the compensating voltage signal.

16. The method as in claim 11, further comprising increasing the compensating voltage signal applied to the rotor shaft with a step-up transformer downstream.

17. The method as in claim 11, wherein the capacitive coupling is defined by a cylinder around the rotor shaft with an air gap defined between the cylinder and the rotor shaft.

18. The method as in claim 17, wherein the cylinder is open-ended and defines a sleeve around the shaft.

19. The method as in claim 11, wherein the electrical machine includes a machine frame, the stator and the rotor housed within the machine frame, the rotor shaft having an end portion extending through an end wall of the machine frame, the method further comprising configuring the capacitive coupler with the end portion of the rotor shaft outside of the machine frame.

20. A wind turbine, comprising the electrical machine of claim 1 configured as an electrical generator.

Images