The present invention relates generally to electromechanical systems, such as an electric motor. Although the following discussion focuses on electric motors, the present invention affords benefits to a number of electromechanical systems and devices that have rotatable elements. For example, the present invention is equally applicable to rotatable elements in gearboxes, press rolls, and conveyor systems, to name a few applications. Indeed, devices comprising shafts that acquire a charge can benefit from the present invention. Additionally, the invention could also be utilized to impose a voltage onto a rotatable shaft, for instance.
Electric motors of various types are commonly found in industrial, commercial and consumer settings. In industry, such motors are employed to drive various kinds of machinery, such as pumps, conveyors, compressors, fans and so forth, to mention only a few. Such motors generally include a stator, comprising a multiplicity of coils, surrounding a rotor, which is supported by bearings for rotation in the motor frame. When power is applied to the motor, an electromagnetic relationship between the stator and the rotor causes the rotor to rotate. Typically, a rotor shaft extending through the motor housing takes advantage of this produced rotation and translates the rotor's movement into a driving force for a given piece of machinery. That is, rotation of the rotor shaft drives the machine to which it is coupled.
Virtually all rotatable motors, generators, etc., develop some degree of rotor shaft-to-ground voltage (Vrg) that can result in bearing currents (Ib). Typically, electric motors have two sources of Vrg: electromagnetic induction and electrostatic coupling. Electromagnetic induction generally results from the electromagnetic relationships between the stator and the rotor. For example, small dissymmetry of the magnetic field in the air gap between the rotor and the stator due to motor construction can cause electromagnetically induced Vrg to develop in the rotor shaft. Electrostatic coupling, however, results from a number of situations in which rotor charge accumulation can occur. For example, ionized or high velocity air passing over a rotor may cause rotor charge accumulation. However, external sources to the motor generally give rise to the lion's share of the Vrg due to electrostatic coupling. For example, modern voltage source inverters, such as pulse width modulated (PWM) inverters, produce stepped voltage waveforms and, as such, high “net voltage” or common mode voltage (CMV) values. Thus, PWM inverters lead to the development of Vrg in the rotor shaft. As another example, direct current motors fed by rectified power sources can also cause Vrg in the shaft. In any case, the greater Vrg, the greater the likelihood of bearing currents (Ib) and arcing within the bearing. That is, Vrg may cause a discharge of current through the bearing. Similar phenomena occur in the case of dc motors fed by rectified sources as well.
Unfortunately, bearing currents (Ib) and/or arcing within the bearing can cause damage to mechanical components of the motor. For example, if Vrg reaches a sufficient threshold value, arcing occurs between the races of the bearing and the rolling elements within the bearing, leading to localized melting and rehardening of the mechanical components of the bearing, for instance. That is, an instantaneous discharge of current (Ib) through the bearing causes an arc and, as such, localized melting and subsequent rehardening of surfaces within the bearing. This melted and rehardened material is of a metallurgical structure known as untempered martensite. This material is not as robust as the original bearing material and can lead to fatigue failure—even under relatively light bearing loads. A bearing surface with untempered matensite leads to pitting and fluting of the bearing components and may cause the bearing assembly to malfunction or to fail prematurely. Additionally, continued bearing currents (Ib) produce heat that, over time, leads to premature degradation of the bearing lubricant, which ultimately can result in higher maintenance costs and downtime. Additionally, the non-zero shaft voltage with respect to ground may arc to any nearby grounded surface, including shaft guards and motor endcaps. In a hazardous location, such arcing has the potential to cause ignition of the hazardous materials.
To prevent such bearing degradation, some electric motors may include a dissipation brush, which bleeds off Vrg by creating a short circuit in the system. However, traditional dissipation brush devices are not suitable for use in hazardous environments. For example, traditional dissipation brush devices fail to sufficiently account for the potential of danger due to electrical arcing between the rotor shaft and the dissipation brush, for instance. Accordingly, traditional dissipation brush devices are not suitable for use in many mining, industrial, and petroleum applications, where the ignition of a combustible atmosphere, for example, is a relevant concern.
Accordingly, there is a need for improved apparatus and methods for dissipation shaft charge for motors operating in hazardous environments.
According to one embodiment, the present invention comprises an apparatus for use with an electrical device, such as an electric motor and/or various rotatable components of the electric motor. However, as discussed above, the present invention is applicable to a number of devices that employ rotatable elements, such as gearboxes, conveyor systems, and belt drives, to name a few. The apparatus comprises an explosion-proof enclosure and a dissipation member nonrotatably disposed in the enclosure. When installed with respect to the electrical device, the dissipation member of the exemplary apparatus generally abuts a rotatable member of the device, which is at least partially disposed within the explosion-proof enclosure. To dissipate charge developed in the rotatable member during operation of the electrical device, the dissipation member of the exemplary apparatus is configured to electrically couple the rotatable member to ground. Advantageously, by dissipating the charge or voltage in the rotatable member, the likelihood of damage due to arcing and bearing currents (Ib) may be mitigated in the bearing assembly that supports the rotatable member.
According to another exemplary embodiment, the present invention provides an electric motor. The electric motor comprises a rotor, stator, and bearing assembly disposed within an explosion-proof motor housing or enclosure. The electric motor also includes a brush member disposed in the explosion-proof motor housing as well. In the exemplary motor, the brush member is configured to generally abut the rotor shaft and to facilitate dissipation of shaft charge developed in the rotor shaft during operation of the motor. Because the brush member is disposed within the explosion-proof motor enclosure, the exemplary motor provides applicability to certain hazardous environments, such as those found in petroleum and mining applications, for example. Moreover, the brush member can be configured to impart a desired voltage signal onto the shaft.
According to yet another exemplary embodiment, the present invention provides an apparatus for use with a device having a rotatable device member. The exemplary apparatus comprises a self-contained explosion-proof enclosure. The apparatus also includes a rotatable shaft at least partially disposed in the explosion-proof enclosure and supported by a bearing assembly that is also housed in the explosion-proof enclosure. To engage with the rotatable device member, the rotatable shaft includes a coupling mechanism configured to couple the rotatable device member and the rotatable shaft mechanically and electrically with respect to one another. The exemplary apparatus also includes a dissipation member nonrotatably housed in the explosion-proof enclosure such that the dissipation member is configured to generally abut the rotatable shaft. To mitigate the likelihood of damage in the device due to charge build-up in the rotatable device member during operation of the device, the dissipation member is configured to couple the rotatable shaft and, as such, the rotatable device member electrically to ground.
As yet another exemplary embodiment, the present invention provides a method for dissipating charge build-up in a rotatable member of the device during operation of the device. The method comprises disposing a dissipation brush in an explosion-proof enclosure such that the dissipation brush is configured to generally abut the rotatable device member that is also at least partially disposed in the explosion-proof enclosure. Additionally, the method comprises electrically coupling the dissipation brush to ground. Advantageously, the exemplary method mitigates the likelihood of damage within a bearing assembly of the device due to arcing and bearing currents, for example.
The foregoing and other advantages and features of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:
As discussed in detail below, embodiments of the present invention provide apparatus and methods for dissipating charge build-up within rotatable members of devices operable in hazardous environments. Although the discussion regarding the present invention focuses on electric motors, the present invention is equally applicable to a number of applications in which a rotatable member develops charge during operation. For example, the present invention is applicable to conveyor systems, gearboxes, and drive mechanisms, to name but a few applications. Additionally, the term “explosion-proof” appears throughout the present discussion to describe various items. As employed herein, the term “explosion-proof” refers to an enclosure that is configured to withstand the pressure of an explosive mixture exploding inside the enclosure and to prevent the propagation of the explosion to the atmosphere surrounding the enclosure. By way of example, associations, such as the National Electrical Manufacturers' Association (NEMA), and regulatory institutions, such as Underwriters Laboratory (UL) and the National Fire Protection Association (NFPA), which propagates the National Electric Code (NEC), provide standards for establishing the specific parameters pertaining to an explosion-proof enclosure. Moreover, these associations and institutions often test various enclosures to ensure that such enclosures meet their respective standards.
Turning to the drawings,
To induce rotation of the exemplary rotor, current is routed through stator windings disposed in the stator. (See
Routing electrical current from the external power source 22 through the stator windings produces a magnetic field that induces rotation of the rotor. A rotor shaft 26 coupled to the rotor rotates in conjunction with the rotor. That is, rotation of the rotor translates into a corresponding rotation of the rotor shaft 26. To support and facilitate rotation of the rotor and the rotor shaft 26, the exemplary motor 10 includes opposite drive end and drive end bearing sets carried within the opposite drive end and drive end endcaps 14 and 16, respectively. (See
In the exemplary motor 10, a rotor 36 resides within this rotor passageway 34. Similar to the stator core 30, the rotor 36 comprises a plurality of rotor laminations 38 aligned and adjacently placed with respect to one another. Thus, the rotor laminations 38 cooperate to form a contiguous rotor 36. The exemplary rotor 36 also includes rotor end rings 40, disposed on each end of the rotor 36, that cooperate to secure the rotor laminations 38 with respect to one another. The exemplary rotor 36 also includes rotor conductor bars 42 that extend the length of the rotor 36. In the exemplary motor 10, the end rings 40 electrically couple the conductor bars 42 to one another. Accordingly, the conductor bars 42 and the end rings 40 comprise nonmagnetic, yet electrically conductive materials. As discussed below, inducing current in the rotor 36, specifically in the conductor bars 42, causes the rotor 36 to rotate. By harnessing the rotation of the rotor 36 via the rotor shaft 26, a machine coupled to the rotor shaft 26, such as a pump or conveyor, may operate.
To support the rotor 36, the motor 10 includes opposite drive end and drive end bearing sets 44 and 46 that are secured to the rotor shaft 26 and that facilitate rotation of the rotor shaft 26 and rotor 36 within the stator core 30. By way of example, the exemplary bearing sets present a ball bearing construction; however, the bearing sets may present a sleeve bearing construction, among other types of bearing constructions. Advantageously, the endcaps 14 and 16 include features, such as the illustrated inner bearing caps 47, that secure the bearing sets 44 and 46 within their respective endcaps 14 and 16. In the exemplary motor 10, the inner bearing caps comprise assemblies that are releaseably secured to the end caps 14 and 16. The bearing sets 44 and 46 transfer the radial and thrust loads produced by the rotor shaft 26 and rotor 30 to the motor housing. Each bearing set 44 and 46 includes an inner race 48 disposed circumferentially about the rotor shaft 26. The fit between the inner races 48 and the rotor shaft 26 causes the inner races 48 to rotate in conjunction with the rotor shaft 26. Each bearing set 44 and 46 also includes an outer race 50 and rolling elements 52 disposed between the inner race 48 and the outer race 50. The rolling elements 52 facilitate rotation of the inner races 48 while the outer races 50 remains stationarily mounted with respect to the endcaps 14 and 16. Thus, the bearing sets 44 and 46 facilitate rotation of the rotor shaft 26 and the rotor 36 while providing a support structure for the rotor 36 within the motor housing, i.e., the frame 12 and the endcaps 14 and 16. To improve the performance of the bearing sets 44 and 46, a lubricant coats the rolling elements 52 and race 48 and 50, providing a separating film between to bearing components, thereby mitigating the likelihood of seizing, galling, welding, excessive friction, and/or excessive wear, to name a few adverse effects.
During operation of the motor 10, current passing through the stator windings 32 electromagnetically induces current in the conductor bars 42, thereby causing the rotor 36 and rotor shaft 26 to rotate. In addition, the common mode voltage (CMV) in the stator windings 32 causes a charge to build up on the inner surface 54 of the stator core 30. As charge builds on the inner surface 54 of the stator core 30, an electric field is produced. In turn, this electric field causes parasitic capacitive coupling between the inner surface 54 of the stator core 30 and the outer surface 56 of the rotor 36. In essence, the air gap between the rotor 36 and the stator core 30 acts as a dielectric, while the inner surface 54 of the stator core 30 and the outer surface 56 of the rotor 36 cooperate as plates of a capacitor, which accumulate charge.
Additionally, the electrical communicativeness between rotor shaft 26 and the bearing sets 44 and 46 causes a charge build-up in the inner race 48 of the respective bearing sets 44 and 46. However, as discussed above, the lubricant coating the rolling elements 52 the races and 48 and 50 of the respective bearing sets 44 and 46 acts as a dielectric. Accordingly, the lubricant facilitates capacitive coupling between the races 48 and 50 and the rolling elements 52. Unfortunately, when charge build-up on the inner race 48 exceeds the lubricant's electric field breakdown threshold, for instance, electrons begin to move between the previously isolated races 48 and 50 and the rolling elements 52. This movement of electrons in the bearing sets is referred to as bearing current (Ib). Often, Ib results in arcing between the rolling elements 52 and the races 48 and 50, thereby causing pitting and fluting on the various surfaces of the bearing sets 44 and 46, for instance. By way of example, the bearing current (Ib) can lead to metallurgical damage of components of the bearing sets. A similar effect occurs in the case of sleeve bearings, but between the shaft and bearing surface, without any intermediate rolling elements.
To dissipate the build-up of charge in the rotor shaft 26 and, as such, to reduce the occurrence of arcing and Ib within the bearing sets, the exemplary motor 10 includes at least one charge-dissipating mechanism, such as the exemplary dissipation assembly 57. In summary, the exemplary dissipation assembly 57 provides an electrical pathway to dissipate the build-up of charge on the rotor shaft 26. By dissipating the build-up of charge on the rotor shaft 26, the likelihood of arcing and bearing current (Ib) occurring in the bearing set is reduced.
In the exemplary embodiment illustrated in
Because the dissipation brush 58 is housed within the motor housing (i.e., the frame 12 and the endcaps 14 and 16), the dissipation brush 58 is housed within an explosion-proof enclosure. Thus, the exemplary explosion-proof motor housing is capable of withstanding an explosion produced by arcing between the dissipation brush 58 and the rotating shaft 26, for instance, and prevents propagation of the explosion to the atmosphere external to the motor housing. That is, the dissipation brush 58 and the motor housing cooperate to provide a mechanism to dissipate the build-up of charge on the rotor shaft 26 produced during operation of the motor 10 in a hazardous environment.
As discussed above, the rotor shaft 26 develops a build-up of charge or Vrg during operation of the motor. However, the dissipation brush 58, via the brush portion 60, provides an electrical pathway to dissipate this charge, thereby mitigating the likelihood of Vrg of surpassing the threshold level of the bearing lubricant and, as such, the likelihood of arcing and bearing current (Ib) occurring, for instance.
Additionally, the member 58 may be configured to impart a voltage onto the shaft 26, i.e., a transmission member. For example, the transmission member 58 may impart a voltage signal onto the shaft 26 that could be indicative of an operating condition, for instance. Moreover, the transmission member 58 may impart a voltage to prevent against corrosion on the shaft 26.
In the exemplary embodiment, the dissipation brush 58 is electrically coupled to various electrical pathways. As one example, the dissipation brush 58 is electrically coupled directly to ground 59, which may be an earth ground or frame ground, for example. However, the dissipation brush 58 also may communicate with other electrical devices and circuits via alternate pathways. For example, the dissipation brush 58 may electrically communicate with voltage sensing circuitry (VSC) 64, which is configured to determine the voltage level (e.g., Vrg) developed in the rotor shaft 26 during operating of the motor 10. Advantageously, the voltage sensing circuitry 64 may include communication circuitry that facilitates communication of the sensed voltage level to a remote location, such as a remote monitoring center 66. As yet another example, dissipating current traveling though the dissipation brush 58 may be harnessed to provide at least some operating power to various electrical devices 68 (e.g., clutch mechanisms, instrumentation, indicators). For example, the dissipation of the rotor shaft 26 charge (Vrg) may produce a current that may be harnessed to provide at least a portion of the operating power to the various electrical devices 68 of the motor 10. Advantageously, these various electrical pathways may include switches 69 that facilitate selective transition between the various electrical pathways. Alternatively, there may also be a plurality of brushes inside the motor housing, each dedicated for its own purpose and circuit.
The explosion-proof enclosure 70 houses either one or a plurality of dissipation brushes 58 that abut the rotor shaft 26, which extends through the enclosure 70 when the dissipation assembly 57 is mounted to a motor 10. Specifically, the dissipation brushes 58 are housed in a series of chambers 78. To maintain abutment between the dissipation brushes 58 and the rotor shaft 26, biasing members, such as the illustrated compression springs 62 located in the chambers 78, bias the dissipation brushes 58 toward the rotor shaft 26. To access the dissipation brushes 58, the exemplary explosion-proof enclosure 70 includes an access panel 82. The access panel 82 is removably mounted to the enclosure 70 and, as such, facilitates the removal or replacement of the dissipation brushes 58 from the explosion-proof enclosure 70, if so desired. It will be recognized that the access panel may be positioned at various locations to most advantageously access the interior of the explosion-proof enclosure 70.
In the exemplary embodiment, the dissipation assembly 57 includes three dissipation brushes 58, each electrically coupled to three different electrical pathways. The dissipation brush 58 located furthest away from the motor 10 is coupled directly to ground 59, which, as discussed above, may be earth ground or frame ground, for example. A second pathway electrically couples the dissipation brush 58 located closest to the motor 10 to the VSC 64 and a display. Advantageously, the VSC determines a voltage level (e.g., Vrg) developed in the rotor shaft 26 during operation. Moreover, the VSC may communicate with the display to display the determined voltage level to a technician or operator. A third electrical pathway electrical couples the third and intermediate dissipation brush 58 to an electronic device 68, such as instrumentation or a clutch mechanism, for example. In the third electrical pathway, the dissipating charge from the rotor shaft 26 produces a current in this pathway that can provide at least some operating power to the electronic device 68. The VSC circuitry 64 and the electrical device 68 may be local to the dissipation assembly 57, or they may be remotely located with respect to both the motor 10 and the dissipation assembly 57, for example.
Advantageously, the embodiment of
The grounding shaft 94 includes features that facilitate coupling of the grounding shaft 94 to the rotor shaft 26 and to the drive shaft 90. For example, a first end 100 of the grounding shaft 94 is configured to mate with the rotor shaft 26. Additionally, a second end 102 of the grounding shaft 94, which is opposite the first end 100, is configured to mate with a drive shaft 90. When mated, the grounding shaft 94 physically couples the drive shaft 90 and the rotor shaft 26 to one another. Accordingly, rotation of the rotor shaft 26 produces rotation in the grounding shaft 94 and, in turn, in the drive shaft 90. Advantageously, the grounding shaft 94 can include coupling features that mitigate the negative effects of misalignments between the various shafts and that improve the transfer of torque between the shafts.
Additionally, the conductive nature of the grounding shaft 94 electrically couples the grounding shaft 94 to the rotor shaft 26 and to the drive shaft 90. Accordingly, the build-up of charge on the rotor shaft 26 and on the drive shaft 90 during operation of the motor 10 and driven machine 92, respectively, is communicated to the grounding shaft 94. To dissipate this build-up of charge on the grounding shaft 94 and, as such, the drive shaft 90 and the rotor shaft 26, the dissipation assembly 57 includes a dissipation brush 58 nonrotatably housed within the explosion-proof enclosure 70. The dissipation brush 58 generally abuts the grounding shaft 94 and, as such, is electrically coupled to the grounding shaft 94. By coupling the dissipation brush 58 to ground 59 (e.g., frame ground, earth ground, etc.), the dissipation brush 58 provides a path to ground to dissipate the build-up of charge on the rotor shaft 26 and the drive shaft 90. Advantageously, dissipating this build-up of charge mitigates the likelihood of damage to bearing sets in both the motor 10 and the driven machine 90 due to arcing and bearing currents, for example.
While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Although the present discussion focused on electric motors, the present invention provides benefits to a number of devices in which a rotating member is employed. Indeed, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.