COPYRIGHT STATEMENT

A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.

FIELD

The present disclosure relates, in general, to methods, systems, and apparatus for cooling the stator of an electric machine, for example an electric motor or electric generator. The present disclosure relates more specifically to the cooling of the stator of a permanent magnet (PM) electric machine.

BACKGROUND

Electric machines employed in commercial and industrial applications are often required to operate at 100% of the applicable power rating over what is typically a 60,000-hour service life. Therefore, a motor or other electric machine employed in a commercial or industrial setting must be both reliable and versatile. Machine reliability and lifespan can be compromised by many factors including but not limited to: 1) high temperature operation resulting in thermally induced component failures, 2) vibration, heat, friction, unbalanced operation, contamination, inadequate material selection or other causes of mechanically induced component failure, or 3) dielectric failure resulting in electrical short circuits or open circuits caused by heat, inadequate material selection or other causes.

Electric machines are often provided with a mechanical cooling system, for example a fan plus a cowling configured to direct air over cooling fins cast into the machine housing. Many such devices are classified as totally enclosed fan cooled (TEFC) devices. TEFC motors suffer from inherent inefficiency since a portion of the total power output by the motor must turn the cooling fan, and therefore is unavailable for output. For example, a TEFC motor attached to a conveyor belt must operate both the cooling fan and the conveyor belt, therefore the portion of the motor output required to turn the fan is not available at the conveyor belt. The fan and cowling apparatus of a TEFC machine is prone to damage, and presents a safety risk in a typical industrial setting. Furthermore, since a TEFC machine drives a cooling fan with one side of the motor shaft, it is impossible or difficult to attach two downstream machines to a single TEFC motor.

Conventional electric machines are made robust and durable usually by implementing the machines with large and heavy components. The larger and heavier components required to promote stable long-term operation can increase machine cost and weight.

Other electric machines, having directly ventilated and therefore partially open housings to promote cooling, can suffer from moisture and particulate matter contamination which in turn can cause mechanical component degradation and wire insulation or electric junction degradation and premature failure. Totally enclosed non-ventilated (TENV) electric machines present unique cooling challenges since TENV devices lack direct ventilation or an external fan moving air over external cooling fins.

The embodiments disclosed herein are designed to minimize one or more of the above problems.

SUMMARY

Embodiments disclosed herein include electric machine cooling apparatus and methods. Other embodiments include elements imparting enhanced robustness and durability to an electric machine. As used herein, the class of devices referred to as electric machines includes both electric generators and electric motors. Certain embodiments described herein are permanent magnet motors having a radial flux configuration. Many of the disclosed methods and apparatus are also applicable to transfer heat or in part robustness to axial flux machines, transfer flux machines, and linear machines. Certain methods and apparatus may be applicable to non-rotating torque motors, transformers, or inductors. Although many specific embodiments are illustrated with respect to permanent magnet motors, the disclosure and claims are not limited to any specific apparatus configuration and are applicable to any type of electric machine.

Certain embodiments disclosed herein include an electromagnet having a stack of laminations defining a tooth and a yoke segment, and an insulating bobbin surrounding a portion of the tooth of each lamination, such that each lamination is held against adjacent laminations by the bobbin. The disclosed electromagnets also have electrically conductive windings surrounding a portion of the bobbin; and an encapsulant fully encapsulating the bobbin and windings, wherein the encapsulant comprises a dielectric material and an additive to increase the thermal conductivity of the encapsulant.

In some embodiments, the dielectric material comprises a polymer and the additive comprises one or more of boron nitride, silicon carbide, silicon; aluminum powder, copper powder, metal oxide, ceramic, and graphene.

In some embodiments, Individual laminations in the stack of laminations may include opposing first and second planar faces, with one face being in thermal contact with a heat transfer layer. Individual laminations in the stack of laminations may further be in physical contact with a dielectric layer, opposite the heat transfer layer.

Other embodiments include a method of fabricating an electromagnet or stator having a plurality of electromagnets as described above. In some embodiments, each yoke segment defined by a stack of laminations further defines a tongue structure and an opposing groove structure. A plurality of electromagnets may be assembled into a stator by mating the tongue structure and the groove structure of each electromagnet with the corresponding tongue structure and the groove structure of adjacent electromagnets; and encapsulating the assembled stator with a thermally conductive encapsulant.

Certain electric machine embodiments disclosed herein minimize the production of heat caused by eddy currents in the permanent magnets, rotor back assembly, electromagnetic cores, or other structures. In addition, methods and apparatus are disclosed providing for the removal of heat from an electric machine by conduction, convection and radiation utilizing various heat paths through the internal cavities and components or subsystems of the machine.

An embodiment disclosed herein is an electric machine having a rotor, a stator and a housing. The rotor includes a shaft defining a lengthwise axis. The shaft is surrounded by a rotor back assembly, also known as a back-iron assembly. The rotor also includes a radially mounted array of permanent magnets positioned around the perimeter of the rotor back assembly. The machine stator includes a plurality of electromagnets radially positioned around the rotor defining an air gap between an exterior surface of the permanent magnets of the rotor and an interior surface of the electromagnets of the stator.

The rotor and the stator are supported by and enclosed within a housing. In certain embodiments the housing is a totally enclosed nonventilated (TENV) housing. In one embodiment, the stator and the housing define a substantially cylindrical rotor cavity within the air gap and bounded by the housing end plates or similar structures. The rotor cavity may further be divided into first and second cavities when the rotor is positioned within the rotor cavity. Specifically, a first cavity exists between the rotor and the housing at one end of the rotor, adjacent to a housing end plate. A second cavity exist between the rotor and housing at the other end of the rotor, adjacent the other end plate. The housing end plates may be separate plates attached to a housing perimeter portion, alternatively one of the end plates and the housing perimeter may be a cast, machined or otherwise unitary or co-formed housing element.

The first and second cavities are connected through the air gap. In addition, one or more ventilation channels can be provided through the rotor back assembly, extending from the first cavity to the second cavity. The first and second cavities, the air gap, and the ventilation channels therefore define a fluid circuit around the exterior of and through the back-iron of the rotor. The rotor may also include an internal fan extending into the first or second cavity. The internal fan is part of the rotor or connected to the rotor and is configured to cause low pressure at either the air gap or the ventilation channels and high pressure at the other of the air gap and ventilation channels. Therefore, when the rotor rotates, air or another fluid, for example an air and oil mixture, is caused to circulate around the fluid circuit from one cavity through the air gap to the other cavity and back to the original cavity through the ventilation channels. Thus, heat generated in the rotor during operation can be transferred to the air or other fluid flowing in the fluid circuit to cool the rotor.

The rotor fan can be a separate structure, or could be rotor fan blades formed in an exterior surface of the rotor back assembly. In addition, the rotor back assembly, fan blades, or fan may be treated to enhance the radiation of heat to the fluid circuit and therefore to the first cavity or the second cavity. Suitable surface treatments include but are not limited to surface roughening, or surface anodization.

Heat is produced during the operation of an electric machine within the rotor primarily by magnetically induced eddy currents in the permanent magnets and magnetically induced eddy currents and hysteresis within the rotor back assembly. In certain embodiments, the production of heat within the rotor can be reduced by implementing both the permanent magnets and the rotor back assembly with a series of laminations. Structural support and advantageous heat transfer characteristics may be provided by binding the permanent magnets to the rotor back assembly with a retainer band surrounding an outer surface of the magnets, facing the air gap.

Heat transfer from the rotor to the fluid circuit and subsequent heat transfer from the first and second cavities to the machine housing may be facilitated with various heat transfer structures. One class of heat transfer structures is mounted to the rotor facing either the first cavity or the second cavity. Another class of heat transfer structures may be mounted to the housing, typically at the end plates, facing into the first cavity and/or the second cavity. Any one of these heat transfer structures may include an array of pins, fins, combination pin/fins, or other structures to increase surface area and turbulence, and therefore promote effective heat transfer to or from the attached rotor or housing structure. In addition, a heat transfer structure may be fabricated from a material such as aluminum or copper having relatively high thermal conductivity. A heat transfer structure may be textured, colored, have a surface treatment, or otherwise fabricated to effectively transfer heat to or from the fluid circuit.

Additional heat may be transferred away from the permanent magnets by including a thermally conductive filler or encapsulant material in the gap between adjacent permanent magnets. The thermally conductive encapsulant material may be a polymer such as an epoxy having an additive suspended within the polymer matrix to increase the thermal conductivity of the rotor encapsulant above the native thermal conductivity of the polymer, epoxy or other rotor encapsulant material. The thermally conductive encapsulant serves to conduct heat away from the sides of the permanent magnets toward the first and second cavities during rotor operation. Any heat transfer structures attached to the rotor can be placed into thermal contact with the thermally conductive encapsulant regions to promote heat exchange with the fluid circuit.

A thermally conductive encapsulant in the gap between adjacent permanent magnets also provides structural rigidity and robustness to the rotor. The encapsulant serves to additionally secure the permanent magnets to the rotor back assembly and prevent the magnets from slipping around the circumference of the rotor under load. In some embodiments, the rotor back assembly may define an anchoring surface between adjacent permanent magnets serving to more securely anchor the encapsulant to rotor back assembly. An anchoring surface may be a groove, protrusion, keyway or the like formed in her extending from the rotor back assembly.

A portion of the heat generated in the permanent magnets or rotor back assembly may be conducted to the shaft and conducted from the shaft to equipment driving or being driven by the electric machine. A machine shaft is typically fabricated from steel or another high-strength alloy that may not have relatively high thermal conductivity. The thermal conductivity of a machine shaft may be enhanced by providing the shaft with a thermally conductive shaft core made of a material, copper for example, having a different composition, and higher thermal conductivity than other portions of the shaft.

In certain embodiments heat may be transferred to the shaft from the housing as well. The shaft is typically supported by bearings at each end of the rotor. The bearings are supported by the housing. In some embodiments portions of the bearing structure, the bearing seals for example, may be fabricated from a material having enhanced thermal conductivity such as copper. Bearing flanges or other housing elements supporting the bearings may also be fabricated from a material having enhanced thermal conductivity. In such embodiments, the thermally conductive shaft core may be made to extend toward the shaft perimeter where the shaft and bearings are in contact.

It may be advantageous in certain embodiments to seal the housing. For example, a TENV motor may be sealed to prevent internal contamination. In such an embodiment the bearings may be accessible from outside the housing to facilitate bearing removal or replacement without requiring the housing to be opened.

Electric machine embodiments also include a stator having a plurality of radially positioned electromagnets. In some embodiments, the stator is encapsulated such that the stator encapsulant is in thermal contact with housing structures, for example the housing end plates. In some embodiments the entire perimeter portion of the housing is in thermal contact with the stator or the stator encapsulant. In an embodiment, the housing includes a perimeter portion, a first end plate at the second end plate. The end plates may be separate structures or co-fabricated with the perimeter portion of the housing. The stator encapsulant may contact the first end plate and the second end plate such that a central region of the first end plate, a central region of the second end plate, and an interior stator surface define the enclosed cylindrical rotor cavity. In some embodiments, substantially no voids will extend from the interior stator surface, the central region of the first end plate, and the central region of the second end plate toward the perimeter portion.

The stator encapsulant provides for device robustness and thermal transfer from the stator to the housing. The thermal conductivity of the encapsulant may be enhanced by mixing an additive to the encapsulant to increase the thermal conductivity of the encapsulant. For example, the encapsulant may be a dielectric material such as a polymer or epoxy and the additive may be boron nitride, silicon carbide, silicon, aluminum oxide, aluminum powder, copper powder, metal oxide, ceramic, graphene, substantially spherical particles or combinations of these or similar materials.

Thermal transfer between the stator and the housing and overall machine robustness may be enhanced by fitting the stator closely to the perimeter portion of the housing. Thermal transfer between the stator and the perimeter portion of the housing may be further enhanced by filling any gap between the stator perimeter and housing with a thermally conductive lubricant or encapsulant.

Thermal transfer from the rotor cavity defined by the stator and housing may be facilitated by providing one or more heat transfer structures in thermal contact with the central regions of the housing or an end plate. The heat transfer structures may include pins, fins, combination pin/fins or other structures extending into the rotor cavity to increase surface area, air turbulence, or otherwise promote heat transfer from the rotor cavity. In addition, any heat transfer structure may be colored, anodized, or have a surface treatment designed to promote effective heat transfer.

Thermal energy transferred to the housing may be removed from the electric machine by conduction, convection or radiation. Heat transfer from the housing may be enhanced by providing the housing with fins, external heat transfer structures, black anodization or other means. In addition, the housing may include feet fabricated from a material having high thermal conductivity, aluminum for example. Heat may be transferred from the feet to a mounting surface, for example a factory floor, shelf, or other equipment. Heat transfer from the feet to the mounting surface may be facilitated by providing an interface having high thermal conductivity between the feet and the mounting surface, for example thermal paste or copper.

Stator embodiments include a radial array of electromagnets. In certain embodiments, each electromagnet includes a core having a stack of laminations defining a tooth portion and a yoke segment. An insulating bobbin may be provided surrounding a portion of the tooth of each lamination. Electrically conductive windings then surround a portion of the bobbin. In one embodiment, each lamination in the stack of laminations is held against adjacent laminations solely by pressure from the bobbin, without the use of screws, welds, pins, crimp joints, glue or other fastening means.

In some embodiments, heat transfer from an electromagnetic core may be enhanced by providing a heat transfer layer in thermal contact with one or both of the planar faces defined by a core lamination. The heat transfer layer may be any material having a higher thermal conductivity than the magnet steel used to fabricate the laminations. Representative heat transfer materials include, but are not limited to, metals such as copper, nickel, silver or materials such as graphene. The heat transfer material must be in thermal contact with the associated lamination, meaning that heat from the lamination may transfer directly to the heat transfer material. Thermal contact may be physical contact. Alternatively, thermal contact may occur through an intermediate material such as a thermal paste. In some instances, the heat transfer material may be deposited on, plated onto, coated onto or otherwise permanently bonded to the lamination.

Electromagnetic cores will also typically include a dielectric layer between laminations. In one embodiment, laminations will have a dielectric layer applied or in contact with one planar face and a heat transfer layer in thermal contact with the opposing planar face. In this embodiment the interface between adjacent laminations will include a dielectric layer from one lamination and a heat transfer layer from the other lamination.

The arc-shaped yoke segment defined by a stack of laminations may in certain embodiments define a tongue structure and an opposing groove structure configured to mate with each other. Thus, a stator may be assembled from a plurality of electromagnets by engaging the tongue structure of the first electromagnet with the groove structure of an adjacent electromagnet and so on until the stator is completed. In certain embodiments, the yoke segments of a plurality of electromagnets is directly supported by a shoulder structure extending from the housing or a housing end plate providing machine robustness and a direct thermal pathway from the stator to the housing.

Alternative embodiments include methods of cooling an electric machine rotor, methods of cooling an electric machine stator, methods of cooling an electric machine, methods of fabricating an electric machine, methods of stabilizing an electric machine, and methods of fabricating an electromagnet for an electric machine.

