WO2022070479A1

WO2022070479A1 - Methods and systems for bonding a heat sink to a stator of an electric motor

Info

Publication number: WO2022070479A1
Application number: PCT/JP2021/011282
Authority: WO
Inventors: Eric HARLAN; Zaher Daboussi; Matthew Todd Keennon
Original assignee: Hapsmobile Inc.
Priority date: 2020-09-30
Filing date: 2021-03-18
Publication date: 2022-04-07

Abstract

Systems, devices, and methods including at least one electric motor (110), including: a stator, including: a stator housing (114); a stator lamination stack (116); and a heat sink with a plurality of cooling fins (154); wherein the heat sink (150) may be bonded to an underside (152) of the stator housing (114)with a high thermal conductivity epoxy providing for transmission of heat from the stator housing (114).

Description

METHODS AND SYSTEMS FOR BONDING A HEAT SINK TO A STATOR OF AN ELECTRIC MOTOR

　　Embodiments relate generally to electric motors and more particularly to bonding of a heat sink to a stator of an electric motor.

Summary

　　A system embodiment may include: at least one electric motor, including: a stator, including: a stator housing; a stator lamination stack; and a heat sink with a plurality of cooling fins; where the heat sink may be bonded to an underside of the stator housing with a high thermal conductivity epoxy providing for transmission of heat from the stator housing.

　　The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principals of the invention. Like reference numerals designate corresponding parts throughout the different views. Embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which:
[Fig. 1] FIG. 1 depicts an unmanned aerial vehicle including an electric motor;
[Fig. 2] FIG. 2 depicts a side perspective exploded view of a stator of the electric motor of FIG. 1, including a stator lamination stack and a stator housing;
[Fig. 3] FIG. 3 depicts a side perspective view of a portion of the assembled stator of FIG. 2;
[Fig. 4] FIG. 4 depicts a bottom perspective view of a portion of the assembled stator of FIG. 2 with injection holes in the stator housing;
[Fig. 5] FIG. 5 depicts a top perspective view of a portion of the assembled stator of FIG. 2 with a heat sink bonded to the stator housing; and
[Fig. 6] FIG. 6 depicts a top perspective view of an alternative heat sink;
[Fig. 7] FIG. 7 depicts a side perspective view of the heat sink of FIG 6 bonded to the stator housing of FIG. 2; and
[Fig. 8] FIG. 8 depicts a top perspective view of the motor of the electric motor of FIG. 1 mounted to a nacelle.

　　A heat sink is bonded to an underside of a stator housing. The heat sink may be made of aluminum. In one embodiment, the heat sink may maximize its surface area in contact with the cooling medium surrounding it, such as the air around the stator. For example, the heat sink may have a plurality of cooling fins, tightly coiled to maximize the surface area of the heat sink. In one embodiment, the cooling fins are wire electric discharge machining (EDM) cuts, whereby the desired cooling fin shape is obtained by using electrical discharges, e.g., sparks.

　　With respect to FIG. 1, an unmanned aerial vehicle (UAV) 100 with at least one motor 110 is depicted. UAVs are aircraft with no onboard pilot and may fly autonomously or remotely. In one embodiment, the UAV 100 is a high altitude long endurance aircraft. In one embodiment, the UAV 100 may have one or more motors 110, for example, between one and forty (40) motors, and a wingspan between 100 feet and 400 feet. In one embodiment, the UAV 100 has a wingspan of approximately 260 feet and is propelled by a plurality of propellers 140 coupled to a plurality of motors, for example, ten (10) electric motors, powered by a solar array covering the surface of the wing, resulting in zero emissions. In one embodiment, the UAV 100 may weigh approximately 3,000 lbs.

　　Embodiments of the present application disclose electric motors. The embodiments may in particular be directed to brushless motors, such as brushless DC motors. A brushless DC motor may consist of two main parts, a stator and a rotor. Generally speaking, a brushless DC motor is a collection of electromagnets on the stator with permanent magnets attached on the movable rotor. The motor can be either an in-runner (magnets on the inside of the coils) or an out-runner (magnets outside the coils). For an in-runner motor, the rotor is a permanent magnet with two poles, while the stator consists of coils. Through application of a desired current, the coils will generate a magnetic field that will attract permanent magnets of the rotors. If each coil is activated one after another, the rotor will keep rotating because of the force interaction between permanent magnets and the electromagnet. In turn, an out-runner brushless motor has the permanent magnets outside the electromagnets. In one embodiment, the motor 110 may be a three-phase inverter powered permanent magnet motor, which may propel a UAV for extended flight. In one embodiment, the motor 110 is an out-runner motor. In another embodiment, the motor 110 is an in-runner motor.

　　Flying at an altitude of approximately 65,000 feet above sea level and above the clouds, the UAV 100 is designed for continuous, extended missions of up to months without landing. The motor 110 may function optimally at high altitude where the motor may be exposed to temperatures between 40 ℃ and -85 ℃, and provides for considerable periods of sustained flight of the UAV 100 without recourse to land.

　　The stator may include a lamination stack, which is a package of individual sheets separated by electrically insulating layers to suppress eddy current losses under dynamic magnetic loading. Generally speaking, electric shorts may occur in the lamination stack of the stator over such extreme temperature ranges described above. More specifically, the stator housing may be made of titanium and the stator lamination stack may be made of iron, which are both electrically conductive. A large coefficient of thermal expansion (CTE) differential between materials may arise, such as a large CTE differential between the stator housing and the stator lamination stack of the motor 110.

　　When mounting of the stator lamination stack onto the stator housing with glue, an electrical short in the lamination stacks may occur, because of features of the stator. More specifically, the stator lamination stack and the stator housing may be made of two different electrically conductive materials, such as iron and titanium, respectively. In one embodiment, a plurality of isolators or "spacers" may be placed between the stator lamination stack and the stator housing. The spacers may provide a bond gap between the stator housing and the lamination stack and prevent electrical shorting. In one embodiment, the spacers may be made of tape, such as Kapton tape. Kapton is a polyimide film that remains stable across a wide range of temperatures. In one embodiment, the spacers not only provide for isolating of the stator lamination stack from the stator housing to prevent electrical shorts, but also for properly lamination stack into position during assembly of the motor. In one embodiment, the spacers may be electrically non-conductive to maintain a bond gap between the stator lamination stack from the stator housing.

　　The stator lamination stack may reach very high temperatures, and it may be desired to dissipate heat through a heat sink. In one embodiment, the heat sink may be a passive heat exchanger that transfers the heat generated by the stator lamination stack, thereby allowing regulation of the stator's temperature.

　　In one embodiment, the heat sink may maximize its surface area in contact with the cooling medium surrounding it, such as the air around the stator. For example, the heat sink may have a plurality of cooling fins, tightly coiled to maximize surface area of the heat sink.

　　In one embodiment, the heat sink may be bonded to the stator housing after the bonding process of the stator to the stator housing. The heat sink may be bonded in with a high thermal conductivity epoxy that allows for transmission of heat from the titanium stator housing.

　　With respect to FIG. 2, a stator 112 of the motor 110 is illustrated. The stator 112 may include a stator housing 114 and a stator lamination stack 116. In one embodiment, the stator lamination stack 116 may be of soft alloys, such as iron-cobalt-vanadium alloys. The lamination stack 116 may include a plurality of slots 119 spaced evenly across the outer circumference of the stator lamination stack 116. In one embodiment, the stator housing 114 may be made of titanium. In one embodiment, the stator lamination stack 116 and the stator housing 114 are electrically conductive.

　　In one embodiment, a plurality of spacers 124 may be spaced equally around an inner surface 123 of the stator lamination stack 116. In one embodiment, 6 spacers 124 may be spaced equally around the inner surface 123. In one embodiment, the spacers 124 are strips of electrically non-conductive Kapton tape. In one embodiment, the spacers 124 may be made of a known thickness to create a bond gap. In one embodiment, the Kapton tape spacers 124 have a 0.125 inch width and are 0.003 inches thick. In one embodiment, the Kapton tape spacers 124 may be used to center the stator lamination stack 116 in the stator housing 114, and to prevent metal-to-metal contact between the stator lamination stack 116 and the stator housing 114, hence, preventing electrical shorting, because the lamination stack 116 in the stator housing 114 are both electrically conductive.

　　In one embodiment, low pressure compressed air may be applied to blow off the lamination stack 116 after the Kapton tape spacers 124 are adhered in place.

　　A large coefficient of thermal expansion (CTE) differential between materials may arise, such as a large CTE differential between the iron stator lamination stack 116 and the titanium housing 114 of the motor 110. The spacers 124 provide a bond gap between the magnets 118 and the lamination stack 116, thereby prevent electrical shorting. In one embodiment, the Kapton tape spacers 124 provide a 0.004 inch thick bond gap.

　　With respect to FIG. 3, the stator lamination stack 116 may be slid around the stator housing 114 after a bonding preparation process, wherein the lamination stack 116 may have a liquid oxide removal treatment and the stator housing 114 may be grit blasted. An epoxy, such as Masterbond EP21TCHT-1, aluminum oxide filler, may be injected into a plurality of injection holes 128 that are located around the circumference of the housing 114. The epoxy filler has a high thermal conductivity (e.g., 1.4 W/m2K) and high electrical insulation. In one embodiment, the housing 114 has 30 injection holes 128. With respect to FIG. 4, the assembled stator 112 is shown with an epoxy applicator 129 for applying the epoxy into the injection holes of the stator housing 114 to secure the stator lamination stack 116 to the stator housing 114. In one embodiment, the applicator 129 may be a syringe with a tip of the syringe 129 bent at a 90 degree angle in order to get improved access to the to the injection holes 128.

　　The stator lamination stack116 may reach very high temperatures, and it may be desired to dissipate heat through a heat sink. In one embodiment, the heat sink may be a passive heat exchanger that transfers the heat generated by the stator lamination stack, thereby allowing regulation of the stator's temperature.

　　With respect to FIG. 5, a heat sink 150 is shown bonded to an underside 152 of the stator housing 114. In one embodiment, the heat sink 150 may be made of aluminum. In one embodiment, the heat sink 150 may maximize its surface area in contact with the cooling medium surrounding it, such as the air around the stator. For example, the heat sink may have a plurality of cooling fins 154, tightly coiled to maximize surface area of the heat sink. In one embodiment, the cooling fins 154 are wire electric discharge machining (EDM) cuts, whereby the desired cooling fin 154 shape is obtained by using electrical discharges, e.g., sparks.

　　In one embodiment, the heat sink may be bonded to the stator housing 114 after the bonding process of the stator lamination stack 116 to the stator housing 114. The heat sink 150 may be bonded to the underside 152 of the stator housing 114 with a high thermal conductivity epoxy that allows for transmission of heat from the titanium stator housing 114. In one embodiment, the epoxy is Masterbond EP3HTSDA-2 silver filler with a high thermal conductivity (e.g., 6.5 W/m2K). This epoxy in the bonding process of the heat sink 150 to the underside 152 of the stator housing 114 is also electrically conductive; however, the electrical conductivity of the epoxy does not affect the electromagnetic properties of the stator lamination stack 116, since the stator housing 114 may be electrically isolated from the stator lamination stack 116. In one embodiment, the epoxy creates a glue bond gap of 0.004 inches.

　　In one embodiment, air may flow through the cooling fins 154 of the heat sink 150, dissipating heat from the stator. More specifically, the stator lamination stack 116 has magnet winding wires that generate heat, and said heat needs to pass through the titanium housing 114 and into the aluminum heat sink 150. The cooling fins 154 of the heat sink 150 coiled to maximize the heat transfer.

　　The aluminum heat sink 150 contracts much more dimensionally than the titanium stator housing 114 at very low temperatures; therefore two expansion slots 156 are introduced to allow the cooling fins 154 to expand and contract into the expansion slots 154 to accommodate the expansion and contraction of the heat sink 150 and to not break the bond between the heat sink 150 and the stator housing 114.

　　In one embodiment, the heat sink 150 includes to spring force ears 158 on either side of the heat sink 150. In the event of breakage of the bond between the heat sink 150 and the stator housing 114, the spring force ears 158 maintain surface contact between the heat sink 150 and the stator housing 114 by spring force. The heat sink 150 may also include an air dam 160 to improve stability, aerodynamic performance, and engine cooling by redirecting the flow of air.

　　With respect to FIG. 6, an alternative heat sink 250 of the electric motor 110 is shown bonded to the stator housing 114 described above. The heat sink 250 may include additional expansion slots 256 to the expansion slots 156 described above. In one embodiment, the heat sink 250 has 7 expansion slots 256. The heat sink 250 may further include a pair of spring force ears 258, such as spring force ears 158 described above.

　　With respect to FIG. 7, the heat sink 250 of the electric motor 110 is shown bonded to the stator housing 114 described above. More specifically, a heat sink 250 is bonded between to the underside of the stator housing 114 and secured between adjacent arms 270 of the stator housing.

　　With respect to FIG. 8, the heat sinks 250 bonded to the stator housing 114 are shown within the electric motor 110, which is mounted to an aircraft motor mount 272. In one embodiment, the majority of the cooling air goes through the cooling fins. In one embodiment, a fraction of the air may pass through a magnetic air gap. In one embodiment, a fraction of the warmed air may pass through a spinner air gap. In one embodiment, warmed air may transmit into a nacelle 274 of the motor 110.

　　It is contemplated that various combinations and/or sub-combinations of the specific features and aspects of the above embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments may be combined with or substituted for one another in order to form varying modes of the disclosed invention. Further, it is intended that the scope of the present invention is herein disclosed by way of examples and should not be limited by the particular disclosed embodiments described above.

Claims

　　A system comprising:
　　at least one electric motor, comprising:
　　　　a stator, comprising:
　　　　　　a stator housing;
　　　　　　a stator lamination stack; and
　　　　　　a heat sink with a plurality of cooling fins;
　　　　wherein the heat sink is bonded to an underside of the stator housing with a high thermal conductivity epoxy providing for transmission of heat from the stator housing.
　　The system of claim 1, wherein the stator lamination stack has magnet winding wires.
　　The system of claim 1 or 2, wherein the cooling fins are coiled.
　　The system of any one of claims 1 to 3, wherein the heat sink includes two expansion slots.
　　The system of any one of claims 1 to 4, wherein the heat sink includes 7 expansion slots.
　　The system of any one of claims 1 to 5, wherein the heat sink includes spring force ears on either side of the heat sink.
　　The system of any one of claims 1 to 5, wherein the heat sink includes a pair of spring force ears.
　　The system of any one of claims 1 to 7, wherein the heat sink includes an air dam.
　　The system of any one of claims 1 to 8, wherein the heat sink is bonded between to the underside of the stator housing and secured between adjacent arms of the stator hausing.