EP3815224A1

EP3815224A1 - Cooling element for an electric motor, electric motor and method for cooling the motor

Info

Publication number: EP3815224A1
Application number: EP19758334.7A
Authority: EP
Inventors: Sebastian Martens; Natale Cosmo Bifano; Sascha Klett; Maximilian MUNZ
Original assignee: Ziehl Abegg Automotive GmbH and Co KG
Current assignee: Ziehl Abegg Automotive GmbH and Co KG
Priority date: 2018-09-18
Filing date: 2019-07-15
Publication date: 2021-05-05
Also published as: DE102018222625A1; US20220029509A1; WO2020057699A1; JP2022514727A; CN112740517A

Abstract

The invention relates to a cooling element for an electric motor, particularly for a wheel hub drive, having a channel, which connects a first connection of the cooling element and a second connection of the cooling element, wherein the channel winds in a spiral around the cooling element. According to the invention, such a cooling element allows a flow state of coolant within the channel to be generated in which secondary turbulences are formed and the cooling performance is significantly improved.

Description

HEAT SINK FOR AN ELECTRIC MOTOR,

ELECTRIC MOTOR AND METHOD FOR COOLING THE

MOTORS

The invention relates to a heat sink for an electric motor, in particular for a wheel hub drive of a commercial vehicle, with a channel which extends between a first connection of the heat sink and a second connection of the heat sink. Furthermore, the invention relates to an electric motor with a corresponding heat sink and a method for cooling the electric motor.

Heat sinks of the type in question have been known in practice for years. Liquid cooling for electric motors is particularly widespread in the field of wheel hub drives in order to provide adequate heat dissipation at high power density. Because despite high efficiency over large areas of the map, there are operating conditions in electric motors designed as wheel hub drives in which a large part of the power is converted into heat, for example in start-up situations with a gradient.

These peak loads, at which high heat flows emanate from the coils, can be absorbed and defused by using comparatively large components with a high mass and a high heat capacity. This can be, for example, ribs of the coil windings, which are made of stainless steel and can absorb a relatively high amount of thermal energy for a short time, which is then continuously dissipated during operation.

Heat sinks are used as heat exchangers, which are generally cylindrical components and are introduced into the interior of a stator pack of the wheel hub drive.

According to a known embodiment, the heat sink is, for example, a cylinder made of aluminum, in which channels are milled. The duct system is made up of coolant on one side of the circumference of the cylinder Coolant circuit supplied. The coolant at the first connection strikes a distributor channel vertically as an impact jet and is introduced from the distributor channel into a multiplicity of channels on the cylinder jacket which are guided parallel to one another. On the opposite side of the circumference of the cylinder, the coolant is collected at the second connection and from there it is returned to a coolant circuit.

This solution results in a pressure distribution that is very uneven along the length of the distribution channel. Since the differential pressure drives the volume flow via the duct system, an uneven pressure distribution in the distributor duct causes a corresponding uneven pressure distribution in the individual ducts. The two outer channels, which are located directly in the impact area of the impact beam, are flowed through more strongly than the other channels. This causes an inhomogeneity of the volume flows through the individual channels and thus also an inhomogeneous temperature distribution between the individual channels and different amounts of heat that each individual channel can dissipate. This creates so-called hotspots, ie areas that become particularly hot during operation and from which the heat is dissipated particularly poorly. The occurrence of these hotspots can damage components.

With the known heat sinks arranged inside the stator package of a wheel hub drive, cooling capacities of approximately 14 kW to 16 kW can be achieved. When designing and modeling such heat sinks according to the prior art, any curvature of the channels is usually neglected and the channels are assumed to be straight lines.

The present invention is therefore based on the object of designing and developing a cooling of the type mentioned at the outset in such a way that a higher degree of efficiency can be achieved and the periods in which individual areas are exposed to high temperatures are reduced. According to the invention, the above object is achieved by the features of claim 1. The duct in question then spirally winds around the heat sink.

With regard to the electric motor, the above object is achieved by the features of the independent claim 10, according to which the electric motor is equipped with a corresponding heat sink. With regard to the method according to the invention, the object is achieved by claim 14, which is further subordinate, according to which coolant is guided within the channel at a flow rate which is sufficiently high to effect the Dean number at least in sections in the channel which is greater than or equal to 50 and preferably greater than 54.

In the manner according to the invention, it was first recognized that the flow behavior of the coolant within the channels can be specifically influenced and optimized by taking the curvature of the channels into account as a function of the other dimensions of the channels.

Due to centrifugal forces, secondary eddies occur when the dimensionless Dean number is increased, as can be seen, for example, from elbows in pipes. The Dean number depends on the one hand on the flow rate and the viscosity of the coolant, but can also be influenced by changing a curvature and by changing the width and height of the channels - or the hydraulic diameter of the channels.

Depending on the geometry of the channels, several double vertebrae can appear distributed across the cross-section of the channel. Due to additional friction on the duct wall, these turbulences on the one hand increase the pressure loss when flowing through the duct. On the other hand, a more uniform pressure distribution and an improved mixing of the flow are achieved. Mixing increases the heat transfer, because the Nußelt number - which describes the heat transfer to the coolant - is strongly dependent on the Dean in a curved channel, even in laminar flow conditions. Number. This increases the amount of heat that can be released from the electric motor to the coolant via the heat sink, that is, the cooling capacity. In addition, the temperature distribution is more homogeneous than with cooling according to the prior art. The formation of hotspots is reduced. With comparable dimensions, the heat sink according to the teaching described here can achieve cooling outputs of approximately 26 kW, which is a considerable improvement compared to the 14 kW to 16 kW that can be achieved with heat sinks according to the prior art.

Since the flow does not flow through several parallel channels but only a spiral channel, the channel length and the flow rate increase. Both increase the pressure loss. The Dean number of the flow through the channels of the proposed heat sink is significantly higher compared to the known heat sinks. In return, a uniform - albeit swirling - flow through the spiral channel is guaranteed at every operating point. Even though the pressure loss across the ducting is increased compared to the parallel ducting, the pressure loss for a given volume flow still appears to be relatively small and therefore negligible.

Preferably, the cooling body is basically in the form of a rotating body. This can be, for example, a truncated cone which has a first base area, a second base area and a jacket, the jacket of the rotary body preferably being provided with a groove forming the channel. Since the channel spirally wraps around the heat sink, the heat sink - with the channel - is not strictly rotationally symmetrical.

According to a particularly preferred embodiment, the heat sink is designed as a hollow circular cylinder. The circular cylindrical shape of the heat sink is suitable for use with electric motors that have a stator and a rotor that can be rotated relatively to it. The stator and rotor are also regularly designed in the form of a first circular cylinder and a hollow second circular cylinder surrounding it. Either the outer hollow second circular cylinder is the non-rotatable stator and the inner first circular cylinder is relative to it rotating rotor (inner rotor), or the inner first circular cylinder is the non-rotatable stator and the hollow second circular cylinder is the rotating rotor (outer rotor) relative to it. While the heat sink according to the teaching described here can be arranged on the outside around the stator of the inner rotor motor in the first case, in preferred embodiments the heat sink is inserted inside the stator of an outer rotor motor. The latter embodiment is particularly suitable for wheel hub drives of vehicles - preferably commercial vehicles. Due to its hollow circular cylindrical shape, the heat sink is adapted to the shape of the stator, which is also circular cylindrical.

The groove forming the channel can be wholly or partially introduced into an outer surface of the casing of the rotary body forming the cooling body, for example by milling. However, the channel is preferably at least partially introduced into an inner surface of the jacket of the heat sink. The heat sink is therefore suitable for use in a permanently excited synchronous motor (PMSM) designed as an external rotor motor and can be inserted inside the stator package.

According to one embodiment, the first connection and the second connection can be located closer to the first base area of the heat sink. The first connection can in particular be used to supply coolant, the second connection serving to return the coolant to the coolant circuit. The coolant can thus be introduced close to the first base area and move along the heat sink through the spiral channel.

The channel can preferably make a U-turn closer to the second base area of the heat sink. The guide of the channel has in particular an angle of almost 180 °. The area of the U-turn can have a particularly high curvature compared to the rest of the spiral guidance of the channel and can lead to a particularly high Dean number, which in turn favors the formation of secondary vortices and the heat absorption of the coolant in the heat sink and thus the cooling capacity increases. After the U-turn, the canal then moves back to the first base area in order to end in the second connection. In this case, the first connection and the second connection preferably lie opposite one another on the circumference. This configuration is particularly advantageous for the use of the heat sink with wheel hub drives, because the coolant can be fed to the heat sink on the inside of the vehicle - close to the first base area - and can also be guided back into the coolant circuit on the same side.

According to a further embodiment, the channel is partially designed in the form of a double helix. In particular, the first connection can be connected to a first strand of the double helix and the second connection can be connected to a second strand of the double helix. The first strand of the double helix thus leads from the first connection to the U-turn. The second strand of the double helix leads from the U-turn to the second connection. The double helix shape alternates turns of the first strand, which lead the coolant in one direction towards the U-turn, with turns of the second strand, which lead the coolant in the opposite direction.

The coolant supplied to the heat sink - which has the lowest temperature when it enters the first connection - therefore first passes through the windings of the first strand and thereby absorbs heat. There is a temperature difference not only in relation to the components to be cooled, but also in relation to the coolant in the turns of the second strand. After the turn, the coolant flows back through the turns of the second strand, but in opposite directions between two turns of the first strand until it has reached the highest temperature when it exits the second connection.

While the temperatures in the windings of the first strand and the second strand are far apart from one another, the temperatures in the windings of the first strand and the second strand in the region of the U-turn near the second base surface are largely the same. Because the particularly cold coolant in the turns of the first Strand near the first base surface has a high temperature difference to the components to be cooled, and the already largely heated coolant in the windings of the second strand near the first base surface has a low temperature difference to the components to be cooled, is the cooling capacity via the axial extent of the heat sink reasonably adjustable.

More preferably, a width of the channel is greater than a third of a height of the channel. The width of the channel is preferably less than three times the height of the channel. The width of the channel can be approximately 5 to approximately 50 mm and in particular 7 mm to 21 mm. The height of the channel can also be approximately 5 mm to approximately 50 mm and in particular 7 mm to 21 mm. The ratio of width to height is preferably between 1/3 and 3, because in these areas the relative pressure loss can be represented universally as a function of the Dean number and thus manipulated in a targeted manner. With a more extreme width-to-height ratio, the effect of increasing the relative pressure drop is reduced because the vortex systems become unstable and the radial flow exchange no longer takes place primarily on the outer walls. The radius of curvature of the channels - that is to say the radius of the casing of the rotating body - can preferably be between 50 mm and 200 mm.

According to a preferred embodiment, the heat sink is made of aluminum, and any material with a suitable thermal conductivity and a suitable specific heat capacity can be used.

According to the teaching described here, an electric motor is also proposed which comprises such a heat sink - with one or more of the features described. This is preferably a permanently excited synchronous motor (PMSM) designed as an external rotor motor. The electric motor is further preferably a wheel hub drive for a vehicle, in particular for a commercial vehicle. The heat sink can be located inside a stator of the electric motor and can dissipate the energy converted into heat during operation. This preferably includes

According to the teaching described here, a method for cooling an electric motor is finally proposed. Coolant is guided at a flow rate in a channel or in the channel of the heat sink described. The flow rate is sufficiently high to cause a Dean number in the channel, at least in sections, which is greater than or equal to 50. The Dean number is preferably greater than 54, so that secondary vortices form in the flow of the coolant through the channel. A heat sink according to the teaching described here is preferably used. The coolant is introduced at the first connection into the spiral channel that winds around the heat sink. Depending on the dimensions of the channel or channels of the heat sink and the viscosity of the coolant, the Dean number can be determined via the flow rate and the formation of secondary vortices can be controlled.

There are now various possibilities for advantageously designing and developing the teaching of the present invention. For this purpose, reference is made on the one hand to the claims subordinate to claim 1 and on the other hand to the following explanation of preferred exemplary embodiments of the invention with reference to the drawing. In connection with the explanation of the preferred exemplary embodiments of the invention with reference to the drawing, generally preferred refinements and developments of the teaching are also explained. Show in the drawing

1a shows an embodiment of a heat sink in longitudinal section;

1b the embodiment of the heat sink viewed in longitudinal section from the opposite side; and

Fig. 2 shows the embodiment of the heat sink in an enlarged section. A heat sink 1 for an electric motor can be seen in general in FIG. 1a (not shown). The heat sink 1 is particularly suitable for cooling permanently excited synchronous motors (PMSM), which are designed as external rotor motors, preferably as wheel hub drives for a vehicle and in particular for a commercial vehicle. In these embodiments, the heat sink 1 is arranged inside a stator or a stator pack of the electric motor.

The heat sink 1 comprises a channel 2, which extends between a first connection 3 a of the heat sink 1 and a second connection (not shown in FIG. 1 a) of the heat sink 1. Since FIG. 1a is a sectional view, only the first connection 3a can be seen. In this embodiment, however, the second connection is designed in a manner that is rotationally symmetrical about the longitudinal axis 4 relative to the first connection 3a. The first connection 3a and the second connection are therefore mutually opposite.

The heat sink 1 is designed in the form of a hollow circular cylinder. The hollow circular cylinder has a first base area 5, a second base area 6 and a jacket 7. The jacket 7 of the circular cylinder is provided with a groove forming the channel 2. The channel 2 wraps around the heat sink 1 in a spiral. In the embodiment shown, the channel 2 is designed in the form of a double helix and is introduced into an inner surface 8 of the jacket 7. In other embodiments, however, the channel 2 can also be introduced into an outer surface 9 of the jacket 7. The first connection 3a and the second connection are closer to the first base 5 of the heat sink 1 than to the second base 6 of the heat sink 1. On the opposite side, closer to the second base 6 of the heat sink 1, the channel 2 makes one U-turn 10, wherein the channel 2 changes its direction a tight curve by about 180 °.

The first connection 3a is connected to a first strand 11 of the double helix. During operation, the first line 11 leads coolant from the first connection 3a via the turns 12, 13 of the first line 11 to the U-turn 10. A second line 14 guides the coolant from the U-turn 10 via the turns 15, 16 of the first strand 11 back towards the first base 5 of the heat sink 1 to the second connection 3b. From there, the coolant can return to a coolant circuit (not shown). The direction of flow of the coolant within the first strand 11 and the second strand 12 of the channel 2 can be understood using the arrows 17.

Fig. 1b shows the embodiment of the heat sink 1 from Fig. 1a also in longitudinal section, but from the opposite side, so that in Fig. 1b that part of the heat sink 1 is shown, which is not shown in Fig. 1a due to the section, and vice versa.

1b, the direction of flow of the coolant can also be traced using the arrows 17. The coolant, which flows during operation from the first connection 3a (not shown in FIG. 1b) via the windings 12, 13 of the first strand 11 to the U-turn (not shown in FIG. 1b) arrives after the U-turn second strand 14 of the channel 2 through its windings 15, 16 to the second connection 3b and from there back into the coolant circuit.

The geometry of the heat sink 1 and in particular the channel 2 can be seen from FIG. 2.

A width 18 of the channel 2 is approximately three times as large as the fleas 19 of the channel 2. The width 18 can be approximately 40 mm, while the height 19 is approximately 13.5 mm. The radius of curvature 20 is approximately 140 mm. With these specified dimensions, the Dean number is only dependent on the flow rate and the viscosity with which the coolant flows through channel 2.

The Dean number is generally described by the equation

with the flow velocity, the viscosity ^v , the radius of curvature 19 and the hydraulic diameter of the channel,. While the hydraulic diameter can be determined with a rectangular cross section from the width 18 and the height 19, the radius of curvature _* 20 is measured from the longitudinal axis 4 of the heat sink 1 to the inner surface 8 of the jacket 7, that is to say to the inner boundary surface of the channel 2 .

With a geometry of the channel 2 given in this way, the Dean number De depends only on the flow velocity ^u and the viscosity ^v . When carrying out a method for cooling an electric motor with a heat sink 1 according to the exemplary embodiment in FIGS. 1 and 2, only a coolant with a suitable viscosity and a suitable flow rate ^{u has to be passed} through the channel 2 in order to achieve a specific Dean number to reach. In order for secondary vertebrae to form at least in sections, a Dean number that is greater than 54 should be achieved.

With regard to further advantageous refinements of the device according to the invention, reference is made to the general part of the description and to the appended claims in order to avoid repetition.

Finally, it should be expressly pointed out that the above-described exemplary embodiments of the device according to the invention only serve to explain the claimed teaching, but do not restrict it to the exemplary embodiments.

Reference list

1 heat sink

2 channel

a First connection b Second connection

4 longitudinal axis

5 First base area

6 Second base area

7 coat

8 Inner surface

9 Outer surface 0 U-turn

1 first strand

2 turns

3 turn

4 Second strand

5 turns

6 turns

7 arrows

8 width

9 height

0 radius of curvature

Claims

Expectations

1. heat sink (1) for an electric motor, in particular for a wheel hub drive of a commercial vehicle, with a channel (2), which is located between a first connection (3a) of the heat sink (1) and a second connection (3b) of the heat sink (1 ) extends

d a d u r c h g e k e n n z e i c h n e t that the channel (2) spirally wraps around the heat sink (1).

2. Heat sink (1) according to claim 1, characterized in that

the cooling body (1) is generally designed in the form of a rotating body, in particular a hollow circular cylinder, which has a first base area (5), a second base area (6) and a jacket (7), the jacket (7) of the rotating body including a groove forming the channel (2) is provided.

3. heat sink (1) according to claim 2, characterized in that

the channel (2) is at least partially introduced into an inner surface (8) of the jacket (7).

4. heat sink (1) according to claim 2 or 3, characterized in that

the first connection (3a) and the second connection (3b) are closer to the first base area (5) of the heat sink (1) and / or

the channel (2) closer to the second base (6) of the heat sink (1) makes a U-turn (10).

5. Heat sink (1) according to one of claims 1 to 4, characterized in that

the first connection (3a) and the second connection (3b) lie opposite one another on the circumference.

6. heat sink (1) according to any one of claims 1 to 5, characterized in that

the channel (2) is partially formed in the form of a double helix.

7. heat sink (1) according to claim 6, characterized in that

the first connection (3a) is connected to a first strand (11) of the double helix and the second connection (3b) is connected to a second strand (14) of the double helix.

8. heat sink (1) according to any one of claims 1 to 7, characterized in that

a width (17) of the channel (2) is greater than one third of a height (19) of the channel (2) and / or less than three times the height (19) of the channel (2).

9. heat sink (1) according to any one of claims 1 to 8, characterized in that

the heat sink (1) is made of aluminum.

10. Electric motor with a heat sink (1) according to one of claims 1 to 9.

11. Electric motor according to claim 10, characterized in that

the electric motor is a permanently excited synchronous motor (PMSM) designed as an external rotor motor.

12. Electric motor according to claim 10 or 11, characterized in that the electric motor is a wheel hub drive for a vehicle, in particular for a commercial vehicle.

13. Electric motor according to one of claims 10 to 12, characterized in that

the heat sink (1) is located inside a stator of the electric motor.

14. A method for cooling an electric motor, preferably with a heat sink (1) according to one of claims 1 to 9, comprising a

Guiding coolant within the channel (2) at a flow rate which is sufficiently high to effect a Dean number in the channel (2) at least in sections which is greater than or equal to

50 and preferably greater than 54.