The present invention relates to an electric machine with a cooling system and a method for cooling an electric machine with the features of the preambles of the independent claims.

From the prior art, there are numerous embodiments of electric machines known, including electric motors and generators comprising a fixed stator and a rotor, that is rotatable with respect to the stator. Based on the design and the mode of operation of the electric machine, variable amounts of heat are emitted. To keep the electric machine within the allowable temperature range for operation, different cooling systems, utilizing vaporous, liquid and two-phase coolants, are used in the prior art. Known cooling systems may be driven actively or passively. Active driving includes a pump or similar driving means for transportation of the coolant through a cooling loop. In passively driven systems, a difference in the density and geodetical height of the coolant in different parts of a cooling loop leads to transportation of the coolant without the necessity of a pump.

Passively driven systems are based, for example, on the principle of a heat pipe or loop heat pipe as disclosed in US 2009/024846, or a thermosiphon.

In a two-phase system, in which a part or the whole thermal energy emitted by the heat source is absorbed by the coolant as latent heat and results in vaporization of a formerly liquid coolant, a large amount of heat can be transported away from the heat source with small temperature changes in the coolant.

US 2005/0194847 A1 discloses an electric machine with a thermosiphon-type cooling system. The cooling system comprises an evaporator in a ring shaped configuration surrounding the rotor shaft such that heat is transferred to the evaporator from the rotor and the stator by convection. The vaporous coolant flows to a condenser where the heat absorbed within the coolant is emitted to the surrounding air by convection so that the vaporous coolant condenses. The condensed coolant flows back into the evaporator. The cooling loop is passively driven by a thermosiphon effect, based on gravity. The condenser is disposed outside the housing at a geodetically higher location than the evaporator, so that the liquid coolant moves downward toward the evaporator and the vaporous coolant flows toward the condenser due to a difference in density between vapor and liquid. To improve heat transfer by convection at both the evaporator and the condenser, blade wheels are attached to the electric machine's rotor to lead outside air through the machine and swirl it around. By vaporizing the coolant upon receiving heat from the electric machine, a large heat flow can be absorbed by the coolant with a relatively low change of its temperature due to latent heat. The amount of coolant being condensed within the condenser has to be equal to the amount of coolant being vaporized within the evaporator. If, due to too large of heat emission by the electric machine, the amount of coolant being vaporized exceeds the amount of coolant being condensed, the cooling system is not able to maintain the cooling performance required to run the electric machine, and the pressure and temperature within the cooling system and the temperature of the machine starts to increase.

To make sure that in each possible operation mode of the electric machine, in particular at strongly varying power output, the temperature within the machine stays below a maximum temperature above which the machine could not be operated safely, the cooling system has to be adapted for absorbing the heat emission of the motor at maximum power, which is typically only used for a fraction of operating time. This includes providing adapted heat transfer devices, a sufficient amount of coolant and a sufficient mass flow of coolant for absorbing the emitted heat at the maximum power of the electric machine for a limited period of time.

US 5,994,092 describes a capillary pumped heat transfer loop that shall be used for cooling of electronic apparatuses and electronic equipment, especially in satellites where gravity cannot be applied to passively drive the cooling loop. The capillary pumping pressure within the loop is generated in a porous material in the evaporator. A part of the heat transfer fluid is stored in a tank that is kept at a lower temperature level than the evaporator to promote a steady flow of the heat transfer fluid. The heat emission of electronic apparatuses is significantly lower and steady in comparison to that of electric motors.

It is an object of the present invention to provide an electric machine with a cooling system that can reliably absorb the heat emitted by the electric machine in different possible operation modes having a simple and reliable construction.

This object is achieved by an electric machine with a cooling system with the features of claim 1 .

By providing a first coolant reservoir, the total coolant mass and heat capacity of coolant within the system is increased. During temporary peak power output of the electric machine and consequently large heat emission, the amount of heat received by the coolant in the evaporator may exceed the amount of heat emitted by the coolant in the condenser. As a consequence, the amount of coolant being vaporized may exceed the amount of coolant being condensed. The evaporator may then be provided with liquid coolant from the coolant reservoir, whereby the additionally produced heat by the electric machine results in an increased enthalpy of the coolant in the reservoir. Thereby, the additional heat amount may be stored by an increased temperature of the coolant within the reservoir or a shift of the ratio between liquid and vaporous coolant within the reservoir resulting in an increased amount of vaporous coolant within the reservoir.

In an operation mode, in which the electric machine emits less heat, the amount of coolant being condensed may exceed the amount of coolant being vaporized, such that the enthalpy of coolant within the reservoir slowly decreases. While being operable in a continuous mode of identical amounts of vaporization and condensing of the coolant, the present electric machine with a cooling system is particularly adapted for non-continuous operation and may temporarily compensate additional generated heat in a case of operating the machine with higher power or even overloading the machine.

Possibly, the cooling system may be passively driven. This allows the saving of power for driving the system while providing a steady and reliant coolant flow.

In one embodiment, the thermal capacity of the cooling system may be increased by the coolant in the coolant reservoir. Thereby, the amount of waste heat stored by the cooling system may be increased. Especially during operation modes with large heat emission, an increase amount of emitted heat can be received and temporarily stored by the cooling system.

In particular, the fill level of the liquid coolant within the first coolant reservoir may be adapted to increase or decrease dependent on the amount of heat flow emitted by the heat source of the electric machine. Thereby, the large amount of heat produced by the heat source and absorbed by the coolant can be stored as latent heat with a comparatively low change in temperature.

In one embodiment, the electric machine may comprise one conduit, which is adapted to conduct coolant from the evaporator to the first coolant reservoir and from the first coolant reservoir to the evaporator. Thereby, a simple and reliable design can be provided that has a low number of parts.

In one further embodiment, the condenser may be provided between the evaporator and the first coolant reservoir in a flow direction of the coolant. By providing the condenser between the evaporator in the first coolant reservoir, a large amount of condensed coolant is entering the reservoir so that a large amount of condensed coolant is stored therein.

Possibly, the condenser and the first coolant reservoir may be integrally formed, or the condenser and the first coolant reservoir may be provided separately. The complexity and size of the cooling system may be reduced by integrally forming the coolant reservoir and the condenser. By providing them separately, the position of these parts relative to each other can be chosen more freely and the device can be better adapted to different kinds of electric machines.

In one further embodiment, the evaporator may comprise a wick, dividing a region containing mostly liquid coolant from a region containing vaporous coolant. The wick can establish a permeable barrier between vapor and liquid at different pressure levels, so that the cooling system can be passively driven by the resulting pressure difference. This may allow choosing the position of the reservoir and the condenser more freely, particularly with respect to the geodetic height. In particular, the wick can divide the evaporator into regions, in which one comprises only liquid coolant and one comprises only vaporous coolant.

In particular, the region containing mostly vaporous coolant can be closer to the heat source of the electric machine than the region containing mostly liquid coolant and/or the wick may serve as a thermally insulating element between the region containing mostly liquid coolant and the region containing mostly vaporous coolant. This can promote a higher heat flow to the vaporous coolant than to the liquid coolant to maintain a stable pressure difference and coolant flow.

Possibly, the wick may be better thermally coupled to the heat source than the region containing mostly liquid coolant, wherein, specifically the region containing mostly liquid coolant may comprise a thermally insulating element. An advantage of such layout is to promote evaporation within the wick so as to lower the temperature of the inlet liquid coolant and thus reduce backpressure and decrease formation of vapor within the region comprising mostly liquid coolant, in order to increase the heat transfer capability of the system.

In a further embodiment, a second coolant reservoir may be coupled to the evaporator and at least partially filled with a coolant. Thereby, a larger amount of coolant may be stored within the cooling system to improve its ability to absorb high heat flows. The two reservoirs may be provided at different positions of the system or at different geodetic heights to promote the passive drive of the cooling system. For example, one reservoir may be positioned before the condenser and one after the condenser to promote continuous flow through the condenser in different operation modes of the electric motor.

In a further embodiment, the electric machine may comprise an additional heat exchanger, adapted to act as a condenser and/or adapted to decrease the temperature of the coolant. Thereby the total amount of heat absorbed by the coolant and transported away may be increased. A larger amount of vaporous coolant may be condensed and/or a lower minimum temperature of the condensed liquid coolant may be reached. Possibly, the electric machine may comprise a housing, whereby the first coolant reservoir may be integrated within the housing or in that the first coolant reservoir may be provided outside of the housing. When providing the coolant reservoir within the housing, the design of the electric machine, including the cooling system, may be compact. When providing the reservoir outside of the housing, the electric machine may be more easily adapted to different applications. For example in the machine, representing the heat source may be better divided from the condenser or an external heat sink.

In a further embodiment, the first coolant reservoir and the evaporator may be integrated into a common cooling loop, whereby coolant can flow from the first cooling reservoir into the evaporator. This provides a good transfer of thermal energy from the evaporator to the condenser, as it is directly transported by coolant.

In a further embodiment, the evaporator and the condenser may be integrated into a primary cooling loop and the first coolant reservoir may be divided from the primary cooling loop but thermally coupled to the condenser or to a heat exchanger in the primary loop. The coolant used in the primary loop, including the condenser and the evaporator, and the coolant within the reservoir are decoupled from each other and may be chosen freely and separately from each other.

The invention's object is also solved by a method with the features of claim 14. By storing a part of the coolant within the first coolant reservoir, the total amount and heat capacity of the coolant within the cooling system can be increased. When the amount of heat emitted by the electric machine increases during high power operation, so that the amount of evaporating coolant exceeds the amount of condensing coolant in parallel and the pressure and temperature of the coolant rise. Thereby, sufficient cooling of the machine can be maintained as liquid coolant from the reservoir can be fed to the evaporator while the pressure and the temperature in the system can be kept in a range allowing safe operation of the motor. During normal operation of the motor, the emitted heat is lower, so that more coolant condenses than evaporates and the pressure and the temperature of the coolant lowers.

Possibly, the fill level of the liquid coolant within the first coolant reservoir may increase or decrease dependent on the amount of heat flow emitted by the heat source of the electric machine. This allows temporarily storing an increased amount of thermal energy emitted by the heat source of the electric motor with a small increase of the coolant temperature. In one embodiment, the coolant flows from the evaporator to the first coolant reservoir and/or from the first coolant reservoir to the evaporator through the same conduit. Thereby, the complexity of the method and number of required parts can be kept low.

In one embodiment, the coolant may traverse the condenser when flowing from the evaporator to the first coolant reservoir. Thereby, a large amount of liquefied coolant enters the reservoir and is stored therein.

In one embodiment, the coolant may traverse a wick within the evaporator, whereby the coolant is mostly liquid before traversing the wick and is mostly vaporous after traversing the wick, wherein, in particular the heat flow received by the coolant within the wick is larger than the heat flow received by the coolant in the region comprising mostly liquid coolant. While the mostly vaporous and the mostly liquid coolant is divided by the permeable wick, a pressure difference can be maintained that can passively drive or at least support passively driving the cooling system. By providing the coolant within the wick with a higher heat flow, it is ensured that the coolant evaporates within the wick so that a stable pressure gradient is sustained to drive the cooling system.

In one embodiment, a part of the coolant may be stored in a second coolant reservoir. The second reservoir may increase the amount of coolant stored within the cooling system so that the ability of the cooling system to provide effective thermal capacity during high power operation of the machine is improved.

In another embodiment, the coolant may flow through an additional heat exchanger. Thereby, an increased amount of heat can be transported to the environment. For example, the additional heat exchanger can act as an additional condenser or decrease the temperature of the condensed coolant.

In the following, exemplary embodiments of the invention will be explained.

Fig. 1 schematically shows an embodiment including an evaporator coupled to an electric motor, integrally formed condenser and coolant reservoir and one conduit connecting the evaporator with the condenser and coolant reservoir.

Fig. 2 schematically shows a coolant reservoir with a pressure relief valve.

Fig. 3 schematically shows a coolant reservoir and a heat exchanger formed integrally. Fig.4 schematically shows a coolant reservoir and a heat exchanger formed integrally with a heat exchanging zone provided above a storage zone.

Fig.5 schematically shows a coolant reservoir and a heat exchanger formed integrally with a heat exchanging zone provided below a storage zone.

Fig. 6 schematically shows a coolant reservoir and a heat exchanger provided separately with a heat exchanging zone provided above a storage zone.

Fig. 7 schematically shows a coolant reservoir and a heat exchanger provided separately with a heat exchanging zone provided below a storage zone.

Fig. 8 shows an embodiment comprising a cooling system, in which the coolant reservoir acts as a condenser and an additional heat exchanger is provided between the coolant reservoir and the evaporator.

Fig. 9 shows an embodiment comprising a cooling system, in which a condenser is provided between the evaporator and the coolant reservoir in flow direction of the coolant.

Fig. 10 shows an embodiment comprising a cooling system with two coolant reservoirs, one before and one after a heat exchanger.

Fig. 1 1 shows an embodiment comprising a cooling system with a wick separating regions containing mostly liquid coolant and a region containing mostly vaporous coolant.

Fig. 12 schematically shows a coolant reservoir divided from a primary cooling loop and thermally coupled to the primary cooling loop.

Identical or corresponding elements have the same reference signs throughout the description.

In one embodiment of the invention shown in figure 1 , the electric machine is represented by an electric motor having a stator 2 and a rotor 1. The stator 2, which is generating heat, is thermally coupled to the evaporator, which comprises channels 3 extending through the stator 2 and/or stator housing 21. The channels 3 are mostly filled with coolant in liquid form. The channels have an outlet 1 1 which directly connects all the channels in the highest geodetical point with a conduit 4. The conduit 4 is also connected to a reservoir 5 filled with coolant which is positioned above the highest point of the cooling channels. During operation, vaporous coolant rises through the conduit from the evaporator channels 3 into the reservoir 5 and liquid coolant 8 sinks back into the condenser channels 3 through the same conduit. The reservoir 5 is thermally coupled with the environment such that heat can be transferred through the outer walls of the reservoir 5 so that vaporous coolant 9 condenses within the reservoir 5 and is stored therein. The rotor 1 and the stator 2 are provided within a housing 21. The coolant reservoir is provided outside the housing 21.

In this embodiment, the cooling system is passively driven. In the present embodiment, the two phase coolant is in a thermodynamically saturated state. This enables the cooling loop to be driven only by the excessive heat generated by the electric machine. In comparison to an actively driven system, this results in a decreased number of moving parts, less energy consumption and noise while at the same time providing reliable heat transport. Other embodiments of the invention may be actively driven, for example by a pump.

Figure 2 shows a coolant reservoir partly filled with liquid coolant 8 and partly filled with vaporous coolant 9. The reservoir is provided with a pressure relief valve 52. In case the internal pressure of the cooling system exceeds a maximum value, the pressure relief valve 52 opens to release a part of the coolant to reduce the pressure within the cooling system in order to avoid potential damage.

Figures 3 to 7 show different options of providing combinations of coolant reservoirs and condensers. In the condenser, thermal energy is drawn from the coolant such that its temperature is decreased and/or its state of matter changes from vaporous to liquid. The condenser acts as a heat exchanger, transferring heat and latent heat from the coolant to an external heat sink thermally coupled to the evaporator, for example by convection, conduction and/or radiation. As in the embodiment of figure 1 , the condenser or coolant reservoir is connected to one conduit at its bottom wall, through which vaporous coolant enters and liquid coolant exits. In alternative embodiments, the coolant reservoirs and condensers could be provided with separated ports for entering vaporous coolant and exiting liquid coolant.

In figures 3 to 5, reservoirs and heat exchangers are provided integrally. Condensation and storage of the coolant is performed within a common hollow space. The reservoir itself may act as condenser, either by providing good heat flow over most of its inner walls as in figure 3 or by providing a condensation zone leading to a high heat flow and a storing zone, which leads to a comparatively low heat flow.

In figure 3, there is no separation between a condensation zone and a storage zone. In figure 4, a condensation zone is provided below a storage zone. The vaporous coolant entering the reservoir condenses and then rises above the condensing zone into the storage zone.

In figure 5, a storage zone for the coolant is provided below the condensation zone. The vaporous coolant traverses the storage zone into the condensing zone above where it condenses and sinks back down into the storage zone.

In figures 6 and 7, condenser and coolant reservoir are provided as different components.

In figure 6, a condenser is provided below the coolant reservoir. The vaporous coolant entering the condenser condenses and then rises above the condenser zone into the reservoir.

In figure 7, a coolant reservoir is provided below the condenser. The vaporous coolant traverses the coolant reservoir into the condensing zone above where it condenses and sinks down into the reservoir.

In another embodiment presented in figure 8, the cooling system comprises an evaporator 2 with separated inlet 41 for liquid coolant and outlet 42 for vaporous coolant. In this embodiment, the vapor outlet is at the geodetically highest point of the evaporator and the liquid inlet is connected to the bottom part of the evaporator. The reservoir 5 is at the highest point of the system, while an additional heat exchanger 6 is provided below the evaporator. The system should contain enough liquid that the reservoir is always partially filled with liquid coolant. All parts of the electric machine, including the coolant reservoirs are provided within a common housing 21 .

In the embodiment of figure 9, the heat exchanger can also be positioned between the evaporation zone outlet 42 and the reservoir 5. In that case, it is advantageous if the heat exchanger 62 is positioned higher than the evaporation zone to allow passively driving the cooling system.

In figure 10, two reservoirs are used, a first coolant reservoir 53 before a heat exchanger 6 and a second coolant reservoir 54 after a heat exchanger 6, as more cooling fluid will be available at a lower system temperature, for non-continuous high power operation. The amount of coolant within both reservoirs 53, 54 may change during operation of the electric machine, dependent on the amount of heat emitted by the machine.

The two coolant reservoirs may contain coolant at different temperature levels. For example, the first coolant reservoir 53 may act as a condenser, similar to the integral reservoir and heat exchanger of figure 3 and contain coolant at condensing temperature. The temperature of the liquid coolant may be further decreased in the additional heat exchanger 6 and stored at a lower temperature in the second coolant reservoir 54.

The cooling system is not limited to having one or two coolant reservoirs and may instead also comprise a larger amount of reservoirs at different positions of the system.

The evaporator and one or more condensers may comprise inner walls that are at least partially covered with hydrophobic, hydrophilic and/or biphilic material to promote bubble or drop formations on the walls to increase heat transfer and feeding capacity.

The concept of passively driving the cooling system, as in figures 8 to 10, is similar to the concept of a thermosiphon, based on driving the cooling system by a difference in density between coolant vapor and liquid.

In another embodiment of the invention, presented in figure 1 1 , the vapor channels 31 and the liquid channels 32 of the evaporator are separated in the evaporation zone by a wick 7. The wick has a good thermal contact to the stator 2, representing a heat source. The system is driven similar to the principle of a loop heat pipe or capillary pumped loop. Cooling liquid from the reservoir 5, which is above or in line with the evaporation zone, enters the evaporation zone through liquid guiding inlet channel 43. The evaporation takes place in the wick 7. The wick 7 has fine pores to develop a capillary pressure that circulates the fluid around the cooling system. Vapor is pushed through the vapor channel 44 to the heat exchanger 6, where it condenses. The position of condenser is not limited to above, or near the evaporation zone. In the shown embodiment, it is represented by an additional heat exchanger 6, provided geodetically below the evaporator. If there is no additional heat exchanger in the system, vaporous coolant can be conducted directly to the reservoir to condense therein.

For efficient operation, the system should be at saturated vapor pressure, whereby there should be no air in the system. The coolant reservoir shall at all times be partially filled with liquid coolant to maintain operation. A system can also include sensors for checking the level of liquid coolant within the coolant reservoir or the pressure within the cooling system. The sensors may be connected to vehicle control devices.

During operation, the temperature of the coolant within the one or more reservoirs may change or the ratio between liquid coolant and vaporous coolant may change. Both these changes of the thermodynamic status of the coolant include a change of the enthalpy. The wick may be made of a porous polymer material, such as polyethylene, polypropylene and PTFE or of a sintered metal powder, for example nickel, titanium or stainless steel.

In a preferred version of the embodiment, the adjacent groups of channels are substantially thermally isolated between each other. The wick may serve as a heat insulating element between the adjacent groups of channels. The channels or regions of the condenser comprising liquid coolant may be thermally decoupled from the heat source, while the channels or regions of the condenser comprising vaporous coolant may be thermally coupled to the heat source. This is achieved by providing the channels containing mostly vaporous coolant closer, for example in direct contact, with the heat source than the channels comprising mostly liquid coolant.

In another embodiment of this invention, presented in figure 12, the reservoir 55 is separated from a primary cooling loop that includes the evaporator and condenser. The channels 45 leading from the stator housing form a closed system and are thermally well connected to the reservoir, however the liquid in the reservoir is not in direct contact with the coolant in the channels. In such embodiment, the reservoir can also be opened towards the environment.