OIL COOLING SYSTEM FOR AN ELECTRIC MOTOR

Description

[0001] The invention relates to cooling systems for electric motors of vehicles and, more specifically, to a cooling and lubrication system for an electric machine used in a motor vehicle.

[0002] Electrical machines typically have a stator and a rotor arranged coaxially within each other. The rotor consists of a rotor body carrying magnetic flux generators, such as permanent magnets or windings. This rotor is typically housed within the stator, which carries magnetic flux generators in the form of windings to generate a magnetic field to rotate the rotor in conjunction with the magnetic field generated by the magnets or windings of the rotor.

[0003] Cooling the windings of an electrical machine plays an essential role because, among the efficiency losses of an electrical machine, a significant part is taken by Joule losses which are proportional to the temperature of the winding wire.

[0004] Systems for cooling electrical machines by circulating an oil-type coolant on the active parts of these electrical machines, in particular their windings, are known, see for example DE19824202C1, JP2016201959A, US2008 / 047799A1, US2011 / 084561A1, EP1841046A1, US2019 / 173359A1, JP2014030296A and EP2213497A1. In particular, document DE 198 24 202 C1 discloses an electric machine provided with a cooling circuit comprising a radiator and a pump, which draws the coolant from the bottom of the engine, passes it through a radiator and presses it through injectors in order to cool the machine.

[0005] Patent document WO2018 / 206890 thus discloses such a system mainly comprising an oil circulation circuit capable of bringing the oil into contact with the active parts of the electrical machine, via injectors making it possible to spray the active parts with oil, a reservoir capable of collecting the oil having cooled these active parts, a pump, making it possible to reinject the oil from the reservoir into the circulation circuit and a heat exchanger, making it possible to maintain the temperature of the oil below a temperature threshold. This arrangement makes it possible to guarantee both the lubrication of the rotating parts of the machine (dynamic seals and bearings) and the cooling of the active parts of the machine (rotor and stator) so that it does not overheat.

[0006] Such a cooling system is generally expensive, particularly due to the presence of the pump, which is an electric pump and not a mechanical pump, because at low speed and high torque demand, the cooling of the machine must be maximum.

[0007] This pump is sized according to the system requirements. In the case of a sealed electrical machine, the seals and bearings must be permanently lubricated. In other words, for lubrication purposes, it is necessary to keep the pump running to ensure a minimum oil flow rate at all times, regardless of the conditions and, in particular, regardless of the oil temperature. Thus, when cold, in negative oil temperature ranges and below a certain threshold, no cooling is required and the oil circulation provided by the circuit pump serves only to lubricate the machine's seals and bearings.

[0008] However, pump sizing depends on the oil viscosity. This viscosity is very high at negative temperatures and decreases rapidly to converge at low viscosities from a temperature threshold. In cold conditions, high oil viscosities impose significant pressure in the circuit, which requires a higher torque demand from the pump and, consequently, oversizing the pump, at the expense of cost.

[0009] Another problem is the noise emitted by the pump, especially at low engine speeds, for example when maneuvering in a parking lot or in traffic jams on a mountain pass. In these conditions, the noise emitted by the pump is greater than the noise emitted by the electric motor, which is obviously not desirable.

[0010] We also know splash cooling systems for the rotor during operation, where the active parts of the machine are cooled and the seals and bearings are lubricated by the oil sprayed by the rotor when it rotates. The rotation speed of the machine is, however, a limiting factor in the proper operation of the machine. Indeed, increasing the rotation speed, from a certain operating speed, significantly increases the friction losses due to the oil in the air gap and on the surfaces of the rotor. In other words, this system can only operate effectively over a relatively limited range of variation of the rotation speed of the motor, without degrading the efficiency of the machine.

[0011] Also, there is a need for a cooling system for an electric machine, in particular an electric traction machine of an electric or hybrid vehicle, which is at least partly free from the limitations mentioned above.

[0012] To this end, the invention relates to a cooling system for an electrical machine comprising a rotor comprising a rotating shaft and a stator fixed on an internal wall of a casing of the machine and surrounding said rotor, the casing comprising at least one bearing housing receiving one end of said rotating shaft via a bearing, said system comprising: a circulation circuit capable of bringing a coolant into contact with active parts of the electrical machine, a main reservoir located in the lower part of the casing capable of reserving the coolant, a pump connected to said main reservoir capable of injecting said coolant into said circulation circuit, said system being characterized in that the volume of liquid in said main reservoir reaches, when stopped, a level of coolant in the casing interfacing a lower part of said rotor, said circulation circuit leading to a secondary reservoir located in the upper part of the casing, said system comprising means for controlling the pump as a function of at least the temperature of the coolant, said control means being adapted to, on the one hand, keep the pump stopped when the temperature of the coolant is below a given threshold, so that said coolant is projected into the casing by splashing of the rotor and accumulated in a receptacle attached to the surface of the casing on the side of said bearing housing capable of guiding said liquid towards said bearing housing to lubricate and cool said bearing and, on the other hand,commanding the pump to start as soon as the coolant temperature reaches said threshold, so that the liquid is drawn by the pump from the main tank into said circulation circuit and to said secondary tank, lowering the coolant level in the crankcase.,

[0013] Thus, thanks to this arrangement, when the temperature of the coolant, typically the oil, is below the given threshold, adapted to correspond to cold operation where the viscosity of the oil is high, for example a threshold of 0°C, the cooling of the rotor and the stator as well as the lubrication of the machine are ensured by the splashing of the rotor alone, which allows the oil to be projected into the casing, in particular into the receptacle adapted to guide the oil towards the bearing housing, without using the pump. The latter therefore does not need to be sized to meet high requirements in terms of oil viscosity, since it is not used during cold operation phases where there is a need for lubrication or cooling.The power required and the maximum torque requested from the pump can thus be reduced, the pump only being used at oil temperatures above the predefined threshold, implying low oil viscosity.

[0014] Thus, as soon as the oil temperature reaches the predefined threshold, mainly for positive temperatures, the pump is started and the oil is thus brought into the circulation circuit which will ensure the filling of the secondary tank, while the oil level in the casing will drop simultaneously, so that the contact between the lower part of the rotor and the oil will be limited, which is particularly favorable to the operation of the electric machine for high rotation speeds due to the significant reduction in frictional losses due to the oil, which results from it. Cooling and lubrication are then essentially ensured by the circulation of the oil by the pump in the circulation circuit intended to water the active parts of the machine.This cooling mode is not limited by the engine speed, which allows the machine's operating speed to be extended without impacting its efficiency, as opposed to the cooling and lubrication mode using only splashing of the rotor.

[0015] In other words, the selection permitted by the system of the invention, depending on the temperature of the oil, between a cooling and lubrication mode with the pump stopped, by simply splashing the rotor in the oil, and a cooling and lubrication mode with the pump started, by essentially spraying oil onto the parts of the machine to be cooled and lubricated, through the circulation circuit and the secondary reservoir, advantageously makes it possible to provide optimal sizing of the pump, while increasing the operating speed of the machine, without impacting its efficiency, mainly at positive temperatures.

[0016] Advantageously, said secondary reservoir comprises at least one discharge arranged on the periphery of the stator, provided with a nozzle with adjustable flow rate capable of spraying cooling liquid onto a portion of the longitudinal external surface of said stator.

[0017] Advantageously, said secondary tank comprises two discharges located opposite one another or a discharge watering a central peripheral part of said stator.

[0018] Advantageously, said circulation circuit comprises nozzles with adjustable flow rate arranged in the casing so as to spray the winding heads of the rotor and said bearing housing.

[0019] Advantageously, said control means are adapted to control the flow rate of the pump as a function of the oil level in the crankcase.

[0020] Advantageously, said control means are adapted to control the flow rate of the pump as a function of the temperature of the active parts of the machine.

[0021] Advantageously, said control means are adapted to control the flow rate of the pump according to the lubrication requirements of the machine.

[0022] Advantageously, said control means are adapted to at least limit the speed of the pump, or even to stop it, when the noise emitted by the pump is greater than the noise emitted by said electrical machine.

[0023] Advantageously, said receptacle is adapted to fit an upper angular portion of the bearing housing on which it is arranged and to communicate, via a channel passing through this upper angular portion of the bearing housing, with a space located between the bearing and a dynamic seal extending the casing wall to seal the machine at the outlet of the rotating shaft.

[0024] The invention also relates to an electric or hybrid motor vehicle comprising an electric traction machine and a cooling system for said machine as described above.

[0025] Other characteristics and advantages of the invention will emerge clearly from the description given below, for information purposes only and in no way limiting, with reference to the appended drawings in which: [ Fig. 1 ] schematically represents in longitudinal section an electric machine cooled by the cooling system according to the invention; [ Fig. 2 ] schematically represents the operation of the control means associated with the pump.

[0026] According to the embodiment of the invention shown in the figure 1 , the cooling system according to the invention is intended to cool an electrical machine 10, in particular an electrical traction machine of an electric or hybrid vehicle. This electrical machine 10 comprises a casing 11, in which is fixed a stator 12, consisting mainly of a stack of magnetic sheets and copper windings inserted into notches in the stack of sheets. In the stator 12 and housed a rotor 13, consisting according to this embodiment of the invention, of a stack of magnetic sheets forming projecting magnetic poles around which copper windings 14 are wound. The rotor 13 is mounted on a rotating shaft 15 fixed on the one hand to the rotor 13 and on the other hand to the casing of the electrical machine.The ends of the rotating shaft 15 are held in bearings 16, in particular ball bearings, mounted on the rotating shaft and housed in substantially cylindrical bearing housings 17 formed in the vertical walls of the casing 11. In particular, the ball bearings 16 are housed in a cylindrical internal part of the bearing housings. A dynamic seal 18 hermetically closes the axial end of the electrical machine at the level of the ball bearing 16.

[0027] A cooling system according to the invention is used to cool the active parts of the electrical machine 10, namely the stator 12 and the rotor 13, as well as to lubricate and cool the seals and bearings.

[0028] For this, the cooling system comprises in particular a circulation circuit 20 making it possible to bring a cooling liquid into contact, by spraying this liquid, preferably oil, onto these elements of the electrical machine 10, the oil being injected into the circulation circuit 20 from a main reservoir 21 located in the lower part of the casing under the machine, by means of a pump 22. A system of strainer 23 and pipes brings the oil from the reservoir 21 to the pump 22, which itself is connected by a pipe bringing the pumped oil to a heat exchanger 24, intended to cool the oil received from the pump 22 before reinjecting it into the circulation circuit 20 making it possible to bring and spray the oil which has been cooled at the level of the heat exchanger 24, directly onto the heating elements of the electrical machine.The oil thus projected recovers the calories produced by the machine and then returns to the tank 21 by gravity.

[0029] The circulation circuit comprises for this purpose a pipe at the outlet of the exchanger 24, which brings the cooled oil to an inlet 25, located on a casing bottom of the electric machine 10, this casing bottom forming a vertical wall of the casing 11 of the electric machine 10. This inlet 25 brings the cooled oil at the exchanger to a passage 26 formed in the casing bottom and leading on the one hand, to a nozzle located opposite the bearing 16 of the rotating shaft 15 of the electric machine 10 and, on the other hand, to a nozzle 26a located opposite the winding heads 14a of the rotor. These nozzles are preferably of adjustable flow rate. The inlet 25 also brings the cooled oil to the level of the exchanger of the upper inlets 41 and 42 of a secondary tank 27 located in the upper part of the casing, at the top of the electric machine. This secondary tank 27 extends over substantially the entire length of the stator, on the periphery thereof.It is intended to be filled with oil and comprises on its lower part a central discharge 28 arranged on the periphery of the stator, provided with a nozzle, preferably with a modular flow rate, making it possible to spray oil on a central portion of the longitudinal external surface of the stator 12. As a variant, the reservoir 27 may comprise two discharges arranged opposite one another, preferably substantially at the level of the respective longitudinal ends of the secondary reservoir 27. Preferably, the reservoir 27 comprises an additional overflow outlet 43 to avoid the reservoir being completely filled.

[0030] The oil from the cooling nozzles arranged opposite the rotor winding heads 14a and the periphery of the stator having cooled the stator and the rotor, descends by gravity into the main oil reservoir 21. In parallel, the oil from the lubrication nozzles arranged opposite the bearings 16 also descends by gravity into the oil reservoir. The flow rate of the lubrication nozzles is preferably adjusted so that it is lower than the flow rate of the cooling nozzles.

[0031] Furthermore, in accordance with the invention, the liquid volume of the main reservoir 21 is adapted so that the oil level in the casing reaches, when the pump is stopped, a level 30 interfacing with a lower part of the rotor. As illustrated in figure 1 , the oil level 30 in the lower part of the electric machine 10 is located at the level of the lower winding of the rotor 13. Thus, the lower part of the rotor is bathed in oil.

[0032] Advantageously, the oil temperature is measured by a temperature sensor 31, installed in the main tank 21.

[0033] We will now describe a first cooling mode, called cold, when the temperature of the oil measured by the sensor 31 is lower than a predefined temperature threshold. This threshold advantageously makes it possible to define the temperatures for which the viscosity level of the oil is such that it would require a high torque demand from the pump to circulate the oil in the circuit. This threshold is for example preferably set at 0°C, the viscosity of the oil being very high at negative temperatures. Also, to avoid having to size the pump accordingly, when the temperature of the oil measured by the sensor 31 is lower than a predefined temperature threshold, the pump is controlled to stop. The oil level in the crankcase being located at level 30 as illustrated in figure 1 , so that the lower part of the rotor is immersed in oil, when the machine is in operation, due to the rotation of the rotor coupled to the rotating shaft, the oil will be projected into the casing by splashing of the rotor. More precisely, the rotational movement of the rotor immersed in the oil brings the oil over the entire cylindrical surface of the rotor, which makes it possible to cool the rotor windings. The oil is further projected by the rotor onto the cylindrical internal surface of the stator, thus cooling the stator windings. A portion of the projected oil is further accumulated in a receptacle 32, attached to the surface of the casing on the side of the bearing housing 17. This receptacle is provided to guide the oil thus recovered towards the bearing housing to lubricate and cool the seal and the bearing mounted therein. The receptacle is for example clipped or screwed onto the surface of the casing on the side of the bearing housing.More specifically, the receptacle 32 is adapted to fit an upper angular portion of the bearing housing on which it is arranged and to communicate, via a channel passing through this upper angular portion of the bearing housing, with a space located between the bearing and the dynamic seal which extends the housing wall to seal the machine at the outlet of the rotating shaft. The oil thus effectively lubricates the ball bearing as well as the dynamic seal. A recess is also provided so that the oil flowing between the dynamic seal and the ball bearing does not completely fill the seal housing and can flow into the lower part of the housing.

[0034] Thus, in this cold cooling mode, the active parts of the machine are only cooled by the oil sprayed by the rotor when it rotates and the bearings of the machine are also lubricated by the oil sprayed by the rotor during its operation. The pump 22 of the oil circulation circuit 20 is here stopped and is therefore not used in this cold cooling mode where the viscosity of the oil is high, which advantageously allows a smaller sizing of the pump, by reducing the power required for it and therefore, a lower cost of the pump for the benefit of the competitiveness of the electric machine cooled by the system of the invention.

[0035] On the other hand, as soon as the oil temperature reaches the predefined threshold, for example 0°C, from which the viscosity of the oil quickly converges to low values, a second cooling mode, called hot, is implemented, in which the pump 22 is started. The start of the pump 22 is therefore advantageously controlled by the oil temperature measured by the temperature sensor 31. The pump 22 being activated, the oil is then brought into the circulation circuit 20 through the strainer system 23 and the heat exchanger 24, then on the one hand, towards the passage 26 provided in the bottom of the casing to water the rotor and the bearing and, on the other hand, towards the secondary reservoir 27 located in the upper part of the casing, which will fill.

[0036] With the start of pump 22 and the filling of the secondary tank 27, the oil level 30 in the crankcase will drop until it reaches the new oil level referenced 40 on the figure 1 , and the contact of the oil with the rotor is thus limited. This advantageously makes it possible to reduce mechanical losses by friction due to the resistive torque generated by the oil in contact with the rotor and in the air gap, in particular when increasing the operating speed of the machine and thus to improve the efficiency of the machine, for oil temperatures above the predefined threshold. In particular, it is possible to increase to rotation speeds above 14,000 rpm, up to 16,000 to 20,000 rpm, without degrading the efficiency, thanks to the pump which will operate at oil temperatures above said threshold.

[0037] Pump 22 can be controlled in an optimized manner based on several criteria. With reference to the figure 2 , control means 100 of the pump 22 are described. The control means 100 are in particular adapted to control the stopping / starting of the pump, as a function of the received measurement of the oil temperature Thuile, as explained above. The control means 100 of the pump are also adapted to control the flow rate of the pump 22 as a function of the cooling requirement of the machine, defined as a function of the temperature of the active parts of the machine and taking into account the oil temperature. The temperature of the active parts of the machine is for example provided by an estimation of the temperature of the rotor Trotor and by a measurement of the temperature of the stator Tstator, acquired by a temperature sensor 33 installed at the stator.

[0038] The control means may also be adapted to control the flow rate of the pump as a function of the oil level in the crankcase. In this regard, the control means 100 are provided to ensure a minimum flow rate of the pump in order to empty the main oil reservoir to reach an oil level suitable for limiting oil-rotor contact.

[0039] The control means can also be adapted to control the pump flow rate according to the machine's lubrication requirements, defined for example from a Carto map integrated into the control means, making it possible to guarantee a minimum flow rate delivered by the pump for lubrication purposes.

[0040] In the case where the noise emitted by the pump is greater than the noise emitted by the machine, the control means 100 can also be adapted so as to limit the speed of the pump, or even to stop it. In order to objectively quantify the noise emitted by the machine or the pump in operation, different noise evaluation methods can be used, for example by analyzing the acoustic power produced. In the case where the operating conditions imply a need for lubrication or cooling of the machine, if the pump is stopped or has its speed limited, the machine normally risks reaching its thermal limits and / or the seals or bearings risk degradation.However, in the system according to the invention, even if the pump is stopped or limited so that its noise is lower than the noise of the machine, any lubrication or cooling defect is advantageously avoided, since the secondary oil reservoir will then empty to cool the machine, so that the oil level increases again in the crankcase to level 30 as illustrated in . figure 1 and the rotor will bathe in oil to ensure proper operation of the rotor splash cooling.

Claims

1. System for cooling an electric machine (10) having a rotor (13) having a rotary shaft (15) and a stator (12) fixed to an internal wall of a casing (11) of the machine and surrounding said rotor, the casing (11) comprising at least one bearing housing (17) which receives an end of said rotary shaft by way of a rolling bearing (16), said system comprising: - a circulation circuit (20) able to bring a cooling liquid into contact with active parts of the electric machine, - a main reservoir (21) situated in a lower part of said casing, said main reservoir being able to hold the cooling liquid, - a pump (22) connected to said main reservoir, said pump being able to inject said cooling liquid into said circulation circuit, said system being characterized in that the volume of liquid in said main reservoir reaches, in the shutdown state, a level (30) of cooling liquid in the casing that interfaces with a lower part of said rotor, said circulation circuit leading to a secondary reservoir (27) situated in an upper part of the casing, said system comprising control means (100) for controlling the pump as a function of at least the temperature of the cooling liquid (Toil), said control means being designed to, for the one part, keep the pump (22) in the shutdown state when the temperature of the cooling liquid is lower than a given threshold, such that said cooling liquid is propelled into the casing by splash lubrication of the rotor and accumulated in a receptacle (32) attached to the surface of the casing on the side of said bearing housing, said receptacle being able to guide said liquid toward said bearing housing in order to lubricate and cool said rolling bearing, and to, for the other part, command the start-up of the pump (22) as soon as the temperature of the cooling liquid reaches said threshold, such that the liquid is drawn in by the pump from the main reservoir into said circulation circuit and toward the secondary reservoir, lowering the level of cooling liquid in the casing.

2. Cooling system according to Claim 1, characterized in that said secondary reservoir (27) has at least one discharge (28, 29) disposed at the periphery of the stator and provided with a variable flow nozzle able to spray cooling liquid onto a portion of the longitudinal external surface of said stator.

3. System according to Claim 2, characterized in that said secondary reservoir has two discharges situated on opposite sides from one another or one discharge spraying a central peripheral part of said stator.

4. System according to any one of Claims 1 to 3, characterized in that said circulation circuit (20) has variable flow nozzles disposed in the casing so as to spray winding overhangs of the rotor and said bearing housing.

5. System according to any one of the preceding claims, characterized in that said control means (100) are designed to control the flow rate of the pump (22) as a function of the oil level (30) in the casing.

6. System according to any one of the preceding claims, characterized in that said control means (100) are designed to control the flow rate of the pump (22) as a function of the temperature (Tstator, Trotor) of the active parts of the machine.

7. System according to any one of the preceding claims, characterized in that said control means (100) are designed to control the flow rate of the pump (22) as a function of the lubrication requirement of the machine.

8. System according to any one of the preceding claims, characterized in that said control means (100) are designed to at least limit the speed of the pump, or even to stop it, when the noise emitted by the pump is greater than the noise emitted by said electric machine.

9. System according to any one of the preceding claims, characterized in that said receptacle (32) is designed to accept an upper angular portion of the bearing housing (17) on which it is disposed and to communicate, by way of a duct passing through this upper angular portion of the bearing housing, with a space situated between the rolling bearing (16) and a dynamic seal (18) extending the casing wall so as to seal the machine at the exit point of the rotary shaft (15).

10. Electric or hybrid motor vehicle comprising an electric traction machine and a cooling system for cooling said machine according to any one of the preceding claims.