FR3071113A1 - COOLING SYSTEM FOR OIL COOLED ELECTRIC MACHINE - Google Patents

Abstract

The invention relates to a cooling system for an electric machine, comprising a cooling circuit capable of bringing a cooling liquid into contact with active parts of said electrical machine, a tank (OT) able to receive said liquid from said active parts and a pump adapted to reinject said cooling liquid into said cooling circuit, the walls of said tank (OT) having a thermal insulation layer (CIT) and the cooling liquid contained in said tank (OT) being in thermal contact with a phase change material.

Description

Cooling system for oil-cooled electric machine

The present invention relates generally to the fields of electrical engineering and the automobile industry, and more specifically relates to a cooling system for an electric machine using an oil type dielectric coolant.

The electric machines used in electric or hybrid vehicles for traction / propulsion being crossed by large currents, they require an efficient cooling system. In particular, the cooling of the active parts of such a machine by injection of a dielectric coolant such as oil, can be envisaged for cooling such machines with very high power density.

However, when an electric vehicle equipped with an oil injection cooling system on the active parts of its electric motor is in the start-up phase, the oil cooling these active parts did not have time to warm up . As at low temperature the oil has a high viscosity, to be able to effectively cool the engine, the oil injection pressure on the active parts must be higher than the oil injection pressure on the active parts when the vehicle has been driving for some time. It is therefore necessary to oversize the cooling circuit in relation to the needs of the electric motor at nominal speed, to take account of the starting phases. In addition, the resistive torque of the oil at low temperature in contact with the rotating parts of the engine generates energy losses which explain a low efficiency for vehicles fitted with such a cooling system in the starting phase.

Document DE102011050164 proposes to reduce the volume of oil cooling the active parts at start-up compared to the volume of oil used under nominal speed, which allows the cooling oil to heat up faster. However, this solution requires a complex cooling system comprising a valve for regulating the oil flow.

Other solutions pool the oil used to lubricate the reducer of a vehicle and the cooling oil of the electric motor of the vehicle. This solution makes it possible, thanks to the passage of oil through the reduction gear, to preheat the oil before it passes through the electric motor. However, in these solutions, the oil from the reducer contains debris from the gears and pinions of the reducer. This debris can harm the insulators of the electric motor windings when the oil transporting this debris is used to cool the active parts of the electric motor. This is the case of document JP2015074325 which also proposes to isolate the oil pan with glass wool or a phenolic derivative. However, this solution is not sufficient to preheat the oil at start-up, a thermo-controlled heater is necessary, which requires the insertion of an electrical component in the tank.

One of the aims of the invention is to remedy at least part of the drawbacks of the prior art by providing a cooling system for an electric traction or propulsion motor of an electric or hybrid vehicle, which has improved efficiency at start-up. of the electric motor, without requiring any additional component requiring electronic control or using oil from the vehicle's reduction gear.

To this end, the invention provides a cooling system for an electric machine, comprising a cooling circuit capable of bringing a coolant into contact with active parts of said electric machine, a tank capable of receiving downstream of said active parts said coolant, and a pump capable of re-injecting said coolant into said coolant circuit, said coolant system being characterized in that walls of said reservoir have a layer of thermal insulation and in that the coolant contained in said reservoir is in thermal contact with a phase change material.

Phase change materials are capable of storing and releasing large amounts of heat. Thus being in the tank when the coolant having been in contact with the active parts of the machine has absorbed a large amount of heat, they increase their temperature to a target temperature defined by the cooling system of the machine electric. They keep this temperature for a period of time depending on the material used and the volume of the phase change material. The choice of the phase change material used depends on the desired melting temperature, which will be chosen within the temperature range of the electric machine in nominal operation, and on the thermo-physical characteristics of the material.

Thanks to the invention, the heat transition from the coolant present in the tank to the ambient air is greatly limited. Thus when the vehicle implementing the invention is stopped, the energy of the liquid is stored in the tank. When the vehicle is restarted, the electric machine will therefore benefit from an already hot coolant. Preferably the layer of insulating material is added to the internal surfaces of the tank.

According to an advantageous characteristic of the system according to the invention, said thermal insulation layer is of nanoporous material with thermal conductivity less than 0.025W.m ' ¹ .K' ¹ (Watt / meter / Kelvin). It is indeed advantageous to use a material with very low thermal conductivity such as a nanoporous material whose thermal conductivity is between 0.005 Wm- ¹ .K- ¹ and 0.025Wm- ¹ .K- ¹ .

According to another advantageous characteristic, said thermal insulation layer is arranged between an external wall of said tank and an internal wall of said tank. This configuration of the reservoir allows good rigidity to be kept thanks to an appropriate choice of material for the external wall, and a sealed surface on the internal wall of the reservoir which is chosen from a material other than the insulating layer.

Advantageously, said phase change material is contained in at least one capsule provided with a frame having concavities capable of leaving a passage for the coolant between said capsule and an inlet or outlet orifice formed in said tank. This conditioning of the phase change material does not block the inlets and outlets of the oil tank.

Preferably, said phase change material is encapsulated in a sphere encircled by two rings orthogonal to each other.

The invention also relates to an electric or hybrid vehicle comprising an electric traction motor and a cooling system of said motor according to the invention.

Other characteristics and advantages will appear on reading a preferred embodiment described with reference to the figures in which:

FIG. 1 represents a cooling system according to the invention, in this preferred embodiment,

- Figure 2 shows a coolant tank of the prior art,

FIG. 3 represents a reservoir of cooling liquid in the cooling system according to the invention, in this preferred embodiment,

FIG. 4 is a sectional view along an axis A-A of the coolant reservoir of FIG. 3,

- And Figure 5 shows a capsule for phase change material used in the cooling system according to the invention, in this preferred embodiment.

According to a preferred embodiment of the invention shown in FIG. 1, the cooling system S according to the invention is capable of cooling an electric traction machine ME shown in section of an electric or hybrid vehicle, this electric machine ME including in particular:

- an STA stator mainly consisting of a stack of magnetic sheets and copper coils inserted in the notches of the stack of sheets,

a ROT rotor mainly constituted in this embodiment of the invention, of a stack of magnetic sheets forming salient magnetic poles, and of copper windings wound around these magnetic poles,

- And a rotor shaft fixed on the one hand to the ROT rotor and on the other hand to a casing CA of the electric machine by means of RO bearings.

An oil cooling system according to the invention is used to cool the active parts of the ME electric machine, which are the rotor ROT and the stator STA. For this, the cooling system includes in particular:

- a cooling circuit OC allowing the oil to come into contact with the active parts of the ME electric machine;

- an OT oil tank which collects the oil which has cooled the active parts of the ME electric machine and descended by gravity into the OT oil tank located at the bottom of the ME electric machine;

- a pump OP which re-injects the oil contained in the oil tank OT into the cooling circuit OC to bring the oil upstream of the active parts of the electric machine ME.

More specifically, the OC cooling circuit brings the oil to the injectors located on each side of the ME electrical machine facing the coil heads of the ROT rotor. The OC cooling circuit also brings the oil to an injector located at the periphery of the stator STA and in height relative to the latter, so that the injected oil spreads by gravity all around the STA sheet metal stack. Other injectors located at the RO bearings inject the cooling oil from the OC cooling circuit onto the RO bearings to lubricate them.

In the prior art, the oil tank in an oil cooling system is generally in the form of a metal tank as shown in FIG. 2. This tank receives the cooling oil lowered by gravity at the level of its open upper part as well as at the level of an inlet opening OE. Oil comes out of the tank through an OS outlet. Such a tank does not maintain the heat of the oil absorbed during the operation of the electric machine as it cools. Consequently, in the prior art, it is sought to preheat the oil before or during the start of the electric machine.

With reference to FIG. 3, the reservoir OT of the cooling system of the electric machine ME according to the invention has not only an external wall PE made of metal, in particular aluminum, but also a layer of thermal insulation CIT disposed between the wall external PE of the OT tank, and an internal wall PI of the OT tank, also made of metal, here made of aluminum. The thermal insulation layer CIT is made of a very light thermal insulation material, or of a nano-porous material with very low thermal conductivity, of the order of 0.005 Wm- ¹ .K- ¹ to 0.025 W .nT ¹ .K ' ¹ .

Preferably, this thermal insulating coating covers the entire interior of the OT tank, with the exception of the OE inlet and OS outlet openings of the OT tank. Figure 4 showing in section the OT tank along the axis A-A shows an alternative embodiment in which the thermal insulating coating is only placed on the side walls of the OT tank.

The OT oil tank contains the oil which has cooled the active parts of the ME electric machine, but also one or more bags or bags filled with a phase change material, such as sodium acetate.

These pockets are made of a thermally conductive material, for example aluminum, and are designed to avoid direct contact between the phase change material and the oil. In addition, in order to prevent them from blocking the inlet OE and outlet OS ports of the OT tank, and blocking the cooling circuit, their casings must be larger than these OE and OS ports and present concavities allowing the oil to pass between the pockets and these OE and OS orifices.

Figure 5 shows an example of such a pocket. It is formed by a spherical CAP capsule comprising the phase change material, this CAP capsule being encircled by two circular rings B1 and B2 arranged orthogonally to one another. The capsule is for example made of aluminum, and the rings are also made of aluminum. In this preferred embodiment of the invention, the rings B1 and B2 form an armature superimposed on the CAP capsule, but as a variant this armature is integrated into the CAP capsule, that is to say molded at the same time as the CAP capsule.

So when the electric machine is running, the heat absorbed by the cooling oil in contact with the active parts of the machine is transmitted to the phase change material. The capsules comprising this phase change material store this heat and gradually return it to the oil when the electric machine is stopped. So the next time the machine is started after an overnight shutdown time, the cooling oil will be at a warm temperature allowing the electric machine to operate efficiently.

Other embodiments of the invention are of course conceivable, in particular another dielectric coolant than oil can be used to cool the electric machine. In addition, the cooling system according to the invention does not necessarily use injectors, the invention also applying for example to a cooling system using a hollow rotor propelling oil during its rotation. In another embodiment of the invention, the arrangement of the thermal insulation layer is different, for example it covers the walls of the oil tank externally.

Claims (6)

1. Cooling system (S) for an electric machine (ME), comprising a cooling circuit (OC) capable of bringing a coolant into contact with active parts of said electric machine (ME), a tank (OT) capable receiving downstream of said active parts said coolant, and a pump (OP) capable of reinjecting said coolant into said coolant circuit (OC), said coolant system (S) being characterized in that walls of said reservoir (OT) comprise a thermal insulation layer (CIT), and in that the coolant contained in said tank (OT) is in thermal contact with a phase change material. 2. Cooling system (S) according to claim 1, characterized in that said thermal insulation layer (CIT) is of nano-porous material with thermal conductivity less than 0.025W.m ' ¹ .K' ¹ . 3. Cooling system (S) according to claim 1 or 2, characterized in that said thermal insulation layer (CIT) is disposed between an external wall (PE) of said tank (OT) and an internal wall (PI) of said tank (OT). 4. Cooling system (S) according to any one of claims 1 to 3, characterized in that said phase change material is contained in at least one capsule (CAP) provided with a frame having concavities capable of leaving a passage for the coolant between said capsule (CAP) and an inlet (OE) or outlet (OS) orifice formed in said tank (OT). 5. Cooling system (S) according to any one of claims 1 to 4, characterized in that said phase change material is encapsulated in a sphere (CAP) encircled by two rings (B1, B2) orthogonal to each other. 6. Electric or hybrid vehicle comprising an electric traction motor and 5 a system (S) for cooling said engine according to any one of claims 1 to 5.