DE102017003610A1 - Method for determining temperatures in an electrical machine - Google Patents

Abstract

The invention relates to a method for determining temperatures (T) in an electrical machine (1), comprising a rotor (R) and a stator (S) comprising stator iron (Fe) and stator windings (W) and a cooling system (2) with cooling oil, wherein in the electric machine (1) also air (L) is located, wherein the rotor (R) rotor-iron and at least one permanent magnet, wherein the temperatures (T) by means of a heat balance model comprising at least three heat balance spaces (W1, W2, W3) are simulated for stator iron (Fe), stator windings (W) and rotor (R), with a heat flow balance for each of the heat balance spaces (W1, W2, W3) according to the first law of thermodynamics Consideration of losses (PVFe, PVW, PVR), cooling temperatures (TOil, TOil (KO)) of cooling media, heat transfer resistances (RFeWvirt, RFeRvirt, RWRvirt) as well as heat capacities (CFe, CW, CR).

Description

The invention relates to a method for determining temperatures in an electrical machine, in particular a permanent-magnet synchronous machine, for example for a traction drive of a motor vehicle.

Permanently excited synchronous machines are electrical machines which usually have one or more permanent magnets on a housing-fixed part of stator windings and stator laminations and on a moving / rotating part of the motor, for example neodymium-iron-boron magnets, samarium-cobalt magnets or other permanent magnets.

Temperature sensors for electric machines in electric drives are used as economically as possible because of the costs, the space required and the reliability. In particular, no suitable sensors for series production are available for rotating elements due to the costs and the insufficient durability.

However, to optimize the life, diagnosability, and availability of electrical machines, stator and rotor temperature information is desirable.

It is known to estimate the rotor temperature based on the voltage equations (u _d , u _q ). However, this is only possible under tight conditions.

It is also known to determine a plurality of temperatures in the electric machine in detail by means of FEM (Finite Element Method) based heat balance models. Due to their complexity, however, these methods are not suitable for on-board use in control units of motor vehicles.

• – es erfolgt eine Temperaturmessung mit Temperatursensoren und folgenden kritischen Bauteilen: an einem Zwischenkreiskondensator, an dem Pulswechselrichter, an einem Schleifringsystem, an einem Leistungshalbleiter, an einem Mikrocontroller oder an einem ASIC (anwendungsspezifische integrierte Schaltung);
• – es erfolgt eine Temperaturberechnung der kritischen Bauteile mit nicht direkt messbaren Temperaturen über Temperaturmodelle;
• – es erfolgt eine Auswertung der gemessenen Temperaturen und der berechneten Temperaturen und deren Vergleich mit kritischen Temperaturen;
• – bei Erreichen der kritischen Temperatur an einem kritischen Bauteil erfolgt eine Zuordnung, welche Stellgröße die Temperatur des zu warmen Bauteils beeinflusst und
• – es erfolgt eine Regelung durch eine Änderung des Sollwertes einer oder mehrerer Stellgrößen, so dass die Temperatur des zu warmen Bauteils sinkt, wobei die jeweilige Stellgröße mit geändertem Sollwert die Eingangsgröße eines Mehrgrößenreglers ist und die Stellgrößen
• – im Pulswechselrichterbetrieb des Generators die Erreger- und die Phasenströme sind,
• – im Diodenbetrieb des Generators der Erregerstrom ist und
• – Starterbetrieb der elektrischen Maschine der Erregerstrom und die Phasenströme sind,
• – wobei bei Erreichen einer kritischen Temperatur der Startstrom und das Startmoment oder die zulässige Startzeit reduziert werden und der Start-Stopp-Betrieb der elektrischen Maschine unterbunden wird.

From the DE 101 06 944 B4 a method for controlling the power output of an electrical machine is known, comprising a field winding and stator windings, which are arranged downstream of a pulse inverter and a DC electrical system, the electric machine can be operated as a generator and as a motor, wherein control technology exceeds the exceeding of a critical temperature of temperature-sensitive components the electric machine is prevented according to the following process steps:

• A temperature measurement is carried out with temperature sensors and the following critical components: on a DC link capacitor, on the pulse inverter, on a slip ring system, on a power semiconductor, on a microcontroller or on an ASIC (application-specific integrated circuit);
• - There is a temperature calculation of the critical components with not directly measurable temperatures on temperature models;
• - there is an evaluation of the measured temperatures and the calculated temperatures and their comparison with critical temperatures;
• - Upon reaching the critical temperature of a critical component is an assignment, which manipulated variable affects the temperature of the component too warm and
• - There is a control by changing the setpoint of one or more control variables, so that the temperature of the component to warm drops, the respective manipulated variable with a changed setpoint is the input of a multi-variable controller and the manipulated variables
• In the pulse inverter operation of the generator, the excitation and the phase currents are,
• - In the diode mode of the generator, the excitation current is and
• - Starter operation of the electric machine, the excitation current and the phase currents are,
• - When reaching a critical temperature of the starting current and the starting torque or the allowable starting time can be reduced and the start-stop operation of the electric machine is suppressed.

The invention is based on the object to provide an improved method for determining temperatures in an electrical machine.

The object is achieved by a method having the features of claim 1 or 2.

Advantageous embodiments of the invention are the subject of the dependent claims.

A method according to the invention for determining temperatures is used in an electrical machine, comprising a rotor and a stator, comprising stator iron and stator windings, and a cooling system with cooling oil, wherein air is also present in the electric machine, wherein the rotor Iron and at least one permanent magnet comprises. In one embodiment of the method according to the invention, the temperatures are simulated by means of a heat balance model comprising at least four heat balance spaces for stator iron, stator windings, rotor and air, wherein a heat flow balance for each of the heat balance rooms according to the first law of thermodynamics taking into account losses in stator Iron, stator windings and rotor, cooling temperatures of cooling media, heat transfer resistances between stator iron and outside air, between stator iron and air, between stator iron and cooling oil, between stator iron and rotor, between stator iron and stator windings , between rotor and air, between rotor and cooling oil, between stator windings and air and between stator windings and cooling oil, and heat capacity is created, with losses in the stator windings from current heat losses, taking into account resistance changes depending on the model The losses of the stator iron are calculated from heat inputs due to eddy current losses and hysteresis losses, losses in the rotor due to hysteresis losses and eddy current losses in the rotor iron and hysteresis losses in the permanent magnet are calculated.

According to a further embodiment of the method, the temperatures are simulated by means of a heat balance model comprising three heat balance spaces for stator iron, stator windings and rotor, wherein heat transfer resistances between stator iron and air, between rotor and air and between stator windings and air by means of a star -Thieck transformation into virtual heat transfer resistances are converted, wherein a heat transfer resistance between stator iron and stator windings is combined with a parallel to the virtual heat transfer resistors, a heat flow balance for each of the heat balance rooms according to the first law of thermodynamics taking into account losses in Stator iron, stator windings and rotor, cooling temperatures of cooling media, the heat transfer resistance between stator iron and cooling oil, the heat transfer resistance between rotor and cooling oil, the virtual Heat transfer resistors and heat capacities is created, taking into account losses in the stator windings from current heat losses taking into account resistance changes as a function of the modeled temperature of the stator windings and the skin effect, wherein losses in the stator iron from heat inputs due to eddy current losses and Hysteresis losses are calculated, whereby losses in the rotor due to hysteresis losses and eddy current losses in the rotor iron and hysteresis losses are calculated in the permanent magnet.

The stator and rotor temperatures are simulated using a heat balance model based on the first law of thermodynamics. In particular, the structure of the heat balance model (number of heat balance rooms) is reduced specifically for on-board use. The parameterization of the model can be done on the basis of test profiles (test bench, vehicle) and suitable identification algorithms.

By means of the method according to the invention, an increase in the service life is achieved by effective component protection for the electrical machine by avoiding thermal overload, in particular with high power requirements.

Furthermore, the diagnostic capability of a possibly existing stator temperature sensor can be improved by detecting deviations from the stator model temperature.

The availability can be increased by substitution with the stator model temperature in case of failure of any existing stator temperature sensor. Further, an increase in performance through better utilization of temperature reserves and possibly by improved design (cooling / operating strategy, saving of material and rare earths) is possible.

By appropriate design of the electrical machine and the operating strategy by taking into account the feedback from the process Temperaturbelastungskollektive an optimization of costs or reliability is possible. An example of this is the saving of rare earths.

The method can be carried out by means of a program provided in a control unit of a motor vehicle, which calculates the required temperatures on-board without additional hardware with the aid of a software simulation model.

Embodiments of the invention are explained in more detail below with reference to drawings.

Showing:

1 a schematic heat balance model of an electric machine with four balance spaces, and

2 a schematic, simplified heat balance model of an electrical machine with three balance spaces.

Corresponding parts are provided in all figures with the same reference numerals.

1 shows a schematic heat balance model of an electrical machine 1 , in particular a permanent magnet synchronous machine with four balance spaces.

The electric machine 1 comprises a stator S and a rotor R with at least one permanent magnet. The electric machine 1 is by means of a cooling system 2 , For example, an oil circuit, cooled. Likewise, ambient air U contributes to cooling. The stator S includes stator iron Fe and stator windings W. In the electric machine 1 is also air L.

The temperatures are simulated using a heat balance model based on the first law of thermodynamics applied to four heat balance rooms: heat balance space W1 for stator iron Fe, heat balance space W2 for stator windings W, heat balance space W3 for rotor R and heat balance space W4 for Air L in the electric machine 1 , The temperatures of the oil in the cooling system 2 are modeled.

• • Unterteilung der elektrischen Maschine 1 in die vier Wärmebilanzräume W1, W2, W3, W4,
• • Aufstellen der Wärmestrombilanzen für jeden Wärmebilanzraum W1, W2, W3, W4 nach dem ersten Hauptsatz der Thermodynamik
  

wobei T = Temperatur, Pv = Verlustwärmeströme, Q . = zugehende- oder abgehende Wärmeströme, C = Wärmekapazität.Starting point are difference equations, which are structured as follows:

• • Subdivision of the electrical machine 1 into the four heat balance rooms W1, W2, W3, W4,
• • Setting up the heat flow balances for each thermal balance room W1, W2, W3, W4 according to the first law of thermodynamics
  

where T = temperature, Pv = waste heat flows, Q. = incoming or outgoing heat flows, C = heat capacity.

The equation (1) for the heat flow balance for the heat balance room W1 is:

Equation (2) for the heat flow balance for the heat balance space W2 is:

Equation (3) for the heat flow balance for the heat balance room W3 is:

Equation (4) for the heat flow balance for the heat balance room W4 is:

P_VFe

Verluste im Stator-Eisen Fe

P_VW

Verluste in den Stator-Wicklungen W

P_VR

Verluste im Rotor R

sowie die als eingeprägt betrachteten kühlenden Temperaturen der Kühlmedien, beispielsweise Öl und Luft:

T_Oil

Temperatur des Kühlmediums im Stator S

T_Oil(K0)

Temperatur des Kühlmediums im Rotor R

T_AmbAir

Temperatur der Umgebungsluft U.

The input variables for the equation system from the equations (1), (2), (3) and (4) are the following heat losses:

P _VFe

Losses in the stator iron Fe

P _VW

Losses in the stator windings W

P _VR

Losses in the rotor R

as well as the cooling temperatures of the cooling media considered as impressed, for example oil and air:

T _Oil

Temperature of the cooling medium in the stator S

T _{Oil (K0)}

Temperature of the cooling medium in the rotor R

T _AmbAir

Ambient air temperature U.

The temperatures of the heat balance spaces W1 to W4 can be considered as state variables, of which T _W and T _{R are} the actually interesting outputs. The heat transfer between the heat balance spaces W1 to W4 via heat transfer resistors, for example between stator iron Fe and stator windings W in the form of R _FeW . Heat storage takes place by means of heat capacities, for example C _Fe for the stator iron Fe.

The losses P _VW in the stator windings W are calculated from the current heat losses in the copper. The change in resistance is taken into account as a function of the (modeled) winding temperature and the skin effect. P _VW = Skinfctr · (I _q ² + I _d ² ) · R _Cp Skinfctr = 1 + f (I _q , I _d ) · n _EM ² Iges ² = Iq ² + Id ² R _Cp = R _{Cp, T0} + R _{Cp, T0} · α _T0 · (T _w - T ₀ ) (5)

The factor Skinfctr for calculating the skin effect is realized by a characteristic field depending on I _d and I _q .

The losses P _VFe in the stator iron Fe are calculated from the heat _inputs due to the eddy current losses and the hysteresis losses in the stator iron Fe. PvFe = PvFe, hysteresis + PvFe, Eddy PvFe, hysteresis = f (Iq, Id) · nEM PvFe, Eddy = f (Iq, Id) · nEM ² (6)

The pre-factors for the calculation of the losses are realized by maps depending on I _d and I _q .

The losses P _VR in the rotor R are calculated from the heat _inputs due to the hysteresis losses P _{VRIron, hysteresis} and eddy current losses P _{VRIron, Eddy} in the rotor iron and the hysteresis losses P _{VRPM, Eddy} in the permanent magnets. PvR = PvRIron, Hysteresis + PvRIron, Eddy + PvRPM, Eddy PvRIron, Hysteresis = f (Iq, Id) · nEM PvRIron, Eddy = f (Iq, Id) · nEM ² PvRPM, Eddy = f (Iq, Id) · nEM ² (7)

The pre-factors for the calculation of the losses are realized by maps depending on I _d and I _q .

• – Die Wärmetransportgesetze zur Wärmeleitung, Wärmestrahlung und Konvektion entsprechen dem Ohm'schen Gesetz. Neben dem linearen Gesetz zur Wärmeleitung (Fourier) gibt es die nicht-linearen Wärmetransportmechanismen Wärmestrahlung (Stefan-Boltzmann) und Konvektion, die einzeln oder kombiniert auftreten. Letztere können aber für den betrachteten eingeschränkten Temperaturbereich näherungsweise linearisiert werden, so dass insgesamt ein linearer Zusammenhang für den Wärmetransport – analog dem Ohm'schen Gesetz – angenommen werden kann.
• – Der Wärmestrom entspricht dem elektrischen Strom.
• – Die Temperaturdifferenz entspricht der elektrischen Spannungsdifferenz.
• – Der Wärmeübergangswiderstand entspricht dem elektrischen Widerstand.
• – Der Wärmeleitwert entspricht dem elektrischen Leitwert.
• – Das Gesetz zur Wärmekapazität entspricht dem Gesetz zur Kondensatoraufladung.
• – Der erste Hauptsatz der Thermodynamik, wonach die Summe der Wärmeströme in Knoten gleich Null ist, entspricht der Kirchhoff'schen Knotenregel.
• – Aus Temperatur (skalare Zustandsgröße) folgt: Die Summe der Temperaturdifferenzen über eine geschlossene Kurve ist gleich Null. Dies entspricht der Kirchhoff'schen Maschenregel.

In 1 For a better understanding of superordinate relationships, equations (1) to (4) are presented in a quasi-electrical network. The well-known general analogies between thermodynamics and electrical engineering are used here:

• - The heat transfer laws for heat conduction, heat radiation and convection comply with Ohm's law. In addition to the linear law for heat conduction (Fourier), there are the non-linear heat transfer mechanisms heat radiation (Stefan-Boltzmann) and convection, which occur individually or in combination. The latter can but for the considered limited temperature range Approximately linearized, so that a total of a linear relationship for the heat transfer - analogous to Ohm's law - can be assumed.
• - The heat flow corresponds to the electric current.
• - The temperature difference corresponds to the electrical voltage difference.
• - The heat transfer resistance corresponds to the electrical resistance.
• - The thermal conductivity corresponds to the electrical conductance.
• - The heat capacity law complies with the law on capacitor charging.
• - The first law of thermodynamics, according to which the sum of the heat flows in knots is equal to zero, corresponds to Kirchhoff's knot rule.
• - From temperature (scalar state variable) follows: The sum of the temperature differences over a closed curve is equal to zero. This corresponds to Kirchhoff's mesh rule.

In a further step, the number of heat balance rooms W1 to W4 can be reduced for on-board use.

2 shows a schematic, simplified heat balance model of an electrical machine 1 with three heat balance rooms W1 to W3.

• a) Das Kühlmedium hat für eine gekapselte elektrische Maschine 1 einen gegenüber der Umgebungsluft U dominanten Kühleffekt. In der Folge kann der Einfluss der Umgebungsluft U vernachlässigt und somit das in 1 gezeigte R_FeAmbAir „hochohmig”, das heißt auf den Wert unendlich, gesetzt werden und somit in 2 unberücksichtigt bleiben.
• b) Bei Kühlung des Stator-Eisens Fe erfolgt der Wärmetransport aus den Stator-Wicklungen W in das Kühlmedium hauptsächlich über das Stator-Eisen Fe, weshalb der Wärmeübergang zwischen Stator-Wicklungen W und Kühlmedium einen Umweg darstellt und daher das in 1 gezeigte R_WOil „hochohmig” gesetzt werden und somit in 2 unberücksichtigt bleiben kann.
• c) Der Wärmetransport vom Stator-Eisen Fe zum Rotor R erfolgt in erster Linie über einen dazwischenliegenden Luftspalt mittels Abstrahlung und Konvektion und nur in weit geringerem Maß über den mechanischen Umweg über Lager des Rotors R mittels Wärmeleitung. Daher kann das in 1 gezeigte R_FeR (in seiner ursprünglichen Bedeutung) „hochohmig” gesetzt werden und in 2 unberücksichtigt bleiben.
• d) Die Wärmekapazität C_EMAir der Luft L in der elektrischen Maschine 1 kann verglichen mit den großen Wärmekapazitäten C_Fe, C_W, C_R von Stator-Eisen Fe, Stator-Wicklungen W und Rotor R in erster Näherung vernachlässigt werden.

To minimize the effort for the parameterization and the required on-board resources are at the in 2 shown model made certain tolerable simplifications with the help of physical considerations under less restrictive conditions. These are used here for a gear-integrated electric machine 1 whose stator S and rotor R have a cooling system 2 is cooled with a cooling medium, for example oil, shown:

• a) The cooling medium has for a sealed electrical machine 1 a relation to the ambient air U dominant cooling effect. As a result, the influence of the ambient air U neglected and thus the in 1 shown R _FeAmbAir "high impedance", that is, set to the value infinite, and thus in 2 disregarded.
• b) When the stator iron Fe is cooled, the heat transfer from the stator windings W into the cooling medium takes place mainly via the stator iron Fe, which is why the heat transfer between the stator windings W and the cooling medium is a detour and therefore the in 1 shown _WOil R are set to "high impedance" and thus in 2 can be disregarded.
• c) The heat transfer from the stator iron Fe to the rotor R takes place primarily via an intervening air gap by means of radiation and convection and only to a much lesser extent via the mechanical detour via bearings of the rotor R by means of heat conduction. Therefore, the in 1 shown R _FeR (in its original meaning) are set to "high impedance" and in 2 disregarded.
• d) The heat capacity C _{EMAir of} the air L in the electric machine 1 can be neglected in a first approximation compared to the large heat capacities C _Fe , C _W , C _R of stator iron Fe, stator windings W and rotor R.

Thus, the node of the thermal balance space W4 can be resolved, in which a star-delta transformation is performed and the remaining resistance R _{Few is combined} with the now parallel-lying resistor.

The result of these simplifications is in the circuit diagram of 2 shown.

The difference equations of the simplified heat balance model follow from the equations (1) to (3) and the above-mentioned simplifications a) to d):

The equations (8) to (10) are particularly suitable for on-board use in a control unit of a motor vehicle, in which the electric machine 1 can be arranged for example as a drive. In equations (8) to (10) are found opposite 1 new parameters R _FeWvirt , R _WRvirt , R _FeRvirt , virtual resistances which formally result from the transformation described in d) and which are obtained from the original physical resistances R _WEMAir , R _FeEMAir , R _REMAir and R _FeW as follows:
Resolution of the node of the heat balance space W4 by star-triangle transformation: R _{FeW, 2} = RFeemair · RREMAir + RREMAir · RWEMAir + RWEMAir · RFeEMAir / RREMAir (11) RFeRvirt = RFeemir · RREMAir + RREMAir · RWEMAir + RWEMAir · RFeemir / RWEMAir (12) RWRvirt = RFeemir · RREMAir + RREMAir · RWEMAir + RWEMAir · RFeemir / RFeEMAir (13)

Integration of R _FeW by parallel connection with R _{FeW, 2} according to (11): RFeWvirt = RFeW · RFeW, 2 / RFeW + RFeW, 2 (14)

Equations (11) to (14) thus provide an off-board transformation that allows starting values and value ranges of the parameters of the simplified model, which are required for the subsequent fine parameterization, to be obtained from physical values. List of abbreviations

LIST OF REFERENCE NUMBERS

1

electric machine

2

cooling system

C _EMAir

Heat capacity of the air in the electric machine

C _Fe

Heat capacity stator iron

C _R

Heat capacity rotor

C _W

Heat capacity stator winding

Fe

Stator iron

L

air

P _VFe

Losses in the stator iron

P _VR

Losses in the rotor

P _VW

Losses in the stator windings

R

rotor

R _FeAmbAir

Heat transfer resistance between stator iron and outside air

R _FeEMAir

Heat transfer resistance between stator iron and air in the electric machine

R _FeOil

Heat transfer resistance between stator iron and cooling oil

R _FeR

Heat transfer resistance between stator iron and rotor

R _FeRvirt

Heat transfer resistance virtually between stator iron and rotor

R _FeW

Heat transfer resistance between stator iron and stator winding

R _FeWvirt

Heat transfer resistance virtually between stator iron and stator winding

R _REMAir

Heat transfer resistance between the rotor and the air in the electric machine

R _ROil

Heat transfer resistance between rotor and cooling oil

R _WEMAir

Heat transfer resistance between stator winding and the air in the electric machine [K / W]

R _WOil

Heat transfer resistance between stator winding and cooling oil

R _WRvirt

Heat transfer resistance virtually between stator winding and rotor

S

stator

T _AmbAir

Temperature of the ambient air

T _EMAir

Temperature of the air in the electric machine (in the air gap)

T _Fe

Temperature stator-iron

T _Oil

Temperature of the cooling medium

T _{Oil (K0)}

Temperature of the cooling medium in the rotor

T _R

Temperature rotor

T _W

Temperature stator winding

U

ambient air

W

Stator windings

W1

Heat balance room

W2

Heat balance room

W3

Heat balance room

W4

Heat balance room

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10106944 B4 [0007]

Claims (3)

Method for determining temperatures (T) in an electrical machine ( 1 ) comprising a rotor (R) and a stator (S) comprising stator iron (Fe) and stator windings (W) and a cooling system ( 2 ) with cooling oil, wherein in the electrical machine ( 1 ) in addition to air (L), wherein the rotor (R) rotor-iron and at least one permanent magnet, characterized in that the temperatures (T) by means of a heat balance model comprising at least four heat balance spaces (W1, W2, W3, W4) for stator Iron (Fe), stator windings (W), rotor (R) and air (L) are simulated, with a heat flow balance for each of the heat balance spaces (W1, W2, W3, W4) according to the first law of thermodynamics taking into account Losses (P _VFe , P _VW , P _VR ) in stator iron (Fe), stator windings (W) and rotor (R), cooling temperatures (T _Oil , T _{Oil (KO)} , T _AmbAir ) of cooling media, heat transfer _resistances (R _FeAmbAir , R _FeEMAir , R _FeOil , R _FeR , R _FeW , R _REMAir , R _ROil , R _WOil , R _WEMAir ) between stator iron (Fe) and outside air, between stator iron (Fe) and air (L) between stator iron (Fe) and cooling oil, between stator iron (Fe) and rotor (R), between stator iron (Fe) and stator windings (W), between rotor (R) and air (L), between rotor (R) and cooling oil, between stator windings (W) and air (L) and between stator windings (W) and cooling oil, as well as heat capacities (C _Fe , C _W , C _R , C _EMAir ), wherein losses (P _VW ) in the stator windings (W) from current heat losses taking into account resistance changes as a function of the modeled temperature (T _W ) of the stator windings (W) and the Skin losses (P _VFe ) in the stator iron (Fe) are calculated from heat _inputs due to eddy current losses and hysteresis losses, with losses (P _VR ) in the rotor (R) due to hysteresis losses and eddy current losses in the rotor iron and hysteresis losses in the rotor Permanent magnets are calculated. Method for determining temperatures (T) in an electrical machine ( 1 ) comprising a rotor (R) and a stature (S) comprising stator iron (Fe) and stator windings (W) and a cooling system ( 2 ) with cooling oil, wherein in the electrical machine ( 1 ), wherein the rotor (R) rotor-iron and at least one permanent magnet, characterized in that the temperatures (T) by means of a heat balance model comprising three heat balance spaces (W1, W2, W3) for stator iron ( Fe), stator windings (W) and rotor (R) are simulated, with heat transfer _resistances (R _FeEMAir , R _WEMAir , R _REMAir ) between stator iron (Fe) and air (L), between rotor (R) and air ( L) and between stator windings (W) and air (L) by means of a star-triangle transformation in virtual heat transfer _resistances (R _FeWvirt , R _FeRvirt , R _WRvirt ) are converted, wherein a heat transfer _resistance (R _FeW ) between stator iron ( Fe) and stator windings (W) with a virtual heat transfer resistors (R _FeWvirt ) connected in parallel _therewith , wherein a heat flow _balance for each of the heat _{balance spaces} (W1, W2, W3) according to the first law of thermodynamics taking into account Losses (P _Vfe , P _VW , P _VR ) in stator iron (Fe), stator windings (W) and rotor (R), cooling temperatures (T _Oil , T _{Oil (K0)} ) of cooling media, the heat transfer resistance (R _FeOil ) between stator iron (Fe) and cooling oil, the heat transfer _resistance (R _ROil ) between rotor (R) and cooling oil, the virtual heat transfer _resistances (R _FeWvirt , R _FeRvirt , R _WRvirt ) as well as heat capacities (C _Fe , C _W , C _R ), taking into account losses (P _VW ) in the stator windings (W) from current heat losses taking into account resistance changes as a function of the modeled temperature (T _W ) of the stator windings (W) and the skin effect where losses (P _VFe ) in the stator iron (Fe) are calculated from heat _inputs due to eddy current losses and hysteresis losses, with losses (P _VR ) in the rotor (R) due to hysteresis losses and eddy current losses in the rotor iron and hysteresis losses in the rotor Permanent magnets are calculated. Method according to one of claims 1 or 2, characterized in that the method for an electrical machine ( 1 ) is used in a motor vehicle.

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