HEAT TRANSFER IN ELECTRIC MACHINESOverview of cooling and simulation techniques in electric machines

JANDAUD Pierre-Olivier

LE BESNERAIS Jean

20th September 2017www.eomys.comcontact@eomys.com

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PRESENTATION OF EOMYS • Innovative Company created in may 2013 in Lille, North of France (1 h from

Paris)

• Activity: engineering consultancy / applied research• R&D Engineers in electrical engineering, vibro-acoustics, heat transfer,

scientific computing • 80% of export turnover in transportation (railway, automotive, marine, aeronautics),

energy (wind, hydro), home appliances, industry

• Diagnosis and problem solving including both simulation & measurements

• Multi-physical design optimization of electrical systems

• Technical trainings on vibroacoustics of electrical systems

• MANATEE fast simulation software for the electromagnetic, vibro-acoustic and

heat transfer design optimization of electric machines

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EOMYS can be involved both at design stage & after manufacturing of electricmachines

EOMYS SERVICES & PRODUCTS

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WEBINAR SUMMARY

• INTRODUCTION

• TYPES OF COOLING TOPOLOGIES

• SIMULATION TECHNIQUES

• CONCLUSION

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INTRODUCTION

• Why is heat management important in an electric machine?

• General introduction to Heat Transfer & Fluid Mechanics

• Types of Losses

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Why is heat management important? • Temperature levels impact directly on the lifetime of a machine• High temperature increases the fatigue of a material

• Each machine has an insulation class for its windings based on the nature of the insulation material

• Basic rule of thumb: lifetime divided by two for each 10°C over the rated temperature, multiplied by two for each 10°C below.

• Temperature levels are also important to avoid demagnetization of the permanent magnets and efficiency reduction

• Heat Management is important for reliable and robust machines

Overheated windings (Reinap, 2015)Demagnetization and characteristic curves of a PM (Neorec53B magnet)

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Why is heat management important?• Temperature levels impact directly on the efficiency of the machine• High temperatures increase linearly the electric resistance of conductors:

𝑅𝑅(𝑇𝑇) = 𝑅𝑅𝑟𝑟𝑟𝑟𝑟𝑟 1 + 𝛼𝛼(𝑇𝑇 − 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟)

• Higher temperatures ⇒ higher Joule losses

• Several studies show the impact of temperature on efficiency of PM machines

• From 25°C to 100°C, the efficiency can decrease up to 5%

• Investing in the cooling system optimization at the design stage of the machine can give significant long-term cost savings

Torque vs Temperature in a PM motor (Lungoci, 2008)

Efficiency vs Temperature for different PM (Wang 2008)

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General introduction to heat transfer in electric machines

• An electric machine is a complex system in terms of heat transfers

• The three kind of heat transfers interact (Conduction, Convection, Radiation)

• Heat is generated by losses in the machine

• Heat always flow from the hottest temperature to the lowest

From Techniques de l’Ingénieur (Bertin, 1999)

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General Introduction: Conductive heat transfer• Conduction occurs inside a body, depends on the thermal

conductivity (𝜆𝜆 in 𝑊𝑊.𝑚𝑚−1.𝐾𝐾−1)

• In a homogeneous body, heat flux (𝝋𝝋 in 𝑊𝑊/𝑚𝑚2) respects a simple PDE the Fourier’s Law, fundamental law for conduction:

𝝋𝝋 = −𝜆𝜆.𝛁𝛁𝑇𝑇

• For an equivalent heat flux, a higher thermal conductivity means a lower temperature gradient i.e. lower temperature levels

• Electric analogy: Ohm’s Law, Temperature is Voltage, thermal conductivity is equivalent to electric conductivity

• Electric insulators are most of the time good thermal insulators.

• Air is one of the best insulator if it’s not moving; if there is air motion, convective heat transfer appears

Material 𝜆𝜆 (𝑊𝑊/m/K)

Air 0.026

PVC 0.15

Epoxy 0.25

Water 0.6

Stainless Steel 30

Cast Iron 50

Aluminum 230

Copper 390

Thermal conductivities of commonmaterials at 20°C

Ex: thermal effect of Vaccum Pressure Impregnation (VPI) when air replaced by resin

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General Introduction: Convective heat transfer• Convective heat transfer occurs in case of a moving fluid on a

solid body• The convective heat flux between a solid and a fluid body is given by

Newton’s Law:𝝋𝝋 = ℎ. (𝑇𝑇𝑠𝑠𝑠𝑠𝑠𝑠 − 𝑇𝑇𝑟𝑟𝑠𝑠𝑓𝑓)

• ℎ is the convective Heat Transfer Coefficient (HTC) in 𝑊𝑊/𝑚𝑚2/𝐾𝐾

• The fluid can be a gas (e.g. air), or a liquid (e.g. water, oil)

• Natural convection: fluid motion due to thermal gradients (e.g. hot air balloon, ocean currents)

• Forced convection: fluid motion due to an external source (e.g. pump, fan)- main method to cool electric machines

Material ℎ (W/m²/K)

Air (naturalconvection)

5-10

Air (forcedconvection)

10-300

Water (forcedconvection)

500 – 10000

Range of convective HTC for air and water

Ex: effect of relative wind on the cooling of outer rotor wind turbine generator

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General Introduction: Radiative heat transfer• Each body emits electromagnetic radiations depending on its

temperature levels (contactless heat transfer)• Bodies are modelled using the gray body theory. The heat flux

exchanged with a body and its environment is:𝝋𝝋 = 𝜎𝜎. 𝜀𝜀. (𝑇𝑇4 − 𝑇𝑇∞4)

• 𝜎𝜎 is the Stefan-Boltzman constant and 𝜀𝜀 is the emissivity of the body

• The emissivity is low for reflective surfaces (polished metals) and depends strongly on the surface finish

• Radiative heat transfer is often neglected inside the machine due to relatively low temperature levels

• Radiative heat transfer can be important as a boundary condition especially in case of natural convection

Material 𝜺𝜺

Aluminum(polished)

0.05

Aluminum(strongly oxidized)

0.25

Black electricaltape

0.95

Cast iron(polished)

0.21

Copper (polished) 0.01

Copper (oxidized) 0.65

Galvanized steel 0.28

Ideal Black Body 1

Matt paint (oil) 0.9-0.95

Water 0.98

Emissivity values for common materialsat 20°C (Fluke)

Ex: alternator in a car exchanging heat with the other parts of the engine

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General Introduction: Fluid Mechanics considerations• Average velocity of the fluid 𝑢𝑢0 (m/s)

• Volume flow rate (𝑄𝑄 in m3/s) through a section S: 𝑄𝑄 = 𝑢𝑢0. 𝑆𝑆. Between 2 points of a circuit, flow rate is constant:

𝑢𝑢1𝑆𝑆1 = 𝑢𝑢2𝑆𝑆2• The pressure of the fluid (𝑝𝑝 in Pa). Between 2 points of a path line, pressure and average velocities

are linked by Bernoulli equation (𝜌𝜌 is the density of the fluid in kg/m3):

𝑝𝑝1 + 12𝜌𝜌𝑢𝑢12 = 𝑝𝑝2 + 1

2𝜌𝜌𝑢𝑢22 + 𝚫𝚫𝑷𝑷

• Δ𝑃𝑃 is the Head Loss or Pressure drop between two points of the circuits. It represents the energy lost due to friction (on walls or due to a singularity). Equation of the hydraulic power:

𝑃𝑃𝐻𝐻 = 𝑄𝑄.∆P

• Hydraulic power is important to evaluate the energy consumption of a cooling system

Ex: cost of cooling power consumption over 25 yrs of a wind turbine generator

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General Introduction: Dimensionless numbers• In Fluid Mechanics and Heat Transfer, most of the phenomena are

studied using dimensionless numbers which are used also in correlations

• The Reynolds number dimensionless number for the velocity. In a channel, for Re < 1500 flow is laminar. For Re > 3000, flow is turbulent.

𝑅𝑅𝑒𝑒𝐷𝐷 =𝑢𝑢.𝐷𝐷𝜈𝜈

• The Nusselt number is for convective heat transfer. In the scientific literature most of the convection correlations have the form: 𝑁𝑁𝑢𝑢 = 𝛽𝛽.𝑅𝑅𝑒𝑒𝛼𝛼

𝑁𝑁𝑢𝑢𝐷𝐷 =ℎ.𝐷𝐷𝜆𝜆

• Pressure drop coefficient is given by: 𝜅𝜅 = �Δ𝑃𝑃 12𝜌𝜌𝑢𝑢2

• Friction factor in a channel of diameter D and length L is given by: 𝑓𝑓 =𝐿𝐿𝐷𝐷𝜅𝜅𝑟𝑟. For laminar flow, given by an analytical expression: 𝒇𝒇 = 𝟔𝟔𝟔𝟔/𝐑𝐑𝐑𝐑. For

turbulent flow, the Moody chart must be used.

Laminar (up) and turbulent (down) rotating flow visualizations at Re=900 and Re=5000 (Bauduin,

2014)

Moody chart for friction factor

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Heat Sources in a Machine: Losses• Heat in the machine is generated by electromagnetic and

mechanical losses• Losses distribution highly depends on machine topology, load and supply

conditions

• Joule losses are generated by electric currents in the windings

• Core losses include hysteresis losses, eddy-current and stray losses, they are located in the laminations of the machine

• Magnet losses are due to eddy currents, they can be high in concentrated winding topologies with surface magnets

• Mechanical losses include friction and windage losses (friction in bearings, aerodynamic friction and drag)

Losses in an 4 poles IM at 50Hz (Yang, 2016)

Losses in an IPM machine (Yang, 2016)

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Heat Sources in a Machine: Joule Losses• Joule Losses are usually the most important sources of losses in an

electric machine• Located in windings/end-windings and rotor bars of IM

• Usually dissipated with convection on end-windings (for stator)

• Temperature dependent: higher temperatures increase electric resistivity

• Joule Losses equation with frequency dependent effects:

𝑃𝑃𝐽𝐽 = 𝑚𝑚. 𝐼𝐼𝑝𝑝2 𝑅𝑅𝐷𝐷𝐷𝐷 + 𝑅𝑅𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑓𝑓 + 𝑅𝑅𝑝𝑝𝑟𝑟𝑠𝑠𝑝𝑝(𝑓𝑓)

Losses in an 4 poles IM at 50Hz (Yang, 2016)

Losses in an IPM machine (Yang, 2016)

Phase number

rms phase current

DC, Skin and proximity components of phase resistance

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Heat Sources in a Machine: Core Losses• Core losses are usually the second sources of losses in a machine• Located in the stator and rotor cores

• Combine two phenomena: eddy-current losses and hysteresis losses

• Modeling of core losses is more challenging than Joule Losses

• Steinmentz equation taking harmonic components into account:

𝑃𝑃𝑐𝑐 = �𝑠𝑠

𝐾𝐾ℎ𝑠𝑠𝐵𝐵𝑠𝑠1,6𝑛𝑛𝑓𝑓 + 𝐾𝐾𝑟𝑟𝑠𝑠𝐵𝐵𝑠𝑠

2 𝑛𝑛𝑓𝑓 2

Losses in an 4 poles IM at 50Hz (Yang, 2016)

Losses in an IPM machine (Yang, 2016)

Harmonic rank

Frequency

Flux density

Hysteresis coeff.

Eddy losses coeff.

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Heat Sources in a Machine: Magnet Losses• Magnet losses can be critical in some topologies• Magnets can be isolated inside the machine (e.g. IPMSM) -> difficulty to

dissipate magnet losses

• Magnet Losses equation for SPMSM (Deeb et al, 2012)

𝑃𝑃𝑚𝑚 =𝑉𝑉𝑚𝑚𝑊𝑊𝑚𝑚

2

24 𝜌𝜌𝑚𝑚�𝑠𝑠

𝐵𝐵𝑠𝑠2𝜔𝜔2𝑛𝑛2

Losses in an 4 poles IM at 50Hz (Yang, 2016)

Losses in an IPM machine (Yang, 2016)

VolumeWidth

ResistivityFrequency

Harmonic id

Flux density

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Heat Sources in a Machine: Mechanical lossesBearings losses.

• They depend on the frictional moment and the rotation speed

• For some applications, an independent cooling system can be needed for bearings (e.g. direct drive wind turbines)

Air friction losses• Caused by the aerodynamic drag, the turbulent structures and the head

losses in the machine

• Neglectable at low speeds: for high peripheral velocity, they can be very important (cf. example)

• For a smooth rotating cylinder of radius R and length L, equation of the air friction losses:

𝑃𝑃𝑟𝑟𝑎𝑎𝑠𝑠𝑟𝑟 = 𝑐𝑐𝑟𝑟𝜋𝜋𝜌𝜌𝑎𝑎𝑠𝑠𝑟𝑟𝜔𝜔3𝑅𝑅4𝐿𝐿

Overall losses (Pd) and friction lossesdue to air (Pfair) in a 100W, 500k rpm PM

machine (Luomi, 2009)

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COOLING ARCHITECTURES OF ELECTRIC MACHINES

• Overview of the different cooling topologies

• Tips for designing a cooling system

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Cooling architectures: IEC standards• Based on standard IEC 60034-6• Primary coolant: coolant directly in contact with the machine (air most of the time)

• Secondary coolant: coolant for a primary coolant

• Designation example of a cooling circuit, designation can be different for rotor and stator if the circuits are different:

IC 4 (A) 1 (A) 6

International Cooling Circuit arrangement0: open circuit4: Frame cooled8: Heat Exchanger

Primary coolantA: Air (omitted)W: WaterU: Oil

Primary circuit0: Free convection1: Self circulation6: Independent system on machine7: Separatecomponent8: Relative displacement

Secondarycoolant

A: Air (omitted)W: WaterU: Oil

Secondary circuit0: Free convection1: Self circulation6: Independent system on machine7: Separatecomponent8: Relative displacement

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Cooling architectures: Open Machines

• In an open machine, air is drawn inside the machine by openings in its housing and directly rejected in its environment.

• Fans can be mounted on the rotor

• Examples of machines: car alternators (Valeo, Bosch, Delphi…)

• Advantages: low-cost system, no need of external power source, high reliability, good cooling of the end-windings

• Drawbacks: highly influenced by the outer environment (external temperature, dirt, etc.), no control of the cooling, almost no air flow in the air gap

Delphi Alternator

Air flow in a Valeo Starter-Generator(Jandaud, 2013)

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Cooling architectures: Self ventilated machines• Totally enclosed machine: air motion in the machine is induced by

rotation of the rotor, a fan can blow air on the outer surface of the machine.

• Fins are often placed on the outer surface of the machine to increase exchange surface

• Very common architecture for low voltage motors

• Not suitable for high power density machines

Full view and cutaway view through the stator of an IM (ABB Motor)

From Techniques de l’Ingénieur (Bertin, 1999)

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Cooling architectures: Axial and Radial cooling circuits

• Air flow is controlled independently and guided inside the machine following either an axial path or a radial path

• Air is guided inside the rotor and stator by radial and axial ventilation ducts

• Topology for air-cooled high power machines like wind-turbines

• Advantages: good cooling inside the stator and rotor laminations, control of the external fans possible depending on the load

• Drawbacks: heat exchanger needed to cool down the air circuit, higher power needed for the cooling

• Axial and radial cooling can be mixed

From Techniques de l’Ingénieur (Bertin, 1999)

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Cooling architectures: Liquid Cooling• For high power density machines, air cooling is not enough and

liquid cooling is needed

• Liquid is generally either water or oil• Two main topologies: water jackets in the housing of the machines or

ducts inside the machine

• Very effective cooling due to the liquid state of the coolant

• High pumping power needed for the system

Water jackets topologies (Satrústegui, 2017)

Water ducts inside a stator (Kim, 2017) Porsche Carrera motor using a water jacket

APM 120R motor for racing cars using oil cooling through ducts (Equipmake)

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Cooling architectures: Other cooling devicesOil jet and sprays cooling• Impinging jets or sprays directly on the end-windings.

• Very good cooling of end-windings.

• Mostly automotive applications

Heat pipes cooling• Heat pipes are passive cooling devices using phase change phenomena

• For high-end applications (expensive) but very effective and reliable• Aerospace and automotive applications

Schematics of spray cooling used by Renault (Davin, 2017)

Tesla Rotor cooling with heat pipes (Putra, 2017)

Heat pipes stator cooling (Putra, 2017)

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Design of a cooling system: How to choose?General considerations• What are is the loss distribution generated of the machine?

• Where are located the critical temperatures of the machine?

• What is the required power density of the machine?

Basic rules of thumb• Based on current density range (Staton, 2014)

CoolingSystem

CurrentdensityA/mm²

Coolingefficiency

Complexity Energy cost

Free convection

1.5 – 5 Low Simple None

Forcedconvection

5 – 10 Medium Medium Low

Liquidcooling

10 – 30 High Complex High

Cooling technologies depending on cooling target (Yang, 2015)

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Design of a cooling system: How to improve convective heat transfer?• Convective (solid to fluid) heat transfer is the main way of cooling electric machines. How to improve it?

• Equation of heat transfer between a fluid and a solid:

Φ = ℎ. 𝑆𝑆 ( 𝑇𝑇𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 − 𝑇𝑇𝑟𝑟𝑠𝑠𝑓𝑓𝑠𝑠𝑠𝑠)

Solid temperature: what wewant to minimize

Temperature of the coolantConvective

Conductance

Total Losses of the machine (W)

ℎ: convective HTC

S: exchange surface

Better EM design to reduce losses

Increase fluidvelocity, change

nature of coolant to increase ℎ

Add fins, add new coolingpaths (ventilation ducts) to

increase exchange surface

Better heat exchanger to reduce coolant

temperature

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Design of a cooling system: Design objectivesGood practices to design a cooling system• Keep in mind the energy cost. For a closed circuit, given by the hydraulic power divided by the electrical

and mechanical efficiency of the pump/fan:

• 𝐸𝐸𝑛𝑛𝑒𝑒𝐸𝐸𝐸𝐸𝐸𝐸 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = Q.ΔP𝜂𝜂𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒.𝜂𝜂𝑚𝑚𝑒𝑒𝑒𝑒𝑚𝑚

What are the losses generated by my machine? The location of the heat sources is important

• A good cooling doesn’t mean the lowest possible temperatures everywhere, it is important to focus on the critical parts of the machine

• Windings should respect the operating temperatures of their insulation classes• Magnet temperature should be far from their demagnetization threshold

-> Fast (magneto-thermal coupling, design iterations) and accurate simulation tools are needed

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THERMAL SIMULATION TECHNIQUES

• Available methods for the thermal simulation of electric machines

• General considerations for the simulation of electric machines

• Brief overview of the different existing software (commercial + free / open-source)

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Available Simulation Techniques for Electric Machines• Electric machines are complex systems to model combining both Heat Transfer and Fluid Mechanics

• Three main techniques with an increasing degree of complexity and accuracy exist: Lumped-Parameter Thermal-Networks (LPTN), Thermal Finite Elements (FEM) Simulations and Computational Fluid Dynamics (CFD).

Lumped Parameters

CFD Simulation

FEM Simulation

Com

plex

ity0D Simulation solving the Heat Equation using electrical analogy

2D/3D FEM Conductive simulation, analytical/empirical boundary conditions

2D/3D Fluid and solid parts are fully solvedNo correlations or empirical data used

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Lumped Parameter Thermal Networks• LPTN are based on the Electrical Analogy

• The machine is divided in small isothermal volumes linked by thermal conductances (G) depending on the nature of the heat transfer:

𝐺𝐺𝑐𝑐𝑠𝑠𝑠𝑠𝑠𝑠 = 𝜆𝜆.𝑆𝑆𝐿𝐿

𝐺𝐺𝑐𝑐𝑠𝑠𝑠𝑠𝑐𝑐 = ℎ. 𝑆𝑆

• Two equations, one for unsteady the other for steady state:

Unsteady: 𝐶𝐶 𝑠𝑠𝑑𝑑𝑠𝑠𝑑𝑑

+ 𝐺𝐺.𝑇𝑇 = 𝑃𝑃 Steady: 𝐺𝐺.𝑇𝑇 = 𝑃𝑃

• Steady state: a simple linear system to solve

• The method is very fast and simple, allowing magnetothermal iterations

• Empirical and/or analytical correlations needed to determine the convection coefficients

Thermal Resistances Network of an electric machine generated by MotorCAD (Boglietti,

2009)

Results of a thermal network on a JeumontElectric machine (Bornschlegell, 2013)

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Thermal FEM Simulation• Only for conductive heat transfer in solid parts of the machine

• Solves the heat equation using Finite Element Method:𝜕𝜕𝑇𝑇𝜕𝜕𝑐𝑐

−𝜆𝜆

𝜌𝜌. 𝑐𝑐𝑝𝑝𝛻𝛻𝛻 = 𝜑𝜑𝑐𝑐

• It can be both solved in steady and unsteady states

• Advantages: detecting potential hot spots as the solution is local – easily coupled with electromagnetic FEM calculations

• Like LPTN, empirical data or correlations for convective boundary conditions are needed, its accuracy depends greatly on them

• Can be solved in 2D or 3D.

• Heat transfer problems in a machine are often fully 3D problems

Results of a 2D thermal FEM simulation in a BPMSM (Yang, 2016)

Results of a 3D thermal FEM simulation of a stator (Kim, 2017)

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Full CFD simulation• In a CFD simulation, heat equation is solved (like FEM) and the

Navier-Stokes equation is added:𝜕𝜕𝒖𝒖𝜕𝜕𝑐𝑐

+ 𝒖𝒖.𝛁𝛁 .𝒖𝒖 = −1𝜌𝜌𝛻𝛻𝑝𝑝 + 𝜈𝜈Δ𝒖𝒖

• Equation usually solved in steady state as computation cost would be too high for unsteady

• Turbulent flow must be modelled, the most common technique is to use Reynolds Averaged Navier-Stokes equations (RANS)

• Computation cost can be very high (several hours/days)

• No correlation/empirical data needed

• Accuracy depends on turbulence modelling knowledge

Velocity field and contours of heat flux dissipated on a machine with external cooling (Boglietti, 2009)

Surface mesh and velocity contours in a Valeo Starter-Generator (Jandaud, 2013)

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Summary of the different techniques• All the techniques are complementary, with pros and cons

• LPTN: ideal for the early stage design of electric machines and for optimization, gives a quick overview of the cooling in the machine

• FEM: ideal to detect eventual hotspots and model more complex geometries (wires in slot)

• CFD: no need of empirical data but very high computation times, can be used for validation of LPTN model

• All of these methods can be combined• Example: CFD can be used to determine convective HTC and flow in isolated

parts of the machines (air-gap, around windings, etc.) the results can be then used for in a thermal network or a FEM simulation.

Hot spot detection due to the air flow using CFD in a salient pole machine

(Lancial, 2017)

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Anisotropy of materials in simulationsWindings modeling• Windings in slots are copper wires with insulation

• Copper is a very good thermal conductor

• Electric insulators are good thermal insulators

• Using an equivalent material, radial and tangential conductivity << axial conductivity

Laminations• Cores are constituted of steel sheets packed with insulation layers between

them

• Axial conductivity < tangential and radial conductivities

Different types of windings arrangement (hairpin, round wires, Litz wires) in slots (Liu,

2017)

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Steady vs Unsteady simulations• Most of the simulation techniques are done using steady state analysis

• Time to reach thermal steady state >> electromagnetic steady state

• For a large machine (ie. wind turbine generator), steady state can be reached in ~10 hours

• For non constant load (car motors, wind turbines), steady state analysis is not enough

• Unsteady calculations need a lot of resources, CFD is often not an option

• For LPTN, unsteady equation is 𝐶𝐶 𝑠𝑠𝑑𝑑𝑠𝑠𝑑𝑑

+ 𝐺𝐺.𝑇𝑇 = 𝑃𝑃

• The capacitance matrix (𝐶𝐶) is very important for short time heat transfer, but it is not easy to obtain from supplier datasheet or tests

• Experimental validation is needed

Material 𝒄𝒄𝒑𝒑 (J/kg/K)

Air 1006

Aluminum 890

Copper 385

Epoxy Resins 1000

Plastics 800-1200

Steel 460

Water 4181

Thermal capacities values for commonmaterials at 20°C

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Typical uncertainties of thermal simulations• A good thermal simulation needs a good EM simulation to calculate losses,

location and values of losses are very important• Boundary conditions need to be as precise as possible (especially for LPTN

and FEM)

• For each methods, mesh / discretization is important, a finer mesh is needed in zones of high temperature gradient

• Contact resistance is important - by default, contact is assumed perfect but real contacts increase thermal resistances, small layers of air can be added to simulate the effect

• Differences between CAD and real geometry (e.g. airflow obstacles)

• CFD models precision +/- 5°C on steady state temperature

• Experimental validation is always important for any type of simulationSame level of accuracy for a fine LPTN and

a 2D FEM simulation (from MANATEE software, www.manatee-software.com)

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Overview of existing softwareExamples of Commercial Software• LPTN: MotorCAD (MDL), SPEED (Siemens)

• FEM (dedicated) : MotorSolve (infolytica),

• FEM (from EM FEM software): Flux (Altair), JMAG (JSOL), Opera (Cobham)

• CFD packages: Ansys Fluent, Ansys CFX, Star CCM+ (Siemens), SC/Tetra (MSC)

Opensource/Free Software• CAD/Meshing: FreeCAD, gmsh, Salome (EDF)

• 2D FEM: FEMM

• 3D FEM: CalculiX, Code_Aster (EDF), Elmer, GetDP

• CFD package: OpenFOAM (ESI)

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CONCLUSIONS• Better cooling means higher efficiency, extended lifetime and lower overall cost

• Cooling must be considered at the early electromagnetic design stage, similarly to noise & vibrations (see tomorrow webinar on 21 Sept 15H CET http://go.leonardo-energy.org/170921MOTORS41_Join.html)

• Simulations methods must be chosen depending on the objectives: Lumped Parameters Network for early design and FEM/CFD for detailed design.

• Thermal simulation workflow must be adapted and coordinated to the electromagnetic and mechanicaldesign workflow

• Experiments should be used to regularly check and improve model behavior (e.g. static pressure loss in cooling chambers, flow rate of heat exchangers, end-winding hot spot, air flow homogeneity)

• Multi-objective optimization algorithms are advised to carry coupled electromagnetic and thermal design of electric motors

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