HEAT TRANSFER IN ELECTRIC MACHINES Overview of cooling and simulation techniques in electric machines
JANDAUD Pierre-Olivier LE BESNERAIS Jean 20th September 2017 www.eomys.com [email protected]
1 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
2 EOMYS SERVICES & PRODUCTS
• 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
EOMYS can be involved both at design stage & after manufacturing of electric machines 3 WEBINAR SUMMARY
• INTRODUCTION
• TYPES OF COOLING TOPOLOGIES
• SIMULATION TECHNIQUES
• CONCLUSION
4 INTRODUCTION
• Why is heat management important in an electric machine?
• General introduction to Heat Transfer & Fluid Mechanics
• Types of Losses
5 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
Demagnetization and characteristic curves of a Overheated windings (Reinap, 2015) PM (Neorec53B magnet) 6 Why is heat management important?
• Temperature levels impact directly on the efficiency of the machine • High temperatures increase linearly the electric resistance of conductors: ( ) = 1 + ( )
• Higher temperatures𝑅𝑅 𝑇𝑇 higher𝑅𝑅𝑟𝑟𝑟𝑟𝑟𝑟Joule𝛼𝛼 losses𝑇𝑇 − 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟 ⇒ Torque vs Temperature in a PM motor (Lungoci, 2008) • 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
Efficiency vs Temperature for different PM (Wang 2008)
7 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)
8 General Introduction: Conductive heat transfer
• Conduction occurs inside a body, depends on the thermal conductivity ( in . . ) −1 −1 Material ( /m/K) • In a homogeneous𝜆𝜆 𝑊𝑊 body,𝑚𝑚 heat𝐾𝐾 flux ( in / ) respects a simple Air 𝜆𝜆 0.026𝑊𝑊 PDE the Fourier’s Law, fundamental law for conduction:2 = .𝝋𝝋 𝑊𝑊 𝑚𝑚 PVC 0.15 Epoxy 0.25 • For an equivalent heat flux𝝋𝝋, a higher−𝜆𝜆 𝛁𝛁𝑇𝑇 thermal conductivity means a lower temperature gradient i.e. lower temperature levels Water 0.6 Stainless Steel 30
Cast Iron 50 • Electric analogy: Ohm’s Law, Temperature is Voltage, thermal conductivity is equivalent to electric conductivity Aluminum 230 Copper 390 • Electric insulators are most of the time good thermal insulators. Thermal conductivities of common • Air is one of the best insulator if it’s not moving; if there is air motion, materials at 20°C convective heat transfer appears
Ex: thermal effect of Vaccum Pressure Impregnation (VPI) when air replaced by resin
9 General Introduction: Convective heat transfer
• Convective heat transfer occurs in case of a moving fluid on a solid body Material (W/m²/K) • The convective heat flux between a solid and a fluid body is given by Air (natural ℎ 5-10 Newton’s Law: convection) = . ( ) Air (forced 10-300 convection) 𝑠𝑠𝑠𝑠𝑠𝑠 𝑓𝑓𝑓𝑓𝑓𝑓 • is the convective Heat Transfer𝝋𝝋 ℎ 𝑇𝑇 Coefficient− 𝑇𝑇 (HTC) in / / Water (forced 500 – 10000 2 convection) • The fluid can be a gas (e.g. air), or a liquid (e.g. water, oil) ℎ 𝑊𝑊 𝑚𝑚 𝐾𝐾 Range of convective HTC for air and • Natural convection: fluid motion due to thermal gradients (e.g. hot air water balloon, ocean currents) • Forced convection: fluid motion due to an external source (e.g. pump, fan)- main method to cool electric machines
Ex: effect of relative wind on the cooling of outer rotor wind turbine generator
10 General Introduction: Radiative heat transfer
• Each body emits electromagnetic radiations depending on its temperature levels (contactless heat transfer) Material 𝜺𝜺 • Aluminum 0.05 Bodies are modelled using the gray body theory. The heat flux (polished) exchanged with a body and its environment is: Aluminum 0.25 = . . ( ) (strongly oxidized) 4 4 Black electrical 0.95 • is the Stefan-Boltzman𝝋𝝋constant𝜎𝜎 𝜀𝜀 and𝑇𝑇 − 𝑇𝑇is∞ the emissivity of the body tape Cast iron 0.21 • 𝜎𝜎The emissivity is low for reflective surfaces𝜀𝜀 (polished metals) and (polished) depends strongly on the surface finish Copper (polished) 0.01 • Radiative heat transfer is often neglected inside the machine due to Copper (oxidized) 0.65 relatively low temperature levels Galvanized steel 0.28 • Radiative heat transfer can be important as a boundary condition Ideal Black Body 1 especially in case of natural convection Matt paint (oil) 0.9-0.95
Water 0.98 Ex: alternator in a car exchanging heat with the other parts of the engine Emissivity values for common materials at 20°C (Fluke)
11 General Introduction: Fluid Mechanics considerations
• Average velocity of the fluid (m/s) 3 • Volume flow rate ( in m /s) 𝑢𝑢through0 a section S: = . . Between 2 points of a circuit, flow rate is constant: 𝑄𝑄 𝑄𝑄 𝑢𝑢0 𝑆𝑆 =
• The pressure 𝑢𝑢of1 𝑆𝑆the1 fluid𝑢𝑢2𝑆𝑆 2( in Pa). Between 2 points of a path line, pressure and average velocities are linked by Bernoulli equation ( is the density of the fluid in kg/m3): 𝑝𝑝 + = + 𝜌𝜌 + 1 2 1 2 • is the Head𝑝𝑝 Loss1 2 𝜌𝜌or𝑢𝑢 1Pressure𝑝𝑝2 2 𝜌𝜌drop𝑢𝑢2 between𝚫𝚫𝑷𝑷 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: Δ𝑃𝑃 = . P
• Hydraulic power𝑃𝑃𝐻𝐻 is 𝑄𝑄important∆ to evaluate the energy consumption of a cooling system
Ex: cost of cooling power consumption over 25 yrs of a wind turbine generator
12 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. Laminar (up) and turbulent (down) rotating flow . visualizations at Re=900 and Re=5000 (Bauduin, = 2014)
𝐷𝐷 𝑢𝑢 𝐷𝐷 • The Nusselt number is for convective𝑅𝑅𝑒𝑒 heat transfer. In the scientific 𝜈𝜈 literature most of the convection correlations have the form: = . . 𝛼𝛼 = 𝑁𝑁𝑁𝑁 𝛽𝛽 𝑅𝑅𝑒𝑒 ℎ 𝐷𝐷 𝑁𝑁𝑢𝑢𝐷𝐷 • Pressure drop coefficient is given by: 𝜆𝜆= 1 2 • Friction factor in a channel of diameter𝜅𝜅 DΔ and𝑃𝑃�2 𝜌𝜌length𝑢𝑢 L is given by: = . For laminar flow, given by an analytical expression: = / . For 𝑓𝑓 turbulent𝐿𝐿 flow, the Moody chart must be used. 𝐷𝐷 𝜅𝜅𝑓𝑓 𝒇𝒇 𝟔𝟔𝟔𝟔 𝐑𝐑𝐑𝐑 Moody chart for friction factor
13 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 Losses in an 4 poles IM at 50Hz (Yang, 2016) • 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 IPM machine (Yang, 2016)
14 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: Losses in an 4 poles IM at 50Hz (Yang, 2016)
rms phase current
= . + + ( ) Phase number 2 𝑃𝑃𝐽𝐽 𝑚𝑚 𝐼𝐼𝑝𝑝 𝑅𝑅𝐷𝐷𝐷𝐷 𝑅𝑅𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑓𝑓 𝑅𝑅𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑓𝑓 DC, Skin and proximity components of phase Losses in an IPM machine (Yang, 2016) resistance 15 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:
Losses in an 4 poles IM at 50Hz (Yang, 2016)
Flux density Harmonic rank
= , + 1 6 2 2 𝑐𝑐 ℎ 𝑛𝑛 𝑒𝑒 𝑛𝑛 𝑃𝑃 � 𝐾𝐾 𝑛𝑛𝐵𝐵 𝑛𝑛𝑛𝑛 𝐾𝐾 𝑛𝑛𝐵𝐵 𝑛𝑛𝑛𝑛Frequency Hysteresis𝑛𝑛 coeff. Eddy losses coeff. Losses in an IPM machine (Yang, 2016)
16 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)
Losses in an 4 poles IM at 50Hz (Yang, Width 2016) Volume Flux density = 24 2 𝑚𝑚 𝑚𝑚 2 2 2 𝑚𝑚 𝑉𝑉 𝑊𝑊 𝑛𝑛 Harmonic id 𝑃𝑃 𝑚𝑚 � 𝐵𝐵 𝜔𝜔 𝑛𝑛 𝜌𝜌 𝑛𝑛 Frequency Resistivity
Losses in an IPM machine (Yang, 2016)
17 Heat Sources in a Machine: Mechanical losses Bearings 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: = 3 4 𝑃𝑃𝑓𝑓𝑎𝑎𝑎𝑎𝑎𝑎 𝑐𝑐𝑓𝑓𝜋𝜋𝜌𝜌𝑎𝑎𝑎𝑎𝑎𝑎𝜔𝜔 𝑅𝑅 𝐿𝐿 Overall losses (Pd) and friction losses due to air (Pfair) in a 100W, 500k rpm PM machine (Luomi, 2009) 18 COOLING ARCHITECTURES OF ELECTRIC MACHINES
• Overview of the different cooling topologies
• Tips for designing a cooling system
19 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 arrangement Primary coolant Primary circuit Secondary Secondary circuit 0: open circuit A: Air (omitted) 0: Free convection coolant 0: Free convection 4: Frame cooled W: Water 1: Self circulation A: Air (omitted) 1: Self circulation 8: Heat Exchanger U: Oil 6: Independent W: Water 6: Independent system on U: Oil system on machine machine 7: Separate 7: Separate component component 8: Relative 8: Relative displacement displacement
20 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…)
Delphi Alternator • 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
Air flow in a Valeo Starter-Generator (Jandaud, 2013)
21 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) 22 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) 23 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 APM 120R motor for racing cars using oil cooling through ducts (Equipmake) • High pumping power needed for the system
Water jackets topologies (Satrústegui, Water ducts inside a stator (Kim, 2017) Porsche Carrera motor using a water 2017) jacket 24 Cooling architectures: Other cooling devices Oil 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 Schematics of spray cooling used by Renault (Davin, 2017) • For high-end applications (expensive) but very effective and reliable • Aerospace and automotive applications
Tesla Rotor cooling with heat pipes (Putra, 2017)
Heat pipes stator cooling (Putra, 2017) 25 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)
Cooling Current Cooling Complexity Energy System density efficiency cost Cooling technologies depending on cooling A/mm² target (Yang, 2015) Free 1.5 – 5 Low Simple None convection Forced 5 – 10 Medium Medium Low convection Liquid 10 – 30 High Complex High cooling 26 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: Solid temperature: what we want to minimize
= . ( )
Total Losses of the machine 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 Temperature of the coolant (W) Φ Convectiveℎ 𝑆𝑆 𝑇𝑇 − 𝑇𝑇 Conductance
Better EM design to Better heat exchanger to reduce losses reduce coolant : convective S: exchange surface temperature HTC ℎ
Increase fluid Add fins, add new cooling velocity, change paths (ventilation ducts) to nature of coolant to increase exchange increase surface
ℎ
27 Design of a cooling system: Design objectives Good 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: . • = . What are the losses generated by my machine? The location of the heat Q ΔP sources is important 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝜂𝜂𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝜂𝜂𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 • 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
28 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)
29 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 0D Simulation solving the Heat Parameters Equation using electrical analogy
2D/3D FEM Conductive simulation, FEM Simulation analytical/empirical boundary conditions Complexity
2D/3D Fluid and solid parts are fully solved CFD Simulation No correlations or empirical data used
30 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: . = = . 𝜆𝜆 𝑆𝑆 • Two equations, one𝐺𝐺𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 for unsteady𝐿𝐿 the other for𝐺𝐺𝑐𝑐𝑐𝑐𝑐𝑐 steady𝑐𝑐 ℎ state:𝑆𝑆
Unsteady: + . = Steady: . = Results of a thermal network on a Jeumont Electric machine (Bornschlegell, 2013) 𝑑𝑑𝑑𝑑 • Steady state: a simple𝐶𝐶 𝑑𝑑𝑑𝑑 𝐺𝐺linear𝑇𝑇 𝑃𝑃system to solve 𝐺𝐺 𝑇𝑇 𝑃𝑃 • 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) 31 Thermal FEM Simulation
• Only for conductive heat transfer in solid parts of the machine
• Solves the heat equation using Finite Element Method:
= . 𝜕𝜕𝑇𝑇 𝜆𝜆 − 𝛻𝛻훻 𝜑𝜑𝑣𝑣 • It can be both solved in steady𝜕𝜕𝑡𝑡 and𝜌𝜌 unsteady𝑐𝑐𝑝𝑝 states
• Advantages: detecting potential hot spots as the solution is local – easily Results of a 2D thermal FEM simulation in a coupled with electromagnetic FEM calculations BPMSM (Yang, 2016)
• 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 3D thermal FEM simulation of a stator (Kim, 2017) 32 Full CFD simulation
• In a CFD simulation, heat equation is solved (like FEM) and the Navier-Stokes equation is added: 1 + . . = + 𝜕𝜕𝒖𝒖 𝒖𝒖 𝛁𝛁 𝒖𝒖 − 𝛻𝛻𝑝𝑝 𝜈𝜈Δ𝒖𝒖 • Equation usually solved𝜕𝜕𝑡𝑡 in steady state𝜌𝜌 as computation cost would be too high for unsteady Velocity field and contours of heat flux dissipated on • Turbulent flow must be modelled, the most common technique is to a machine with external cooling (Boglietti, 2009) 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
Surface mesh and velocity contours in a Valeo Starter-Generator (Jandaud, 2013) 33 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) 34 Anisotropy of materials in simulations Windings 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 Different types of windings arrangement (hairpin, round wires, Litz wires) in slots (Liu, • Axial conductivity < tangential and radial conductivities 2017)
35 Steady vs Unsteady simulations
• Most of the simulation techniques are done using steady state analysis Material (J/kg/K)
• 𝒑𝒑 Time to reach thermal steady state >> electromagnetic steady state Air 𝒄𝒄 1006
• For a large machine (ie. wind turbine generator), steady state can be reached Aluminum 890 in ~10 hours Copper 385
• For non constant load (car motors, wind turbines), steady state analysis is not Epoxy Resins 1000 enough Plastics 800-1200
• Unsteady calculations need a lot of resources, CFD is often not an option Steel 460
• For LPTN, unsteady equation is + . = Water 4181 𝑑𝑑𝑑𝑑 • The capacitance matrix ( ) is very 𝑑𝑑important𝑑𝑑 for short time heat transfer, but it Thermal capacities values for common 𝐶𝐶 𝐺𝐺 𝑇𝑇 𝑃𝑃 materials at 20°C is not easy to obtain from supplier datasheet or tests 𝐶𝐶 • Experimental validation is needed
36 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 Same level of accuracy for a fine LPTN and • Experimental validation is always important for any type of simulation a 2D FEM simulation (from MANATEE software, www.manatee-software.com)
37 Overview of existing software Examples 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)
38 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 mechanical design 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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