L10 Thermal Design. EIEN20 Design of Electrical Machines, IEA, Forced cooling. Heat dissipation. Previously.

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1 L Thermal Design Forced cooling Heat dissipation Life span of e electrical machines Cooling channel 36 mm long mm wide.5 mm parallel plates Convection - W/m K Empiric vs FEM Flow rate Previous - m 3 /min Temperature D FEM conjugate heat transfer P/Q=constant for out = o C vo R Design of Electrical Machines Energy loss in electric circuits q e =ρj Energy loss in mechanic circuits q ω = Previously Energy loss in magnetic circuits q Φ =C h B f+c e (Bf) vo R Design of Electrical Machines 3 Conduction cooling Radiative cooling Next Convection cooling Thermal circuit and ermal design Heat sources, heat sinks and heat flow. vo R Design of Electrical Machines 4 EIEN Design of Electrical Machines, IE, 6

2 L Thermal Design Content Equivalent circuit relations Heat transfer (transport) vs heat and mass transfer Conduction Convection and advection Radiation Temperature distribution and limitations Insulation systems and realisations Thermal design Coolant and cooling ducts Conduction vs convection Relation Potential Flow Conductive element Ohm s Law Electrical circuit U=E l I=J G=γ /l U=I R Magnetic circuit N I=H l Φ=B G=μ /l N I=Φ R Thermal circuit =G l Q=q G=λ /l =Q R Cooling circuit P= l Q=v G= /l P=Q R vo R Design of Electrical Machines 5 vo R Design of Electrical Machines 6 λ l Thermal conductivity Q x Q l Q l Conduction is heat transfer by diffusion in a stationary medium due to a temperature gradient. The medium can be a solid, a liquid or gas Diffusion rough e substance Thermal conductivity Material λ [W/mK] Material λ [W/mK] ir Cast iron -46 Nomex. Stainless-steel 5-3 Kapton. Laminated iron - Mica.4-.6 Copper 3 Bonding epoxy.64 luminum - vg.ins.system. NdFeB 9 SmCo vo R Design of Electrical Machines 7 vo R Design of Electrical Machines EIEN Design of Electrical Machines, IE, 6

3 L Thermal Design hot α l α Q Convection x Q Q q n n k h Convection is heat transfer between eier a hot surface and a cold moving fluid or a cold surface and a hot moving fluid. Convection occurs in liquids and gases Movement of e substance l hot Ph Q c W 55 K m P 7.v loss cool Transport of heat..7 kw.5 m Q - e required flow rate, m 3 /s, P h - required cooling power, W, ρ - e density of e heat carrier, kg/m 3, c - e specific heat capacity, J/kg C, Δ - e temperature difference between incoming and outgoing temperature C Natural convection Forced cooled plane surface by air speed v Empirical cooling capability vo R Design of Electrical Machines 9 vo R Design of Electrical Machines High Performance Cooling Conjugate heat transfer Electronics-cooling.com Spray and jet cooling, continuous and fluctuating Single-phase and two-phase flows, phase changing materials Micro and minicahnnels, higher intensity cooling in Lh L P cool cq out Pheat hcool win Flow rate Q [L/min] Flow speed v=q/ [m/s] Re=inertia force / viscous force dcool dcond Heat transfer and pressure drop in e cooling channel is determined by flow Flow characterisation Development: laminar, unstable or transitional or turbulent entrance leng, boundary layer Dimensionless quantities Reynolds number characterizes e flow and Mach number illustrates e compressibility of e flow. vo R Design of Electrical Machines vo R Design of Electrical Machines EIEN Design of Electrical Machines, IE, 6 3

4 L Thermal Design Estimation of heat transfer Coolant The character of flow is described by Reinolds number, Re vdh Q e heat transfer is expressed by Nusselt number h qdh Nu Dh k k in wall D h bulk and e coolant is described by Prandtl number Pr c p k The hydraulic diameter is related to e geometric layout of e cooling channel area D h 4 perimeter, degc λ,mw/mk, upas 6 ir H 7 C H Ideal coolant = high ermal capacity & low viscosity Hydrogen is used in large turbo generators bility to store and carry heat = mass density times specific heat capacity reduces wi temperature Coolant steam Tr Oil c, kj/kgk , kg/m vo R Design of Electrical Machines 3 vo R Design of Electrical Machines 4 hot α l α Q Radiation x Q c rad Radiation is heat transfer between cooling surface at temperature and ience at temperature via electromagnetic waves rad c rad hot α l Transient heat flow α Q P Q S Q D Steady state temperature Heating time constant Temperature rise during e transient heating x Q P Q P C S Q D d dt R d P V c dt P R m C R P V c t m e vo R Design of Electrical Machines 5 vo R Design of Electrical Machines 6 EIEN Design of Electrical Machines, IE, 6 4

5 L Thermal Design Transient heat flow Heat transfer problem formulation for electrical devices - machines C R P d P dt C R C av Thermal model representing a physical model Maematical formulation Many simplifications and approximations Heat is not internally generated in e body Losses are applied to specific node-point Heat sources and sinks Temperature distribution and limits vo R Design of Electrical Machines 7 vo R Design of Electrical Machines Design target - Thermal limits Temperature dependence The most critical component in e electrical machine is insulation and temperature dependent is magnet. Insulation lifetime is shortened radically if temperature exceeds e limit and at is due to accelerated oxidation process in e insulation material. Δ= K -> ½ lifetime Materials temperature dependence is taken account wi material ermal coefficients coil coil coil coil coil BRmagn B R magn Brmagn magn magn H Cmagn H C magn Hcmagn magn magn vo R Design of Electrical Machines 9 vo R Design of Electrical Machines EIEN Design of Electrical Machines, IE, 6 5

6 L Thermal Design Evaluation of ermal loading Machine slots Heat transfer Input: heat sources and cooling conditions Outcome: temperature distribution Computational tools nalytic, empiric, numeric FE, CFD, lumped circuits for heat transfer and fluid flow Material characterization Sub-model validation Total conductor area 5.7 mm insulated slot area 5.4 mm Specific conductor losses 4 W/mm 3 reduced for winding.77 W/mm 3 Slot impregnation. W/mK selected equivalent ermal conductivity.4 W/mK vo R Design of Electrical Machines vo R Design of Electrical Machines Complexity Thermal design Electrical machine is complex 3D electromagnetic structure complex spatial fluid dynamic structure wi cooling medium In order to determine e temperature distribution good estimate of losses has to be known Properties of e cooling process has to be known The ermal characteristics and properties has to be known n optimized ermal design can help increase machine rated power substantially Good estimate of losses e spatial and temporal distribution of heat sources Waveform of a loss origin Distribution of heat sources Duty cycle operational cycle time often much shorter an ermal time constant Short time operation Intermittent Thermal characteristics of materials Temperature dependence Temperature limits Heat dissipation ermal circuit and cooling system Thermal efficiency Cooling conditions (normal, forced) Maximum allowed loading according to e ermal limits at cooling capability vo R Design of Electrical Machines 3 vo R Design of Electrical Machines 4 EIEN Design of Electrical Machines, IE, 6 6

7 L Thermal Design Heat transfer Thermal circuit at steady state Steady state and transient Heat transfer problem according to temperature (potential) and heat balance between source, sink and storage x y z Q x y z x y z Q c p x y z t heat transfer convectiondiffusion equation c p t k c u Q incompressible Navier- Stokes equations for fluid dynamics u u u p u F t u vo R Design of Electrical Machines 5 p cooling heating Node points i, Q i [W], i [K] 5. Coil loss and temperature 4. Too loss and temperature 6. Yoke loss and temperature 7.. mbience temperature Thermal conductivity elements G ij [W/K] From coil to too G 54 From coil to yoke G 56 From too to yoke G 46 From yoke to ience G 67 vo R Design of Electrical Machines 6 Equivalent circuit Thermal modelling example I Determine heat sources in regions Specify cooling conditions over cooling surfaces Find heat balance i.e. temperature distribution vo R Design of Electrical Machines 7 vo R Design of Electrical Machines EIEN Design of Electrical Machines, IE, 6 7

8 L Thermal Design Thermal circuit ermal contacts Thermal circuit heat carrier bad electric conductor is usually also a bad ermal conductor No air-gaps in electrical circuit, many air-gaps in ermal circuit Thermal contact between stator core and housing. mm +5K. mm +K Experience from 3 good electric conductor is usually also a good ermal conductor Interested in hotspots: % conductor in e middle of winding Heat is taken from endwindings: conduction, convection or bo vo R Design of Electrical Machines 9 vo R Design of Electrical Machines 3 Thermal model Model development k sa k sa k sa k k wsa ws k ms k k ws mws k mw k ms surf k Qwin mw win k Qpm mw pm pm win surf Geometry of a PMSM Material & ermal loading Winding Permanent magnets Surface & cooling Natural convection Temperature nodes Nodes of interest Thermal circuits Heat transfer raer an flow network Thermal resistances Focus on ermal air-gaps Sorces and loads Conductor losses Convection cooling D heat transfer pproximate rating Extraction of elements 3D heat transfer Extrucion from D Focus on end turns Heat exchange rough endturns Thermal conduction vo R Design of Electrical Machines 3 vo R Design of Electrical Machines 3 EIEN Design of Electrical Machines, IE, 6

9 L Thermal Design Thermal modelling example II Multi-physics FEM Calculating flux (and current) density waveform Estimating losses densities in e symmetric part of machine Calculating temperature distribution according to heat sources and sinks u 6 y u 5 N 3(x 3,y 3 ) 3 u 4 u N (x,y ) u N (x,y ) x J x, y, z, t p cu B x, y, z, t p fe u 3 Different problems in physics share e same geometry Calculate for a single element The variation of loss origin RMS power loss MEN temperature field equation is solved for e finite size of volume boundaries suppose to specify a potential (essential), flow naturally given. vo R Design of Electrical Machines 33 vo R Design of Electrical Machines 34 Thermal modelling example III Thermal design Cooling power, p=c p Q( out - in ) [W] Directly cooled laminated windings Peak heat sources J m =.3. /mm p=. 6.6 W/cm 3 P=.9 kw Thermal management Limit winding, wall and outlet temperature L/min =.5 m/s per div FEM heat transfer Contribution from conduction and natural convection wall temperature, out [C] vo R Design of Electrical Machines 35 vo R Design of Electrical Machines 36 EIEN Design of Electrical Machines, IE, 6 9

10 L Thermal Design Mapping operation points Heat transfer analysis Cooling power, p=c p Q( out - in ) [W] Driving parameters for cooling P=f( out,q) at in Flow (Re) and coolant (Pr) characterization Heat transfer correlations (Nu) and coefficient h Wall and winding temperature Pressure across cooling channel Power for supply Expected cooling power P=f( w,q) at in Reynolds number, Re=d h Q/() [-] h across boundary, P cool /(h cool ) [C] p out - in ) [W] Nusselts number, Nu=f(Re,Pr) [-] Heat transfer coefficient, h=nu k/d [W/(m K)] Temperature Pressure drop, dp [Pa] Ideal cooling supply power, dpq [-] Cooling power, p=c Q( outlet temperature, out [C] outlet temperature, out [C].4 outlet temperature, out [C] Ideal coil geometry and cooling conditions non cooled spots overheated terminal leads & small cross-section layers close to e air-gap outlet temperature, out [C] outlet temperature, out [C] outlet temperature, out [C] outlet temperature, out [C] wall temperature, out [C] 5 5 cooling intensity -- flow rate -- control over hot-spot temperatures vo R Design of Electrical Machines 3 vo R Design of Electrical Machines 37 Parallel plates, laminar flow, Defining designing cooling channels Narrow cooling channels allow higher surface speed, us higher cooling capability for e same flow rate Driving parameters for cooling P=f( out,q) at in Flow (Re) and coolant (Pr) characterization Heat transfer correlations (Nu) and coefficient h Wall and winding temperature Pressure across cooling channel Power for supply Expected cooling power P=f( w,q) at in cooling power, p=c p Q( out - in ) [W] Reynolds number, Re=d h Q/() [-] Narrow channels results higher pressure drop and is difficult to secure in production 7kW@ o C&4m 3 /min h Nusselts number, Nu=f(Re,Pr) [-]. heat transfer coefficient, h=nu k/d [W/(m K)].6. temperature drop across boundary layer, P cool /(h cool ) [K)].4.6. pressure drop, dp [Pa] ideal cooling supply power, dpq [-] winding temperature, T w =T out +P cool /(h cool ) [C] outlet temperature, out [C] 9 7 Lack of cooling (flow leakage) results high risk for overheating Slide sow: L from 5 mm to m/s 7 5 L=5 mm c=. mm L=5 mm c=.4 mm L= mm c=.6 mm L= mm c=. mm vo R Design of Electrical Machines vo R Design of Electrical Machines 39 EIEN Design of Electrical Machines, IE, 6

11 L Thermal Design Thermal circuit cooling circuits Cooling Concepts Natural and Forced Integrated cooling as a result of machine integrated construction Slotted stator operates as a cooling circuit Directly cooled heat sources Cooling ducts, cooling jackets, cooling channels Cooling capability Maximize e cooling surface area Improve cooling medium parameters and velocity Smallest temperature rise is e goal when designing a ermal circuit Structure Where e energy conversion, heat transfer and temperature drop (Δ) takes place Heat sources Energy converted to heat Cooling sources Heat dissipation Cooling concepts arrangement of heating and cooling sources Indirect Cooling (high Δ) Direct cooling (low Δ) vo R Design of Electrical Machines 4 vo R Design of Electrical Machines 4 Summary Thermal constrains and dependences Thermal circuits, heat sources and cooling options Heat transfer model and modelling Learning skills from e assignments vo R Design of Electrical Machines 43 EIEN Design of Electrical Machines, IE, 6