ICEM 2012 - TUT3 - Improving Motor Effiency and Motor Miniaturisation - The Role of Thermal Simulation

7/28/2019 ICEM 2012 - TUT3 - Improving Motor Effiency and Motor Miniaturisation - The Role of Thermal Simulation

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Improving Motor

Efficiency and MotorMiniaturisationThe Role of Thermal

SimulationDr David Staton

Motor Design [email protected]


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Todays Topics

Motor Design Ltd (MDL)Need for improved tools for thermal analysis of electric machines

Important Issues in Thermal Analysis of ElectricMotors

Examples of use of thermal analysis to optimiseelectric machine designs


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Motor Design LtdBased in Ellesmere, Shropshire,UK

On England/Wales borderSouth of Chester and Liverpool

MDL Team:Dave Staton (Software Development & Consultancy)

Mircea Popescu (Consultancy)Douglas Hawkins (Software Development & Consultancy)

Gyula Vainel (Motor Design Engineer)Lyndon Evans (Software Development)James Goss (EngD Motor-LAB)Lilo Bluhm (Office Manager)

Many University Links:Sponsor 3 Students in UK at presentMany links with universitiesthroughout world

Bristol, City, Edinburgh, MondragonSheffield, Torino,


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Dave StatonApprentice/Electrician - Coal Mining Industry (1977 - 1984)

BSc in Electrical Engineering at Trent Polytechnic (Nottingham)PhD at University of Sheffield (1984 - 1988)

CAD of Permanent Magnet DC Motors (with GEC small machines) 95% electromagnetic aspects and less than 5% on thermal aspects

Design Engineer - Thorn-EMI CRL (1988-1989) design of electric motors for Kenwood range of food processors

Research Fellow - SPEED Laboratories (1989 - 1995) help develop SPEED electric motor design software predominantly on electromagnetic aspects

Control Techniques (part of Emerson Electric) (1995 - 1998) design of servo motors More involved in thermal analysis as we were developing radically new motor

constructions (segmented laminations) that we had no previous experience.Set up Motor Design Ltd in 1998 to develop heat transfersoftware for electric machine simulation

there was no commercial software for thermal analysis of motors such analysis was becoming more important in the design process

(volume/weight minimisation, energy efficiency, etc.)Given many courses on motor design and thermalanalysis of electric machines worldwide


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Motor Design Ltd (MDL)

set up in 1998 to develop software for designof electric motors and provide motor designconsulting and training

distributeSPEED

,Motor-CAD

,FLUX

andPORTUNUS software

complete package for electric motor and drive

simulationsoftware package also used in our consultingwork which helps with development


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Todays TopicsMotor Design Ltd (MDL)

Need for improved tools for thermal analysis of electric machines


Examples of use of thermal analysis to optimiseelectric machine designs


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Thermal and Electromagnetic DesignTraditionally in electric motors design the thermal

design has received much less attention than theelectromagnetic design electric motor designers tend to have an electrical

engineering background rather than mechanical?

CAD tools for thermal design have tended to be very

specialized and require extensive knowledge of heat transferMDL have developed Motor-CAD + Portunus thermal/flowlibrary to make it easier to carry out thermal analysiswithout being a thermal expert

FEA and CFD software are starting to have featuresincluded to make it easier to set up electric motorthermal models

There is a similar situation in electronics cooling Electronics designers tend to have an electronics

background with little knowledge of heat transfer


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Temperature of motor is ultimate limit on performance andshould be given equal importance to the electromagnetic design

Life of motor is dependent upon the winding temperature

1M

100K

10K

1K

100

A v e . e x p e c

t e d l i f e [ h o u r s

]

ClassA

Class B

Class H

Class F

Total winding temp. [deg C]60 120 240180

Need for thermal analysis of electric motors


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There is a requirement for smaller, cheaper and more efficient motors soan optimised design is required Losses depend on temperature and temperature on losses

Simple sizing method based on such inputs as limiting winding currentdensity are no good for optimisation (see next few slides)

depend on users experience for the manufacturing process andmaterial used and so tend to become very inaccurate when tryingsomething new in the design

Give the designer no indication of where to concentrate design effortto reduce temperature

Mix of analytical and numerical methods required Many applications do not have a steady-state operation so in order to obtain

a reasonable transient calculation time lumped circuit techniques arerequired

CFD/FEA are useful to help set up accurate models

Part models best. e.g. heat transfer through winding, fan model, etc.

Need for thermal analysis of electric motors


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Traditional Motor Sizing Methodssizing based on single parameter

thermal resistance housing heat transfer coefficient winding current density specific electric loading

thermal data from simple rules of thumb

5 A/mm2, 12 W/m2/C etc. tests on existing motors competitor catalogue data

can be inaccurate single parameter fails to describe

complex nature of motor cooling

poor insight of where toconcentrate design effortAlternatives are analytical lumpedcircuit thermal analysis andnumerical thermal analysis

h [W/(m 2.oC)]

Twinding Tambient

R TH [oC/W]P [W]

Thermal Resistance

Heat Transfer Coefficient


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Typical Rules of Thumb Air Natural Convection h = 5-10 W/(m 2 .C)

Air Forced Convection h =10-300 W/(m 2 .C)

Liquid Forced Convection h = 50-20000 W/(m 2 .C )

Wide range of possible values makes past experience very important Otherwise the design will not be correctly sized Values may not be valid if change manufacturing process, material, etc. Tables taken from: SPEED Electric Motors, TJ E Miller


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Numerical Thermal AnalysisTwo basic types available that subdivide problem into small element orvolumes and temp/flow solved:

Finite element analysis (FEA)Useful to accurately calculate conduction heat transfer

Computational fluid dynamics (CFD)Simulation of fluid flow involves the solution of a set of coupled, non-linear, second order, partial differential equations conservationequations (velocity, pressure & temperature)

Many of the Fluent CFD slides are from theExample from University of Nottingham

FEA

CFD


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Motor-CAD Integrated Thermal FEA Solver Just a few seconds to generate a mesh and calculate Help improve accuracy through calibration of analytical model


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Computational Fluid Dynamics (CFD)The expected accuracy is notas great as withelectromagnetic FEA

Due to complexities of geometryand turbulent fluid flow

Impossible to model actualgeometry perfectly

But trend predictions andvisualisation of flow are useful

Can be very time consumingto construct a model andthen calculate

Can be several weeks/months

Best use of results tocalibrate analyticalformulations


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Motor-CAD Software Analytical network analysis package

dedicated to thermal analysis of electric motors and generators input geometry using dedicated editors select cooling type, materials,

etc and calculate steady state ortransient temperatures

all difficult heat transfer datacalculated automatically

easy to use by non heat

transfer specialists provides a detailed

understanding of cooling andfacilitates optimisation

what-if & sensitivity analysis


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Motor-CAD Motor Types

Brushless Permanent MagnetInner and outer rotorInduction/Asynchronous

1 and 3 phase

Switched reluctancePermanent magnet DCWound Field SynchronousClaw pole


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Cooling TypesMotor-CAD includes proven models for anextensive range of cooling types

Natural Convection (TENV)many housing design types Forced Convection (TEFC)

many fin channel design types Through Ventilation

rotor and stator cooling ducts Open end-shield cooling Water J ackets

many design types (axial and circumferential ducts)stator and rotor water jackets

Submersible cooling Wet Rotor & Wet Stator cooling

Spray Cooling Direct conductor coolingSlot water jacket

ConductionInternal conduction and the effects of mounting

RadiationInternal and external


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Housing Types

Many housing designs can be modeled and optimized the designer selected a housing type that is appropriate for the cooling

type to be used and then optimizes the dimensions, e.g. axial findimensions and spacing for a TEFC machine


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Steady-State & Transient Analysis

Some application are steady state and some

operate with complex transient duty cycle loads


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Thermal Lumped Circuit Modelssimilar to electrical network so

easy to understand by electricalengineers thermal resistances rather

than electrical resistances power sources rather than

current sources thermal capacitances rather

than electrical capacitors(not shown here)

nodal temperatures ratherthan voltages

power flow throughresistances rather thancurrent

Place nodes at importantplaces in motor cross-section


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Thermal Lumped Circuit Modelsthermal resistances placed in the circuit to model

heat transfer paths in the machine conduction (R = L/kA)

path area (A) and length (L) from geometrythermal conductivity (k) of material

convection (R = 1/hA)heat transfer coefficient (h W/m 2 .C) from empiricaldimensionless analysis formulations (correlations)

many well proven correlations for all kinds of geometry inheat transfer technical literature

radiation (R = 1/hA)h = 1 F1-2 (T 1 4 T 2 4 )/ (T 1 T 2 )emissivity ( 1 ) & view factor (F 1-2 ) from surface finish & geometry

power input at nodes where losses occurthermal capacitances for transient analysis

Capacitance = Weight Specific Heat Capacity


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Heat Transfer Ohms Law:In a thermal network the heat flow is given by:

T VP = electrical circuit I =R R

P = power [Watts]T = temperature difference [C]

R = thermal resistance [C/W]

PR

T

Fluid Temperature Rise:Power Dissipated

T = Volume Flow Rate x Density x C pT = temperature difference [C]Cp = Specific Heat Capacity [J/kg C]


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Examples of use of thermal analysis to optimise

electric machine designs


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Important Issues in Thermal Analysisof Electric Motors

Conduction, Convection and Radiation

Losses

Winding Heat Transfer

Interface Thermal Resistance

Accuracy and Calibration


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Conduction Heat Transfer Heat transfer mode in a solid due to molecule vibrationGood electrical conductors are also good thermal conductors

Would like good electrical insulators that are good thermal conductorsmaterial research to try to achieve this Metals have large thermal conductivities due to their well ordered

crystalline structurek is usually in the range of 15 400 W/m/C

Solid insulators not well ordered crystalline structure and are oftenporous

k is typically in the range of 0.1 1W/m/C (better than air with k 0.026W/m/C)

Conduction thermal resistance calculated using R = L/kA Path length (L) and area (A) from geometry, e.g. tooth width and area Thermal conductivity (k) of material, e.g. that of electrical steel for

toothOnly complexity is in the calculation of effective L, A and k forcomposite components such as winding, bearings, etc.

Motor-CAD benefits from research using numerical analysis and testing todevelop reliable models for such complicated components


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Convection Heat Transfer Heat transfer mode between a surface and a fluid due tointermingling of the fluid immediately adjacent to the surface

(conduction here) with the remainder of the fluid due to fluid motion Natural Convection fluid motion due to buoyancy forces arisingfrom change in density of fluid in vicinity of the surface

Forced Convection fluid motion due to external force (fan andpump)

Two types of flow

Laminar Flow, streamlined flow at lower velocities Turbulent Flow, eddies at higher velocities

Enhanced heat transfer compared to laminar flow but a largerpressure drop

Convection thermal resistance is calculated using:

RC = 1/ (A h) [C/W]A = surface area [m 2 ] h = convection heat transfer coefficient [W/m 2 /C]

Need to predict h rule thumb, dimensionless analysis, CFD?


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Convection Heat Transfer Coefficienth C can be calculated using empirical correlations based ondimensionless numbers (Re, Gr, Pr, Nu)

Just find a correlation with a similar geometry and coolingtype to that being studied

Cylinder, flat plate, open/enclosed channel, etc.

Dimensionless numbers allow the same formulation to be usedwith different fluids and dimensions to those used in theoriginal experiments

Hundreds of correlations available in the technical literatureallowing engineers to carry out the thermal analysis of almostany shape of apparatus

Motor-CAD automatically selects the most appropriatecorrelation that matches the cooling type and surfaceshape


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Convection Analysis Dimensionless Numbers

The dimensionless numbers are functions of fluid properties,size (characteristic length), fluid velocity (forced convection),temperature (natural convection) and gravity (naturalconvection)

v = fluid velocity [m/s]

= surface to fluid temperature [C]L = characteristic length of the surface [m] = coefficient of cubical expansion of fluid [1/C]g = acceleration due to gravity [m/s2]

Reynolds number (R e ) Grashof number (G

r)

Prandtl number (P r) Nusselt number (N

u)

h = heat transfer coefficient [W/m 2C]

= fluid dynamic viscosity [kg/s m] = fluid density [kg/m 3]k = thermal conductivity of the fluid [W/mC]cp = specific heat capacity of the fluid [kJ/kg.C]

R e = v L / Gr = g 2 L3 / 2Pr = c p / k N u = h L / k


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Natural and Forced Convection Correlations Natural convection is present over most surfaces

Present even over surfaces that are designed for forced convection

e.g. TEFC machine with axial fins (at low speed the fan will not providemuch forced air and natural convection will dominate)

Large set of correlations typically required for complex shapes For very complex shapes area averaged composite correlations are

used where the complex geometry (e.g. finned housing) is divided into aset of simpler shapes for which convection correlations are known

For instance a cylinder with axial fins is divided to various cylindricaland open fin channel sections of different orientation

A positive aspect is that if extremely small fins are attached to thecylinder the same results are given as for a cylindrical correlation

Forced convection is present over surfaces with fluid movement due to afan or pump (or movement of the device or wind)

A more limited set of correlations are required to calculate forcedconvection in electrical machines (Flat Plate, Open Fin Channel,Enclosed Channel, Rotating Airgap, End Space Cooling)


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General Form for Convection CorrelationsNatural Convection

General form of natural convection correlation :

Nu = a ( G rPr) b a & b are curve fitting constants

Transition from laminar to turbulent flow:

107 < GrPr < 109 (GrPr = Ra Rayleigh number)

Forced ConvectionGeneral form of convection correlation for forced convection:

Nu = a (Re) b (Pr) c a, b & c are curve fitting constants Internal flow laminar/turbulent transition R e 2300

(fully turbulent R e > 5 x 10 4 ) External flow laminar/turbulent transition R e 5 x 10 5


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Horizontal Cylinder (Natural Convection)

A formulation for average Nusselt number of a horizontalcylinder of diameter d:

Nu = 0.525 (G r Pr) 0.25 (10 4 < G rPr < 10 9 ) LaminarN

u= 0.129 (G

rP

r) 0.33 (10 9 < G

rP

r< 10 12 ) Turbulent

h = N u k / d

Fluid properties at mean film temperature (average of surfaceand bulk fluid temperatures)


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Vertical Cylinder (Natural Convection)

A formulation for average Nusselt numberof a vertical cylinder of height L:

Nu = 0.59 (G r Pr) 0.25 (10 4 < G rPr < 10 9 ) Laminar

Nu = 0.129 (G r Pr) 0.33 (10 9 < G rPr < 10 12 ) Turbulent

h = N u k / L

Fluid properties at mean film temperature(average of surface and bulk fluidtemperatures)


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Vertical Flat Plate (Nat Convection)

A formulation for average Nusseltnumber of a vertical flat plate of height L:

Nu = 0.59 (G r Pr) 0.25 (10 4 < G rPr < 10 9 ) Laminar

Nu = 0.129 (G r Pr) 0.33 (10 9 < G rPr < 10 12 ) Turbulent

h = Nu k / L

fluid properties at mean filmtemperature (average of surface andbulk fluid temperatures )


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Horizontal Flat Plate (Nat Conv)

Upper Face:Nu = 0.54 (G r Pr) 0.25

(10 5 < G rPr < 10 8) Laminar

Nu = 0.14 (G r Pr) 0.33

(10 8 < GrP

r) Turbulent

Lower Face:Nu = 0.25 (G r Pr) 0.25

(10 5 < G rPr < 10 8) Laminar

h = N u k / LFluid properties at mean film temperature(average of surface and bulk fluidtemperatures)

upper

lower


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Horizontal Servo Housing (Nat Conv)

Average of the following:

Horizontal Cylinder Corner Cutout [%]/100 Horizontal Square Tube {1 - Corner Cutout [%]/100}

As corner cut-out approaches 100% then the cylindercorrelation predominates and as it approaches 0% the tubecorrelation predominates


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Vertical Fin Channel (N Conv)Ref [1] gives a formulation for Nusselt number of u-shaped vertical channels (laminar flow):

h = N u k / ra = channel aspect ratio fin_spacing/fin_depthr = Characteristic Length (fin hydraulic radius)

= 2 fin_depth fin_spacing /(2 fin_depth + fin_spacing)L = fin height

The fluid properties are evaluated at the wall temperature (except volumetriccoefficient of expansion which is evaluated at the mean fluid temperature).

[1] Van De Pol, D.W. & Tierney, J.K. : Free Convection Nusselt Number for Vertical U-Shaped Channels, Trans. ASME, Nov. 1973.

( )( )( )

( )

0.75

33

3

0.51 exp

0.171 0.483 exp24

1 1 exp 0.83 9.141 2 exp 465 fin_spacin 0.61

r r

ur r

r G P LLN ZZ r G P

aZ

a aag

= =

+ +


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Horizontal FinChannel (Nat Conv)Ref [1] gives a formulation for Nusseltnumber of u-shaped horizontal channels(laminar flow):

h = N u (s) k / ss = fin spacing used as characteristic dimension

[1] Jones, C.D., Smith, L.F. : Optimum Arrangement of Rectangular Fins on HorizontalSurfaces for Free-Convection Heat Transfer, Trans. ASME, Feb 1970.

Horizontal Fin Channels

1.70.447640

( ) 0.00067 ( ) ( ) 1 exp ( )u r r r

r

N s G s P s PG s

=


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The axial finned motors above are designed for a shaft mounted fan (TEFC)When used as a variable speed drive at low speed there is little forcedconvection as natural convection dominates

Therefore we must be able to calculate such surfaces with natural convection

Use of composite calculations based on averages of all the different simpleshapes found in the more complex shape can give accurate results

Good agreement between Motor-CAD with default data and measureddata for a range of TEFC motors operating at zero speed (work carriedout by Boglietti at Politecnico di Torino) see graph above

Axial Finned Housing with Natural Convection


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Flat PlateForced Conv(External Flow)

Ref [1] gives a formulation for average Nusseltnumber of flat plate (or horizontal cylinder) length L:

N u = 0.664 ( R e ) 0.5 ( P r ) 0.33 ( R e < 5 x 10 5 )

N u = [0.037 ( R e ) 0.8 871] ( P r ) 0.33 ( R e > 5 x 10 5 )h = N u k / L

Fluid properties at mean film temperature (average of surfaceand bulk fluid temperatures)

[1] Incropera, F.P & DeWitt, D.P.: Introduction to Heat Transfer, Wiley, 1990.

flow


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Enclosed ChannelForced Convection

The following formulations are used to calculateh from enclosed channelsRe = Dh x Fluid Velocity / Kinematic ViscosityDh = Channel Hydraulic Diameter

Dh = 2 Gap [Concentric Cylinders]Dh = 4 Channel Cross Sectional Area / Channel Perimeter[Round/Rectangular Channels]

h = Nu (Fluid Thermal Conductivity) / (Dh)

The flow is assumed to be fully laminar when Re < 2300 inRound/Rectangular Channels and when Re < 2800 in ConcentricCylindersThe flow is assumed to be fully turbulent when Re > 3000 (in practicethe flow may not be fully turbulent until Re > 10000)A transition between laminar and turbulent flow is assumed for Revalues between those given above


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Enclosed Channel (Forced Conv)Laminar Flow

Concentric Cylinders (adaptation of formulation for parallel plateswhich includes entrance length effects):

The 2 nd term in the above equation is the entrance length correctionwhich accounts for entrance lengths where the velocity and temperatureprofiles are not fully developed .

Round Channels (which includes entrance length effects):

Rectangular Channels (adaptation of formulation for round channels):

where H/W = Channel Height/Width Ratio

23

7.54 0.03 1 0.016h hu e r e r D D

N R P R PL L

= + +

23

3.66 0.065 1 0.04h hu e r e r D D

N R P R PL L

= + +

( ) ( )2 3

23

7.49 17.02 22.43 9.94

0.065 1 0.04

u

h he r e r

H H HN W W W

D DR P R P

L L

= + + +


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Enclosed Channel (Forced Conv)Turbulent Flow

Calculated using Gnielinski's formula for fully developed turbulentflow,i.e., 3000 < R e < 1 10 6 :

Friction Factor and for a smooth wall is:f = [0.790 Ln( R

e) - 1.64] -2

Transition from Laminar to TurbulentFlow

N u is calculated for both laminar andturbulent flow using the aboveformulations. A weighted average (basedon R e ) is then used to calculate N u .

It is noted that h increases dramaticallyas the flow regime changes from beinglaminar to turbulent flow.

( ) ( ) ( ) ( )0.5 2 38 1000 1 12.7 8 1u e r r N f R P f P = +


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Fin Channel (Open) Forced ConvHeiles [1] calculates h for a forced cooled open fin channel:

h = Air Cp Air D Air Velocity / (4 L) x [1-exp(-m)]m = 0.1448 L0.946 / D 1.16 {k Air / ( Air Cp Air AirVelocity])} 0.214

D = Hydraulic Diameter = 4 x channel area / channel perimeter (including open side)L = Axial length of cooling fin

This assumes isothermal wall, laminar air flow with air propertiescalculated at the film temperature = (T free-stream + T wall )/2.Heiles recommends the use of a Turbulence Factor to directly multiplyh by his tests indicate typical turbulence factors in the range 1.7 -1.9 which seem independent of the flow velocity

[1] Heiles, F. : Design andArrangement of Cooling Fins,Elecktrotecknik undMaschinenbay, Vol. 69, No. 14,

J uly 1952 .


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Axial Fin Channel Air LeakageTypical form of open axial

channel air leakage shownComplex function of Cowling design

GapFin overhang

Fan design Speed Motor size Fin design Fin roughness Restrictions etc.


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Mixed ConvectionThe total heat transfer coefficient due to convection(h MIXED) is the combined free and forced convectionheat transfer coefficients.

h MIXED is calculated using:

The Motor Orientation determines the sign ( ):+ in assisting and transverse flows

in opposing flows

3 3 3MIXED FORCED NATURALh h h= +


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End Space CoolingComplex area of machine cooling due to flow around end-windingsTurbulent flow which depends upon:

Shape & length of end-windings Fanning effects of internal fans, end-rings, wafters/rotor wings,

etc. Surface finish of the end sections of the rotor

Several authors have studied such cooling and most propose the useof the following formulation:

h = k1 [1 + k2 velocity k3 ]


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Airgap Heat Transfer Airgap heat transfer is often calculated using aformulation developed from Taylors work onconcentric cylinders rotating relative to each other

Heat transfer by conduction when flow is laminar Increase in heat transfer when the airgap R e

number increases above a certain critical value at

which point the flow takes on a regular vortexpattern (vortex circular rotational eddy pattern) Above a higher critical R e value the flow becomes

turbulent and the heat transfer increases further(turbulent is more of a micro eddy flow)

Taylor, G.I.: 'Distribution of Velocity and Temperature betweenConcentric Cylinders', Proc Roy Soc, 1935, 159, PtA, pp 546-578


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Typical Air/Fluid Cooling Types

Blown Over

Wet Rotor

Water J acket

Through Ventilation


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Flow Network Analysis Separate topic not covered in this tutorial

where a set of analytically based flow

resistances are put together in a circuit topredict the pressure drops and flow Pressure drops due to duct wall friction and

restrictions to flow (bend, expansion,contraction, etc.).

f


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Radiation Heat Transfer The heat transfer mode from a surface due to energy transferby electromagnetic waves

Convection thermal resistance is calculated using:RR = 1/ (A h R) [C/W]

h R can be calculated using the formula:

F1-2 (T 1 4 T 2 4 )h R = [W/m 2 .C]T1 T 2

h R = radiation heat transfer coefficient [W/m 2 .C]A = area of radiating surface [m 2 ] = Stefan-Boltzmann constant (5.669x10 -8 W/m 2 /K 4 )T1 = absolute temperature of radiating surface [K]T2 = absolute temperature of surface radiated to (ambient) [K] = emissivity of radiating surface ( 1)F1-2 = view factor (F 1-2 1) calculated from geometry

Typical Emissivity ( ) Data


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Typical Emissivity ( ) DataMaterial Emissivity Material Emissivity

Aluminium Iron

black anodised 0.86 polished 0.07 0.38 polished 0.03 0.1 oxidised 0.31 0.61

heavily oxidized 0.20 0.30 Nickel 0.21

sandblasted 0.41 Paints

Alumina 0.20 0.50 white 0.80 0.95

Asbestos 0.96 grey 0.84 0.91

Carbon 0.77 0.84 black lacquer 0.96 0.98

Ceramic 0.58 Quartz, fused 0.93

Copper Rubber 0.94

polished 0.02 Silver, polished 0.02 0.03 heavily oxidized 0.78 Stainless Steel 0.07

Glass 0.95 Tin, bright 0.04

From the Stokes Research Institute


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Convection/Radiation Example Calculations

For the following conditions, calculate h for the abovehorizontal cylinder

Natural convection with air as the fluid Forced convection with 5m/s of air flowing axially over the cylinder Forced convection with 5m/s of water flowing axially over the

cylinder Radiation from cylindrical surface with emissivity of 0.9

Not realistic problem as we already know the temperature Usually calculate h to calculate temperature (non-linear system)

100mm

100mm

100C 20C


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Natural Convection (fluid = air)Laminar or turbulent flow? (Gr Pr), L= 0.1m

Gr = g 2 L3 / 2 Pr = c p / kAir properties at mean film temperature = (20 +100)/2 = 60C

k = 0.0287 W/m K = 1.06 kg/m 3 = 1.996 10 -5 kg/m scp = 1008 J/kg K

Gr = 1/(273 + 60) 9.81 (100 20) 1.062

0.13

/ (1.996

10-5

)2 =

6.65 10 6 Pr = 1008 1.996 10 -5 / 0.0287 = 0.701GrPr = 6.65 10 6 0.701 = 4.66 10 6

Horizontal Cylinder correlation with Laminar Flow from 10 4 < GrPr < 10 9

Nu = 0.525 (Gr Pr)0.25

(laminar flow)Nu = 0.525 (4.66 10 6 ) 0.25 = 24.39

Heat Transfer Coefficienth = N u k / L = 24.39 0.0287 / 0.1h = 7.0 W/m 2 /C

F d C i (fl id i )


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Forced Convection (fluid = air)Laminar or turbulent flow? (R e at 5m/s, L = 0.1)

Re = v L /

Air properties at mean film temperature = (20 +100)/2 = 60Ck = 0.0287 W/m C = 1.06 kg/m 3 = 1.996 10 -5 kg/m scp = 1008 J/kg C

Pr = c p / k = 1008 1.996

10-5

/ 0.0287 = 0.701Re = 1.06 5 0.1 / 1.996 10 -5 = 2.66 10 4

Flat Plate correlation (external flow) with Laminar Flow as Re 5 x 10 5

Nu = [0.037 (R e ) 0.8 871] (P r) 0.33 (R e > 5x10 5 )

Nu = [0.037 (1.05

106

)0.8

871] (3.02)0.33

= 2242Heat Transfer Coefficient

h = N u k / L = 2242 0.651 / 0.1h = 14595 W/m 2 C


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Radiation

Painted surface with emissivity = 0.9View factor = 1 as unblocked view of outside world by surfaceh R = 1 F1-2 (T 1 4 T 2 4 ) / (T 1 T 2 )h R = 5.669x10 -8 0.9 1 [(100+273) 4 (20+273) 4 ] / (100-20)

h R = 7.6 W/m 2 C In this case the radiation is larger than that for natural

convection (it was 7.0 W/m 2 C)

100mm

100mm

100C 20C

Electrical Losses


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Electrical LossesMost important that the losses and their distribution be known inorder to obtain a good prediction of the temperaturesthroughout the machine

Some losses are associated with the electromagnetic designCopper (can have high frequency proximity loss)Iron (difficult to calculate accurately due to limitations of steel manufacturers data)

Loss data from material manufacturer may not have the samesituation as real machine

Different waveformsStress of manufacture on laminationsDamage to interlamination insulation due to bursWork underway to try and produce better loss dataCalibration using tests on actual machine best if available

Magnet (circumferential and axial segmentation tominimize) Sm-Co 1-5 (5 x 10 -8 ohm-m), Sm-Co 2-17 (90 x 10 -8 ohm-m), Sintered Nd-Fe-B

(160 x 10 -8 ohm-m), Bonded Nd-Fe-B (14000 x 10 -8 ohm-m)


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Mechanical LossesSome losses are associated with the mechanicaldesign

friction (bearings)windage (air/liquid friction in gap/fan)

Losses used in thermal model may be calculatedand/or measured

Losses are inputs in a thermal model and are notdiscussed in much detail in this tutorial

L V i ti ith T t


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Loss Variation with TemperatureWe can model the variation in copper loss with temperaturedirectly in the thermal model by knowing the electrical resistancevariation with temperature

= 20 [1 + (T 20)] mwhere 20 = 1.724 10 -8 m

= 0.00393 / C for copper 50 C rise gives 20% increase in resistance

140 C rise gives 55% increase in resistanceWe can also account for the loss in flux due to magnettemperature rise directly in the thermal model typical temperature coefficients of Br

Ferrite = -0.2 %/C (-20% flux for 100C rise, 1.56 x I 2 R)

Sm-Co = -0.03 %/C (-3% flux for 100C rise, 1.06 x I 2 R)Nd-Fe-B = -0.11 %/C (-11% flux for 100C rise, 1.26 x I 2 R)

We can also account for the variation in windage loss usinganalytical windage formulations and the variation of viscosity with temperature

C l l t d FEA


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Complex loss types and FEAMore complex loss mechanisms benefit from FEA analysis

Automated links from lumped circuit thermal solver to the FEA codecan speed up with this analysis

Often if a loss is going to take days/weeks to set up andcalculate then it may be tempting to make estimates based onprevious experience rather than by calculation (less accurate)

Wi di H T f


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Winding Heat Transfer The aim is to form a set of thermal resistances, powersources and thermal capacitances that model the

thermal behavior of the windingNeed to set area (A), length (L) and thermalconductivity (k) in R = L/kA for a complex slot shapeholding a variety of componentsVarious other modelling strategies have beendeveloped to model the heat transfer within a windingbut they usually are limited to one or more of thefollowing:

A particular placement of conductors A simple slot shape A particular slot fill

Such methods typically require test or FEA solutions tocalibrateThe layered winding model used by MDL has theadvantage that:

It has some physical meaning and can be fully set upfrom slot geometry and known wire number and size

L d Wi di g M d l


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Layered Winding Model

Winding modelled using copper/insulation (liner, enamel andimpregnation) layersTo calculate a thermal resistance require layer thickness (L), layer area

(A) and layer thermal conductivity (k) R = L/kA, A is layer periphery xstack lengthModel has same quantity of components as in the actual machineThe model gives details of temperature build up from slot wall to the hot-spot at the centreThe copper loss is distributed between the layers accordingto their volume

C lib ti f t d d i di d l


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Calibration of stranded winding modelsIf we can make an estimate of the placement of the conductors in aslot we can create a finite element thermal model and check the

conduction heat transfer in the slot matches with the layered windingmodel

C lib i f f d i di


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Calibration of form wound windingThe form wound winding has rectangular wire with its associatedlayers of insulation

Automated links to FEA for winding thermal resistance calibrationare useful in this case

Motor CAD Integrated Thermal FEA Sol er


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Motor-CAD Integrated Thermal FEA Solver Just a few seconds to generate a mesh and calculate Help improve accuracy through calibration of analytical model



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Interface Thermal ResistanceTouching surfaces have an effective thermal resistance:

Contact resistance due to imperfections in touching

surfaces (uneven surfaces lead to voids) contactarea is typically smallTwo parallel resistances, conduction for touching spots & conduction/radiation for voidsCan be crucial, especially in heavily loaded machinesMDL model them using an effective airgap (R = Gap/kA)

Gap dependent upon materials and manufacturing processes used Air assumed as interface fluid (can scale gap if other material) Easy for non thermal expert to input as they soon get a feel for what

is a good and what is a bad gap (physical dimension) Alternative used by thermal experts is to input a value of Contact

Resistance [m2.C/W] or Interfacial Conductance [W/C.m2]

More difficult for the non thermal expert to gain a feel for what isa good and what is a bad value Values are displayed in Motor-CAD as useful for thermal experts

I t f Th l R i t


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How rough is a surface? - published work

gives a Mirror Finish = 0.0001mm andRough Finish = 0.025mm (root-mean-square of deviations of a surface from areference plane) Average interface gap is not just a

function of roughness

Softer materials have smaller averageinterface gaps as peaks are squashedand will fit together better

Complex function of material hardness,interface pressure, smoothness of thesurfaces, air pressure, thermal

expansion, etc.

Interface Gaps published data


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Interface Gaps published dataA book by Holman gives the following values for roughnessand contact resistance with air as fluid medium

materials pressure original roughness resistance x m 2

416 ground Stainless 3-25 atm 0.0025 mm 2.64 m 2 .C/W 10 4 304 ground Stainless 40-70 atm 0.0011 mm 5.28 m 2 .C/W 10 4 ground Aluminum 12-25 atm 0.0025 mm 0.88 m 2 .C/W 10 4 ground Aluminum 12-25 atm 0.00025 mm 0.18 m 2 .C/W 10 4

Lower value of contact resistance is better

Conversion of Holman data to equivalent airgaps m 2 .C/W x W/m.C x 1000 = mm k for air = 0.026 W/m/C

materials pressure original roughness effective gap416 ground Stainless 3-25atm 0.0025mm 0.0069mm304 ground Stainless 40-70atm 0.0011mm 0.0137mm

ground Aluminum 12-25atm 0.0025mm 0.0023mmground Aluminum 12-25atm 0.00025mm 0.0005mm

Materials used in electric machines have effective gap of between0.0005mm and 0.014mm (average of around 0.007mm)softer material have smaller effective gap

Interface Gaps published data


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Interface Gaps published dataA book by Mills gives values of interfacial conductances (atmoderate pressure and usual finishes)

Stainless Steel Stainless Steel 1700-3700 W/m 2 /C

Aluminum Aluminum 2200-12,000 W/m 2 /C Stainless Steel Aluminum 3000-4500 W/m 2 /C Iron Aluminum 4000-40,000 W/m 2 /C

Higher value of interfacial conductance is betterConversion of Mills data to equivalent airgaps

1/(m2.C/W) x W/m/C x 1000 = mm k for air = 0.026 W/m/C

Stainless Steel Stainless Steel 0.0070-0.0153 mmAluminum Aluminum 0.0022-0.0118 mmStainless Steel Aluminum 0.0058-0.0087 mmIron Aluminum 0.0006-0.0065 mm

Materials used in electric machines have effective gap of between0.0006mm and 0.015mm (average of around 0.0075mm)

softer material have smaller effective gap

Similar gaps to Holman data

Stator Lamination Housing


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Stator Lamination Housing

Data from Boglietti -Politecnico di Torino,Italy

A problem in gaining reliable results is that thelaminated surface roughness is dependent uponthe manufacturing processes used

Larger interface gaps than usual are typical dueto laminated surface

Best to calibrate to suit motors, materials andmanufacturing practices used

Other Practical Results 142mm diameter BPM servo motor - 0.01mm (cast aluminium housing) 335mm & 500mm diameter IM's - around 0.015mm (cast iron frames) 128mm diameter IM - around 0.02mm (aluminium housing)

Bearings


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BearingsBearings are a complex composite structure Inner and outer race, balls, grease

Can be modeled by an effective interface gapProblem is to know what this gap should be

Calibration testing has been done Boglietti showing atypical effective interface gap between 0.2mm and0.4mm

Dealing with difficult areas of thermal analysis


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Dealing with difficult areas of thermal analysisMany manufacturing uncertaintiessuch as:

Goodness of effective interfacebetween stator and housing

How well the winding is impregnatedor potted

Leakage of air from open fin channelblown over machines

Cooling of the internal parts in aTENV and TEFC machine

Heat transfer through the bearings etc.

Test program with leadinguniversities over the past 15 years

to help develop data to help withsuch problems Set default parameters in Motor-CAD

giving good level of accuracy withoutthe user having done extensivecalibration using testing of their ownmachines

I d A U i C lib ti


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Improved Accuracy Using CalibrationIt has been seen that there are many complexissues (some manufacturing issues) when settingup an accurate thermal model for a motorCalibration of models helps to increase accuracy

Also useful to gain an insight of how your machine compares toother machines in terms of manufacturing goodness and qualityof design

Often the thermal model identifies a problem with theelectromagnetic calculation, i.e. hot rotor temperature showinghigher than expected magnet loss, etc.

Various specialist tests can be made on a machineto help with model calibration

Fixed DC Current (known loss) into stator winding (all phases inseries) with various temperatures measured

Help calibrate interface gaps and winding impregnation, etc.

Sensitivity analysis is also a good way to identifythe main design issues so concentration can begiven to those

d


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Examples of use of thermal analysis to optimise

electric machine designs


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75

Previous Thermal ProjectExamples

Most projects details are provided by MDLcustomersOnly examples are shown that havepermission to publish from the user

Most have technical papers associated withthem that can be examined for moredetails

Segmented Motor Miniaturization


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Segmented Motor MiniaturizationPapers published in 2001

Existing Motor: 50mm active length 130mm long housing traditional lamination overlapping winding

New Motor: 50mm active length 100mm long housing 34% more torque for same temperature rise segmented lamination

non-overlapping windingIn order to optimize the new design aniterative mix of electromagnetic andthermal analysis was performed



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Old motor

inserted mush winding54% slot-fill80mm diameter18-slots, 6-poleslarge end windings whichoverlap each other

New Motor

precision bobbin wound82% slot-fill80mm diameter12-slots, 8-polesshort end windings that areare non-overlapping




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Traditional Windinginserted mush winding54% slot-fill

Concentrated Windingprecision bobbin wound82% slot-fill

Mixed EM/Thermal design: Iron losses have an easier path to the housing Optimum thermal design requires correct balance between

copper & iron losses complex function of size, speed, materials, etc.


Improved winding insulation system


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Improved winding insulation system

Developed improved winding insulation in newdesigns New potting/impregnation materials with k = 1W/m/C

previous materials have k = 0.2W/m/Cabove designs show 6%-8% reduction in temperature

Above potted end-winding design shows a 15% reducedtemperature compared to non-potted design

Vacuum impregnation can eliminate air pocketsabove design shows 9% decrease in temperature inperfectly impregnated motor compared to oldimpregnation system

Improved housing for TENV motor


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Improved housing for TENV motor

Motor-CAD analytical formulations for convection used tooptimise the fin spacing

small fin spacing has large surface area but reduced air flow large fin spacing has maximum air flow but reduced surface

areaAccuracy validated using tests on series of rectangular andcircular discs and internal heaterAlso did CFD which gave same optimum but took alot longer to set up the model and calculate

optimum fin spacing

oC

spacing

A i l C li g Fi O ti i ti


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Axial Cooling Fin Optimization

motors with shaft mounted fan or blower unit large thermal benefits possible with correct fin design large selection of fin types available in Motor-CAD

Selection of TEFC machines where axial


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Selection of TEFC machines where axialfins optimized using thermal analysis

315mm Shaft Height, Cast Housing 200mm Shaft Height, Cast Housing

80mm Shaft Height, Aluminium Housing

Through Ventilation Motor


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Through Ventilation Motor

ventilation paths

Through ventilated motor optimised using amix of heat transfer and flow network analysis

More details in paper from ICEM 2002

1150hp IM Tw(test) = 157C Tw(calc) = 159C

Through Ventilation


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gFew of the many ducting systems available for usein through ventilation thermal analysis

flow circuit automaticallycalculated

User can input the fancharacteristic and thesystem flow resistance iscalculated

pressure

flow

fancharacteristic

system resistance

flowcalculation

Underwater Camera Motor


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Underwater Camera Motor

Analysis carried out on motors driving propellers on a small submersiblecraft fitted with a camera

Analysis to make sure that the housing temperature did not get too hotwhen the motor turned off and removed from the waterLosses now zero and winding cools down but this heats up the housingwhich is now not liquid cooled

winding

housing

turn off & take out of water (no loss/less

cooling)

Minimization of Motor Size/Weight


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Minimization of Motor Size/Weight Aerospace Duty Cycle Analysis

the motor needed tooperate on the duty cyclesand just get to 180C

duty-cycle analysis on anaerospace application with ashort term load requirement

1 2

3

4

12 3

4 Nm

rpm

time

winding

magnet

Validation of Duty Cycle Thermal


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y yTransient Analysis (Aerospace)

the complex loadmodelled in Motor-CAD isshown below:

excellent agreementbetween the calculatedthermal response andmeasured temperatures

Thermal Modeling of a Short-Duty Motor


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g y

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8-60

-40

-20

0

20

40

60

Ratio of parameter variation from calibrated value

T e m p e r a

t u r e v a r a

t i o n

f r o m c a

l i b r a

t e d v a

l u e

( K )

Impgrenation goodnessWire insulation thicknessSlot liner thicknessSlot liner-lamination gap

Motor designed to have minimum size and weight for a highperformance short-duty cycle application.

Optimization of thermal performance is critical for minimizedweight and size Motor-CAD used for transient analysis and size optimisation Excellent agreement with test data when prototype built

Winding temperature rise with DC currents (0.2 to 1.1 p.u.) - Test & Motor-CAD

IPM Traction Motor


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IPM Traction Motor Optimisation of water jacket for a traction motorExcellent agreement with test data when prototype built showing

level of confidence in default values for manufacturing issues likethe stator-lamination interface gap

ECCE 2011

BPM Traction Motor


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Motor-CAD used to optimise cooling of traction motorMotor had open endcap with local convection cooling of

end-windingsExcellent accuracy shown with fast calculation times

Automotive PMDC - Improved Winding


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ICEM 2008 electro-hydraulic brake Optimisated impregnation process and

slot liner to allow a longer operation time Two transients shown

Same load of 20A locked rotor One has Nomex liner and the other a

powder liner Powder liner allows 4 minute load

rather than 1.8 minutes Measured and simulated results shown

p gInsulation System

20

40

60

80

100

120

140

160

180

200

220

0 2 4 6 8 10

time [min]

T e m p e r a

t u r e

[ C ]

Twinding [Test] Trotor [Test] Tmagnet [Test] Tcomm [Test] Thousing [Test] Twinding [Calc] Trotor [Calc] Tmagnet [Calc] Tcomm [Calc] Thousing [Calc]

20

40

60

80

100

120

140

160

180

200

220

0 2 4 6 8 10 12 14 16 18time [min]

T e m p e r a

t u r e

[ C ]

Twinding [Test] Trotor [Test] Tmagnet [Test] Tcomm [Test] Thousing [Test] Twinding [Calc] Trotor [Calc] Tmagnet [Calc] Tcomm [Calc] Thousing [Calc]

NomexLiner

PowderLiner

Automotive PMDC Brush Model


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Improved brush model developed with aid of extensive testing

120.00

130.00

140.00

150.00

160.00

170.00

0.00 2.00 4.00 6.00 8.00 10.00time [s]

T e m p e r a

t u r e

[ C ]

-ve Brush [TEST]

+ve Brush [TEST]

Brush [CALC]

100.00

110.00

120.00

130.00

140.00

150.00

160.00

0.00 20.00 40.00 60.00 80.00 100.00 120.00time [s]

T e m p e r a

t u r e

[ C ]

-ve Brush [TEST]+ve Brush [TEST]Brush [CALC]

Servo Motor Duty Cycle Analysis


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Prediction of a motor thermal performance on a duty cycle load isimportant to match the motor to the load

excellent agreement between the prediction and test data shown

winding

housing

flange3 x full-load / 0.5 x full-load

short timeconstant of

windingleads to

rapidheating

long timeconstant of motor bulk

calculation

test



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y y yRange of tests with different duty cyclesused to proved model symbols = Motor-CAD model lines = test data



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Servo Motor Duty Cycle AnalysisAgain for another servo motor design excellentagreement between test and calculation

PEMD 2006

Induction Motor Locked Rotor Analysis


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yexcellent agreement between test and calculation of induction machine transient on locked rotorIterative calculation between SPEED and Motor-CADWork done by Dave Dorrell at Glasgow University

IECON 2006

Outer Rotor Brushless Motor


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Project with University of Bristol tooptimise the cooling of this outer rotorBPM machine

More complex cooling than traditionalinner rotor machine

TEFC & TENV Synchronous Gens.


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Range of machines tested and modelled with goodlevel of agreement

Mostly


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Wind generator (University of Cantabria)FLUX Users meeting 2010

Excellent results with Motor-CAD v TestFault Analysis also performed

Motor size and weight reduction


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Motor-CAD enabled a sizable reduction insize and weight compared to if no thermalanalysis was done at the design stage Weight reduction 31% Volume reduction 39%

UK Magnetics Society Meeting 2008


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Thank you for your attention!

Are there any questions?

Documents

ICEM 2012 - TUT3 - Improving Motor Effiency and Motor Miniaturisation - The Role of Thermal Simulation