2012 Practical Strategies for Improved Cooling of Electrical Motors and Generators Dave Staton INDUCTICA TECHNICAL CONFERENCE

8/9/2019 2012 Practical Strategies for Improved Cooling of Electrical Motors and Generators Dave Staton INDUCTICA TECHNICAL CONFERENCE

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Practical Strategies for Improved Cooling of Electrical Motors and Generators

Dave Staton, Douglas Hawkins and Mircea Popescu

Motor Design Ltd., Ellesmere, SY12 0EG, U.K.

www.motor-design.com

Abstract- Thermal analysis is becoming a more important aspect of the electric motor and generator

design process due to the push for reduced weights, costs and increased efficiency. This paper

reviews recent published examples of use of software to improve the cooling of various motor and

generator designs. Details of the cooling system used are given in the form of thermal models to

highlight the most important cooling paths and features of the particular designs. Comparisons are

made with test data where applicable. Brushless PM, asynchronous, dc, wound field synchronous

machine types are covered together with various cooling types such as TENV, TEFC, through

ventilation, water jackets, open/closed constructions, mounting conduction and radiation.

Applications with both steady-state and transient duty cycle loads are included.

Introduction:

Thermal analysis is an important design aspect and becoming a more important component of theelectric motor design process due to the push for reduced weights and costs and increased

efficiency. To help designers optimise their cooling system design Motor Design Ltd (MDL) has

developed the Motor-CAD software. Motor-CAD is the most sophisticated and easy to use software

simulation program for thermal analysis of electric motors. It combines the best aspects of

analytical lumped circuit and numerical analysis methods to provide a fast and accurate way to

analyse and optimise the design fully accounting for thermal aspects. It has been developed

continuously over the past 15 years and is now at version 7.1. It is developed by motor designers

rather than pure software engineers such that it features functions that machine designers find very

useful. Its continual development benefits from the valuable feedback from it hundreds of users. It

is used extensively in such industries as automotive, aerospace, servo, transport, renewables, etc.

and by leading universities. A few of the prestigious names using Motor-CAD are ABB, BAESystems, Bosch, BMW, Caterpillar, Daimler, Ford, Goodrich, Magneti Marelli, Otis, Siemens,

Thales, Williams, etc.

Fig 1: Motor-CAD Radial and Axial Cross-Section Editors and 3D Model Viewer


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Motor-CAD Software:

Motor-CAD is a unique piece of software dedicated to thermal analysis of electrical motors and

generators. It is mainly based on network (lumped circuit) analysis but also has a fully integrated

thermal finite element analysis module to give the best combination of calculation speed and

solution accuracy. Nodes are automatically placed at important points on the motor cross-section,

such as the stator back iron, tooth, winding hotspot, etc. These are linked by conduction, convection

and radiation thermal resistances. Losses are input at the relevant nodes. Thermal capacitances areadded when thermal transient analysis is performed. Flow network analysis is used to calculate

pressure drops and predict flow rates. Fig 2 shows the typical heat transfer and flow network

schematics as set up and calculated by Motor-CAD. The vast electric motor design and thermal

analysis experience of the Motor-CAD development together with close links with universities and

industry [1 - 23] has been utilised to make sure that the networks developed give an accurate

representation of all the important heat transfer and flow paths in the machine. Fig 2 shows the new

integrated thermal FEA solver that was introduced in v7.1. This takes just a few seconds to generate

a mesh and calculate the temperature rise in a slot filled with either stranded conductors or a form

wound windings.

Fig 2: Motor-CAD Heat Transfer and Flow Network Schematic Diagrams and the Integrated

Thermal FEA Solver

Editors are provided in Motor-CAD to make data input and interpretation of the results as easy as

possible. For instance in Fig 1 we see the radial and axial cross-section editors, which have pre-

setup parameterised geometries for a wide range of motor types (induction, inner and outer rotor

brushless BPM, PM commutator, switched reluctance, synchronous and claw pole), housing types

(a few of the many types are shown in Fig 1, i.e. the axial cross-section shows a machine with oil

spray cooling and a circumferential water jacket, the radial cross-section shows an axial water

jacket and internal rotor cooling ducts, the 3d-model viewer shows axial fins with fan cowling and

rotor endring wafters/wings), magnet shapes (the v-shaped magnet is shown in Fig 1), induction

motor cage shape, etc. A new feature of Motor-CAD v7.1 is option to indicate on the cross-section

drawing the flow paths for the cooling system selected, i.e. the axial cross-section in Fig 1 includes


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Motor-CAD has powerful inbuilt multi-parametric sensitivity analysis capabilities. Sensitivity

analysis is very useful for gaining an in-depth understanding of the main constraints to dissipation,

allowing informed design decisions to be made to improve the cooling. For instance, in Fig 4, we

see the sensitivity of a particular design to variation in the effective interface gap between stator

lamination and housing and impregnation goodness factor (amount of air in the impregnation

system). Motor-CAD gives valuable information on what typical values are expected for such

manufacturing quantities for different machine and cooling types and manufacturing processes. The

unrivalled experience built into Motor-CAD has been gathered from the extensive research MDLhas done in thermal analysis of electrical machines over the past 15 years and the wealth of

feedback from the hundreds of Motor-CAD users in industry. This allows new users to be able to

produce accurate results without the need to build up their own experience of such manufacturing

issues. The impregnation goodness factor is figure of merit for the winding impregnation system. A

value of 1 means the winding is perfectly impregnated and contains no air. A value of 0 means that

the winding is not impregnated. We can vary the impregnation goodness factor between 0 and 1 to

investigate all possible mixtures of impregnation and air. Such meaningful parametric quantities are

typical in Motor-CAD. But, in order to quantify the upper and lower limit of such parameters and

give typical values for a particular manufacturing process some experience is required. This is

where Motor-CAD is a big assistance to the designer, as the default values are set to quantities

typical in an average machine. Also, help is give as to what the maximum value could be withdifferent manufacturing techniques.

Fig 4: Typical Motor-CAD Sensitivity Analysis

Motor-CAD also has an in-built circuit editor which is useful for including thermal models of othercomponents attached to the machine, such as flanges mounted gear boxes, etc. The software is fully

compatible with the ActiveX standards for transfer of data between programs under the Windows

operating system. This makes it possible to automatically run optimization from such packages as

Excel and Matlab. Motor-CAD also has advanced data links with the Speed software making it easy

to automatically transfer geometry, losses and temperatures between the packages.

Previous Motor-CAD Projects:

Next we will give examples from some of the users of Motor-CAD on how useful the software has

been in modelling a variety of machine types. They also give an indication of the good level of

accuracy that can be achieved with little or no heat transfer knowledge of the designer. We onlyhave space to show a few highlights of the many paper published.


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cooling. The full load stator winding temperature was measured to be 157C, which compared well

with the Motor-CAD prediction of 159C.

A joint project with the University of Edinburgh and Cummins Generator Technologies used

Motor-CAD to model the temperature rise of a range of TENV, TEFC and through ventilated

synchronous generators [19,20]. In general the Motor-CAD error for prediction of nodal

temperatures is less than 5% so could be used reliably to aid in design optimisation. Its powerful

multi-parametric sensitivity analysis capabilities were also used to help set up robust design/6Sigma models.

Fig 7: 1150hp induction machine with through ventilation axial ducting and flow calculation

Fig 8 shows some work done by Siemens VDO Automotive to optimise the transient performance

of a Permanent Magnet DC Motor for an electro-hydraulic anti-lock brake (ABS/EPS) [18]. Motor-

CAD was used to optimise the slot insulation system with the aim of maximising the time that the

motor could operate with a locked rotor current without exceeding the winding insulation

temperature limit. A simple change to the insulation system allowed the operation to be extended

from 1.8 minutes to 4 minutes. Both designs were built and showed that Motor-CAD was giving

very accurate predictions of the transient so could be relied upon for such design optimisation.

Work was also done at that time to give improved prediction of brush temperatures, this work also

being validated with test data. The brush/commutator thermal model has been found useful in the

analysis of motor life expectancy and for understanding the main design criteria required to keepthe brushes cool.

20

40

60

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100

120

140

160

180

200

220

0 2 4 6 8 10time [min]

Temperature[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]

Temperature

[C]

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

Trotor [Calc]Tmagnet [Calc]Tcomm [Calc]Thousing [Calc]

Fig 8: PMDC motor with measured and Motor-CAD predictions of thermal transients

Fig 9 shows some work done by the University of Bristol [21] to optimise the water jacket cooling

and stator insulation system for a IPM brushless PM motor for an electric vehicle application.

There was excellent agreement between predicted and measured stator and rotor temperatures.


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Fig 9: Thermal modelling of a traction motor using Motor-CAD

Fig 10 shows some work done by Goodrich Power Systems on the optimisation of a short duty

cycle rated motor [22]. Motor-CAD was used to optimise the quality of the slot impregnation as itwas found to play a significant role in the transient performance. Optimisation was facilitated by the

in-built multi-parametric sensitivity analysis module in Motor-CAD, Fig 10 showing the results of

sensitivity analysis on various slot insulation parameters, i.e. how well the winding is impregnated,

the wire enamel thickness, the slot liner thickness and the thickness of the gap between the liner and

the lamination. In this case the impregnation goodness (amount of air left in the impregnation

system) was found to be the most important factor and the motor winding was optimised to account

for this. Thermal analysis also showed that thermal protection with temperature sensors was not

reliable due to delays in temperature measurement and that a predictive protection was essential.

The analysis was confirmed by 2-dimentional finite element analysis and prototype motor testing,

excellent agreement with measured thermal transients for a range of loads being shown in Fig 10.

Fig 10: Thermal Modelling of a Short-Duty Motor using Motor-CAD

Fig 11 shows some work done by Electro Kinetic Designs on the optimisation of motor for an

aircraft wheel retraction system [23]. Without thermal analysis the designer would need to rely on

their previous experience to size the motor. By using Motor-CAD to model the particular duty cycle

for this application the motor was designed to be 39% and 31% smaller in volume and weight

respectively. This was achieved by an iterative process of first doing the electromagnetic design

followed by thermal analysis using Motor-CAD, shrinking the motor down until it performed thedesired electromagnetic performance and just reached 180C with the desired transient duty cycle.


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Fig 11: Optimisation of a high power density motor for a very short duty aerospace actuator

References:[1] Staton, D.A., Pickering, S.J, Lampard, D : 'Recent Advancement in the Thermal Design of Electric Motors', SMMA 2001 Fall Technical

Conference "Emerging Technologies for the Electric Motion Industry", 3-5 Oct 2001, Raleigh-Durham, North Carolina, USA

[2] Staton, D.A., Boglietti, A., Cavagnino, A.: 'Solving the More Difficult Aspects of Electric Motor Thermal Analysis in Small and Medium Size

Industrial Induction Motors'; IEEE Transactions on Energy Conversion, Volume 20, Issue 3, Sept. 2005 Page(s): 620628

[3] Staton, D.A.: 'Servo Motor Size Reduction - Need for Thermal CAD', Drives & Controls 2001 Conference, ExCeL - Docklands, London, UK,

13-15 March 2001

[4] Staton, D.A., So, E: 'Computer Aided Design of Brushless Servo Motors', UK Magnetics Society Seminar 'Developments in Electromagnetic

CAD for industrial Applications', RIBA, London, UK, 28 Feb. , 2001

[5] Staton, D.A.: 'Thermal Computer Aided Design - Advancing the Revolution in Compact Motors', IEEE - IEMDC Conference, MIT,

Massachusetts, USA, 17-20 June 2001

[6] Boglietti, A., Cavagnino, A., Staton, D.A.: 'Thermal Analysis of TEFC Induction Motors', Industry Applications Conference, 2003. 38th IAS

Annual Meeting. Volume 2, 12-16 Oct. 2003 Page(s):849 - 856 vol.2

[7] Boglietti, A., Cavagnino, A., Staton, D.A.: 'Thermal Sensitivity Analysis of TEFC Induction Motors', IEE PEMD, Edinburgh, April 2004

[8] Boglietti, A., Cavagnino, A., Staton, D.A.: 'TEFC Induction Motors Thermal Models: A Parameter Sensitivity Analysis', IEEE Transactions onIndustry Applications, Volume 41, Issue 3, May-June 2005 Page(s): 756763

[9] Boglietti, A., Cavagnino, A., Staton, D.A., Popescu, M., Cossar, C., McGilp, M.I.: 'End space heat transfer coefficient determination for

different Induction Motor enclosure types', Industry Applications Conference, 2008. Edmonton, October 2008

[10] Boglietti, A., Cavagnino, A., Pastorelli, M., Staton, D.A., Vagati, A. : 'Thermal Analysis of Induction and Synchronous Reluctance Machines',

IEMDC 2005, San Antonio, USA, May 2005

[11] Chin, Y.K., Nordlund, E., Staton, D.A.: 'Thermal Analysis - Lumped Circuit Model and Finite Element Analysis', Sixth International Power

Engineering Conference (IPEC2003), pp. 952 - 957, Singapore, 27 - 29 November, 2003

[12] Chin, Y.K., Staton, D.A.: 'Transient Thermal Analysis using both Lumped-Circuit Approach and Finite Element Method of a Permanent

Magnet Traction Motor', IEEE Africon, pp. 1027 - 1035, Gaborone, Botswana, September 2004

[13] Dorrell, D.G., Staton, D.A., McGilp, M.I., 'A Combined Electromagnetic and Thermal Approach to the Design of Electrical Machines' IECON

2006, Paris, Nov 2006

[14] Dorrell, D.G., Staton, D.A., Hahout, J., Hawkins, D., McGilp, M.I., 'Linked Electromagnetic and Thermal Modelling of a Permanent Magnet

Motor', PEMD 2006, Dublin, April 2006.

[15] Dorrell, D.G., Staton, D.A., McGilp, M.I., 'Design of Brushless Permanent Magnet Motors - A Combined Electromagnetic and ThermalApproach for High Performance' IECON 2006, Paris, Nov 2006

[16] Wrobel, R., McNeill, N., Staton, D. A., Booker, J. D., Mellor, P. H.: 'Torque Dense, External Rotor Hub-Drive for a Hybrid Solar Vehicle',

2006 IEEE Vehicle Power and Propulsion Conference VPPC 2006, Windsor, UK, 6-8 Sept., 2006

[17] Al'Akayshee, Q., Staton, D.A. : 1150hp Motor Design, Electromagnetic & Thermal Analysis, ICEM 2002, Brugge, Belgium, 25-28 Aug. 2002

[18] Junak, J., Ombach, G., Staton, D.A : 'Permanent Magnet DC Motor Brush Transient Thermal Analysis', ICEM 2008, Portugal, Sept 2008

[19] Mejuto, C., Mueller, M., Staton, D.A., Mebarki, S., Al-Khayat, N. : 'Thermal Modelling of TEFC Alternators', IECON 2006, Paris, Nov 2006

[20] Mejuto, C., Mueller, M., Shanel, M., Mebarki, S., Reekie, M., Staton D.A : 'Improved Synchronous Machine Thermal Modelling', ICEM 2008,

Vilamoura, Portugal, Sept 2008

[21] Mellor, P., Wrobel, R., Mlot, A., Horseman, T., Staton, D.A., Influence of Winding Design on Losses in Brushless AC IPM Propulsion

Motors, ECCE 2011, Phoenix, 2011

[22] Sawata, T., Staton, D.A., Thermal Modeling of a Short-Duty Motor, IECON 2011, Melbourne, 2011

[23] Flew, A., 'Practical Application of CAD in a High Power Density Motor for a very short duty Aerospace Actuator', UK Magnetics Society

Seminar, Derby, Nov 2008

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2012 Practical Strategies for Improved Cooling of Electrical Motors and Generators Dave Staton INDUCTICA TECHNICAL CONFERENCE