An Alternative Cooling Arrangement for the End Region of a Totally Enclosed Fan Cooled (TEFC) Induction Motor. c. Micallef*, S.J. Pickering t , K.A. Simmons t , K.J. Bradley# *Department of Mechanical Engineering, University of Malta Email: [email protected]tSchool of Mechanical, Materials and Manufacturing Engineering, University of Nottingham #School of Electrical and Electronics Engineering, University of Nottingham Keywords: End Winding, Cooling, TEFC, Induction motors. Abstract An alternative cooling arrangement for the end winding of a high voltage, strip wound, totally enclosed fan cooled induction motor is proposed. The study is undertaken using Computational Fluid Dynamics (CFD) techniques and this is validated up by experiment. The cooling arrangement proposed gives a better distribution of heat transfer on the end windings as well as enhanced heat transfer on the end region frame, thus reducing the overall thermal resistance in the end region 1 Introduction Due to industry demands for cost reduction and in order to retain their presence in this highly competitive market, electric motor manufacturers are developing electric machines to their limits. The size of the machines is constantly being reduced and they are have increased loads. These effects are creating a thermal challenge since the power density of electric machines is constantly increasing and, as a consequence, electric machines are experiencing higher temperatures. This affects the performance and life expectancy of the machine. The maximum temperature permitted in an electric machine is determined by the insulation material of the windings. If the maximum permitted insulation temperature is exceeded, the insulation breaks down or deteriorates more rapidly, giving rise to sudden failure or reduced life expectancy. Thermal analysis of electric machines is therefore essential in the design of modem electric machines. 2 Problem Identification In large TEFC induction motors, the highest temperature is often reached in the end windings. This paper will therefore investigate fluid flow and heat transfer in the end region of a high voltage, strip wound induction machine to increase understanding and suggest improved cooling arrangements. The investigations were done through a series of CFD analyses. Experimental methods were then used to validate the results generated from CFD. The validation was performed by comparing the CFD predictions of the; 1. nature of air flow field in the end region; 2. rate of heat transfer from the end windings to the frame; and 3. windage loss with the experimental results. 3 Literature review Numerous papers have been published dealing with the thermal modelling of electric machines. Each individual component of the machine has been, in some way or another, thermally investigated. However it is recognized that the end region is the most difficult to predict and understand as the heat transfer is dominated by the complex air flow pattern. A number of papers have been published on heat transfer in the end region of electric machines. A summary of the results relating to heat transfer coefficients on end windings is given in Table 1 [1, 6, 7, 8, 10, 11 & 13]. Heat transfer coefficients are found to depend upon, the machine speed, rate of ventilation through flow (if present), detail of the end winding topology and the length and number of any wafter blades. Apart from the heat transfer coefficients on the end windings, the fluid flow in the end region was also investigated in some detail. There is a general agreement in the published literature [3, 4, 6, 9 & 12] relating to the nature of fluid flow field in the end region of TEFC machines; this may be described as consisting of two main recirculating flows superimposed on the main swirling flow. A strong recirculating toroidal vortex flow is present over the wafters. A weaker recirculating flow penetrates the end windings near their base, flows upwards behind the end windings and passes over the tips of the end windings to combine with the other main recirculating flow. This is clearly shown in Figure 1 below. The flow field in through ventilated machines was also investigated [1, 7 & 8]. Other papers investigated the effect of geometric changes on the flow field and heat transfer in the end region of electric machines [2, 5, 11 & 13]. In spite of this published literature, there is still a lack of information for the effective thermal design of the end region of a machine and this is largely because the investigations were made on particular machines and the results are not 305
Transcript
An Alternative Cooling Arrangement for the End Region of aTotally Enclosed Fan Cooled (TEFC) Induction Motor.
c. Micallef*, S.J. Pickeringt, K.A. Simmonst, K.J. Bradley#
*Department ofMechanical Engineering, University of MaltaEmail: [email protected]
tSchool of Mechanical, Materials and Manufacturing Engineering, University ofNottingham#School of Electrical and Electronics Engineering, University ofNottingham
Keywords: End Winding, Cooling, TEFC, Induction motors.
Abstract
An alternative cooling arrangement for the end winding of ahigh voltage, strip wound, totally enclosed fan cooledinduction motor is proposed. The study is undertaken usingComputational Fluid Dynamics (CFD) techniques and this isvalidated up by experiment. The cooling arrangementproposed gives a better distribution ofheat transfer on the endwindings as well as enhanced heat transfer on the end regionframe, thus reducing the overall thermal resistance in the endregion
1 Introduction
Due to industry demands for cost reduction and in order toretain their presence in this highly competitive market,electric motor manufacturers are developing electric machinesto their limits. The size of the machines is constantly beingreduced and they are have increased loads. These effects arecreating a thermal challenge since the power density ofelectric machines is constantly increasing and, as aconsequence, electric machines are experiencing highertemperatures. This affects the performance and lifeexpectancy of the machine.
The maximum temperature permitted in an electric machine isdetermined by the insulation material of the windings. If themaximum permitted insulation temperature is exceeded, theinsulation breaks down or deteriorates more rapidly, givingrise to sudden failure or reduced life expectancy.
Thermal analysis of electric machines is therefore essential inthe design of modem electric machines.
2 Problem Identification
In large TEFC induction motors, the highest temperature isoften reached in the end windings. This paper will thereforeinvestigate fluid flow and heat transfer in the end region of ahigh voltage, strip wound induction machine to increaseunderstanding and suggest improved cooling arrangements.
The investigations were done through a series of CFDanalyses. Experimental methods were then used to validate
the results generated from CFD. The validation wasperformed by comparing the CFD predictions of the;
1. nature of air flow field in the end region;2. rate of heat transfer from the end windings to the
frame; and3. windage loss
with the experimental results.
3 Literature review
Numerous papers have been published dealing with thethermal modelling of electric machines. Each individualcomponent of the machine has been, in some way or another,thermally investigated. However it is recognized that the endregion is the most difficult to predict and understand as theheat transfer is dominated by the complex air flow pattern.
A number of papers have been published on heat transfer inthe end region of electric machines. A summary of the resultsrelating to heat transfer coefficients on end windings is givenin Table 1 [1, 6, 7, 8, 10, 11 & 13]. Heat transfer coefficientsare found to depend upon, the machine speed, rate ofventilation through flow (if present), detail of the end windingtopology and the length and number ofany wafter blades.
Apart from the heat transfer coefficients on the end windings,the fluid flow in the end region was also investigated in somedetail. There is a general agreement in the published literature[3, 4, 6, 9 & 12] relating to the nature of fluid flow field in theend region of TEFC machines; this may be described asconsisting of two main recirculating flows superimposed onthe main swirling flow. A strong recirculating toroidal vortexflow is present over the wafters. A weaker recirculating flowpenetrates the end windings near their base, flows upwardsbehind the end windings and passes over the tips of the endwindings to combine with the other main recirculating flow.This is clearly shown in Figure 1 below. The flow field inthrough ventilated machines was also investigated [1, 7 & 8].
Other papers investigated the effect of geometric changes onthe flow field and heat transfer in the end region of electricmachines [2, 5, 11 & 13].
In spite of this published literature, there is still a lack ofinformation for the effective thermal design of the end regionof a machine and this is largely because the investigationswere made on particular machines and the results are not
håCOl!r =20 + 8.5vo.7 V - peripheral velocity of the rotor vanes.
Pickering et al. h Fan End = 326 W/m2K (1500 rpm) Through ventilated 4-pole strip wound induction[8] h Inlet End = 210 W/m2K (1500 rpm) motor; frame C280.
h varies with (motor speed)o.75Hay et al. [1] h Inlet End = --250 W/m2K (1500 rpm) Through ventilated low voltage lap-wound
h varies with (motor speed)o.78 electric motor; frame C280. Fan end valueswere around half those of inlet end.
Pickering et al. hinletend = --450 W/m2K (1500 rpm) Through ventilated low voltage concentric[7] h Inlet End varies (motor speed) 1.11 wound induction motor; frame C280.
hfan end = --230 W/m2K (1500 rpm)h Fan End varies (motor speed)o.8
Oslejsek [6] h varies with (motor speed)o.Ho Enclosed machines.
expressed in a sufficiently general form. The heat transfer isdominated by the airflow in the end region and although thisis generally understood qualitatively, quantitative assessmentsof the effects of changes in design on air flow, particularly ifnovel configurations are considered, are difficult unless CFDis used. The use of CFD has now developed to a stage whereit can be used with confidence in the thermal design ofelectric machines [2, 9].
In most of the literature reviewed only existing motorconfigurations were considered and investigations to developimproved cooling arrangements were not made. There istherefore plenty of scope for the investigation of improvedcooling arrangements and these can be done effectively usingCFD.
I!i
Shaft ., ;0;-..:,..I!I
I,i
Figure 1 - Typical flow field in the end region ofTEFC motor
4 Experimental Facility
The experimental facility used consisted of a set of endwindings taken from the non-connection end of a 2-poleTEFC induction motor enclosed in a Perspex 'frame' asillustrated in Figure 2. A model of the shaft, rotor, rotor bar
extensions and end ring was driven by means of a floatingframe induction motor. The end windings were heated bymeans of a DC current. This setup represented a typicalcooling arrangement commonly found in the end region ofTEFC machines.
Figure 2 - Photo showing experimental test rig
The instrumentation recorded local heat fluxes and surfacetemperatures at various locations on the heated end windingusing micro foil heat flux sensors. The locations of these heatflux sensors are clearly shown in Figure 4. The torque,rotational speed of the rotor and local air speed at variouslocations in the end region were also recorded.
5 CFD Analysis
The general CFD software FLUENTTM V6 was used toanalyse the flow field as well as the heat transfer in the endregion. The rotating reference frame technique was used toaccount for the rotation of the rotor. This includes theacceleration of the coordinate system within the flowequations so that the fluid is steady with respect to therotating (non-inertial) reference frame, and a steady stateanalysis was performed. Two reference frames were set up,one containing all rotating parts, i.e. the shaft, end ring,
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wafters, etc. and another stationary reference framecontaining the motor frame, endshield and the end windings.
conditions in a machine, but the thermal resistancescalculated by CFD are not strongly dependent on temperature.
The end windings may be considered as a resistance to airflow. The base and tips of the end windings both have a lowresistance to air flow, as they have a relatively large degree ofopenness, while the middle part has a very high resistance toair flow, due to the presence of packing pieces and thebinding ring (see Figure 2). Therefore, in order to circulatefluid around the end windings (not necessarily penetrating theend windings) the base and tip parts of the end windings canbe used as part of the air flow path.
A validation exercise was performed in order to gainconfidence in the CFD modelling. This was done by solving aCFD model representing the experimental setup of the endregion. The CFD results were then compared with thoseacquired experimentally. The velocity profile on a planelocated between the rotor and the end windings and local heattransfer coefficient at various locations on the end windingswere compared. The results are illustrated Figure 5 and Figure6 respectively.
Figure 4 - Locations of heat flux sensors
SfUtft
140U1Uld.i.~lrU
Figure 3 - Schematic illustrating main components anddimensions of model used for CFD validation.
Due to the complex geometry involved in the end windings,an unstructured mesh consisting of tetrahedral cells wasconstructed. The standard k-e turbulence model was usedtogether with standard wall functions.
An initial grid size in the order of 3 nun was generated andafter arriving at an initial solution, adaption techniques basedon velocity gradient, volume changes and y+ values in cellswith close proximity to the walls, was employed. The y+
values of cells near the wall are chosen so that the boundarylayers near the walls can be correctly represented by wallfunctions within the CFD code. The resulting refined gridwas then solved again. This process was repeated until thesolution was grid independent. Generally the grid independentsolution was of the order of 800,000 cells.
All models employed were run at 1700 rpm. The endwindings surfaces were set at a constant temperature of 140°Cwhile the frame temperature was set to 20°C. Thesetemperatures were chosen to represent typical boundary
the reduction in the thermal convection resistance at the frame(65% reduction).
-'
H I ill I all i iI j li i I!
Jj 1~ III III I I I I I I
mAlternative cooling~ Typical cooHng
300
:i-ei 2501:.I~ 200!ut 150
~I 100
] 50
Additionally if any rotating blades or wafters are positionedin the vicinity of both the end winding surfaces and the framesurface, the heat transfer will be enhanced on both surfaces,thus lowering the total resistance to heat flow. In order tokeep windage losses to a minimum, any rotating blades mustbe kept as close as possible to the shaft (minimum rotatingradius).
A configuration which takes all these points intoconsideration is one having rotating blades attached to theshaft near the end shield as shown in Figure 7. The bladeswill be in close proximity to the end shield and the tips of theend windings thus enhancing the heat transfer in these areas.Additionally it is anticipated that the jet of fluid emergingradially from the blades will travel over the end of the endwindings and then deflect towards the base of the endwindings where it penetrates them to complete the path.
Figure 7 - Photo showing alternative cooling arrangement.
8 Results
Figure 8 - Chart comparing experimental values of local heattransfer coefficients on the end winding of conventional
and alternative cooling arrangements
The summary of CFD results (Table 2) shows that heattransfer enhancement has been achieved in the alternativecooling arrangement. This is attributed to the higher velocitiespresent in the vicinity of the end windings and frame. Thetotal thermal resistance decreased by 33% compared to theconventional cooling arrangement. This was mainly due to
Figure 9 - Air flow field present in alternative coolingarrangement
The thermal resistance of the end windings experienced onlya slight decrease; however the distribution of heat transfercoefficients on the end winding (Figure 8) is better in this
End WindingTotal Thermal Thermal Frame Thermal
Windage Loss Resistance Resistance Resistance End Winding htc Frame htcModel (W) (K/W) (K/W) (K/W) (W/m2K) (W/m2K)
Conventional end 310 0.559 0.298 0.262 75 28.6region
Alternativecooling 368 0.376 0.283 0.093 79 108
arrangement
Table 2 - Summary ofCFD results comparing the two cooling arrangements
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alternative cooling arrangement than in the conventionalarrangement. This is because the heat transfer coefficients aremore evenly distributed and the tips of the end windings,which usually suffer from hot spots, are being cooled muchmore effectively. The windage loss in the alternative coolingarrangement was slightly higher (19%) than the typical case.
As expected, the secondary recirculating flow was reversedby this change. The air now flows downwards behind the endwindings as shown in figure 9. Also a high speed jet of airmoves over the tips of the end windings, increasing the heattransfer performance in the area, where it is needed most.
These CFD results were all successfully validatedexperimentally.
9 Conclusions
The CFD predictions were quite reliable even though the endregion experienced a substantial configuration change. Thisgives confidence in the use of CFD techniques in the designoffice and is an important outcome from this work, sincefurther development may take place without the need forexperimentation. It is anticipated that if future motor designsincorporate CFD analysis techniques, a substantial reductionin development costs will result.
The alternative configuration proposed in this work,employing a shaft mounted fan in the vicinity of the endshield, gives a substantial reduction in the thermal resistanceof the end region and would be worth using in a real machine.
3 Mugglestone, J., Lampard, D and Pickering S.J., 1998.Effects ofend winding porosity upon the flow field andventilation losses in the end region of TEFC inductionmachines. lEE Proceedings Electrical Power Applications,Vol. 145, No.5, September 1998. pp 423-428.
4 Mugglestone, J, Pickering, S.J. and Lampard, D, 1999.Prediction of the heat transfer from the end winding of aTEFC strip-wound induction motor. Proc ofIEMDC'99 IEEEInternational Electric Machines and Drives Conference,Seattle, USA, May 1999. pp 484-486.
5 Mugglestone, J., Pickering, S.J and Lampard, D, 1999.Effect ofgeometric changes on the flow and heat transfer inthe end region of a TEFC induction motor. lEE ConferencePublication, No. 468, 1999. pp 40-44.
6 Oslejsek, O. 1972. The cooling of the end windings ofsmall enclosed electric machines. Elektrotech Obzor, Vol. 61,No. 10. pp 548-556
7 Pickering, S.1., Lampard, D., Hay, N. and Roylance, T.F.,1995. Heat transfer from the stator end-windings ofa lowvoltage concentric-wound induction motor., lEE ConferencePublication, No. 412,1995. pp 477-481.
8 Pickering, S.J., Lampard, D, Hay, N. and Roylance T.F,1998. Heat transfer in a through-ventilated induction motor.lEE Proceedings - Electrical Power Applications, Vol. 145,No.5, September, 1998. pp 429-433
It is recommended though that a full thermal analysis of theh ld b
...c. d . d . h f£ f 9 Pickering, S.J, Lampard, D., Mugglestone, J., Shanel, M.motor s ou e peJlorme In or er to ascertaIn tee ect 0 and Birse, D., 1999. Using CFD In The Design Of Electricthe enhanced end region cooling on the overall thermal Motors And Generators. Computational Fluid Dynamics:performance of the motor. Technical Developments and Future Trends, 13-14
December, 1999.
Acknowledgement
The financial support of the International Office at theUniversity ofNottingham and the Office ofHuman Resourcesat the University of Malta through their respective scholarshipawards programmes is gratefully acknowledged.
References
1 Hay, N., Lampard, D., Pickering, S.J. and Roylance, T.F.,1994. Heat transfer from stator-nd-windings ofa low-voltagelap-wound electric motor. Proceedings of 10th internationalHeat Transfer conference, 1994, Vol. 3. pp 197-202.
2 Micallef, C., Pickering, S. J., Simmons, K. and Bradley, K.,2005. Improvements in air flow in the end region ofa largetotally enclosed fan cooled induction motor. Proc. ofIEMDC'05 IEEE International Electric Machines and DrivesConference, San Antonio, Texas, USA, May 2005.
10 Roberts, T.J., 1969-70. Determination of the thermalconstants of the heat flow equations of electrical machines.Proceedings IMechE, Vol 184, Part 3E, 1969-70. pp84-92
11 Schubert, E., 1968. Heat Transfer Coefficients at EndWindings and Bearing Covers of Enclosed AsynchronousMachines. Elektrie, Vol. 22, April 1968. pp160162.(Translation ERA/IB 2846).
12 Takahashi, K., Kuwahara, H., Kajiwara, K. and Obata, T,2002. Airflow and Thermal Conductance in a TotallyEnclosed Induction Motor. Heat Transfer - Asian Research,Vol. 31, No.1, January, 2002. pp 7-20.
