Hemeida, Ahmed

Mohamed, Abdalla

Sergeant, Peter

Vansompel, Hendrik

Ghent University Academic Bibliography

1Transient Lumped Parameter Thermal Model BasedLoadability Studies for The YASA Axial FluxPermanent Magnet Synchronous MachinesAbdalla Hussein Mohamed, Hendrik Vansompel, Ahmed Hemeida and Peter SergeantAbstract—In this paper, the short time duty and the intermit-tent duty of a 4 kW yokeless and segmented armature (YASA)axial flux permanent magnet synchronous machine (AFPMSM)are calculated based on the transient lumped parameter thermalnetwork (LPTN) model of this machine. The evaluation ofthese duties is crucial for efficient utilization of the machine inapplications where the load is fluctuating or requires high powerfor a short time. The results show that the 4 kW machine can beloaded up to 2.7 of its continuous rating for 30 minutes and upto 2.5 of its continuous rating for 60 minutes without exceedingthe insulation breakdown temperature or the demagnetizationtemperature of the permanent magnets (PMs). Also, results revealthat for periodic loads of period 10 minutes and duty cycle 0.6the machine can be loaded up to 3 times its continuous ratingand more than that for lower duty cycles.Index Terms—YASA, AFPMSM, LPTN, Intermittent rating,Transient model.I. INTRODUCTIONMany types of axial flux permanent magnet synchronousmachines exist. These types differ in the number of stators androtors and the location of them in the machine. Some of thesetypes are axial flux internal rotor (AFIR), toroidally woundinternal stator (TORUS), axial flux internal stator (AFIS) andyokeless and segmented armature (YASA) [1]. The absence ofthe stator yoke in the YASA type makes it more efficient asit means that the staor core has less iron and hence, less corelosses. The complete design and optimization of the YASAmachine or any other machine needs accurate electromagnetic,thermal and mechanical models for the machine. Two methodsfor studying thermal modeling exist, the finite element method(FEM) and the lumped parameter thermal network method(LPTN). References [2]–[5] are some of the papers presentingthe FEM thermal modeling technique for the YASA machinewhile [6]–[8] studied the LPTN modeling method. The LPTNmethod has the advantage of being fast enough while onthe other hand the FEM method provides a more detailedtemperature distribution in every part but on the cost ofthe time. The LPTN developed in [8] has the advantage ofproviding the transient thermal response of the machine that iswhy it is chosen to study the short time ratings and intermittentratings of the YASA machine in this paper.A. Hussein Mohamed, H. Vansompel and P. Sergeant are with the Depart-ment of Electrical Machines, Metals, Mechanical Constructions and Systemsand EEDT, Flanders Make, the research centre for the manufacturing industry.A. Hussein Mohamed and A. Hemeida are also with the department ofelectrical power and machines, Cairo University, Giza 12613, EgyptHousingWindingCoreShaftRotor diskPMsFig. 1: Cross-section view of the YASA prototypeII. YASA LUMPED PARAMETER THERMAL NETWORKA brief description of the lumped parameter thermal modeldeveloped by the authors and adopted for studies in this paperis given in this section. The details of that model can be foundin [8]. In this model the rotor and the stator are representedby two different networks coupled by the analytical convectionheat transfer coefficients in [9], [10]. The lumped parameterthermal network (LPTN) is a network of thermal resistors,capacitors and heat sources. The solution of the network givesthe average temperature of each element represented by a nodein this network. The values of these resistors and capacitorsare driven from the material properties and geometry of themachine under study. The geometry of the YASA machineunder study is shown in Fig. 1 while, the material propertiesare given in Table I.The heat sources come from the power losses in themachine. The different power losses in the machine are thewinding losses, core losses, PM losses and rotational losses.These losses are calculated from the electromagnetic modelof the YASA machine in [11],[12] and computational fluid2TABLE I: Material properties.Material K (W/mK) Cp (J/kgK) ρ (kg/m3)copper 385 392 8890aluminum 167 896 2712epoxy 0.4 600 1540nylon 0.25 1600 1140Nd-Fe-B 9 500 7500dynamics based model in [13]. The stator and rotor LPTNsare shown in Figs. 2 and 3 respectively.The reference temperature appearing in the LPTNs repre-sents the temperature of the air near to the convected surface.III. TEMPERATURE PROFILES AT CONTINUOUS RATINGThe geometrical parameters of the 4 kW YASA axial fluxmachine under study are listed in Table II. The continuousrated power and speed along with the different power lossesat these rated values are listed in Table III.TABLE II: YASA Geometrical ParametersParameter Value Unitnumber of poles 16 -teeth number 15 -outer diameter housing 195 mmouter diameter active 148 mmPM thickness 4 mmrotor outer radius 74 mmslot width 11 mmTABLE III: Continuous Rated ValuesParameter Value Unitrated power 4 kWrated speed 2500 RPMrated torque 15.27 N.Mcopper losses 59.027 Wcore losses 44.973 WPM losses 16.783 WThe winding insulation class is B which breaks down at130◦C [14] while, the demagnetization temperature of therotor magnets is 150◦C [15]. So, at any operating conditionsthe winding temperature should be kept lower than 130◦C andthe permanent magnets temperature below 150◦C.The winding Twdg , stator core Tcore and PM TPM averagetemperature variation with time at the operating conditions inTable III is shown in Fig. 4 assuming an ambient temperatureof 25◦C. The steady state average temperature of the windingand the core is 47.5◦C while the steady state average temper-ature of the PMs is 29.5◦C.Many remarks can be deduced from Fig. 4 as follows,1) The steady state temperature of both winding and PMs ismuch lower than the critical values which means that themachine can be loaded with a much higher load withoutthermal problems.2) The winding and core temperatures are almost the samedue to the high thermal conductivity of the core materialand the winding material and the good thermal contactbetween the core and the winding.3) The thermal time constant of the PM is almost half ofthat of the winding. This returns to the lower specificheat capacitance of the PMs and the smaller volumecompared to the winding.4) The temperature variation is almost exponential withtime.IV. EVALUATION OF THE SHORT-TIME DUTY OF THEMACHINEThe transient time taken by the machine to reach its finalsteady state temperature can be utilized by loading the machinewith a higher power than its continuous rated one but for ashort time. This amount of over power is calculated in a waythat the machine doesn’t reach its critical temperature afterthat short time. Usually the short-time duty of the machine isreferred to by S2 followed by the allowed short time.A. S2-60 ratingThe flow chart in Fig. 5 shows how the short time dutyrating of the machine is calculated for any given short time.Table IV shows the final calculated load power and differentpower losses at that load.TABLE IV: S2-60 rating and lossesParameter Value Unitload power 10.34 kWcopper losses 384.34 Wcore losses 70.836 WPM losses 89.246 WAs indicated in Table IV, the machine can be loaded upto 2.58 of its continuous rating for one hour without thermalproblems. Fig. 6 indicates the temperature variation at the S2-60 rating of the machine.From Fig. 6, the winding temperature reached its limit afterone hour while, the PM temperature is still far from its limit.B. S2-30 ratingTable V has the S2-30 power rating of the machine alongwith the power losses at that loading.TABLE V: S2-30 rating and lossesParameter Value Unitload power 10.89 kWcopper losses 421.56 Wcore losses 73.715 WPM losses 97.5 WIt is interesting to see from Table V that the S2-30 ratingof the machine is only 5.3% higher than that of its S2-60 one.This can be explained by noticing from Fig. 7 the differencein rate of rise of the machine temperature resulting from dif-ference in values of heat sources and reference temperatures.It worth mentioning that the above two short time ratingsare both evaluated at the rated speed of the machine.3Pwdg_end_uphc4Tref4Upper finsEpoxy between winding and upper finsUpper end winding NylonStator core PstNylonPwdg_end_downLower end winding Epoxy between winding and bearingsBearingAir ContactPwdgEffective winding Epoxy between winding and finsRadial fins12345678910111213Upper fins1hc2Tref214Epoxy Resin12hc215Nylon4hc2Tref216hc2Tref2Pst_tipsStator core tooth tips175Nylon6hc2Tref218Epoxy Resin18hc219Bearing29hc3Tref320Fig. 2: LPTN of the stator.V. EVALUATION OF THE INTERMITTENT DUTY OF THEMACHINEThe intermittent duty is usually referred to by S3 followedby the duty cycle of the operation for 10- minute cycles. Eachcycle includes some time of machine operation and some timethe machine in rest.A. S3-60% ratingThe flow chart in Fig. 8 describes the method used forcalculation of the S3 rating of the machine at any given dutycycle.At steady state the temperature of the different machineparts oscillates between two fixed maximum and minimumvalues. Here, the S3 rating is calculated in a way that themaximum temperature of any part doesn’t exceed the criticalvalue. Table VI contains the S3-60% rating of the machineand also the different losses.Fig. 9 shows the winding temperature variation with timealong with the copper-loss variation. The temperature fluctu-ates between 100◦C and 130◦C.By inspecting Fig.10 the PM temperature is still below thecritical value.TABLE VI: S3-60% rating and lossesParameter Value Unitload power 12.295 kWcopper losses 537.37 Wcore losses 82.58 WPM losses 123.16 WIt is useful to mention here that the rate of change ofthe machine temperature depends on the values of the heatsources, the reference temperatures, the thermal resistancesand the thermal capacitances. And, due to the different heatsources, reference temperatures and convection coefficients incase of machine in operation and machine at rest, the rate oftemperature rise is different than the rate of temperature fallFigs. 9 and 10.B. S3-25% ratingThe S3 rating at 0.25 duty cycle is also computed to havea feeling of the variation of the intermittent rating with theduty cycle. The S3 25% rating along with the machine lossesare listed in Table VII. The 58 % reduction in the duty cycleresulted in 37% increase in the rating.4Fig. 3: LPTN of the rotor.0 20 40 60 80 100 120Time [mins]253035404550T[oC]CoreWindingPMFig. 4: Winding, Core, PM average temperature variation atrated conditionsTABLE VII: S3-25% rating and lossesParameter Value Unitload power 16.85 kWcopper losses 1.01 kWcore losses 118.6504 WPM losses 227.8626 WVI. EFFECT OF SPEED VARIATIONIt is very important to mention that the ratings in theprevious sections are calculated assuming that the machineis operating at fixed speed which is the rated one. But, incase of variable speed applications care should be taken whencalculating these ratings. Because the reduction of the machinespeed has the effect of reducing the core and PM losses and inthe same time reducing the convection coefficients. So, at theend this may result in a higher or a lower machine temperature.To clarify this point, Fig. 11 is generated at S2-60 ratingreported in Table IV at different speeds. As indicated in theStartRead load powerCalculate power lossesCalculate temperature after short time from LPTNIs the temperature within 0.5% of the critical?Record load power and lossesYesIncrease load power by 1%NoEndFig. 5: Flow chart for calculating short-time duty0 10 20 30 40 50 60Time [mins]20406080100120140T[oC]CoreWindingPMFig. 6: Winding, core and PM average temperature at S2-60rating of the machinefigure, at 16% of rated speed the winding temperature exceedsthe critical limit. So, at every short-time and intermittent ratingthere is a limit on the lowest speed.VII. CONCLUSIONIn this paper, the transient thermal model of the YASAmachine is used in the calculation of the intermittent andshort time duties of the machine. Also, the effect of thespeed variation on the values of these ratings is clarified andstudied for one case. The results show that the machine can beoverloaded safely up to 2.5 times its continuous rating for onehour of operation at the rated speed and slightly higher thanthat for half an hour. And, it is shown also that the intermittentrating for a cycle of 10-minute and duty cycle of 0.6 can be 3times the continuous rating and slightly higher for lower dutycycles.ACKNOWLEDGMENTThis research was done in the framework of the Hipercoolproject from Flanders Make.50 5 10 15 20 25 30Time [mins]20406080100120140T[oC]Winding Temperature S2-30Winding Temperature S2-60Fig. 7: Winding Temperature at S2-60 and S2-30StartRead load power,duty cycleCalculate power lossesCalculate max.,min. Temperature in first cycleCalculate max.,min. Temperature one more cycle∆Tmax,∆Tmin within 0.25 degrees?NOIs the temperature within 0.5% of the critical?NOYesRecord load power and lossesEndYesFig. 8: Flow chart for calculating S3 ratingREFERENCES[1] F. Giulii Capponi, G. De Donato, and F. Caricchi, “Recent advances inaxial-flux permanent-magnet machine technology,” IEEE Transactionson Industry Applications, vol. 48, no. 6, pp. 2190–2205, 2012.[2] Z. Tan, X.-g. Song, B. Ji, Z. Liu, J.-e. Ma, and W.-p. Cao, “3D thermalanalysis of a permanent magnet motor with cooling fans,” J. ZhejiangUniv. Sci. A, vol. 16, pp. 616–621, Aug. 2015.[3] M. Polikarpova, P. Ponomarev, P. Lindh, I. Petrov, W. Jara, V. Naumanen,J. A. Tapia, and J. Pyrhonen, “Hybrid Cooling Method of Axial-FluxPermanent-Magnet Machines for Vehicle Applications,” IEEE Trans.Ind. Electron., vol. 62, pp. 7382–7390, Dec. 2015.[4] F. Marignetti, V. Delli Colli, and Y. Coia, “Design of Axial FluxPM Synchronous Machines Through 3-D Coupled Electromagnetic100 105 110 115 120 125 130 135 140 145 150Time [mins]0100200300400500600T,Pwdg[oC,W]Winding TemperatureWinding LossesFig. 9: Winding temperature variation at S3-60% rating alongwith the copper losses100 105 110 115 120 125 130 135 140 145 150Time [mins]384042444648505254T[oC]PMFig. 10: PM temperature variation at S3-60% rating0 10 20 30 40 50 60Time [mins]20406080100120140Twinding[oC]0.16 rated speed0.25 rated speed0.5 rated speedrated speedFig. 11: Winding temperature variation for S2-60 rating atdifferent speedsThermal and Fluid-Dynamical Finite-Element Analysis,” IEEE Trans.Ind. Electron., vol. 55, pp. 3591–3601, Oct. 2008.[5] F. Marignetti and V. Colli, “Thermal Analysis of an Axial FluxPermanent-Magnet Synchronous Machine,” IEEE Trans. Magn., vol. 45,pp. 2970–2975, Jul. 2009.[6] N. Rostami, M. R. Feyzi, J. Pyrhonen, A. Parviainen, and M. Niemela,“Lumped-Parameter Thermal Model for Axial Flux Permanent MagnetMachines,” IEEE Trans. Magn., vol. 49, pp. 1178–1184, Mar. 2013.[7] C. Lim, J. Bumby, R. Dominy, G. Ingram, K. Mahkamov, N. Brown,A. Mebarki, and M. Shanel, “2-D lumped-parameter thermal modellingof axial flux permanent magnet generators,” in 2008 18th Int. Conf.Electr. Mach., pp. 1–6, IEEE, Sep. 2008.[8] A. H. Mohamed, A. Hemeida, A. Rashekh, H. Vansompel, A. Arkkio,and P. Sergeant, “A 3D Dynamic Lumped Parameter Thermal Networkof Air-Cooled YASA Axial Flux Permanent Magnet Synchronous Ma-chine,” Energies, vol. 11, 2018.[9] A. Rasekh, P. Sergeant, and J. Vierendeels, “Convective heat transferprediction in disk-type electrical machines,” Appl. Therm. Eng., vol. 91,pp. 778–790, dec 2015.6[10] A. Rasekh, P. Sergeant, and J. Vierendeels, “Fully predictive heat transfercoefficient modeling of an axial flux permanent magnet synchronousmachine with geometrical parameters of the magnets,” Appl. Therm.Eng., vol. 110, pp. 1343–1357, 2017.[11] A. Hemeida and P. Sergeant, “Analytical modeling of surface PMSMusing a combined solution of Maxwell’s equations and Magnetic Equiv-alent Circuit (MEC),” IEEE Trans. Magn., vol. 50, no. 12, p. 7027913,2014.[12] A. Hemeida, P. Sergeant, and H. Vansompel, “Comparison of Methodsfor Permanent Magnet Eddy Current Loss Computations With andWithout Reaction Field Considerations in Axial Flux PMSM,” IEEETrans. Magn., vol. 9464, pp. 1–11, Sept. 2015.[13] A. Rasekh, P. Sergeant, and J. Vierendeels, “Development of correlationsfor windage power losses modeling in an axial flux permanent magnetsynchronous machine with geometrical features of the magnets,” Ener-gies, vol. 9, no. 12, pp. 1–17, 2016.[14] J. Pyrhonen, T. Jokinen, V. Hrabovcova, and H. Niemela, Design ofRotating Electrical Machines. Wiley, 2008.[15] H. Vansompel, P. Sergeant, L. Dupre´, and A. Van Den Bossche,“Axial-flux PM machines with variable air gap,” IEEE Transactions onIndustrial Electronics, vol. 61, no. 2, pp. 730–737, 2014.

Transient lumped parameter thermal model based loadability studies for The YASA axial flux permanent magnet synchronous machines

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Transient lumped parameter thermal model based loadability studies for The YASA axial flux permanent magnet synchronous machines

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