Thermal management of an axial flux permanent magnet machine considering heat pipes

Scowby, Seath (2003-03)

Thesis (MScEng) -- Stellenbosch University, 2003.

Thesis

ENGLISH ABSTRACT: Axial Flux Permanent Magnet (AFPM) machines have become attractive because of significant improvements in permanent magnets over the past decade, improvements in power electronic devices, and the ever increasing need for more efficient machines in electric vehicle systems. In comparison with the cylindrical radial flux motor, the AFPM machine is better in a number of aspects: short frame; compact construction; high efficiency; brush less construction; good starting torque and high-power density. The common modes of failure and typical operating conditions of AFPM machines are discussed further. The focus of this research project is a prototype AFPM machine developed by the Electrical Engineering Department of The University of Stellenbosch. The machine considered has a power rating of 300 kW and an operating efficiency of 95 % at a speed of 2300 rpm. This specific machine is used as an example to illustrate the thermal characteristics of geometrically similar AFPM machines. The thermal characterization was achieved with the use of two numerical computer models. Firstly a fluid model was specially developed and experimentally verified. The objective of the fluid model was to calculate the mass flow rate of air through any geometrically similar AFPM machine. The fluid model was further used to investigate the effects of different magnet thickness and axial gaps between the stator and the rotor plates on the mass flow rate of air through the machine. The fluid model was verified with experimental testing that was done on a half-scale Perspex model. During the experimental testing the magnet thickness was varied between 2.5 mm, 5.0 mm, and 7.5 mm along with axial gaps of 6.5 mm, 7.5 mm, 8.5 mm, and 9.5 mm. The fluid model showed a correlation to within 10 % of the experimental mass flow rates. The results of these tests showed that the magnet thickness and axial gap between the stator and the rotor plates had no significant effect on the mass flow rate of air. The fluid model was based on one-dimensional, steady-state, and incompressible flow. The second numerical computer model was a thermal model. This model was used to calculate the transient temperature response of the AFPM machine. The model was based on a twodimensional transient finite difference solution technique. Experimental temperatures taken from the prototype AFPM machine were used to verify the thermal model. Correlations between the experimental and theoretical temperatures were within 5.8 % of each other. The thermal model was used to investigate the effect of geometrical changes on the temperatures in the AFPM machine. It was found that these geometrical changes had no significant effect on the temperatures in the AFPM machine. It was also established that increasing the air mass flow rate over about I kg/s had no further effect on lowering the temperatures. The stator was also identified as being the most critical component as it reached its maximum temperature limit before any other component. Heat pipes were considered as an alternative thermal management technique. The location of the heat pipe was limited to the stator. Further simulations were done to investigate the effect of the heat pipe properties on the amount of heat removed from the stator. Recommendations were made concerning the thermal management of the current and possible future prototype AFPM machines. It was recommended that a further more detailed investigation into the use of heat pipes be considered. This recommendation is substantiated by the fact that in this research project only one type of heat pipe was considered and its location was limited to within the stator.

AFRIKAANSE OPSOMMING: AFPM masjiene het meer aantreklik geword weens betekenisvolle verbeteringe in permanente magnete gedurende die laaste dekade, verbeteringe in elektroniese toestelle en die vraag na meer effektiewe masjiene in elekriese voertuigstelsels. Die AFPM masjien is beter as die Silindriese Radiale Fluksie Motor wat die volgende aspekte betref: die kort raamwerk; kompakte konstruksie; hoe effektiwiteit; borsellose konstruksie; goeie aanvangsdraaimoment; en hoe-krag digtheid. Die algemene vorms van faling en ook die tipiese werkstoestande van die AFPM word verder bespreek. Hierdie navorsingsprojek fokus op die prototipe AFPM masjien wat ontwikkel is deur die Elektriese Ingenieurs Departement van die Universiteit van Stellenbosch. Die masjien onder bespreking wek 300 kW per uur op en is 95% effektief teen 'n spoed van 2300 rpm. Hierdie masjien word gebruik om die termiese kenmerke van geometries-gelyksoortige masjiene te illustreer. Die termiese eienskappe is bepaal deur die gebruik van twee numeriese rekenaarmodelle. Eerstens is 'n vloeistofmodel spesiaal ontwerp en eksperimenteel geverifieer. Die doel van die vloeistofmodel was om die massa vloeitempo van lug deur enige geometries-gelyksoortige AFPM masjien te bereken. Die vloeistofmodel is verder gebruik om die uitwerking van verskillende magneetdiktes en aksiale gapings tussen die stator en die rotorplate op die massa vloeitempo van lug deur die masjien te ondersoek. Die vloeistofmodel is geverifieer deur eksperimentele toetsing wat gedoen is op 'n halfskaal Perspex model. Tydens die toetsing het magneetdiktes gewissel tussen 2.5 mm, 5.0 mm en 7.5 mm en die aksiale gapings tussen 6.5 mm, 7.5 mm en 9.5 mm. Die vloeistof model het 'n korrelasie van binne 10 % van die eksperimentele massa vloeistempo getoon. Die resultate van hierdie toetse het getoon dat die magneetdiktes en die aksiale gapings tussen die stator en die rotorplate geen noemenswaardige uiterking op die massa vloeitempo van lug gehad het nie. Die vloeistofmodel is gebaseer op een-dimensionele, gestadigde, onsamedrukbare vloei. Die tweede numeriese model was 'n termiese model. Hierdie model is gebruik om die transiente temperatuur respons van die AFPM masjien te bereken. Die model is gebaseer op 'n tweedimensionele, transiente eindige-verskil oplossingstegniek. Eksperimentele temperature gemeet op die prototipe AFPM masjien is gebruik om die termiese model te verifeer. Die eksperimentele en teoretiese temperature het binne 5.8% met mekaar gekorrelleer. Die termiese model is gebruik om die uitwerking van geometriese veranderinge op die temperatuur in die AFPM masjien te ondersoek. Daar is gevind dat hierdie geometriese veranderinge geen noemenswaardige uitwerking op die temperature van die AFPM masjien gehad het nie. Daar is ook vasgestel dat 'n vermeerdering in die lug massa vloeitempo yerby I kg/s geen verdere uitwerking het op die verlaging van die temperatuur gehaad het nie. Die stator is ge-identifiseer as die mees kritiese komponent aangesien dit sy maksimum temperatuur limiet bereik het voor enige ander komponent, Hittepype is oorweeg as 'n alternatiewe termiese bestuurstegniek. Die plasing van die pype is tot die stator beperk. Verdere simulasies is uitgevoer om die uitwerking van die hittepyp eienskappe op die hoeveelheid hitte wat verwyder word van die stator te ondersoek. Aanbevelings is gemaak m.b.t die termiese bestuur van die huidige en moontlike toekomstige prototipes van AFPM masjiene. Daar is aanbeveel dat daar in meer besonderhede ondersoek ingestel word na die gebruik van hittepype. Die rede hiervoor is dat daar in hierdie studie net gebruik gemaak is van een tipe hittepyp en dat die plasing daarvan beperk is tot binne die stator.

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