8/14/2019 Design and Construction of a Permanent Magnet Axial Flux Synchronous Generator (For micro generation system).pdf

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Des ign and Construction of a Permanent MagnetAxial Flux Synchronous Gene ratorRogel R. Wallace* Thomas A. Lipo** Luis A . Moran* Juan A . Tapia** Department of Electrical Engg.University of Concepcion

P.O. Box 53-CConcepcion - CHILEAbstract- This paper presents the design characteristics,

special features, and manufacturing aspects of a permanentmagnet axial flux synchronous generator. This machine isaimed for applications in the low power low speed range (5kVA 500 rpm). A central stator configuration is selected forthe construction of the axial flux generator, so that the statorleakage inductance is significantly reduced.

I. INTRODUCTIONElectric generators using axial flus conf$yation were

developed almost 150 years ago. However, their applicationshave always been limited to fractional power due to theconstruction difficulties, specially the heat dissipation fromthe armature windings [11431. However, compared totraditional machines using radial fl us distribution, asia l f l u sgenerators present the following advantages:i) Better efficiency.ii) They operate with similar magnetic lus density inall the magnetic circuit, which translates in a betterutilization factor of the magnetic core. Also the axial flusconstruction takes advantage of the anysotropiccharacteristic of the oriente d grai n silicon iron [?iii) Axial f l u configuration allows the construction ofcompact electric generators with a large number of poles(low rated frequency).So far no significant effort has been done to develop axialflux permanent magnet synchronous generator for operationin micro-generating systems. In this paper, a completedesign procedure for such electric mach ine is developed andthe manufacturing aspects for the construction of a 5 kVA500 rpm 50 Hz prototype is shown. T he innovative aspectsof the axial flus synchronous generator presented in thispaper are the following:i) The bulk and the lateral covers of the generator aremade with a non ferromagnetic alloy. This reduces the statorleakage inductance and the corresponding magnetic losses.A bronze-aluminum material is used (90 copper and10 alumin um), since this material is more resistant andlighter than the traditional bronze. Also, the bronze-aluminum is easier to cast in induction or Morganfurnaces.ii) Preliminary studies have shown that the efficiencyimprovement of the axial flus generator is around 5 .

** Electrical & Computer Engg.University of Wisconsin1415 Johnson DriveMadison,WI 53706 - U.S.A.

Moreover, due to the absen ce of the increa sing speedmechanism, the overall system efficiency of the micro-generating station increases between 20 and 50%.iii) Permanent rare earth magnets (neodymium-iron-boron) are used in order to avoid the slip rings to supplythe field rotor wind ings. This allows the constru ction ofshielded and more compact units.It is important to emphasize that even though small untsof axial flus generators have been implemented and reportedin the technical literature [l] they are inadequate forindustrial applications. The principal problems of suchprototypes dealt with the reduced number of poles whichresults in larger stator windings making impossible theconstruction of power shielded units. The design of axiaflus generators with a larger number of poles forapplications in micro-generating systems represents a newtechnological development and design of these machmes.

11 GENERAL DESCRIPTION OF THE GENERATORFor the construction of the axial flus generator a hvoairgap central s tato r configuratio n was chosen in order tocancel the longitud inal forces over the rotor pieces.In Fig. 1 the central stator topology of the generator isshown. Th e perm anent magnets are embedded in the rotorlateral pieces mounted on the shaft. A yoke of silicon ironsheets closes the magnetic circuit by the back side of themagnets.The stator is com posed by teeth formed by stacks of siliconiron sheets arranged in radial form in which the three-phase

Do

Figure 1:Topology of the Axial Flu s G enerator

0 7803 3946 0/97/S10.00 1997 EEE. MA1-4.1

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winding is houscd. Th csc tcctli are fis cd by two bronzc disksin their ends, this gcomctry allows to obtain opcn slotswith low leakage reactanccs and minimal losses of matcrialat the moment of the pattcrn-making. Morcovcr. a skcwingof the teeth with one slot pitch is considered i n order toavoid the cogg ing torque.A . Windings

The stator windings must fulfill tlic followiiigAvoid wire crossing spccially i n tlic cstcrnal part of tlieReach th e best copper utilization factor in tlic slots.Obtain a sinusoidal distribution of the nimf i n order to

-The winding factor has to be high enough so that the

requirements:end winding.

avoid torque pulsqt' ons.rated power of the generator is not reduccd.

Figure 2: Linear reprcscntatioli of the windingsTh e construction of two coricciitric windings a s sliowi i nFig. 2 satisfies most of the abovc rcquircmcnts. Tlic i i imrgcneratcd by the winding can bc csprcsscd by tlic Fouricr

series:nm( ( x ) = c b,,sin nx) (1)

n = 1.3, ...

For a masitnum value of mmf(s) equals to 1 tlic Fouricrcocflicients are shown in Table I.TN3LE

FOURIER COEFFICIENTS OF T11E MMT: I I l3Y A I IIASEWINDING

whcre:l is the distribution factor

f2 is tlie pitch factorq = 2q = 0 elcctrical degrees corresponding to oiic sloty = 1T 6

number of slots per phasc and pcr polccoil span in numbcr of slotspolar pitch in numbcr of slots

In tliis case fl 0.966 and fi = 0.866. Thcse factors werecalculatcd for a two layer winding corrcsponding to tlicconventional generator. Finally the winding factor f3) isgivcn by:

It must bc noticed that tlic winding docs not gcneratc athird harmonic and that the fifth and scvcnth harmoniccoiiiponcnts arc small. Thus tlic f lus distribution rcsullsaliiiost sinusoidal since tlie dominant rcluctance harmonicgencratcd by the slot (due to tlic salicncy of the stator tccth)is tlie sixth. When tlie reluctance harmonic and theharmonic component gcneratcd by the winding havc tliesame frequency tlie cogging to rque increascs.Figurc 2 shows that the coppcr mire ocuppies about 213the space of the slots which results in an utilization factorcquals to 0.46 for wire with circular cross scction and 0.60for wirc with squarc cross scction. In convcntionalgaicrators tlie w inding u tili 7~t ion actor is in tlic range or0.50 to 0.55 for wircs with c ircula r cross scctioii.B. Electrwiingnetic Design

The magnctic circuit corresponding to a pair of poles isfornicd by tlic tccth of tlie stator, two airgaps, fourpcrniancnt magn cts an d two rotor yokes.Tlic dctcrmination of the perniancnt magnct width wasdone by using traditional clectromagnctic calculations. Alsotlic rotating ficld powcr was masimizcd. ~~4iichcsults i ncqual cross scctions of tcctli and slots111 CONI:IGUIIATION F I E ANAL FLUS GENERATORwrrri

PERhlANENT h4AGNE TSn b n I? n h,,1 1.065 0 17 0 0173 11 0.097 19 0.0155 0.OS7 13 -0.82 21 -07 0.04 1 1 5 0 23 0.046

Th e winding factors for tlic fiind;imcntal coniponcnt o f tliemmf are dcfined by the following csprcssions:

Figrirc 3 sliows tlic gencral drawing of tlic prolotypc. Tlicprincipal parts of thc generator arc tlic followings:

1. Bulk of tlic gciicrator2 . Latcral cover 7. Shaft3. Pcrmancnt magncts4. Rotor yoke5 . Rolor Iatcral piccc

6 . Bearing8. Holding stator disk9 . Stator tccthIO. Stator holding ring

M A 1 4 2

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