PM GENERATOR TOPOLOGY

The objective of this paper is to provide a comparison among permanent magnet (PM) wind generators of different topologies.

I. Introduction

Most low speed wind turbine generators are PM machines. PM generators can be divided into radial-flux macines and axial-flux machines.

II. Machine topologies investigated

Conventional Inner Rotor Radial-flux Machine

A Permanent magnet poles rotating inside stationary armature windings.Stator made from electrical grade steel laminations.Rotor is cylindrical shaped with a shaft for the bearingsTwo magnets provide MMF required in pair of poles.

Radial-flux Machine with Outer Rotor

B Wound stator in the outer rotor configuration is stationary.Magnets are mounted evenly along the inner circumference of the rotating drum.Blades of the wind turbine are bolted to the face of the drum.Improved cooling condition.

Double Stator Slotted Axial-flux Machine

C Pancake shaped stator and rotor.Two external stators and one inner rotor.Permanent magnets axially magnetized.Stator iron core is laminated.

Double Rotor Slotted Axial-flux Machine

D One stator two rotors.Stator located in the middle.An iron flux path is needed on the rotor back of the yoke.

Single sided Axial-flux Machine with Stator Balance

E Only one stator and only one rotor.Special thrust bearing must be used and an additional stator.Permanent magnets are on the opposite side of the rotor.Stator is laminated.

Single Sided Axial-flux Machine with Rotor Balance

F Adds an additional rotor.Stator yoke length is extended.An iron flux path is located n the additional rotor back.

Axial flux Machine with Toroidal Winding

G Torus machine.Slotless, double-sided, axial flux, permanent magnet, disc-typed machine.Coils have rectangular shape, active conductor lengths are two radial portions.

III. Basis for comparison

We have used 3-bladed, direct-drive Horizontal Axis Wind Turbine taking into consideration the relation between radius, speed and power rating.

Rated phase voltage 220 VPower ratings for output power 1,10,20,50,100,150,200Rated speed 100 rpm for machines below 50 KW and 50

rpm for machines above 50 KW1200 rpm machines Gearbox and 60Hz frequency, 6 poles

Speed is related to the EMF frequency f, number of pole pairs p, armature diameter D and pole-pitch т

- low speed is acquired by increasing number of pole-pairs along with increasing armature diameter and reduction of pole-pitch.

- enlargement of armature diameter increases volume, pole-pitch reduction meets the feasibility limit.

- slot fill is calculated with a fixed slot insulation thickness and using square cross sectional wire.

- ratio between outer radius to inner radius is 2.5.

- tooth and yoke are 1.5 for all designs.

- current density is 4x106 A/m2. Three phase windings are Y connected.

- Magnets used are NdFeb with remanance of 1.0 T and coercivity of 750 KA/m.

Critical coefficients magnetic field waveform coefficient, leakage coefficient and armature reaction coefficient have been derived analytically.

- amplitude of air-gap flux density.

- fundamental amplitude of air-gap flux density.

IV. Results

Direct Drive Machines High-speed machinesTorque/volume and Torque/weight vs. Power

D – double rotor slotted axial flux configuration has the highest torque densityE,F – single sided machines with stator and rotor balance have the lowest values

D – has the highest valueE, F – have the lowest ones

Magnet Weight vs. PowerG – axial flux construction requires maximum weight Double sided axial flux topologies can make best use of the magnets.

- for the same output power high speed machines use less magnet as the speeds of these machines are much higher

Copper weight vs. powerMaximum copper is required by inner rotor radial flux machineSingle sided axial flux configuration uses the least copper

The increase of the lamination use for axial flux configurations in this case is because an enlarged axial length is needed to provide a path for the magnetic flux in the stator yoke as the pole number is lower

Lamination Weight vs. PowerMaximum value of lamination is used by the single side axial flux machinesTorus machine uses the minimum amount of laminations because of the absence of stator teeth.

Increased lamination used for axial flux configurations in this case is because an enlarged axial length is needed to provide a path for the magnetic flux in the stator yoke, as the pole number is lower

Total Volume vs. PowerRadial flux configurations take the maximum space.The double stator axial-flux slotted machine requires the least space

Single sided axial-flux constructions need more room due to the lower pole number and the increased axial length.

Active Material Volume vs. PowerMaximum active materials are spent by single sided axial flux machinesThe double rotor uses the minimum amount of active materials

Increased active material for the Torus machine because enlarged axial length is needed because of the lower pole number.

Efficiency vs. PowerHighest efficiency exists in the double rotor axial-flux machineRadial flux configurations have the lowest efficiency

Single sided axial-flux constructions are not efficient

Direct Drive Machines High-speed machinesOuter Radius vs. PowerSingle sided axial flux constructions need the largest outer radius as there is only one stator in this constructionRadial flux configurations have the minimum outer radiusTotal Length vs. PowerRadial flux construction have a much longer axial length.Axial length for axial-flux machines is much shorter than the radius

V. Conclusions

Axial slotted machines have the best volume/power ratio.Two-sided axial-flux configuration is superior to the one side axial-flux configuration.Outer rotor radial-flux construction is superior to the inner rotor radial-flux.The simplest construction is Torus.Low speed constructions are superior to high speed constructions.Multi-pole PM generators are preferred in the application of small, gearless, low speed wind systems.