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Permanent Magnet Machines for Distributed Generation:A Review

Paper Number: 07GM0593

Authors:Tze-Fun Chan, EE Department,The Hong Kong Polytechnic University, Hong Kong, ChinaLoi Lei Lai, School of Engineering and Mathematical Sciences, City University, UK

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Outline of this presentation

IntroductionTypes of permanent-magnet synchronous generator (PMSG)Radial-flux PMSG for isolated operationLinear PMSGAxial-flux PMSGVariable-speed operation

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Full exploitation of local energy resourcesAutonomous operation reduces the need for grid connection and transmission lossesImproves reliability /security of supplyGenerators types:

Induction generators Wound-field synchronous generatorsPM synchronous generatorsSwitched-reluctance generators

Advantages of distributed generation

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Advantages and disadvantages of PMSG

Advantages:Brushless constructionLight weight amd small sizeHigh reliabilitryHigh efficiencyLess frequent maintenance

DisadvantagesExcitation is fixedOutput voltage varies with load

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PM materials and PM machines

Types of PM material:Alnico (since 1940s)Ferrites (since 1950s)SmCo (since 1960s)NdFeB (since 1980s)

Types of PM machines:Surface magnet typeInterior magnet typeSurface inset type

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Radial-flux PMSG for isolated operation

Research work:New machine configurations (Binns 1983)Performance analysis using 2-axis modelPerformance using finite element method (FEM) (Chen, Nayar and Xu,1998; Chan, Yan and Lai, 2004, 2005)Voltage regulation improvement by capacitor compensation (Rahman, 1996)Voltage regulation improvement by exploiting rotor inverse saliency

Interior type (Chalmers, 1994)Surface inset type (Chan, Yan and Lai, 2004)

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A surface inset PMSG

Stator core

Rotor core

Permanentmagnet

T2T1

LSoft-iron pole piece

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Phasor diagrams of PMSG

(a) Unity power factor (b) Lagging power factor

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Analysis (1)

From phasor diagram for lagging power factor the following equations may be written

From the equations:

RIXI E V qdd −−=δcos

RIXI V dqq −=δsin)sin( φδ + IId =

)cos( φδ + IIq =

φφφφ

δcossin

sincostan

R XZ R X

qL

q

++−

=

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Analysis (2)

The output voltage is

For given load current and power factor angle, the following formula may be using to find V:

)sin()cos(cos.

φδφδδ X + R ZZE V

dL

L

+++=

)()cossin(2

)(sin)(cos22222

222

qq

qdqd

XRI R XVI.V

XXRI XXVI VI.R V = E

++++

+++++

φφ

φφ

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Condition for zero voltage regulation

Zero voltage regulation occurs when

For machine with zero resistance

where r = Xq/Xd = inverse saliency ratioZero voltage regulation possible when r >2

)tan()tan(

2tan

φδφδδ + RX

+XR q

d

−+

=

rr 2

2tan −

=

δ

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Effect of speed on voltage of PMSG with surface inset rotor (Chan, Yan, Lai, 2004)

Parameters at nominal speed: E = 66.44 V, R = 0.295 Ω, Xd = 0.88 Ω, Xq = 2.23 Ω.At 4 times nominal speed a level voltage char. is obtained.

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Air gap flux density obtained by finite element method (Chan, Yan, Lai, 2005)

-1.2-1

-0.8-0.6-0.4-0.2

00.20.40.60.8

11.2

0 100 200 300 400 500 600 700

Electrical angle (deg.)

Flux

den

sity

(T)

Strong flux through interpolar soft iron results in increased output Voltage and hence improved voltage regulation

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Experimental characteristics of PMSG with inset rotor (Chan, Yan and Lai, 2005)

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High-speed PMSGs

Suitable for DG driven by micro-turbinesApplicable in regions with abundant natural gas resourceGenerator designed to run at high speedsMain technical issues (Wang, 2002):

Electromagnetic designReduction of iron / stray lossesBearings for high-speed operationCooling problem

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Rotor of a high-speed PMSG

Retaining ring or tape

Permanent magnetRotor

yoke

Shaft

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Linear PM machines

Linear electric machines (LEMs) involve translational motion instead of rotational motionFor power generation, short-distance and oscillatory motions are involvedTypical application is as a wave generator that captures the energy of perpetual wave motion, e.g., the Archimedes Wave Swing (AWS) (Polinder, 2004)Different machine designs are possible, e.g. transverse flux PMSG (Polinder, 2005), tubular PMSM (Amara, 2005)

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A simple linear PMSG

Permanent magnet

Coil

Nonmagneticstop

Core

Nonmagnetic collarLinear, oscillatory

motion

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Axial-flux PMSG

Electric machine with flat annulus air gapMain flux in axial direction, active conductors in radial directionsSuitable for low-speed, direct drive wind energy systems because large number of poles can be accommodated

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Types of axial-flux PMSG

Modular PM generator with toroidal stator winding (Muljadi, 1999)‘Torus’ generator with double-sided stator (Wu and Chalmers, 1995 and 1999)Axial PMSG with toothed stator core (Hwang, 2004; Parvianen, 2005)Axial PMSG with coreless air gap winding (Chan and Lai, 2007)

Outer rotor, single-sided designZero iron loss, cogging torqueZero magnetic pullSuitable for both vertical or horizontal axis wind turbines

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Test rig for outer-rotor axial-flux PMSM(Chan and Lai , 2007)

The outer-rotor axial-flux PMSG is driven by a dc motor through a belt drive in order to emulate the wind turbine

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Test results

The experimental machine can deliver 350 W when delivering rated current at a speed of 600 r/minOutput voltage is almost sinusoidal

Line voltage waveform of axial-flux PMSG on load

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Variable-speed PMSG connected to grid

Variable-speed operation needed to optimize energy capture from the windFrequency converter is required between generator and the power network Maximum power control strategy needs to be devised

Using rectifier, boost chopper and inverter (Amei, 2002)Using PWM rectifer, intermediate dc circuit and PWM inverter (Chinchilla, 2006)

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Maximum power control of PMSG with grid connection

Variable-speedwind turbine

PMSGPWM

rectifierPWM

inverterStep-up

Transformer

Maximumpowercontrol

Grid

Pitch control

Duty cycle control

DClink

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Conclusions (1)

PMSG is suitable for distributed generationAdvantages are compactness, light weight, and high efficiencyVariable machine configurations are possible

Radial or axial flux machinesLinear machinesOuter-rotor designLight-weight design (e.g., spoke-wheel concept)Torus machinesToothless or coreless designs

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Conclusions (2)

Future work on PMSG might include:Refined performance analysisLoss or thermal modelsDesign optimizationApplication of finite element analysisControl for grid integrationNiche application areas, such as hybrid EVs, micro-turbine generators, etc.