Study of TFPM machines with toothed rotor applied to direct-drive

generators for wind turbines

Maxime R. DuboisLEEPCI, Dept. of Electrical Engineering

Université Laval, Québec, Canada

Henk PolinderLab. of Electrical Power Processing

Delft University of Technology, Delft, The Netherlands

Overview

1- Introduction

2- Advantages of TFPM machines and review of main topologies

3- TFPM machine with toothed rotor

4- Optimization of TFPM machine with toothed rotor and conventional PM synchronous machine

5- Comparison between TFPM machine with toothed rotor and conventional PM synchronous machine

6- Conclusion

Introduction

Direct-drive Geared drive-train- avoided costs of the gearbox- no oil change- lower number of bearings less greasing- less moving parts increased reliability - less acoustical noise and vibrations - avoided friction losses of the gearbox

- lower generator mass, size and costs - power electronics converter rated 30% of nominal power, with related cost and losses

Introduction

Direct-drive Geared drive-train- avoided costs of the gearbox- no oil change- lower number of bearings less greasing- less moving parts increased reliability - less acoustical noise and vibrations - avoided friction losses of the gearbox

- lower generator mass, size and costs - power electronics converter rated 30% of nominal power, with related cost and losses

MOST IMPORTANT ARGUMENT (for now)

Advantages of Transverse-Flux PM machines

100

1000

10000

0 1 2 3 4

Machine Outer Diameter (m)

Co

st

/ To

rqu

e (

EC

U/k

Nm

)

Conventional PM

TORUS

TFPM

Conventional PMwith FC

- According to literature: TFPM machines obtain lower cost of active material- NORPIE 2000: summary of machines designs taken from literature- However: Numerous Machines = Numerous Constraints !!

Review of main TFPM topologies

Stator

Rotor

Stator

Surface-Mounted TFPM vs Flux-Concentrating TFPM

-High Current loading in both cases (typical 300 kA/m)-Strong leakage flux between magnets in surface-mounted TFPM-Higher magnetic loading and torque/mass in flux-concentrating TFPM

Review of main TFPM topologies

Stator

Rotor

Stator

Surface-Mounted TFPM vs Flux-Concentrating TFPM

-High Current loading in both cases (typical 300 kA/m)-Strong leakage flux between magnets in surface-mounted TFPM-Higher magnetic loading and torque/mass in flux-concentrating TFPM

Preferred for cost reduction

Review of flux-concentrating TFPM topologies

Stator

Rotor

Stator

Problems

Double-sided

Difficult rotorStacking

A lot of powderediron

TFPM machine with toothed rotor

Single-sided

Easy rotorinsertion

Laminated Stator

TFPM machine with toothed rotorStator before winding

Stator completed and 1 rotor Phase mounted

Optimization of TFPM machine with toothed rotor & conventional PM synchronous machine

Machine rotational speed as a function of the generator outside diameter.Generator outside diameter (m)

0.5 1.0 2.0 3.0

Wind turbine power range (kW)

10 - 30 30 - 100 100 - 200 400 - 600

Nominal rotational speed (rpm)

130 75 46 34

For a thorough comparison, we optimize both machine types with the same constraints:

-- machine outer radius-- efficiency at full load-- rotational speed

Optimization of TFPM machine with toothed rotor & conventional PM synchronous machine

Optimization procedure:

-Optimization program calculates cost/torque of thousands of designs of TFPM machines with toothed rotor for = 90% and 95%.

-The program identifies the design having the lowest cost/torque.

- Best design is fed into a 3-D finite element software for validation.

-Torque and efficiency are adjusted accordingly.

-Optimization program calculates thousands of designs of conventional PM synchronous machines having the same torque value as optimized design of TFPM machine with toothed rotor

-Identification of the conventional PM synchronous machine with the lowest cost of active material.

Optimization of TFPM machine with toothed rotor & conventional PM synchronous machine

Main assumptions of the optimization procedure:-- copper: 6 Euros/kg // lamination and powdered iron: 6 Euros/kg // PM: 40 Euros/kg

-- Manufacturing and magnetically-inactive material are not considered in the cost calculations

-- Number of phases is 3

-- In Convent. PMSM. : slots are deep (hs/bt = 4), q =1 and winding is double layer full-pitched

-- Sinusoidal terminal voltage v(t), no-load voltage e(t) and phase current i(t)

-- Sufficient forced air or liquid cooling is provided

-- PM = Nd-Fe-B with Br = 1.1 T

-- steels have linear B(H) characteristics mrFe = 1000 up to the point of saturation of 1.8 T

-- the air gap thickness g is equal to 1/1000th of the machine outside diameter

-- the slot fill factor is set to 0.6 for diameters larger than 2 m and to 0.4 for diameters below 2 m.

-- the specific eddy current losses in Fe-Si laminations at 50 Hz/1.5 T are set to 1.0 W/kg

-- the specific hysteresis losses in Fe-Si laminations at 50 Hz/1.5 T are set to 4.0 W/kg

Modeling of the TFPM machine with toothed rotor

Conventional PM Synchr. Mach.: flux lines are straight in the air gap

TFPM machine modeling: bending of flux lines cannot be neglected. We use lumped reluctances and equivalent magnetic circuits

Aligned position

Modeling of the TFPM machine with toothed rotor

Unaligned position

Tp

FF

R R

Ps pn l

s

ap up

F e

m

2 2

2 4

1 12m ax

m axco s sin

Fsmax, pnl, Rap, Rup are determined from the equivalent magnetic circuit

Comparison between TFPM machine with toothed rotor and conventional PM synchronous machine

10

100

1000

10000

0 1 2 3Diameter (m)

Co

st/

To

rqu

e (

Eu

ro/k

Nm

)

TFPM w TR Conv PM SM

Comparison between TFPM machine with toothed rotor and conventional PM synchronous machine

10

100

1000

10000

0 1 2 3Diameter (m)

Co

st/

To

rqu

e (

Eu

ro/k

Nm

)

TFPM w TR Conv PM SM

For DD WEC of 600 kW, active material = 23,000 Euros…..about 4% of WEC cost !

Conclusion

The cost/torque comparison between TFPM machines with toothed rotor and conventional PM synchronous machines was investigated,using innovative optimization and modeling tools.

For diameters of 1.0 m and below, lower cost/torque is obtained with the TFPM machine with toothed rotor. Diameters larger than 1.0 m favor conventional PM synchronousmachines, when air gap is set to 1/1000th of machine diameter.

Efficiency plays a dominant role in the cost/torque of both machinetopologies.

More attention must be paid to the optimization of the mechanical design and to manufacturing costs.