Study of TFPM machines with toothed rotor applied to direct-drive generators for wind turbines Maxime R. Dubois LEEPCI, Dept. of Electrical Engineering Université Laval, Québec, Canada Henk Polinder Lab. 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 - 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 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 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 Problems Double-sided Difficult rotor Stacking A lot of powdered iron

TFPM machine with toothed rotor Single-sided Easy rotor insertion Laminated Stator

TFPM machine with toothed rotor Stator 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) Wind turbine power range (kW) Nominal rotational speed (rpm) 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 (h s /b t = 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 B r = 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 F smax,  pnl, R ap, R up are determined from the equivalent magnetic circuit

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

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 synchronous machines, when air gap is set to 1/1000 th of machine diameter. Efficiency plays a dominant role in the cost/torque of both machine topologies. More attention must be paid to the optimization of the mechanical design and to manufacturing costs.