Dr.-Ing. Andreas Vath Bosch Rexroth AG Wind Turbine Drive Train ...

Bosch Rexroth AG 

Wind Turbine Drive Train Innovations 

and Load Reduction Solutions 

Dr.-Ing. Andreas Vath 

Bosch Rexroth AG 

1 

A. Vath | Wind Turbines Innovations | Sept. 2012 | © Bosch Rexroth AG 2010. All rights reserved, also regarding any disposal, exploitation, reproduction, 

distribution, as well as in the event of applications for industrial property rights.

Content 

• motivation 

• combining benefits of existing drive trains for wind turbines 

• direct drive 

• high speed 

• medium speed 

• differential gearbox benefits in medium speed concepts 

• Bosch Rexroth load reduction solutions 

• Active Torque Control (ATC) 

• Individual Pitch Control (IPC) 

• summary 

2 



Motivation for drive train innovations and load reduction solutions 

Design challenge: Reducing cost of energy 

Onshore: Larger rotors 

1.5 - 3 MW 

series 

4 MW 

development 

Offshore: Larger turbines 

3 MW 

series 

5 -7 MW 

prototype 

>10 MW 

transport 

restrictions 

future 

higher drive train requirements 

drive train innovations 

reliable 

efficient 

low cost 

load reduction solutions 

for wind turbines 

3 



Content 










• summary 

4 



Comparison of drive train efficiencies 

direct drive high speed medium speed 

direct 

drive 

drive train efficiency 

medium speed concept 

high speed concept 

direct drive 

efficiency 

medium speed concept 

high speed concept 

wind speed in m/s 

Medium speed concepts: 

- high partial load efficiency 

- high drive train full load efficiency of 94% 

(gearbox >98.0%; generator 98.3%; 

frequency converter 97.5%) 

⇒ high energy yield at high average 

wind speeds 

Medium-speed concepts: High efficiency leads to high energy yield 

5 



Drive train weight and raw material cost of a 3 MW wind turbine 

direct drive high speed medium speed 

weight in tons 

drive train weight 

magnets 

copper 

iron 

cost in t€ 

raw material cost for drive train* 

rare earth 

copper 

iron 

0 

direct 

drive 

medium 

speed 

high 

speed 

direct 

drive 

medium 

speed 

high 

speed 

*Neodymium 120 €/kg, Dysprosium 1300 €/kg, Copper 7 €/kg, Iron 1 €/kg 

low amount of raw materials 

low drive train material costs 

Drive train weight and cost advantage with gearbox concepts 

6 



Cost of energy comparison for an example onshore turbine 

Main assumptions 

• rated power 3MW 

• rotor diameter 115 m 

• 6.5 m/s average wind speed 

• 20 years design life time 

• raw material cost: 

- copper 7€ per kg 

- steel 1€ per kg 

- magnets 100€ per kg 

• complete wind farm cost 1700 to 

1900 € kW depending on drive 

train concept 

Cost of energy [€/kWh] 

8.5 

8.0 

7.5 

7.0 

6.5 

6.0 

5.50 

Innovations 

for further 

cost 

reduction 

1 2 3 4 5 

• 97% availability of all concepts 

• operation & maintenance costs for 

complete turbine (additional costs 

for gearbox oil change considered) 

• same blade, hub and tower design 

of different drive train concepts 

• higher cost of energy of direct drives due to the high 

amounts of raw and rare earth material 

• lower cost of energy with high speed and medium 

speed solutions 

FC: frequency converter; SESG: separately excited synchronous generator, PMSG: permanently excited synchronous generator; 

DFIG: doubly fed induction generator; HTS-SG: high-temperature superconducting synchronous generator, 

7 



Content 










• summary 

8 



Bosch Rexroth differential gearbox for medium speed concepts 

torque splitting stage 

differential stage 

for torque summary 

i = 10 -15 

input 

torque: 

i = 3-4 

output 

torque 

torque splitting topology 

with equal load distribution 

to 6 planets 

+ equal load distribution to 6 planets 

+ statically defined system 

+ reduced loads on bearings/toothing 

i total = 30-60 

proven concept in 

high speed drive train concepts 

for more than 7 years and 

more than 1300 installed units 

+ higher reliability 

+ higher power density 

+ less material consumption 

High gearbox ratio possible with torque splitting and differential stage 

9 



Effect of higher gear ratio for medium speed concepts 

Investment costs 

100 % 

generator 

gearbox 

i=40 

generator 

gearbox 

i=60 

• higher generator speed means less 

torque at same rated power 

• generator costs are proportional to 

the generator torque 

⇒ generator cost reduction with 

Bosch Rexroth differential 

gearbox REDULUS GPV 

conventional 

design 

Bosch Rexroth 

Design REDULUS GPV 

⇒ Reduced drive train costs with differential gearbox REDULUS GPV 

10 



Content 










• summary 

11 



Load reduction solutions 

Active Torque Control (ATC) 

Active damping of 

dynamical loads 

1 

Active torque control 

1 

2 

Active torque 

control 

Individual pitch 

control 

• Dynamic control of 

drive train torque to 

reduce peak loads and 

Future wind turbines: 

Load-reduced operation 

torque oscillations 

2 

1 

⇒ higher drive train 

lifetime and reliability 

Preventing and reducing 

loading by means of 

innovative approaches 

⇒ lower cost of energy 

Load reduction for torque with active torque control 

12 



Sensor and control system for Active Torque Control 

SCU 

Control unit 

Sensor 

Drive train 

torque 

Generator torque 

adjustment 

• Dynamic measurement of 

drive train speed by using 

adapted automotive 

sensors 

• Calculation of the drive 

train torque 

Patented by Bosch Rexroth 

• Control strategy to reduce 

drive train torque and 

speed oscillations by 

generator torque 

adjustment 

13 



Torque [kNm] 

Measurement results of Active Torque Control 

Drive train torque and speed 

1750 

1500 

1250 

1000 

750 

500 

250 

without 

ATC 

Drive train torque 

with ATC 

0 

0 10 20 30 40 

Time [s] 

Speed [rpm] 

15 

14.9 

14.8 

Drive train speed 

with ATC 

without ATC 

14.7 

15 16 17 18 19 

Time [s] 

Reduction of drive train 

speed oscillations as a 

result of less drive train 


High reduction of drive train torque and speed oscillations 

14 



Active Torque Control of wind turbines 

Drive train torque 

without ATC 

with ATC 

1-10% 

F 

without 

ATC 

with 

ATC 

Force at tooth x 

Damaging loads 

1-10% 

F 

Bearing force 

without 

ATC 

Damaging loads 

Reduction of equivalent damage loads at toothing and bearings 

with 

ATC 

15 

A. Vath | Active Torque Control| Sept. 2012 | © Bosch Rexroth AG 2010. All rights reserved, also regarding any disposal, exploitation, reproduction, distribution, as 

well as in the event of applications for industrial property rights.

Active Torque Control of wind turbines 

Drive train benefits 

Benefits of active torque control 

• Increase of nominal power when using active torque control 

⇒ higher energy yield 

• Less blade oscillations and therefore better aerodynamics 

⇒ higher energy yield 

• Advantages for gears and 

bearings due to lower 


• Structural advantages due 

to less deformations 

caused by oscillations 

• Increase of bearing 

lifetime because of less 

speed oscillations 

16 



Drive train innovations and load reduction solutions 

Load reduction: Individual pitch control 

Active damping of 

dynamic loads 

2 

Individual Pitch Control 

1 

2 

1 

Active torque 

control 

Individual pitch 

control 

Preventing and reducing 

loading by means of 

innovative approaches 

2 

• Measurement of bending 

moments in each blade 

• Controller for bending 

moment reduction 

• Continuously pitching of 

each blade 

Bending moment 

reduction in % 

blades tower 

main 

shaft 

Future wind turbines: 

Load-reduced operation 

Effect on wind turbines: 

Reduced weight and cost and 

increased energy generation 

Load reduction for bending moments up to 25% with individual pitch control 

17 



Content 










• summary 

18 



Summary 

Drive train innovations 

• Wind turbines with reliable high speed and medium speed drive concepts 

will provide the lowest cost of energy 

• High drive train cost reduction with the Bosch Rexroth differential gearbox 

REDULUS GPV in medium speed solutions 

Load reduction solutions 

• Bosch Rexroth active torque control significantly reduces drive train 

torque and speed oscillations higher reliability, higher energy yield 

and reduction in cost of energy 

• Individual pitch control contributes to load reduction 

• Load-reduced operation necessary for further optimization of future wind 

turbines 

19 



Bosch Rexroth: Components and solutions for wind turbines 

electric 

pitch systems 

gearbox cooling & filtration 

gearboxes 

service 

hydraulic 

pitch systems 

rotor blade 

condition monitoring 

brake and hydraulic 

power Units 

Thank you very much for your attention! 

See you at our booth! Hall3 D12 

20

Dr.-Ing. Andreas Vath Bosch Rexroth AG Wind Turbine Drive Train ...

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