Gasch R., Twele J. (Eds.) Wind Power Plants: Fundamentals, Design, Construction and Operation

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3.2 Drive train

Rotor blade

Tower

Blade adapter

Brake

Main carrier

Yaw drive

Winch

Generator/ rotor

Generator/ stator

Rotor blade,

pitch controlled

Pitch motor

Axial pin

Spinner

Rotor blade

Tower

Blade adapter

Brake

Main carrier

Yaw drive

Winch

Generator/ rotor

Generator/ stator

Rotor blade,

pitch controlled

Pitch motor

Axial pin

Spinner

Fig. 3-30 E-66, gearless drive train (ENERCON)

Gearbox

Generator

Platform

Hub

Yaw drive

Yaw brakes

Tower

Hydraulic

supply

Mechanical brake

Housing

Gearbox

Generator

Platform

Hub

Yaw drive

Yaw brakes

Tower

Hydraulic

supply

Mechanical brake

Housing

Fig. 3-31 N 3127, integrated design of the drive train (SÜDWIND, 1996 – today SUZLON)

3 Wind turbines - design and components

As a mixed design type, the partially integrated drive train became established as

wind turbine size increased in the mid 1990s, beginning with the 500 to 600 kW

class wind turbines. Nearly all wind turbine manufacturers chose the so-called

three-point support of the rotor shaft, Fig. 3-33. The main rotor bearing close to

the rotor bears most of the gravitational loads of the rotor as well as the axial

thrust. A second separate bearing does not exist, but the forces are transferred

from the rotor shaft to the low-speed shaft of the gearbox with a hydraulic or me-

chanical clamping unit. The forces are then transferred to the nacelle frame via the

torque support on each side of the gearbox which has a rubber-bonded-to-metal or

elastomer mounting.

The type of power limitation is not relevant when choosing the drive train con-

cept. All concepts are found in both stall and pitch controlled wind turbines. If one

analyses the different drive train concepts in terms of their chronological devel-

opment, one can see that there was a large variety at the beginning of the 1990s

with partially integrated tending to dominate in the mid 1990s. However, current

trends in the Multi-MW class once again reflect greater variety.

Spur gearbox

Safety brake

Generator

Wind sensors

Pitch cylinder

Hydraulic supply

Nacelle frame

Sound damping

Yaw drive

Blade pitch

bearing

Hub

Hydraulic

pitch system

Spur gearbox

Safety brake

Generator

Wind sensors

Pitch cylinder

Hydraulic supply

Nacelle frame

Sound damping

Yaw drive

Blade pitch

bearing

Hub

Hydraulic

pitch system

Fig. 3-32 D4, integrated design of the drive train (DeWind)

3.2 Drive train

Yaw drive

Safety brake

Torque support Left elastomer bearing Support

Yaw drive

Rigid hub

Gearbox

Torque support Right elastomer bearing

Rotor main bearing

Yaw drive

Safety brake

Torque support Left elastomer bearing Support

Yaw drive

Rigid hub

Gearbox

Torque support Right elastomer bearing

Rotor main bearing

Fig. 3-33 N-60, partially integrated design with main shaft supported at three points (Nordex)

Pitch cylinder

Ultrasonic

anemometers

Rotor blade

Yaw drives

Spinner

Hub

Blade bearing

Oil cooler

Water cooler

for generator

Transformer

Top controller

and converter

Overhead

crane

Generator

Gearbox

Pitch cylinder

Ultrasonic

anemometers

Rotor blade

Yaw drives

Spinner

Hub

Blade bearing

Oil cooler

Water cooler

for generator

Transformer

Top controller

and converter

Overhead

crane

Generator

Gearbox

Fig. 3-34 V-90, D = 90 m, P = 3 MW, integrated drive train design (Vestas 2003)

For years the company Vestas, e.g., preferred the partially integrated drive train

concept up to the 2 MW class (V-80). However, the following 3 MW wind turbine

(V-90) shows an integrated drive train concept, Fig. 3-34. The company REpower

Systems abandons the partially integrated drive train concept of their smaller

Multi-MW wind turbines but introduces their 5MW wind turbine (5M) with a

modular drive train, Fig. 3-35. The rotor shaft is supported by two bearings in

order to reduce rotor loads entering the gearbox; moreover, the replacement of the

gearbox without dismantling the rotor is facilitated.

3 Wind turbines - design and components

Safety brake

Yaw drives

Hub

Gearbox

Rotor blade

Blade bearing

Nacelle frame

Jib crane

Rotor bearings

Generator

Coupling

Safety brake

Yaw drives

Hub

Gearbox

Rotor blade

Blade bearing

Nacelle frame

Jib crane

Rotor bearings

Generator

Coupling

Fig. 3-35 5M, D = 126 m, P = 5 MW, modular drive train design (REpower 2004)

Safety brake

Torque support Elastomer bearing Yaw drive

Hub

Rotor main bearing Rigid coupling Gearbox

Yaw drive

Nacelle frame with screwed joint

Cooler Generator

Coupling

Vibration

damping

Safety brake

Torque support Elastomer bearing Yaw drive

Hub

Rotor main bearing Rigid coupling Gearbox

Yaw drive

Nacelle frame with screwed joint

Cooler Generator

Coupling

Vibration

damping

Fig. 3-36 D8, partially integrated drive train design with split, cast nacelle frame and main shaft

supported at three points (DeWind 2002)

The nacelles frames of wind turbines are either casted or welded, Fig. 3-36. For

the MW class wind turbines the nacelle frame is often split in two parts due to

production requirements of the big part dimensions and high masses. The maxi-

mum capacity of foundries is reached if the weight of casted parts for Multi MW

wind turbines reaches up to 100 t.

The wind turbines of the Finnish wind turbine manufacturer WinWind and the

Multibrid M5000 of the company Multibrid GmbH (formerly Prokon Nord) repre-

sent a mixed type for the nacelle frame The latter operates with a medium speed

multi-pole generator of up to 150 rpm, so there is no need for separate spur gear

stages as required in most other wind turbine gearboxes. The drive train and the

supporting nacelle frame are very compact thereby reducing the nacelle weight

significantly compared to other wind turbine concepts both with and without gear-

box.

3.2 Drive train

Nabe Planentengetriebe Generator

Multibrid M5000

Nabe Planentengetriebe Generator

Multibrid M5000

P = 5590 kW i = 9,93

= 14,8 1/min n

= 147 1/min

= 3607 kNm

Hub Planetary gear Generator

Multibrid M5000

Nabe Planentengetriebe Generator

Multibrid M5000

Nabe Planentengetriebe Generator

Multibrid M5000

P = 5590 kW i = 9,93

= 14,8 1/min n

= 147 1/min

= 3607 kNm

Hub Planetary gear Generator

Multibrid M5000

Rotor blade Hub Main bearing Planetary gear GeneratorRotor blade Hub Main bearing Planetary gear GeneratorRotor blade Hub Main bearing Planetary gear Generator

Fig. 3-37 Multibrid M5000, D = 116 m, P = 5 MW (Multibrid GmbH/Prokon Nord, 2004

3 Wind turbines - design and components

3.2.2 Gearbox

Preliminary remarks

The power transmitting gearbox in wind turbines changes the rotational speed

from rotor speed to the speed required by the driven work machine or generator.

The torque changes correspondingly due to P = T



n. As for the post wind

mill, the low rotor speed was increased by the lantern gear to drive the faster run-

ning millstones, cf. Fig. 2-5b. The machine size determines the required transmis-

sion ratio. Since the maximum tip speed ratio and the corresponding blade tip

speed is more or less given (cf. section 3.1), the rotor size determines the rotor

speed, which, in most cases, is significantly lower than the speed of the driven

work machine or generator. The speed of the generators’ rotor is mainly deter-

mined by the grid frequency and the pole number, in particular for the directly

grid connected asynchronous generators. For other work machines, the speed

range results from the range of best efficiencies since the total system efficiency

should be maximized.

Table 3.1 shows both gear types and other types of torque and speed converters.

The bevel gear is required at wind pumping systems, cf. chapter 10, to change

the orientation of the rotation axis from horizontal to vertical. A belt drive is then

used to optimise the torque-speed characteristics of the driven centrifugal pump in

relation to the wind turbine rotor characteristics. Early wind turbines for power

generation based on the Danish concept with a direct grid connection were

equipped with two generators. When the wind speed was small, a belt drive be-

hind the gearbox was used to transmit power to the smaller generator. Later, this

concept was replaced by pole switchable generators, cf. Fig. 13-2. A chain drive

allows only a relatively small chain speed, so it is only found in historical wind

turbines for power generation, e.g. the Gedser wind turbine, cf. chapter 2.

During the 1980s, hydrodynamic converters and couplings were used in several

big prototype wind turbines (e.g. MOD-0A, WTS-3 and WWG-0600) for damping

the load peaks and the “impacts” in synchronous generator (e.g. caused by the

interaction of the rotor blade with the tower wake or by drive train oscillations).

This is possible because the driving and the driven shaft are flexibly coupled by

the fluid. But the disadvantages are smaller part load efficiencies and the require-

ment of additional oil coolers. The hydrodynamic converters were no longer

needed when AC-DC-AC converters were introduced for speed-variable wind tur-

bines allowing the rotor speed to be independent from the grid frequency. Due to

technical progress, hydrodynamic converters are now being reused again in some

modern wind turbines with a variable rotor speed but relatively constant generator

speed (e.g. DEwind D8.2). Moreover, it should be mentioned that the part load

efficiency of AC-DC-AC converters are also moderate to small.

3.2 Drive train

Table 3.1 Torque and speed converters for wind turbines

smallhighsmallhighsmallerhighNoise

generation

High at best

point

smallmediummediumhighesthighEfficiency

Bad part load

efficiency

Only for low

rotational speed

Slip reduces

efficiency

90° angle

between shafts

Cost effective

for big WT

Cost effective

for smaller WT

Remark

Yes, hydraulic

slip in the fluid

No, to be rea-

lised externally

Yes, slip and

slipping

No, to be rea-

lised externally

No, to be rea-

lised externally

No, to be rea-

lised externally

Overload safty

machanism

Prototype with

synchronous

generator

Historical wind

pumps, do

yourself

Wind pumping

system,

Danish concept

Wind pumping

system

Electricity

generation,

> 500 kW

All applicationsApplication

Hydraulic

power

transmission

Form fitFrictionForm fitForm fitForm fitTransmission

principle

variable<

1:5< 1:3< 1:5< 1:7i = n

< 1:5

Transmission

ratio at wind

turbine

application

Scheme

Hydrodynamic

converter

(coupling)

Chain drive Belt driveBevel gearPlanetary gearSpur gearTransmission

type

smallhighsmallhighsmallerhighNoise

generation

High at best

point

smallmediummediumhighesthighEfficiency

Bad part load

efficiency

Only for low

rotational speed

Slip reduces

efficiency

90° angle

between shafts

Cost effective

for big WT

Cost effective

for smaller WT

Remark

Yes, hydraulic

slip in the fluid

No, to be rea-

lised externally

Yes, slip and

slipping

No, to be rea-

lised externally

No, to be rea-

lised externally

No, to be rea-

lised externally

Overload safty

machanism

Prototype with

synchronous

generator

Historical wind

pumps, do

yourself

Wind pumping

system,

Danish concept

Wind pumping

system

Electricity

generation,

> 500 kW

All applicationsApplication

Hydraulic

power

transmission

Form fitFrictionForm fitForm fitForm fitTransmission

principle

variable<

1:5< 1:3< 1:5< 1:7i = n

< 1:5

Transmission

ratio at wind

turbine

application

Scheme

Hydrodynamic

converter

(coupling)

Chain drive Belt driveBevel gearPlanetary gearSpur gearTransmission

type

Pump

Turbine

Pump

Turbine

3 Wind turbines - design and components

In contrast to other common technical applications, the loads and operating states

of the wind turbine and its gearbox vary considerably [10]. Respecting also the

allowed sound pressure levels, this leads to high requirements for the gear teeth

system, the bearing and the lubrication.

The size of the gearbox is determined by the required transmission ratio

between rotor shaft and generator shaft. It is given by the rotor speed (calculated

from the rotor diameter and the rated blade tip speed) and the generator speed.

A stall-controlled Danish concept wind turbine with a four-pole asynchronous

generator, e.g., has a generator speed of approx. 1,500 rpm. In reality it is slightly

above 1,500 rpm depending on the slip of the generator. The maximum blade tip

speed is, according to Fig. 3-4, around 70 m/s for common stall wind turbines.

Table 3.2 shows the required transmission ratio for different rotor diameters. The

values are quite high, as is the power to be transmitted. Hence only gearboxes with

toothed wheels are suitable as torque and speed converters.

Table 3.2 Required transmission ratio for a stall-controlled Danish concept wind turbine

Tip

= 70 m/s) with a four-pole asynchronous generator

Rotor diameter D in m 20 40 60 80

Rotor

in rpm 66.8 33.4 22.3 16.7

Generator

in rpm 1500 (slip not considered)

Transmission ratio i 22.4 44.9 67.3 89.8

Power in kW (approx.) 100 600 1.300 2.300

Spur gears have parallel axes of the paired gear wheels, cf. table 3.1 and 3.4. In a

helical spur gear, Fig. 3-38, there are always two teeth pairs in contact, so there is

less noise emission, and the expected life time is higher due to a better load distri-

bution [11]. So this gear type has become increasingly established despite higher

manufacturing costs.

But the higher the transmission ratio, the larger the distance required between

the two shafts. Gearboxes using only spur gears were still cost effective in wind

turbines of the 500 kW class, whereas at least one planetary gear is suitable for

larger wind turbines. For the same required transmission ratio, its dimensions,

costs and noise emissions are smaller. Table 3.3 shows a comparison study of size,

weight and costs of a 2.5 MW wind turbine showing the advantages of a multi-

stage planetary gearbox. Table 3.4 characterizes the differences of spur gears and

planetary gears.

In most cases, the first (slow speed) stage of the gearbox is a planetary gear

with a hollow shaft, Figs. 3-39 and 3-40. This allows the rotary feedthrough of

hydraulics, electrics and electronics to the hub.

3.2 Drive train

Generator

Coupling

Two-stage

spur gear

Hub of the

two-bladed

rotor

Oil pump

Central

pitch

Brake

Generator

Coupling

Two-stage

spur gear

Hub of the

two-bladed

rotor

Oil pump

Central

pitch

Brake

Fig. 3-38 Small wind turbine (two-bladed downwind rotor) with two-stage helical spur gear

Planetary gears have, arranged concentrically around the sun wheel, three (or

more) planet wheels with teeth contact at both the sun wheel and the hollow wheel

[11…13]. The planet wheels are mounted in the planet carrier which itself is either

immobile or also rotating, table 3.4. The transmission ratio differs slightly, de-

pending on whether the hollow wheel or the planet carrier is fixed, Figs. 3-39,

3-40 and 3-41.

If the hollow wheel rotates and the planet carrier is fixed the transmission ratio

i = n

Sun

/ n

= r

/ r

Sun

If the hollow wheel is fixed and the planet carrier rotates, the situation is a little

more complicated. Fig. 3-41 shows that the length of AA

equals BB

, so the

transmission ratio in this case is

i = n

Sun

/ n

=1 + r

/ r

Sun

3 Wind turbines - design and components

Table 3.3 Comparison of different gearbox types for a 2.5 MW wind turbine with a gearbox

ratio of 1:60, according to [6] in [18]

Number of

stages and

configura-

tion

Scheme and dimensions Weight

in t

Relative

costs in

2 stages:

spur gears

70 180

3 stages:

spur gears

77 164

2 stages:

1 Planetary

gear

1 spur gear

41 169

3 stages:

2 Planetary

gears

1 spur gear

17 110

3 stages:

Planetary

gears

11 100