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Limiting mechanisms for the torque density of wind turbine drivetrains

Kooloos, Wichert
Master thesis
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URI
http://hdl.handle.net/11250/2621693
Date
2019
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  • Institutt for marin teknikk [2351]
Abstract
 
 
As wind turbines continue to increase in size, so do the loads acting on the drivetrain.

To handle the higher loads the mass of the drivetrain needs to increase. A better understanding

of this mass increase can result in lighter drivetrain design and provide

an insight which drivetrain design has the lowest mass potential for large rotor diameters.

In this thesis the limiting mechanisms, mass and torque density of three

wind turbine drivetrains are investigated for an increasing rotor diameter. The limiting

mechanisms in this thesis provide a theoretical limit to the minimum required

mass of a drivetrain for increased loading. Through an extensive literature study the

limiting mechanisms are found and three scaling models are developed to calculate

the torque density of the drivetrains. The drivetrains that are being considered in this

thesis are the drivetrain with gearbox and high-speed generator, the direct drivetrain

and the hydraulic drivetrain. The following research questions are answered in this

thesis:

• What are the limiting mechanisms of the three investigated drivetrain types?

• What are the achievable torque densities of the three investigated drivetrain

configurations for increasing rotor diameters?

Two limiting mechanisms are found for the drivetrain with gearbox and high-speed

generator: Tooth flank stress (1.72 GPa) and Root bending stress (0.24 GPa). The

Tooth flank stress is found to be governed by the Hertzian contact strength while the

root bending stress is governed by the flexural strength. Two limiting mechanisms

for the RFPMSG are found for the airgap flux density (1 T) and the current loading

(30 - 200 kA/m) in the generator windings. The airgap flux density is limited by the

saturation of the stator teeth material in the generator whereas the current loading

is limited by the heat dissipation of the windings and stator laminations. For the

hydraulic pump, the limiting mechanism is found at the interaction of the camring

and the cam roller. The maximum stress that is allowed at this point is determined

by the Hertzian contact strength of the material and is found to be (1.72 GPa). Using

the limiting mechanisms of the three drivetrain configurations, scaling models for

the mass and torque density of the three drivetrains have been developed.
 
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NTNU

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