2 research outputs found
Analisi elettromagnetica di una macchina sincrona a poli sporgenti mediante formulazioni analitiche delle permeanze
Nel seguente documento si descrive uno strumento analitico che permette di fornire un'analisi elettromagnetica preliminare con un'accuratezza che si ritiene adeguata per eseguire un dimensionamento preliminare della macchina. Il tutto senza richiedere l'utilizzo di programmi agli elementi finiti.openEmbargo temporaneo per motivi di segretezza e/o di proprietà dei risultati e/o informazioni sensibil
Direct drive wind turbines: the effect of unbalanced magnetic pull on permanent magnet generators and bearing arrangements
Wind energy has been the fastest emerging renewable energy source over the last decade. The
overriding provisos to minimise greenhouse emissions and increasing concerns regarding
energy security have been the major inducements for many countries to make a resolute
transition to new and non-conventional power sources.
Direct-drive systems for wind turbines are potentially a more reliable alternative to gearbox
driven systems. Gearboxes are liable to significant accumulated fatigue torque loading with
relatively high maintenance costs. It is with this in mind that the primary focus of this research
is on direct-drive wind turbines.
Generators in direct-drive wind turbines tend to be of large diameter and heavier due to the
support structure required to maintain as small air-gap as possible between the stationary and
rotating parts of the generator. Permanent magnet generators (PMGs) are the most common
type to be used within direct-drive wind turbines nowadays.
Generators and other drive-train components in wind turbines experience significant varying
loads, which may lead to a bearing failure. These varying loads can lead to misalignment within
the drivetrain producing eccentricity between the generator rotor and stator. Rotor eccentricity
generates a magnetic force referred to as Unbalanced Magnetic Pull (UMP). The induced UMP
for the same rotor eccentricity is much higher in PMGs than induction generators because of
the higher permanent magnet magnetic field. UMP is an important issue requiring further
research. A part of this study provides a more detailed treatment of UMP under varying rotor
eccentricity regimes for various permanent magnet machine topologies.
The effect of UMP in direct-drive PMGs on the lifetime of the main bearing is a topic that
requires more research aimed at proposing design improvements and solutions. The hope being
that the availability of such solutions can be applied to practical reductions in operating costs.
In brief, identification of the root causes of failure and impacts on component lifetime remain
a subject of research. Establishing analytical tools for studying the impact of UMP on
component lifetime in direct drive wind turbines and identifying the prospects for air gap
winding machines using single bearing configuration are the two key areas for further research.
Firstly, this research aims to establish the relationship between bearing forces and different
types of eccentricities and UMP in direct drive machines. It is intended to use such models for
predicting bearing wear and fatigue. Secondly, this research aims to establish the analytical
tools for studying static, dynamic and tilting eccentricity in air-gap winding direct drive
generators. Such tools are used to increase the understanding of the dynamics of direct drive
PM generators. The final step of this study is using a multi-body simulation software
(SIMPACK) to initiate investigations and comparison by providing assessments of
electromagnetic interaction and internal drive-train loading for four possible designs for a
proposed 5MW direct-drive wind turbine in response to the loads normally seen by a wind
turbine. The four designs include: (a) iron-cored PM direct-drive generator supported by two
main bearings, (b) airgap winding PM direct-drive generator supported by two main bearings,
(c) iron-cored PM direct-drive generator supported by a single main bearing, (d) airgap winding
PM direct-drive generator supported by a single main bearing. An aero-elastic simulation code
(HAWC2) is used to extract the hub loads for different wind speeds corresponding to the
normal operation of the wind turbine. The dynamic eccentricity and its influence on the
electromagnetic interaction and consequential effects on bearing loading for all four designs is
examined to determine the most optimal support structural configuration for a direct-drive
system.
In summary, the main aim of this thesis is studying the effect of different types of rotor
eccentricities in different types of direct drive PMGs on the main bearing arrangements. The
results show that static rotor eccentricity has the maximum impact compared to the other types
of eccentricities. The main result of an eccentricity is the induced UMP which applies directly
as an extra force on the bearings. The influence of UMP on bearing wear is studied. This
influence is found to be significant in PM machines and should be considered when designing
the bearing stiffness. A 20% static rotor eccentricity in a PM machine is found to induce an
UMP that roughly equals third the total weight of the machine. A single bearing design for a
direct-drive wind turbine is proposed and compared with a conventional two-bearing design.
The results show that the Iron-cored PM direct-drive generator supported by two main bearings
design and airgap winding PM direct-drive generator supported by a single main bearing design
have advantages over the other two designs in this study