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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Innovative concepts for drive train components' taking place on Thursday, 13 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Kaswar Mostafa The University of Edinburgh, United Kingdom
Kaswar Mostafa (1) F P Latha Sethuraman (1) Markus Mueller (1)
(1) The University of Edinburgh, Edinburgh, United Kingdom

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Unbalanced magnetic pull comparison of air-cored and iron-cored permanent magnet machines for direct drive wind turbines


Wind energy is one of the fastest growing sectors in the energy market and continues to grow annually despite recent global financial decline. It is known that the wind speed is varies through the time and it is related to the height above the ground. Different horizontal forces, therefore, can be generated on the turbine’s blades, which causes extra mechanical forces on the drive train and could lead to bearing wear and rotor eccentricity.


When the rotor in an electrical machine is eccentric, a net radial force is generated, which will pull the rotor off centre leading to an asymmetric load on the bearings. Such a force is referred to as unbalanced magnetic pull (UMP).
Direct drive systems have been proposed as a potentially more reliable alternative to gear box driven induction generators used in most of the early prototypes. However, direct drive generators tend to be of large diameter and heavy due to the support structure required to maintain a small air-gap between the stationary and rotating parts of the generator. Reduction of UMP would greatly reduce bearing loads and subsequently reduce bearing wear. For wind turbines, it is essential that components are replaced or repaired prior to failure since a failed component can cause other components or other parts of the systems to fail. Hence, upon detection of imminent bearing failure it is desirable to reduce UMP in order to increase time-to-failure. The permanent magnet generator is being favored by the wind industry due its high power density and high part load efficiency. Such generators exhibit high magnet attraction forces between the permanent magnet rotor and iron-cored stator. Alternatively the coils could be supported directly in the air-gap in a non-magnetic structure – a so called air-gap winding or air-cored machine. The advantage of these machines is that the magnetic attraction forces a less than the conventional iron cored machines. If the rotor is eccentric in either topology then UMP will be produced, but as yet no comparison between the two has been published.

Main body of abstract

There have been many papers published on UMP in electrical machines, with some dating back more than a century, the theory of UMP was initially developed in 1955 by Summers, using rotating field components [1]. Subsequently, Frohne explained that UMP is generated as a result of the interaction between two magnetic fields with pole-pair numbers differing by one [2]. This has been shown through graphical representation in [3]. It has proven to be a challenge to develop models that are able to cope with air gap variation and therefore provide accurate calculations. However, modern computational power offers the prospect of using finite element methods to aid UMP calculation and to understand the key factors in UMP generation. Finite elements can also be used for verification in the development of analytical models.
Two-dimensional finite element method (FEM) open-source software has been used to study the effect of rotor eccentricity on the magnetic flux density in the two types of generator’s air gaps and hence the induced UMP for different levels of eccentricity. The software is particularly suitable for solving low frequency electromagnetic problems on two-dimensional planar and axisymmetric domains [4]; therefore, no-load cases for permanent magnet machines are simple to model and simulate. However, loading cases are more complex to simulate and required additional instruction using a scripting language. The meshing size is controllable and 30° minimum meshing angle has been chosen for accurate analysis. A two-dimensional diagram of the studied machines showing the flux density lines is illustrated in for the air gap winding machine and for the iron cored machine. shows the tapered bearing system which has been used to study the effect of the UMP for both generator’s types.
In this paper the authors will investigate the UMP generated in iron cored and air gap winding permanent magnet generators, and provide a qualitative discussion on the impact on bearing wear.
An analytical model has been developed to calculate the radial force generated in both iron cored and air gap winding PM machine due to static rotor eccentricity. The model has been verified with 2D electromagnetic finite element analysis. Results of both analytical and numerical models will be presented for a rating of 5 MW.
It is shown in [5] that an increase in static eccentricity causes an increase in the magnitude of the permeance in one side of the air gap and a decrease on the opposite side. That leads to a corresponding change in the magnitude of the air gap flux density, which induces UMP. The relationship between static rotor eccentricity and UMP for the studied generators using FEM software has been found to be linear.
Potential causes for rotor eccentricity will be discussed. Multi-body modelling software “Simpack” will be used to estimate the degree of eccentricity due to turbine loads, which in turn will then be used in the analytical model to compare the UMP for each generator case. Recommendations will be made regarding the choice of machine and the drive train design in order to reduce potential bearing wear due to UMP.


Electrical generators and other drive train components experience significant varying loads in wind turbines. This can lead to bearing failure due to unbalanced forces caused by misalignment and rotor eccentricity. For wind turbines to operate effectively; components, such as bearings, should be repaired or replaced prior to failure. This paper presents FEM modelling of two permanent magnet electrical generators for wind turbines. 5MW of both air gap cored PM generator and iron cored PM generator have been studied in order to investigate the unbalanced forces induced by static rotor eccentricity and compare the effect on a tapered bearing system in a wind turbine. No load case and loaded case have been simulated for both generator’s types in order to investigate the armature reaction. Analytical models for UMP calculation for both generator’s types have been developed and the results have been verified by the FEM models. The results show a linear relationship between the rotor eccentricity and the unbalanced magnetic pull and the effect of the armature reaction in both types seems to be very small and can be neglected. The static eccentricity and the induced UMP seem to have small effect on the bearing lifetime because of the high stiffness of the bearing. Work from the paper will lead towards a drive-train bearing wear model where unbalanced forces can be reduced and failures minimized. Most of the work presented in the paper is transitional but nonetheless a necessary step towards the understanding of the effect of unbalanced forces on bearings in wave and tidal devices and how wear and failure can be mitigated through design and control.

Learning objectives
- Study the cause and effect relationship between rotor eccentricity and bearing wear.
- Show the armature reaction effect on the induced unbalanced magnetic pull because of rotor eccentricity
- Compare the induced UMP because of same rotor eccentricities in two types of generators; air-gap wounded and iron-gap PM generators.

[1] E.W. Summers, “Vibration in 2-Pole Induction Motors Related to Slip Frequency,” Transactions of the American Institute of Electrical Engineers - Power Apparatus and Systems, Part III., vol. 74, pp. 69-72, 1955.
[2] H. Frohne, “The practical importance of unbalanced magnetic pull, possibilities of calculating and damping it,” Conti Elektro Berichte, vol. 13, pp. 81-92, 1967.
[3] D.G. Dorrell, “Experimental behaviour of unbalanced magnetic pull in 3-phase induction motors with eccentric rotors and the relationship with tooth saturation,” IEEE Transactions on Energy Conversion, vol. 14, pp. 304-309, 1999.
[4] Finite Element Method Magnetics (Version 4.2), 2010 [Online] Available:
[5] J.R. Cameron, W.T. Thomson, and A.B. Dow, “Vibration and current monitoring for detecting airgap eccentricity in large induction motors,” Electric Power Applications, IEE Proceedings B, vol. 133, no. 3. pp. 155–163, 1986.