Conference programme 

Nurul Azim Bhuiyan University of Strathclyde, United Kingdom
Co-authors:
Nurul Azim Bhuiyan (1) ^{F P} Kerri Hart (1) Alasdair McDonald (1)
(1) University of Strathclyde, Glasgow, United Kingdom

Assessment of the suitability of ferrite magnet excited synchronous generators for offshore wind turbines

Introduction

In recent years the use of permanent magnets (PM) for wind turbine generators has increased significantly.
The price of rare earth metals such as neodymium is very unstable and increased more than 350% from August 2009
to August 2011[1]. This means that wind turbine manufacturers (who use permanent magnet generators) are faced with
a significant cost uncertainty. The large price fluctuations force us to look at alternative PM materials.
In this paper, a generator design (for a 6 MW offshore turbine) using ferrite magnets is presented and compared
with a generator using NdFeB magnets, in terms of costs, efficiency, availability and cost of energy.

Approach

This paper focuses on the potential downsides of using ferrite magnets such as increased generator mass,
increased rotor inertia and variation in loss mechanisms. This paper compares the cost of energy of ferrite
magnet machines with a more conventional permanent magnet machine for two turbines: one is a direct drive
turbine and the other has a medium speed drivetrain.

Main body of abstract

This paper builds on the work of Sandra Eriksson and Hans Bernhoff [1] and Kerri Hart et al. [2] with
an emphasis on a typical 6 MW offshore wind turbine. In order to assess the suitability of ferrite magnets,
two 6 MW wind turbine were designed: one with a surface mounted NdFeB (rare earth magnet) rotor and one with
a buried mounted ferrite magnet rotor. The electromagnetic design is done using a lumped parameter magnetic
circuit model which is verified using finite element software. For the same amount of flux crossing the air gap,
the difference in rotor masses, rotor losses, moment of inertia are examined in this paper. This paper further
evaluates the suitability of medium speed and direct drive wind turbines and losses at different wind speeds,
based on a comparison with PM machines from Kerri Hart et al. [2].

The effect of increased inertia of drivetrain is modelled in Bladed software run with same input wind speed
time series. The impact on top head mass (and its effect on tower and foundation cost, lifting costs) is also
investigated in this paper. To compare the cost of energy, this paper considers the turbine capital costs
(variation in generator cost, tower or foundation costs and lifting costs), operation and maintenance costs
and AEP (efficiency curve and effect of inertia on rotor energy capture).


Conclusion

It has been shown that a machine with similar electromagnetic properties can be built with two different
rotor designs, mainly differing in rotor mass and inertia. The main factor for choosing between is the
relative magnet material price.


Learning objectives
1. It will help wind turbine manufacturers who are evaluating different generator types.
2. It can provide tools for finding improved generator designs (in terms of masses, losses, inertia and efficiency).


References:
[1]S. Eriksson and H. Bernhoff, "Rotor design for PM generators reflecting the unstable neodymium price," XXth International Conference, 2012, pp. 1419-1423.
[2]K. Hart, A McDonald, H.Polinder, E.Corr, and J.Carroll,"IMPROVED COST OF ENERGY COMPARISON OF PERMANENT MAGNET GENERATORS FOR LARGE OFFSHORE WIND TURBINES,"2014.