Study of demagnetization risk for a 12 kW direct driven permanent magnet synchronous generator for wind power

Abstract

One of the main aspects when designing a permanent magnet (PM) generator is to choose suitable PMs, both in terms of achieving the required flux in the generator but also of withstanding high demagnetizing fields, that is, having sufficiently high coercivity. If the coercivity is too low, the magnets are at risk of demagnetizing, fully or partially, at the event of a short circuit and/or an increase in temperature. This study aims to determine the risk of demagnetization for a 12 kW direct driven permanent magnet synchronous generator. Furthermore, as the prices on PMs have increased drastically the last few years the possibility to use smaller and/or cheaper PMs of different grades has been investigated. A new proprietary finite element method (FEM) model has been developed, which is also presented. The study is based on simulations from this FEM model and is focused on NdFeB magnets. Results show that the reference magnet can withstand a two-phase short circuit at both the temperatures tested and in both geometries. The use of cheaper magnets, smaller air gap and in the event of a two-phase short circuit often results in partial irreversible demagnetization. However, magnets with lower coercivity are easier demagnetized.

Experimental Verification of a Simulation Model for Partial Demagnetization of Permanent Magnets

“…The aim is to experimentally verify a finite element method (FEM) model originally presented in [4]. The demagnetization model is based on an exponent function model presented by Ruoho et al [5] and the approach is similar to Kang et al [6] and Kim et al [7] .…”

Investigation of Permanent Magnet Demagnetization in Synchronous Machines during Multiple Short-Circuit Fault Conditions

“…The two models were verified against a stationary model based on the same principle, which, with good agreement, has been compared to experiments [20,21]. A three-legged iron core where the middle leg had an air gap, one leg had a permanent magnet and one leg had a coil of 100 turns was used for the comparison.…”

“…The machine studied in this paper is a modification of the 12 kW generator in [20,26,27], which was designed as a prototype generator for a variable speed wind turbine. For the sake of this study, the air gap has been decreased from 10 to 3 mm and the height of the permanent magnets was decreased from 14 to 5 mm.…”

“…Previous work on demagnetization within this project includes a study on single short-circuit events on a 12 kW generator using a stationary model [20] as well as experimental verification of the stationary demagnetization model [21]. Furthermore, a study has been made on working point ripple in permanent magnets when the generator is connected to an external electrical circuit [22].…”

A Review of Research on Large Scale Modern Vertical Axis Wind Turbines at Uppsala University

“…The demagnetization risk has been analysed for the 12 kW generator installed in the Marsta wind turbine, and has shown no risk of demagnetization [51]. A simulation model studying partial demagnetizing of permanent magnet has been developed and experimentally verified [52]. The risk for partial demagnetization for the ferrite 12 kW generator has been investigated and compared to the risk for the NdFeB generator, showing that both generators have a reliable design if operated as intended [53].…”