Sami Ruoho

Interdependence of Demagnetization, Loading, and Temperature Rise in a Permanent-Magnet Synchronous Motor

The demagnetization of permanent magnets in permanent-magnet machines has been under discussion in many publications lately. Demagnetization models have been used, for example, to optimize the machine structures but there have been no studies on how the demagnetization is coupled with the loading and temperature-rise of the machine and how this coupling should be modeled. In this paper, we model the dynamics of the demagnetization of a dovetail machine under a constant load torque. We show that the thermal model should be included in the demagnetization calculations. The demagnetization will cause an increase of the copper losses, which will increase the temperatures of the machine. This will cause more demagnetization and might lead even to a stall in a situation in which a model neglecting the thermal effects predicts stable operation without additional demagnetization.

Comparison of Demagnetization Models for Finite-Element Analysis of Permanent-Magnet Synchronous Machines

We have studied a few simple demagnetization models, which are quick and easy to implement in finite-element calculations, and compared them with measured recoil behavior of Nd-Fe-B magnet material, also using a hysteresis model for comparison. The models are used to estimate post-demagnetization performance of an overloaded surface magnet synchronous machine. Two of the simple models, the sloped linear model and the exponent function model, give the most accurate results without significantly increasing the computation time.

Partial Demagnetization of Permanent Magnets in Electrical Machines Caused by an Inclined Field

We report a study of partial demagnetization of axially pressed sintered Nd-Fe-B magnets by inclined pulse demagnetization measurements, including a recoil behavior of the Nd-Fe-B material. From these measurements, we develop a simple empirical model for demagnetization of Nd-Fe-B magnets caused by an inclined field. We calculate demagnetization of a simple surface-magnet machine and a two-pole high-speed machine by using an exponent function-based model, taking also the inclined demagnetizing field into account. We show that it is not enough to consider only antiparallel demagnetizing field components in accurate demagnetization calculations.

Modeling Magnet Length In 2-D Finite-Element Analysis of Electric Machines

Two-dimensional finite-element-method (2-D FEM) calculations are widely used in electric machine modeling instead of three-dimensional calculations because of their faster calculation time and simplicity. However, the 2-D calculations ignore end effects, causing a large error in calculating eddy currents in permanent magnets of synchronous machines. In this paper, we develop three analytical models and one curve-fitting model based on numerical calculations. The models improve the eddy-current loss calculation accuracy in 2-D FEM. The method adjusts the resistivity of a magnet material according to magnet dimensions. The adjustment takes into account the resistivity, the temperature dependence, and anisotropy of the resistivity of rare-earth magnet materials. We compare the models against FEM calculations in two and three dimensions and show that all the models improve the eddy-current loss calculation accuracy significantly, especially when the time-harmonic caused eddy-current losses in permanent magnets are considered.

Temperature Dependence of Resistivity of Sintered Rare-Earth Permanent-Magnet Materials

We studied the resistivity of rare-earth permanent-magnet materials over the temperature range -40°C to +150°C. We investigated three different materials from four manufacturers, including Nd2 Fe14 B, SmCo5 , and Sm2 Co17 , and measured their resistivities and temperature coefficients. We found that rare-earth permanent-magnet materials show an anisotropic resistivity behavior. In fact, the resistivity anisotropy causes larger resistivity difference than the temperature variation within the range studied. In many applications, such as permanent-magnet motors, this behavior has to be taken into account in design.

Hysteresis Losses in Sintered NdFeB Permanent Magnets in Rotating Electrical Machines

Permanent magnet (PM) materials are nowadays widely used in the electrical machine manufacturing industry. The eddy-current loss models of PMs used in electrical machines are frequently discussed in research papers. In magnetic steel materials we have, in addition to eddy-current losses, there are hysteresis losses when alternating current or a rotating flux travels through the material. Should a similar phenomenon be also taken into account in calculating the losses of PMs? Every now and then, authors seem to assume that some significant hysteresis losses are present in rotating machine PMs. This paper studies the mechanisms of possible hysteresis losses in PMs and their role in PMs when used in rotating electrical machines.

Demagnetization Testing for a Mixed-Grade Dovetail Permanent-Magnet Machine

A dovetail machine is a novel design developed to solve the strength problems of traditional buried magnet machines. A mixed-grade construction can be easily applied to a dovetail machine, because a dovetail machine has several magnets in a single pole in different positions. The basic idea of the mixed-grade construction is to use high intrinsic coercivity material in the positions of the high demagnetization risk and high remanence material in the positions of low demagnetization risk. We have developed a demagnetization model that takes into account the temperature dependence of the properties of the permanent-magnet materials to model a dovetail permanent-magnet motor with mixed-grade construction. We compared the model with a real motor. By comparing the testing and the calculations, we show that our demagnetization model can predict the demagnetization of the permanent magnets with reasonable accuracy. We discuss the benefits of the mixed-grade construction in a dovetail machine.

Hysteresis Losses in Different Types of Permanent Magnets Used in PMSMs

In permanent magnet synchronous machines (PMSM), depending on the machine application, different types of permanent magnets (PM) can be used. The most common PMs are ferrite magnets, neodymium iron boron magnets (NdFeB), and samarium cobalt magnets (SmCo). The selection of a suitable magnet for a particular machine design depends on the magnet properties: remanence, conductivity, mechanical rigidity, losses, and demagnetization characteristics. Usually, the possibility of hysteresis losses in PM materials is neglected. In this paper, however, it is demonstrated that possible hysteresis losses have to be evaluated in the machine design. It is shown by measurements and simulations that in some machine designs, hysteresis losses in NdFeB, SmCo, and ferrite magnets can be a source of significant additional ac losses that may lead to too high PM operating temperatures and a reduction in the machine efficiency.