Neo magnets Neodymium Magnet

Accepted for publication. Electric

Manufacturing and Coil Winding '99 Conference, October 1999.

EFFECTIVE USE OF NEODYMIUM IRON BORON MAGNETS, CASE STUDIESS. R. Trout and Yuriy ZhilichevMagnequench, Inc.

Abstract: Magnets based on neodymium iron boron (NdFeB) alloys have been available for about 15 years, yet many design engineers are frustrated in their attempts to incorporate this material into new devices. These are problems that cannot be solved by research focused on higher energy product materials, nor are they the result of the three well-known objections to NdFeB magnets: higher cost per kilogram than ferrite, limited maximum operating temperature and poor corrosion resistance when uncoated. We address situations encountered by design engineers trying to use magnets correctly, reviewing several common design situations and offering ways to improve device performance through more effective use of NdFeB.

Key words: Permanent Magnets, Neodymium Iron Boron, NdFeB, Rare Earth Magnets, Magnetic Circuit Design.

I. INTRODUCTION

The advent of neodymium iron boron magnets in the early 1980s was met with great enthusiasm. [1,2,3] Compared to samarium cobalt magnets of the day, the simultaneous increase in energy product and reduction in raw material cost of NdFeB was seen as breakthrough technology. Fifteen years later we see just a fraction of the potential fulfilled. Even as the magnetic properties gradually improve and the cost gradually decreases, as was originally predicted, the use of neodymium magnets has not risen accordingly.

In these applications, the magnetic material is not the root cause. We present three common, yet subtle, design problems, in hopes of offering solutions based on practical experience. The basic magnetic properties of NdFeB magnets are summarized in Table I, given as a point of reference.

II. EXAMPLE 1

The first example is the conversion of a 2-pole motor design of ferrite with a 4 MGOe energy product to a bonded NdFeB with a 10 MGOe energy product. The motor drives a fuel pump. The ferrite magnets in the original designs were unable to meet a performance specification at low temperatures. Unlike all other permanent magnet materials, the Hci of ferrite magnets decreases as the temperature falls, an awkward characteristic.

The original approach was a direct replacement of the ferrite magnet with an identically sized bonded NdFeB magnet, keeping the original 2-pole design. There was a slight increase in flux and the performance specification was met, although marginally. The magnetic circuit was analyzed using the finite element method, specifically Maxwell 2-D Field Simulator from the Ansoft Corporation

[5].A two dimensional solution is satisfactory for highly symmetric situations. As can be seen from Figure 1, the return path is saturated, a clear sign that there is too much magnet in the circuit. Performance issues appear to have slowed their growth.

After closer review,

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