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Magnetics 101 - Design Guide

Magnet Design, Engineering & Manufacturing Considerations

1.0 Introduction

Magnets are an important part of our daily lives, serving as essential components in everything from electric motors, loudspeakers, computers, compact disc players, microwave ovens and the family car, to instrumentation, production equipment, and research. Their contribution is often overlooked because they are built into devices and are usually out of sight.

Magnets function as transducers, transforming energy from one form to another, without any permanent loss of their own energy. General categories of permanent magnet functions are:

Mechanical to Mechanical - attraction and repulsion. Examples of applications that utilize this are magnetic separators, holding devices, magnetic torque drivers and magnetic bearings.
Mechanical to Electrical - the conversion of motion to electrical energy. Example applications are generators, magnetos, and microphones.
Electrical to Mechanical - the conversion of electrical energy to motion. Example applications are motors, meters, relays, actuators, loudspeakers, and charged particle deflection, traveling wave tubes, ion pumps, and cyclotrons.
Mechanical to Heat - the conversion of motion to heat energy. Example applications are eddy current heaters.
Special Effects - such as magneto resistance, Hall effect devices, and magnetic resonance.

The following sections will provide a brief insight into the design and application of technical permanent magnets. The design & engineering team at Integrated Magnetics will be happy to assist you further in your applications, contact us or send us a request for quote and let us know how we can help.

2.0 Modern Magnet Materials

There are four classes of modern commercialized magnets, each based on their material composition. Within each class is a family of grades with their own magnetic properties. These general classes are:

NdFeB and SmCo are collectively known as rare-earth magnets because they are both composed of materials from the Rare Earth group of elements.

Neodymium Iron Boron (general composition Nd₂Fe₁₄B, often abbreviated to NdFeB) is the most recent commercial addition to the family of modern magnet materials. At room temperatures, NdFeB magnets exhibit the highest properties of all magnet materials.
Samarium Cobalt is manufactured in two compositions: Sm₁Co₅ and Sm₂Co₁₇ - often referred to as the SmCo 1:5 or SmCo 2:17 types. 2:17 types, with higher H_ci values, offer greater inherent stability than the 1:5 types.
Ferrite, also known as ceramic magnets (general composition BaFe₂O₃or SrFe₂O₃) have been commercialized since the 1950s and continue to be extensively used today due to their low cost. A special form of Ferrite magnet is "Flexible" material, made by bonding Ferrite powder in a flexible binder.
Alnico magnets (general composition Al-Ni-Co) were commercialized in the 1930s and are still extensively used today.

These materials span a range of properties that accommodate a wide variety of application requirements. The following is intended to give a broad but practical overview of factors that must be considered in selecting the proper material, grade, shape, and size of magnet for a specific application. The chart below shows typical values of the key characteristics for selected grades of various materials for comparison. These values will be discussed in detail in the following sections. top

Table 2.1 Magnet Material Comparisons

Material	Grade	Br	Hc	Hci	BHmax	Tmax (°C) *
NdFeB	39H	12,800	12,300	21,000	40	150
SmCo	26	10,500	9,200	10,000	26	300
NdFeB	B10N	6,800	5,780	10,300	10	150
Alnico	5	12,500	640	640	5.5	540
Ferrite	8	3,900	3,200	3,250	3.5	300
Flexible	1	1,600	1,370	1,380	0.6	100

*T_max (maximum practical operating temperature)is for reference only. The maximum practical operating temperature of any magnet is dependent on the circuit the magnet is operating in.

3.0 Units of Measure

Three systems of units of measure are common: the cgs (centimeter, gram, second), SI (meter, kilogram, second), and English (inch, pound, second) systems. Here we have used the cgs system for magnetic units, unless otherwise specified. top