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Contents 

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 - such as attraction & repulsion.
  • Mechanical to electrical - such as generators & microphones.
  • Electrical to mechanical - such as motors, loudspeakers, charged particle deflection
  • Mechanical to heat - such as eddy current & hysteresis torque devices.
  • Special effects - such as magneto resistance, Hall effect devices, & magnetic resonance.

The following sections will provide a brief insight into the design and application of permanent magnets. The Design Engineering team at Magnet Sales & Manufacturing will be happy to assist you further in your applications.

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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:

  • Neodymium Iron Boron
  • Samarium Cobalt
  • Ceramic
  • Alnico 

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 Hci values, offer greater inherent stability than the 1:5 types. Ceramic, also known as Ferrite, magnets (general composition BaFe2O3 or SrFe2O3) have been commercialized since the 1950s and continue to be extensively used today due to their low cost. A special form of Ceramic magnet is "Flexible" material, made by bonding Ceramic 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. 

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Table 2.1 Magnet Material Comparisons
 

Material

Grade

Br

Hc

Hci

BHmax

Tmax (Deg 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

Ceramic

8

3,900

3,200

3,250

3.5

300

Flexible

1

1,600

1,370

1,380

0.6

100

* Tmax (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. This catalog uses the cgs system for magnetic units, unless otherwise specified. 

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Table 3.2 Conversion Factors

Multiply By To obtain
inches 2.54  centimeters 

lines/in2

0.155  Gauss 
lines/in2 1.55 x 10-5 Tesla 
Gauss  6.45 lines/in2
Gauss  0-4 Tesla 
Gilberts  0.79577  ampere turns 
Oersteds  79.577 ampere turns/m 
ampere turns  0.4 Gilberts 
ampere turns/in  0.495  Oersteds
ampere turns/in  39.37 ampere turns/m 

4.0 Design Considerations

Basic problems of permanent magnet design revolve around estimating the distribution of magnetic flux in a magnetic circuit, which may include permanent magnets, air gaps, high permeability conduction elements, and electrical currents. Exact solutions of magnetic fields require complex analysis of many factors, although approximate solutions are possible based on certain simplifying assumptions. Obtaining an optimum magnet design often involves experience and trade offs.

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4.1 Finite Element Analysis

Finite Element Analysis (FEA) modeling programs are used to analyze magnetic problems in order to arrive at more exact solutions, which can then be tested and fine tuned against a prototype of the magnet structure. Using FEA models flux densities, torques, and forces may be calculated. Results can be output in various forms, including plots of vector magnetic potentials, flux density maps, and flux path plots. The Design Engineering team at Magnet Sales & Manufacturing has extensive experience in many types of magnetic designs and is able to assist in the design and execution of FEA models. 

 

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4.2 The B-H Curve

The basis of magnet design is the B-H curve, or hysteresis loop, which characterizes each magnet material. This curve describes the cycling of a magnet in a closed circuit as it is brought to saturation, demagnetized, saturated in the opposite direction, and then demagnetized again under the influence of an external magnetic field. 

 

The second quadrant of the B-H curve, commonly referred to as the "Demagnetization Curve", describes the conditions under which permanent magnets are used in practice. A permanent magnet will have a unique, static operating point if air-gap dimensions are fixed and if any adjacent fields are held constant. Otherwise, the operating point will move about the demagnetization curve, the manner of which must be accounted for in the design of the device.

The three most important characteristics of the B-H curve are the points at which it intersects the B and H axes (at Br - the residual induction - and Hc - the coercive force - respectively), and the point at which the product of B and H are at a maximum (BHmax - the maximum energy product). Br represents the maximum flux the magnet is able to produce under closed c