Arnold Magnetics Technology Center - Magnetizing and Measuring Equipment, Introduction

TECHNOLOGY CENTER

Equipment - Introduction

HOME : TECHNOLOGY CENTER : EQUIPMENT : INTRODUCTION

INTRODUCTION TO
MAGNETIZING and MEASURING EQUIPMENT
Two of the most frequently asked questions regarding magnetic materials are:
1. "Where can I get equipment to magnetize my magnets?" and
2. "How much magnetizing force does it take to fully magnetize them?"

The answer to the first question can be found at the end of this tech note. Several manufacturers of magnetizing equipment are listed. You should have the following information available when you contact a supplier to be sure of getting suitable equipment.

The type of material to be magnetized. Provide a second quadrant hysteresis curve of the material, if possible.

How the magnet is to be used: open circuit or with pole pieces.

Required magnetizing pattern: axial or radial alignment; single pole or multiple poles; allowable deadband between adjacent poles; will the useful flux be on the inside or the outside of a ring magnet; etc.

Drawings of the magnet and magnetic circuit will also be advantageous to provide to the potential equipment supplier.

The equipment suppliers will work with you to determine what equipment and fixtures are required for your needs. Your application will indicate whether a stock unit is suitable or if a custom-manufactured device is required.

The second question is a little more complex, and the term "saturation" will play an important role in your discussion. To reach the maximum energy output of the magnet, it should be saturated, that is magnetized fully even though the magnet may be later stabilized either thermally or using a "knockdown" (or calibrating) field.

The magnetizing force required to saturate a magnet depends on the coercivity of the magnetic material and to a lesser extent, physical characteristics of the magnet and components to which it may be fastened during the magnetizing.

The general rule is that to saturate a magnet, one must apply a peak field of between 2 and 2.5 times the intrinsic coercivity. For example, an Hci of 20,000 oersteds will require at least 40,000 oersteds to saturate.

In the case of magnets attached to conductive fixtures, eddy currents are established in the material that set up a reverse magnetic field during the extremely short magnetizing pulse. This prevents the magnetizing flux from fully penetrating the conductor, perhaps even the magnet, and reduces the field the magnet sees and sometimes also the flux path (direction of the flux) in the magnet. In these cases, it is necessary for the equipment manufacturer to adjust the LC (inductance capacitance ratio) of the magnetizing circuit to extend the magnetizing pulse width. An extended pulse generates more heat which slows the production magnetizing rate. So a careful compromise must be reached.

REQUIRED MAGNETIZING FIELDS
Bonded rare earth magnets pose another problem. The magnetic powder is separated with non-magnetic binder which affects the penetration of flux through the magnet during the pulse and requires a higher magnetic field than one would expect from the rule above. For example, MQP-B powder with an Hci of 10,000 Oersteds requires at least 30,000 Oersteds to saturate. Most magnet manufacturers should be able to provide you with a curve of peak applied field versus percent of saturation for induction, Hci or percent of maximum flux output.

Approximate required magnetizing fields for various magnet types to reach at least 98% of maximum output are listed below. These are general and affected by fixturing and LC circuit, etc.

MATERIAL	OERSTEDS	kA/m
Alnico 5, 6, 8 and 9	3,000 - 7,000	239 - 557
Ceramic (Hard Ferrite)	10,000 - 12,000	796 - 955
Neodymium-iron-boron, motor grade	35,000 - 50,000	2,786 - 3,980
Neodymium-iron-boron, high energy grade	30,000 - 40,000	2,388 - 3,184
Neodymium-iron-boron, most bonded	30,000 - 40,000	2,388 - 3,184
Neodymium-iron-boron, high temp bonded	35,000 - 60,000	2,786 - 4,776
SmCo 1-5	30,000 - 40,000	2,388 - 3,184
SmCo 2-17	35,000 - 50,000	2,786 - 3,980
SmCo low Hci bonded grade	20,000 - 30,000	1,592 - 2,388
SmCo high temperature grade	35,000 - 50,000	2,786 - 3,980

Accuracy Statement
While we make every effort to ensure that the information contained in these documents is complete and accurate, we make no warranty regarding this. Please see our disclaimer statement. We would appreciate your input regarding corrections, additions and suggestions for improvement ().