Introduction to Magnetising

How much magnetising force does it take to saturate (fully magnetise) magnets?

To reach the maximum energy output of a magnet, it should be saturated, that is ,magnetised fully even though the magnet may be later stabilised either thermally or using a “knockdown” (or calibrating) field.

The magnetising 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 magnetising.

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 magnetising pulse. This prevents the magnetising 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 magnetising circuit to extend the magnetising pulse width. An extended pulse generates more heat which slows the production magnetising rate. So a careful compromise must be reached.

Required Magnetising 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 magnetising 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.

MATERIALOERSTEDSkA/m
Alnico 5, 6, 8 and 93,000 - 7,000239 - 557
Ceramic (Hard Ferrite)10,000 - 12,000796 - 955
Neodymium-iron-boron, motor grade35,000 - 50,0002,790 - 3,980
Neodymium-iron-boron, high energy grade30,000 - 40,0002,390 - 3,180
Neodymium-iron-boron, most bonded30,000 - 40,0002,390 - 3,180
Neodymium-iron-boron, high temp bonded35,000 - 60,0002,790 - 4,780
SmCo 1-530,000 - 45,0002,390 - 3,580
SmCo 2-1740,000 - 50,0003,180 - 3,980
SmCo low Hci bonded grade25,000 - 30,0001,990 - 2,390
SmCo high temperature grade45,000 - 55,0003,580 - 4,375

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