IEC TR 62517 pdf download – Magnetizing behaviour of permanent magnets

IEC TR 62517 pdf download – Magnetizing behaviour of permanent magnets
1 scope
lt is within the scope of this technical report to describe the magnetizing behaviour ofpermanent magnets in detail. Firstly，in Clause 3 the relationship between the appliedmagnetic field strength and the effectively acting internal field strength is reviewed. In Clause4 the initial state prior to magnetization is discussed. Then，in the main Clause 5, themagnetizing behaviour of all common types of permanent magnets is outlined.The clause issubdivided according to the dominant coercivity mechanisms, namely the nucleation type forsintered Ferrites，RE-Fe-B and SmCo magnets，the pinning type for carbon steel andSm,Co17 1 magnets and the single domain type for nano-crystalline RE-Fe-B,AInico and Cr-Fe-Co magnets.Finally, the recommended magnetizing field strengths for modern permanentmagnets are compiled in a comprehensive table.
Effective magnetizing field strength
For magnetization of permanent magnets，the internal magnetic field strength Hint in themagnet is the critical parameter.The internal field strength is determined by the applied fieldstrength Happl and the self-demagnetizing field strength Hemag of the magnet or the magnetassembly.The self-demagnetizing field strength depends on the dimensions of the magnet orthe load line of a magnet assembly and the polarization of the magnet material, see equation(1):
Hnt = Happl – Hdemag =Happl- N·J/1o
(1)
N denotes the demagnetization coefficient and J the polarization of the magnet material.
Most advanced magnets are magnetized by a short pulse field，achieved by discharging acapacitor bank through a copper coil. The duration of the field pulse must last sufficientlylong, in order to overcome the eddy currents at the surface of the magnets,in particular forlarge blocks. In general, a pulse duration of 5 ms to 10 ms is sufficient for completepenetration.The penetration depth a , see equation (2), depends on the electrical resistancep, the permeability u of the magnet material and the frequency f of the field pulse [1]2:
(2)
K denotes a constant.
Preferably，magnets will be magnetized after assembly,since handling of unmagnetizedmagnets is easier and prevents contamination by ferromagnetic particles. in addition chippingof magnet-edges due to the mutual attraction of magnet parts is avoided.
3 Initial magnetization state For nucleation type ferrite, SmCo5 and REFeB magnets, the initial state prior to magnetizing is usually the state after the final heat treatment, i.e. after sintering. This state shows no net remanent magnetization and is often called the thermally demagnetized, or virgin, state. Ferrite and REFeB magnets, once magnetized, may be reset to the initial state by heating them to above the Curie temperature. This will return them to the thermally demagnetized state without permanent loss of properties. SmCo 5 magnets can be reset to the initial state only by repeating the full final heat treatment. To prevent chemical changes which can lead to surface damage and permanent loss of properties, rare earth magnets shall be protected in an inert atmosphere during this procedure. For anisotropic Alnico and CrFeCo magnets, where heat treatment in a magnetic field and tempering are involved, some residual magnetization may remain in the magnets. These magnets may be completely demagnetized from any degree of magnetization by applying a slowly reducing alternating magnetic field. The same holds for any pinning or single domain type magnet such as Sm 2 Co 1 7 and rapidly quenched or HDDR-treated REFeB magnets.
4 Magnetizing behaviour of permanent magnets 4.1 General The magnetizing behaviour of permanent magnets is closely related to their coercivity mechanisms, therefore they need to be discussed. Modern permanent magnets may be divided into three groups with respect to their coercivity mechanism. The principal magnetization behaviour for these groups, the nucleation type, the pinning type and the single domain particle type is illustrated in Figure 1 .
4.2 Nucleation type magnets, sintered Ferrites, RE-Fe-B, SmCo 5 4.2.1 General The commercially very important sintered Ferrites, RE-Fe-B and SmCo 5 magnets are nucleation type materials. In the following discussion, the magnetization behaviour of nucleation type magnets will be discussed using anisotropic sintered RE-Fe-B magnets as an example.