Magnetic materials

Soft magnetic materials are characterised by its characteristic magnetic values as hysteresis losses, coercivity, magnetic polarisation, saturation polarisation, induction, permeability, susceptibility and remanence.

In the figure above the determination of typical magnetic values from the hysteresis curve is shown.

Depending on the respective application the focus is set to one or more values. For example a transformer requires a high magnetic induction and permeability of the core material. In this case the coercivity and hysteresis losses should be as small as possible. Unfortunately there is no existing material known, which meet all requirements. This results in a compromise as good as possible.

Unfortunately nearly all magnetic values depend on the magnetization frequency. Therefore not all materials can be used in the same frequency range.

In the animation at the left side of the picture the widening of the hysteresis loop with increasing magnetization frequency is shown. The enclosed area represents the hysteresis losses of the magnetic material. The animation is done of data extracted from grain orientated iron-silicon electrical steel sheet. Of course an appropriate frequency dependence of the losses can be observed for all magnetic materials.

The frequency dependent hysteresis losses are plotted on the right side of the picture. The hysteresis losses are divided into the static losses Ph, the classical eddy current losses Pcl and the anomalous losses Pan. The static hysteresis losses caused by material properties like dislocation density, particle size, contaminations etc. The classical eddy current losses can be calculated by the Maxwell equations and depend on the form of the induced signals, sample geometry, magnetic polarization, frequency and the electrical resistance of the magnetic material. The anomalous hysteresis losses depend on the domain structure of the material.

The coercivity for the static case (the magnetizing frequency converges to 0 Hz) and the dynamic case H(f) are given exemplarily on the hysteresis curve at the left side of the picture. The difference between the dynamic and the static coercivity is called excess field Hexc(f), which is the result for the movement of the domain walls in during the magnetization process.

The magnetic values are measured by a computer-controlled digital hysteresis measuring system.

With the hysteresis measuring it is possible to measure any shape of samples by adapting the measurement coils. The coil system consists of a field coil and a induction coil. A similar compensation coil system is connected anti-parallel to the measurement coils. In this way the vacuum portion of the magnetic induction is compensated automatically and the polarization is measured directly. A programmable wave form generator (WFG) generates arbitrary wave forms at varying amplitudes and frequencies up to 15MHz is connected to a programmable power amplifier which supplies the coil system with a current signal. In the coils an appropriate magnetic field is generated by the applied current. The signal of the induction coils and the current signal (tapped by a shunt) are sent to an analog to digital converter (ADC) and cached in an internal memory. The ADCs digitize the time-resolved measurement and are connected with an autarchic EC-bus with a Z80-based processor. Via a serial interface (RS232), the hysteresis measuring system communicates with the connected personal computer (PC) with the evaluation software. The power amplifier is equipped with a programmable circuit board which makes it possible to control signal forms and the degree of amplification of the induction signal. The personal computer controls the wave form generator (via GPIB) and addresses the Z80 processor for activating a measurement. After the measurement the measurement data is transferred to the PC and evaluated with a special software to evaluate the magnetic properties.

 

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