L5.Properties of Fero and Ferimagnetic Materials

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  PROPERTIES OF FERO AND FERIMAGNETIC MATERIALS UNDER “HIGH SIGNAL” REGIME   Puropse of the experiment The purpose of this experiment is to determine the dependancy of the complex relative magnetic permeability of feromagnetic materials as a function of frequency, the highlighting of the hysteresis curve, which characterises these materials, and the influence that the airgap has on the properties of the material. Theoretical background    Some important metals or their alloys that are representative for the feromagnetic materials class are: Fe, Ni, Co    The “high signal” regime, in which the value of the field H is high enough, allows the material to describe a hysteresis cycle.    Due to energy losses through Foucault currents, hysteresis, magnetisation, the magnetic permeability of the material is defined as: 0 0 m  j  B B j e H H                     Through calculus, we find out that the airgap leads to a decrease of the complex permeability, especially of the losses, resulting therefore an increase of the quality factor of the equivalent material.      The losses through hysteresis are under the form of energy and are proportional to the surface of the hysteresis cycle.  Rtg  L     is the tangent of the loss angle (R is the series resistance)  Apparatus: -   a signal generator   -   a power amplifier   -   an oscilloscope  Laboratory Proceedings 1. The dependancy of the permeability with respect to the frequency and of the induction with respect to the applied magnetic field The measuring principle : In order to characterise the “high signal” regime, we will visualise on the oscilloscope, the dynamic cycle of hysteresis for a core using the Lissajous figures technique. We will visualise on the screen of the oscilloscope 3 hysteresis cycles: o   one of a core made out of feromagnetic material (closed magnetic circuit) o   one of a core made out of ferimagnetic material (closed magnetic circuit) o   one of a core made out of ferimagnetic material (magnetic circuit with airgap) After the calibration of the oscilloscope in order to get a better view on the curve, we change the values of the generator, accordingly. We fill in data Table 5.1  Observation: Before the signal generation, its amplitude will be reduced to the minimum value, to be increased progresively afterwards. The value of 250 mV will not be surpassed at the output of the generator. The generator will be coupled after  the frequency and amplitude are set.  2. The visualisation of the magnetisation curve under an “increasing field” regime   We stop the signal generator by pressing the Output button. We set the frequency at the desired value, we modulate MA with a triangular signal UPRAMP having a 10Hz frequency. The shape of these cycles are those that matter to us. 3. The calculus for the thickness of the airgap for the Tr 3  core We determine the effective relative magnetic permeability  ef   for the Tr 3  core by measuring the slope of its hysteresis characteristic. Knowing 1 ' e ' '        and using the aproximation l        we determine the thickness of the airgap  , using for   the max value of the curve  2  = f(H).
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