Eddy-current losses

When an alternating magnetic field is applied to a ferromagnetic material, it induces an electromotive force, denoted emf, in volts, which generates an eddy current. The Joule s heating, that is, the power released per unit mass of magnetic material, denoted P, due to the eddy current is termed eddy-current losses also known as Foucault-current losses and it is expressed in W/kg. For a low frequency, /, expressed in Hz, the flux penetration into the material is complete and proportional to / and to the reciprocal of the electrical resistivity. 

In that case, for instance, the eddy-current losses in a ferromagnetic sheet can be calculated by the following equation  

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Spinel ferrites, isostmctural with the mineral spinel [1302-67-6] MgAl204, combine interesting soft magnetic properties with a relatively high electrical resistivity. The latter permits low eddy current losses in a-c appHcations, and based on this feature spinel ferrites have largely replaced the iron-based core materials in the r-f range. The main representatives are MnZn-ferrites (frequencies up to about 1 MH2) and NiZn-ferrites (frequencies 1 MHz). 

Ferrites aHowing for operation at frequencies well above 1 MH2 have also become available, eg, 3F4 and 4F1 (Table 6). Other newer industrial power ferrites are the Siemens-Matsushita N-series (28,97) the TDK PC-series (28,100), and the Thomson B-series (28,103). While moving to higher frequencies, the ferrites have been optimized for different loss contributions, eg, hysteresis losses, eddy current losses, and resonance losses. Loss levels are specified at 100°C because ambient temperature in power appHcations is about 60°C plus an increase caused by internal heat dissipation of about 40°C. 

Eddy current losses, caused by leakage flux... 

V = Loss or active component supplying the hysteresis and eddy current losses to the stator core. 

In all the above conditions, the rotor would heatup much more rapidly than the stator due to its low thermal time constant (t), and its smaller volume compared to that of the stator, on the one hand, and high-frequency eddy current losses at high slips, due to the skin effect, on the other. True motor protection will therefore require separate protection of the rotor. Since it is not possible to monitor the rotor s temperature, its protection is provided through the stator only. Separate protection is therefore recommended through the stator against these conditions for large LT and all HT motors. 

Excessive healing of magnetic cores, as a result of harmonic frequencies due to hysteresis and eddy current losses (equations (1.I2) and (1.13)). 

An induction furnace, where the heating is due to eddy current losses itiduced by the magnetic field. 

For MS enclosures, which will have both hysteresis loss B and eddy current loss a higher derating factor must be... 

The Enclosure is of non-magnetic material, therefore it will be devoid of hysteresis and eddy current losses. 

The enclosure is constructed of non-magnetic material, generally aluminium, in view of its low cost and weight as compared to copper The nonmagnetic material eliminates hysteresis and eddy current losses in the enclosure, as i result of mutual induction,... 

The influence of a induced field on a metallic (magnetic) structure is in the form of closed magnetic loops, which cause hysteresis and eddy current losses. These closed loops cannot be broken by insulating magnetic structures at bends or joints or any other locations. (Refer to Figure 28.32 for more clarity.) There is thus no treatment that can be applied to such structures or bodies in the vicinity of an IPB to protect them from the magnetic effects of the field if present in the space. 

The eddy current loss is a much smaller loss than the hysteresis loss, but increases significantly with the operating frequency. It is shown in Equation 4.7. 

The major losses within any core material are the hysteresis loss and eddy current loss. These losses are typically lumped together by the core manufacturer and given in a graph of watts lost per unit volume V5. the peak operational flux density (5max) and frequency of operation. Hysteresis loss is given as... 

As one can see, both losses increase dramatically with the increasing levels of 5max and the eddy current loss increases drastically with the frequency of operation. These losses cause an increase in the size of the inductor or transformer for an increased frequency of operation. Increasing the frequency of operation of a switching power supply does not necessarily reduce the size of the core. 

The magnetic bearing is made up of a series of electromagnets located circumferentially around the shaft to form the radial bearing. The electromagnets (Figure 5-42) are laminated to limit the eddy current losses. The shaft must be fitted with a laminated sleeve (see Figure 5-43) for the... 

Nearly no eddy current losses occur in electrically insulating magnetic materials. This is one of the reasons for the importance of oxidic materials, especially of spinels and garnets. Another reason is the large variability of the magnetic properties that can be achieved with spinels and garnets of different compositions. The tolerance of the spinel structure to substitution at the metal atom sites and the interplay between normal and inverse spinels allow the adaptation of the properties to given requirements. 

The real part is the magnetic permeability whereas the imaginary part is the magnetic loss. These losses are quite different from hysteresis or eddy current losses, because they are induced by domain wall and electron-spin resonance. These materials should be placed at position of magnetic field maxima for optimum absorption of microwave energy. For transition metal oxides such as iron, nickel, and cobalt magnetic losses are high. These powders can, therefore, be used as lossy impurities or additives to induce losses within solids for which dielectric loss is too small. 

EDDFA analysis, 70 327t, 331 Eddy-current losses, in spinel ferrites, 77 64... 

Energy losses in soft magnetic materials arise due to both hysteresis and eddy currents, as described in the previous section. Eddy current losses can be reduced by increasing the electrical resistivity of the magnetic material. This is one reason why solid-solution iron-silicon alloys ( 4% Si) are used at power frequencies of around 60 Hz and why iron-nickel alloys are used at audio frequencies. Some magnetically soft ferrites (see Section 6.2.2.1) are very nearly electrical insulators and are thus immune to eddy current losses. Some common soft magnetic materials and their properties are listed in Table 6.19. Soft magnetic alloys are described further in Section 6.2.1.6. 

There are some notable differences apparent in Fig. 11.14 between the extinction curves for aluminum spheres and those for water droplets. For example, av is still constant for sufficiently small aluminum particles but the range of sizes is more restricted. The large peak is not an interference maximum aluminum is too absorbing for that. Rather it is the dominance of the magnetic dipole term bx in the series (4.62). Physically, this absorption arises from eddy current losses, which are strong when the particle size is near, but less than, the skin depth. At X = 0.1 jam the skin depth is less than the radius, so the interior of the particle is shielded from the field eddy current losses are confined to the vicinity of the surface and therefore the volume of absorbing material is reduced. 

Wiih a good grade of silicon electrical sieel. ihe hysteresis loss can he cm in half and the eddy-current loss reduced even more because of the high resistivity compared with iron. This results in much more efficient operation of alternating current equipment. 

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