Magnetic fields, enclosures

These balanced enclosure currents also induce electric fields into nearby structures, RCC beams and columns in the same way as the main conductors, and hence nullify most of the space magnetic fields. These space fields (fields outside the enclosure) are otherwise responsible for causing eddy current and hysteresis losses in the metallic (magnetic) structures, RCC beams and columns in the vicinity. The electrical bonding of enclosures thus... 

The electrodynamic and electromagnetic forces between the conductors and the structures are also reduced to only 10-15% or even less. These two advantages are not available to this extent in a non-continuous enclosure. The induced current causing the magnetic field in the space is now reduced to only,... 

Figure 31.7 Nullifying of magnetic field in space in continuous enclosures...

To contain the proximity effect, in the metallic structures existing in the vicinity, it is essential that the IPB enclosures be at least 300 mm from tdl structures exi,sting parallel and 1.30 mm existing across the enclosures. A distance of 300 mm is sufficient to contain the proximity effect in view of the substantially reduced magnetic field in the space. 

The above two curves will help optitnize the thick-nes.s, /, of the enclosure foi a total minimum cost of the system. Enclosure losses may not be lowest at this thickness as shown in Figure 31.12, but they wotild maintain the tetnperature rise of the enclosure within limits. The magnetic field in the space, being already very low, would require no other measure. Moreover, it small field in the space may ciiusc only a small amount of heat in nearby structures, which may be... 

Skeats, W.F. and Sverdlow, N.. Minimising the magnetic field surrounding isolated phase bus by electrically continuous enclosures , Trans. IEEE, 81, February, 655-657 (1963). 

The setup for ESR spectroscopy is a cross between NMR and micro-wave techniques (Section 5.8). The source is a frequency-stabilized klystron, whose frequency is measured as in microwave spectroscopy. The microwave radiation is transmitted down a waveguide to a resonant cavity (a hollow metal enclosure), which contains the sample. The cavity is between the poles of an electromagnet, whose field is varied until resonance is achieved. Absorption of microwave power at resonance is observed using the same kind of crystal detector as in microwave spectroscopy. Sensitivity is enhanced, as in microwave spectroscopy, by the use of modulation The magnetic field applied to the sample is modulated at, say, 100 kHz, thus producing a 100-kHz signal at the crystal when an absorption is reached. The spectrum is recorded on chart paper. 

Figure 6.69 gives an example for an optical current sensor. The light path is wound around a current-carrying conductor equidirectionally with the azimuthal magnetic field of the current. The rotation of the plane of the electric vector is not detectable on its own and is converted to light intensity variations by a polarizer/analyser combination. A photo diode is used as a light intensity detector. The optical sensor itself is installed in the - e - compartment, the electronics shall be protected in an adequate type of protection, e.g. in a small flameproof - d - enclosure or in encapsulation - m -. In the special case of an energy distribution system with combined - e - and - d - compartments, the optical fibres may enter the d-compartment to the electronics inside via bushings complying with d -standards EN 50018 or IEC 60079-1 respectively (Fig. 6.70). The evacuation of the sensors into the e-compart-ment results in additional available space in the more expensive d-compart-ment, compared with increased safety - e -. ...

A common material used for the construction of enclosures for shielding against magnetic fields is mumetal, a high-permeability alloy. 

The transfer of electrons from dynode to dynode is likely to be affected by magnetic fields in the vicinity of the detector. For this reason, photomultipliers are magnetically shielded, typically by incorporating a cylinder of a high-permeability material, such as pi-metal (a nickel-iron alloy), around the photomultiplier and within the outer enclosure. 

Permanent magnets are used for some of the less expensive spectrometers, but suffer from the disadvantage that the stability of the field depends on extremely accurate control of the temperature of the magnet enclosure this temperature may be affected by variable-temperature operations, unless the probe is thermally well-insulated.37... 

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