Electrical Engineering ⇒ Topic : Permanent Magnet Type
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Gopal
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Permanent Magnet Type Generally, these instruments are used for direct current measurements. The main working principle of permanent magnet moving coil instrument is similar to the D'Arsonval galvanometer. It is shown in Figure (a). There is a power magnet with soft iron pole pieces. A cylindrical iron core is mounted in between the two poles of the magnet in such a way that there is a narrow air gap.The sides of pivoted rectangular coil lies in the air gap. The rectangular coil is wound of many turns of fine wires on light aluminium or copper former and works as moving coil. The current is fed into and out of the coil by means of phosphor bronze hair springs provided at both the ends. Controlling torque is provided by these springs
figure (a) F = NBIL .......... (1) The deflecting torque is given as: TD = F x 2r = 2NBILr ........... (2) From Eq. (2) it is clear that if the flux density in the air gap is constant, then deflecting torque becomes proportional to the current, i.e., TD ∝ I ............(3) Since such instruments are spring controlled, therefore, Controlling torque Tc ∝ θ ............ (4) In steady state deflection position, Tc ∝ TD θ ∝ I .............. (5) Equation (5) indicates that these instruments have uniform scale. Since eddy currents are induced in the aluminium former, the eddy current damping is provided. | |
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