Explain with neat sketch the construction and working principle of megger.

Explain with neat sketch the construction and working principle of megger.
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Answer :

Megger:

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Construction of Megger : Two coils: the current coil and pressure coil are mounted at an angle on the same spindle and form the part of the moving system. These coils are connected to a small hand driven generator, with polarities such that torque produced by them will act in opposition to each other. The coils being placed in the air gap of a permanent magnet will move in it, the potential coil is connected in series with a fixed control resistance and the current coil is in series with a resistance to control the current flowing through it and the resistance under test. When the resistance under test is infinity no current flows through the current coil, the pressure coil will therefore set itself perpendicular to the magnetic axis, and the pointer indicates infinity on dial. If the resistance under test is very low, the high current will flow through the current coil, it makes the pressure coil; to lie in the direction of axis of permanent magnet, as the effect of pressure coil will be negligible, the position of the pointer in this case is marked as zero. For value in between the pointer will indicate values in between zero and infinity. The dial is marked with values of resistances in mega ohms by calibration. When the instrument is not working the pointer may rest at any position on the dial.

Working principle of Megger: It works on the principle of electromagnetic attraction. When a current-carrying primary coil is placed under the influence of a magnetic field it experiences a force. This force produces a torque that is used to deflect the pointer of the device which gives some reading.

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