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  Insulation Tester

Introduction to Insulation Tester (Megger)

The tester applies DC (direct current) voltage to the insulation system and calculates the current that results. The results of the test demonstrate if the insulation is working well, or if it is permitting current to leak.

Megger, the insulation tester is a mixture of hand driven generator and a true ohmmeter in similar case as displayed in the above diagram. The magneto generator generates 500 V dc (1000 V in another design) and supplies to megohm meter section. The internal parts are displayed in the below diagram. Two fixed magnet bars with four iron pole shoes give magnetic flux for both the generator and ohmmeter setup. Pressure coil and current coil are located at right angle to each other on aluminum 'C' creator of peculiar shape. The pressure coil is connected beyond the sourced by resistor of 0.1 MW as displayed in the below diagram.

While the generator handle is rotated at a speed of 160 rpm, current flows via a pressure coil (Electronic Megger extends 1000 V with dry cell batteries with no generator and handle) and sets up magnetic field that tends to be plane of which of main magnetic field because of that the pointer which attached on the axis of moving system deflects on the 'infinity' (the high megohms). The current coil completes the circuit of it, by series resistance and external resistance within test and by the shunt that divides the current coil circuit to increase deflection for small insulation resistance. A guard system is set up to avoid surface leakage from high potential lead inside instrument and at the specimen. If there is current in external circuit because of inadequate insulation resistance of the specimen, a current will flow via the current coil in such a direction which it turns the moving system in the direction of the right of zero scale. So the deflection of pointer is relies upon external circuit resistance, and independent of magneto generator voltage, as similar voltage is delivered both to the controlling pressure coil and the current cool. In specific design of megger, the magneto generator is determined by a clutch arrangement controlled through a centrifugal governor, that's why voltage cannot rise above a definite maximum, while the crank is turned sufficiently fast, a constant testing voltage is acquired.

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