OR
Operating principle of Meager:-
1. Deflecting[CC] & Control coil [PC]: both coils mounted at right angle to each other and maintain polarities in such a way to produced torque in opposite direction. And Deflecting coil or current coil connected in series and allows flowing the electric current taken by the circuit being tested. The control coil also known as pressure coil is connected across the circuit.
2. Permanent Magnets: Produce magnetic field to deflect pointer with North South pole magnet.
3. D.C generator or Battery connection: Testing voltage is produced by hand operated D.C generator for manual operated Megger. Battery / electronic voltage charger is provided for automatic type Megger for same purpose.
4. Pressure coil resistance (PCR) and Current coil resistance (CCR) : Current limiting resistor (CCR & PCR ) connected in series with control & deflecting coil to protect damage in case of very low resistance in external circuit.
5. Voltage for testing produced by hand operated megger by rotation of crank in case of hand operated type, a battery is used for electronic tester.
6. Torque of the megger varies in ration with V/I, (Ohm's Law :V= IR or R=V/I). Electrical resistance to be measured is connected across the generator & in series with deflecting coil.
7. As the voltage increases in external circuit the deflection of pointer increases and deflection of pointer decreases with a increases of current Hence, resultant torque is directly proportional to voltage & inversely proportional to current.
8. When resistances under test is open, torque due to voltage coil will be maximum & pointer shows ‘infinity’
9. If resistances under test is short circuit pointer shows ‘zero’, which means ‘NO’ resistance
Megger:
Working: 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.