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Draw construction of Megger and write operating principle of it.

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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

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