A megger is a portable device which is used for the measurement of high resistance. The component of megger are
- Permanent magnet
- C shaft
- Hand driven generator
- Resistances RC and RD
- Control coil (Cs)
- Deflection Coil (Cd)
As current provided by the hand driven generator flow through the coil C because of which the coil will tend to set itself at right angle to the magnetic field. The wire on the coil C is wound in such a way that the torque generated is counterclockwise. Coil D is wound with the help of wire in such a way that the torque generated in clockwise.
Initially, the test terminal is open, giving a very high value of resistance by pointing to the infinite point. In this condition there will not be any current flow in coil D. Thereby coil C will allow the motion of the pointer. When the test terminal is short circuited the current flow through the coil D and generated clockwise torque and the pointer of the ohm meter rest at 0Ω.
When a resistance with unknown value is connected between the test terminal, current from the hand generator flow through both coils. In this situation torque in coil C and torque in coil D comes in equilibrium and a particular value of resistance can be seen over dial. So in this manner we can find out the value of unknown resistance.
0 Ω ZD>ZC
R Ω ZC=ZD
It is possible.
An external power supply is not required.