Megger(working principle)

MEGGER

Megger is the most portable insulation tester. It is used to remove very high resistances of the order of meg ohms.

PRINCIPLE:
The instrument works on the principle of ratiometer/ohmmeter. The required deflection torque is produced by both the system voltage and the current. Due to interaction between the magnetic fields produced by the voltage and the current, the deflecting torque is produced. The required coils are so positioned that the deflecting torque is proportional to the ratio, V/I.

CONSTUCTION:
It consists of
i. A small hand drives d.c. generator
ii. A moving element which has 2 coils, a deflecting coil (or current coil) and a controlling coil (or potential coil)
iii. Calibrated scale in mega ohms.
iv. Pointer and
v. Permanent Magnet.
The two coils are rigidly mounted at right angles to each other. They are connected to the small hand driven generator. The coils move in the air gap of a permanent magnet. To protect the coils under short circuit, a limiting resistor is connected in series with the coils.




OPERATION:
Resistance to be measured is connected across the test terminals i.e. connected in series with the deflecting coil and across the generator. When currents are supplied to the coils, then they have torques in opposite directions.
If the resistance to be measured is high, no current will flow through the deflecting coil. The controlling coil will, therefore, set itself perpendicular to the magnetic axis and hence, sets the pointer at infinity.
If the resistance to be measured is small, a high current flow through the deflecting coil and the resulting torque sets the pointer to zero.
For intermediate values of resistances, depending upon the torque production, the pointer is set at a point between zero and infinity.
The hand driven generator is of permanent magnet type and it is designed to generate from 500 to 2500 volts.