insulation tester Megger and working principle

 

Megger works on the principle of electromagnetic induction

 

Megger insulation tester

The insulation tester Megger and working principle is used for measuring the insulation resistance (IR) value of motors, cables, panel-mounted components, bus bars, etc. Megger is available for different voltage ranges like  50 volts, 500 volts, 1000 volts, 2500 volts (2.5kv), and 5 kV. IR value measurement is very important in electrical work to ensure the safety of working personnel and equipment.

Construction and working:-

It consists of a permanent magnet cross coil dynamometer type indicating instrument and a hand-operated DC generator. There is a permanent magnet as shown in fig. Two moving coils are placed within the magnetic field of interest supported by pivots. The two coils are placed at 90 degrees to each other. The first coil is known as a pressure coil connected in parallel to the output voltage of the generator. it provides torque to the spindle in an anti-clockwise direction hence moving the pointer toward infinity. The second coil is connected in series with testing resistance, it produces torque in the opposite direction which moves the pointer towards 0 ohms. So when Megger is used for measuring resistance, the Megger pointer indicates the ratio of V/I that is R of the circuit.

Hand-operated generator:-

 It is a tiny generator with a permanent magnet field. the armature is rotated with the help of a handle and gear system. A clutch is provided so that the handle starts slipping after 120 pm. So when the handle is rotated at full speed the armature speed is limited to 120 rpm and the output voltage remains constant.
Uses:- LT motors are Meggered with 1000  volt Megger. HT motors are merged with 2.5 kV Megger. The minimum IR value should not be less than the voltage rating of a machine of 1000 ohms which means 1000 ohms per volt of machine rating. In HT motors and HT cables, the R15/R60 ratio is also considered. R15 and R60 are begging values at 15 seconds and 60 seconds respectively. The R60 value must be 1.3 to 1.5 times higher than the R15 value.