Basics of Megger and How to Use it For Testing Motors

Megohmmeter (famous as Megger*) is an electrical instrument used to measure the electrical resistance of insulators.  Very often a megger is used to check the insulation of cable jackets, motors, and other high voltage (HV) equipment to ensure the safety of equipment.

During its working, the object under test is connected with the probes (terminals) of megger and voltages are generated, usually, these voltages are between 1kV – 5kV. The resistance of the object is measured, A high resistance value such as 50 MΩ or higher indicates that object is properly insulated. A small value or zero indicates that object is not safe and it is short.

Types of Megger

  1. Hand driven
  2. Automatic

The hand-driven megger has a rotatory knob across which we manually generate voltages. Internally it involves a permanent magnet generator which generates voltage ranging b/w 0.5kV – 2.5kV. On the other hand, the automatic megger generates voltages on its own.

Ohmmeter vs Megohmmeter

The megohmmeter is actually a resistance meter, one might think why we can’t use the normal multimeter or ohmmeter to measure the resistance. To understand this let’s consider revise the working of the ohmmeter: It contains a 9V battery which generates electrical current and actuates the electromechanical meter movement. While such setting works perfectly for small values of resistance such as those of 500kΩ or 1MΩ, or even 5 MΩ, it is insufficient for creating deflection for high values such as 50MΩ.

How to test a motor using Insulation Tester (Megger)

The video below illustrates the testing of motor using different voltage settings lets understand the working:

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