What is a meggar?
Meggar is also known as megohmmeter**, The megger insulation tester is an instrument used for measuring high resistances of the order mega ohms and for testing the insulation resistance of an insulated body.
For its operation, it uses a DC generator or a battery along with the electronic circuits. It is able to generate an output voltage up to 5000V (depending on the model of the instrument). The results depends upon the insulation resistance, which has an insulated element relating to an active element or a conductor.
What is the working principle of meggar?
Working principle of a megger is based on the working principle of moving coil instruments, which states that when a current carrying conductor is placed in a magnetic field, a mechanical force is experienced by it.
The magnitude and direction of this force depend upon the strength and direction of the current and magnetic field.
Construction of a Megger
The parts of meggar are as shown below.
• It consists of a hand driven DC generator and a direct reading ohm meter.
• There are two coils A and B which are fixed together at some angle and are free to rotate about a common axis between the poles of a permanent magnet.
• The coils are connected in the circuit by means of flexible leads (or ligaments) which exerts no restoring torque on the moving system.
The armature of the generator is rotated by the hand driven crank lever.
The clutch mechanism is designed to slip at a predetermined speed.
This facilitates the generator to maintain a constant speed and hence the constant voltage while testing.
The two coils A and B constitutes a moving coil voltmeter and an ammeter. Both are combined to form one instrument.
The hot terminal of the equipment whose insulation resistance has to be measured is connected to the testing terminal X. The terminal Y is connected to the body of the equipment, which is generally grounded.
How does a meggar works?
When the crank handle is rotated, a voltage is generated in the generator. This generator voltage is applied across the voltage coil A through a resistance R1.|
This generator voltage is applied across the voltage coil A through a resistance R1.
When the terminal X & Y are free initially, no current flows through the coil B.
The torque produced by the coil A rotates the moving element to show infinity.
• While testing, the terminals X & Y are connected across the terminal and the body of the machine for measurement.
• The deflecting torque produced by the coil B interacts with the torque of coil A and rotates the moving element to indicate the resistance value.
• The volt generated by this instrument is around 500volts.
• Meggars are available to generate 100 volts, 2500 volts, and 5000 volts also. High voltage meggars are either motor-operated or power operated.
Meaning of Zero and Infinity Reading of Megger
• When a magger is showing zero reading, it means the unknown resistance, which is being measured, has very low value.
• If it is showing zero value while measuring insulation resistance, it is an indication of insulation failure or short circuit.
• Whereas, when it is showing infinity reading, it means the unknown resistance, which is being measured, has very high value (or open circuit).