Megger is used for measuring the insulation resistance. The parts of megger are shown below. The armature of the generator is rotated by the hand driven crank lever. The clutch mechanism is designed to slip at a expecting speed. This facilitates the generator to maintain a constant speed and hence the constant voltage while testing. The two coils A and B constitute 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, which is grounded. When the crank handle is rotated, a voltage is generated in the generator.
The generated voltage is applied across the voltage coil A through a resistance R1. When the terminal X and Y are free initially, no current flows through coil B. The torque produced by the coil A rotates the moving element to show infinity. While testing, the terminals X and Y are connected across the terminal and body of the machine for measurement. Now the current passes through the deflecting coil B. The deflecting torque produced by coil B interacts with the torque of coil A and rotates the moving element to indicate the resistance value. Voltage generated by this instrument is around 500 volts. Meggers are available to genearte 1000 volts, 2500 volts and 5000 volts also. High voltage meggers are either motor operated or power operated.
APPLICATIONS OF MEGGER
Megger is used to measure the insulation resistance of the cable
Insulation resistance of the transformers
Winding resistance of the transformers and winding resistance machines (DC and AC machines).
ADVANTAGES OF MEGGER
Very high accuracy level.
It has digital type of IR value and easy to read.
Even at very congested space its works perfectly.
Why DC used in megger
Only DC is used because to measure the resistance of the winding insulation. When we use AC to conduct the insulation test,we mwasure the impedance instead of the reistance as the insulation has a substantial capacitive reactance. the winding is best to indicated by its resistance but not its impedance. thats why we use Dc to measure the insulation resistance.
TYPES OF MEGGER INSULATION TESTS:
- Short time or Spot-reading test
- Time-resistance method
- Dielectric absorption ratio
Why Zero and infinity scale is used in megger :
The voltage generated in megger ranges from 100 to 2.5kv. In this phenomenon low resistance ohmmeter,the megger scale indicates the infinitywhen measuring with an open-circuit, its indicates zero on short circuit and it indicates half-scale when standard resistor is equals to unknown resistance inside the meg0hmmeter.
The minimum acceptable value of insulation resistance is the rule may stated as insulation resistance is approximately 1mega ohms for each thousand volts of operating voltage, with minimum value of 1MΩ. Example, the motor motor rated at 2.4kv should have minimum insulation resistance is 2.4 MΩ.
DISADVANTAGES OF MEGGER:
The external source is required to energies like dry cell.
It could be costlier in market.
Impact on result due to unstable placement of megger.