Megger - Types of Megger - Construction - Working Principle - Megger Tests - Applications of Megger | Freely Electrons

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Megger



Megger is an electrical instrument used to measure the resistances of circuits in megaohms. It is a portable instrument. It is connected to the outside terminals of any electric circuit. It is a voltage generator and produces voltages and then measure the resistance and do various tests in the electric circuits. It is also used to test the huge electric circuits such as insulation tests, earth leakage test, short circuit test etc.
Construction of Meggers
Megger consists of hand driven DC generator. It is a clutch mechanism in which armature is designed to move at a constant speed. In this way the generator speed and voltage remain constant.
The generator voltage is applied across the fixed resistance and across deflecting coil through a current limiting resistance. The external resistance is connected across testing terminals. These two coils consist of a moving coil voltmeter and ammeter combined into one instrument.
Suppose testing terminals are open circuited and the generator voltage is applied across the coil and it flows through it. The producing torque starts moving the element of the Megger until the scale points to infinity through the coil. The external current is too large for the instrument to measure when the testing terminals are closed through a distance, then a large number of current passes through the deflecting coil. The deflecting torque then produced and produce opposing torque of coil and rotates the moving element until the needle points to zero and the external resistance is too small for the instrument to measure.
Working Principle of Megger
Meggers are cross coil type instruments which shows two coils mounted rigidly at right angles to each other and are free to rotate in a magnetic field. When the current passes in it, these two coils produce torque in it, in an opposite direction.
The torque of the first coil is proportional to I1 Cos Θ and the torque of the second coil is proportional to the I2 Sin Θ. These two coils come to an equilibrium state at a particular stage where two torques are equal but in opposite direction, then
I1 Cos Θ = I2 Sin Θ                or                    tan Θ = I1/I2

These two coils are connected across a common voltage source i.e. battery or any other source. The fist coil is connected directly to the voltage and is called as voltage coil. Its current is I1 = V/R1 . The second coil is called the current coil as it is connected to the current source and its current is I2 = V/R2. These two coils are free to rotate in permanent magnetic field. The deflection Θ of the instrument is proportional to I1/I2 which is equal to Rr/R1. If R1 is fixed then the scale is calibrated to read resistance directly. The value of voltage remains constant and in large amount to give suitable currents with the high resistance to be measured.

Types of Megger
Usually meggers have two types according to its function or according to its working principle. These meggers are separated with two main categories or types.
  • Manual(Hand-driven) Meggers 
  1. Analog Meggers
  2. Digital Meggers


Analog Meggers

Analog meggers are those meggers which consists of pointer and a scale. These meggers deflect the pointer on its scale and shows its reading. It works according to the same working principle of it and is used for the testation of different electric circuits and to measure the large amount of resistances of electric circuits.
Digital Meggers

  

These meggers have the screen and shows its ratings on the digital screen. These meggers work according to the same working principle but some electronic components are added in digital meggers to show its reading on a digital screen. These meggers are also used for the same purposes that the analog meggers are used.

Electronic Type Meggers (Battery-operated)

These are the meggers, which are made up of electronics components and these meggers are automated and battery operated. These have only two connecting leads through which it produces current and voltage. These meggers are button operated and are not hand driven. It consists of a button, through which it starts and stop generating voltages and current.
Advantages of Manual Meggers
It is also used in high scale industries as it is an 0ld technology.
No external source is required to operate.
These are cheap.
Advantages of Electronic Meggers
Accuracy is very high.
It is operate-able easily.
Safe to use.
Works perfectly.
Megger Tests
Insulation Resistance Test:
Insulation resistance test is a major test for the analysis of electric circuits. This test includes short circuit test, insulation resistance test, open circuit test and voltage drop test and test of transformers and electric motors. The method of testing of various electric circuits such as transformers, electric motors, Electric Wiring of a circuit is given below:
Insulation resistance test is done with the help of megger. Megger has two output leads which are connected to the electric circuit terminals. When we start moving its handle megger starts producing high voltages and its dial shows its rating in megaohms. If its reading remains constant (10 megaohms) then its insulation is correct and its insulation resistance is low and is able to use. If the reading shows up to 25 megaohms then insulation of the electric circuit is bad and it has a poor condition also short circuit is occurred and voltage drops are occurred at this rating.
If the dial of megger cannot display its reading on its scale then open circuit is occurred.   
Applications of megger
Megger is used to measure the resistance of a circuit between zero and infinity. It is a high resistance measuring device. It is impossible to measure the small resistances accurately with the help of megger. This instrument is very simple and very easy to operate. Some of these meggers are independent from the external supplies for its operation.

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