Megger is an instrument used for measuring high resistances of the order of mega ohms and for testing of insulation resistance of the cables.
The working principle of megger is similar PMMC, when a current carrying conductor is placed in a magnetic field; a mechanical force is exerted by it. The magnitude and direction of this force depends upon the strength and direction of current and magnetic field.
It consists of hand driven dc generator and a direct reading ohm meter. There are two coils potential coil (PC) and current coil (CC) are fixed together at some angle and are free to rotate about a common axis between the poles of a permanent magnet. The current coil (CC) is similar to that of the permanent magnet moving coil. The coils are connected in the circuit by means of flexible leads which exert no restoring torque on the moving system. The current coil is connected in series with a resistance R1 generator terminal and test line terminal. There another coil is that is called compensating coil. The pressure coil is connected across the generator terminals in series with a compensating coil and protection resistance R2. Compensating coil is connected to obtain better scale proportions. The terminal ‘G’ known as guard terminal is provided by means of which the guard ring can be connected to a guard wire on the insulation under test.
The resistance under test is connected between the test terminal (L) and guard terminal (G) the generator handle is, then steadily turned at a uniform speed till the pointer gives a steady reading. The working of merger is explained in three simple steps:
When the test terminals are open, the resistance to be measured is infinite. In this case, when the generator handle is rotated, voltage is induced in the generator. This induced voltage sends current through the potential coil and no current flows through the current coil. Therefore, the moving system rotates in such a direction that the pointer points at infinity.
If the rest terminals are short-circuited and the generator handle is rotated, it sends a large current through current coil and very small currents flows through the potential coil. Therefore, the resultant torque so produced turns the pointer to zero end of the scale.
If we want to measure unknown resistance, then it is connected between the test terminals. This result, an appreciably amount of current flows in both the coils. The actual position taken up by the pointer depends upon the ratio of currents in two coils, i.e. upon the unknown resistance