Construction and Working of Megger

Construction and Working of Megger Points : Megger Definition, Construction and Working of Megger Megger Definition Megger is an instrument used for measurement of high resistance and insulation resistance. Practically, it is used in operation of continuity, short circuit and open circuit tests of electrical installation. Construction and Working of Megger A simplified diagram of electrical connections of megger is shown in fig. A field for both generator and ohm meter is provided by permanent magnets. The moving element of the ohm-meter consists of three coils, known as:
(i) Current or deflecting coil
(ii) Pressure or control coil
(iii) Compensating coil

These coils are mounted rigidly to a pivoted central shaft and are free to rotate over a stationary C-shaped iron core. The coils are connected to the circuit by means of flexible leads (or ligaments), that exert no restoring torque on the moving element. Hence the moving element may take up any position over the scale, when the generator handle is stationary.

The current or deflecting coil is connected in series with the resistance R’ between one generator terminal and the test terminal market “L”. The series R’ protects the current or deflecting coil in case the test terminals are short-circuited and also controls the range of the instrument. The pressure or control coil in series with a compensating coil and protection resistance R is connected across the generator terminals. The function of compensating coil is to provide a better scale proportions and to make the instrument static.

When the current from the generator flows through the pressure coil, the coil tends to set itself at right angles to the field of the permanent magnet. With the test terminals open, corresponding to infinite resistance, no current flows through deflecting coil. The pressure coil thus govern the motion of the moving element, causing it to move to its extreme counter-clockwise position. The point on the scale indicated by the pointer under this condition is marked infinite resistance.

The current coil is wound to provide clock-wise torque on the moving element. With the test terminals marked “L” and “E” short-circuited, corresponding to zero external resistance, the current flowing through current coil is large enough to produce enough torque to overcome the counter-clockwise torque of pressure coil. This moves the pointer to its extreme clockwise pointer under this condition be marked zero resistance.

When a resistance under test is connected between the test terminals “L” and “F’. the opposing torque of the coils balances each other so that the pointer comes to ret at some intermediate point on the scale. The guard ring is provided to shunt the leakage current over the test terminals or with in the tester itself to the “-v”, terminal of the generator without passing through the current or deflecting coil of the instrument and thus eliminates errors due to it. Usually terminal known as guard terminal is provided by means of which this guard ring may be connected to a guard wire on the insulation under test.

A test voltage (usually 250, 500, 1000, 2500 volts) is generated by the generator “G” which, in many portable sets, is driven by means of the hand operated crank. The higher test voltages are used in the instruments with the higher resistance ranges variations in generator voltage in a given instrument do not appreciably affect the readings unless the apparatus under test has considerable capacitance. To avoid the effect of charging and discharging currents, which are due to variation in applied voltage, a special type of slipping clutch is fitted to the handle so that the generator speed and out put voltage remains constant even when the handle speed is variable.