We know that the ratiometer ohmmeters may be designed to cover a wide range of resistances. The principle of ratiometer ohmmeters is particularly adapted to application in portable instruments measuring insulation resistance. This principle forms the basis of insulation testing instrument known as Meggar.
- Megger for measurement of high resistance
Construction and working of Megger:
The main parts of the Megger are shown in the figure.
The current coil is same as that of permanent magnet moving coil instrument. V1 and V2 these are the two potential or voltage coils. The voltage coil V1 embraces the annular magnetic core. As shown in figure voltage coil V1 is in weak magnetic field when the pointer is at infinity and hence this coil exerts very little torque.
The torque exerted by this voltage coil increases as it moves into a stronger field and this torque will be maximum when it is under the pole face and under this condition the pointer will be at its zero end of the resistance scale.
In order to modify further the torque in the voltage circuit, another voltage coil V2 is used. This coil is also located in such a way it cam move from infinity to zero position of the resistance scale. The coil finally embraces the extension H of the pole piece.
The combined action of the two voltage coils V1 and V2 may be considered as though the coils constituted a spring of variable stiffness ,being very stiff near the zero end of the scale where the current in the current coil is very small (on account of unknown resistance Rx is very large).
Thus this effect compresses the low resistance portion of the scale and opens up the high resistance of the scale. This is a great advantage since this instrument is meant to be used as “insulation tester” as the insulation resistances are quite high.
The voltage range of the Meggar can be controlled by voltage selector switch. This can be done by varying the resistance R connected in series with the current coil.
The test voltages usually 500,1000 or 2500 V can be generated using hand driven generator G. A centrifugal clutch is incorporated in the generator drive mechanism which slips at a predetermined speed so that a constant voltage is applied to the insulation under test. This voltage provides a test of strength of low voltage insulation as well as a measure of its insulation resistance since it is sufficient to cause breakdown at faults. Such breakdowns are indicated by sudden motion of the pointer off scale at zero end. As the same magnet system supplies magnetic fields for both instrument and generator, and as current and voltage coils moves in a common magnetic field, the instrument indications are independent of the strength of the magnet.
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