13-04-2013, 03:05 PM
MEGGER
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INTRODUCTION
Megger is a portable instrument used for measuring insulation resistance(high resistance) of electrical machines, electrical appliances and electrical installations. The instrument works on the principle of ohmmeter. As the insulation resistance measured are usually in the range of a few mega-ohm the instrument is named as megger.
Construction
Diagram shows the constructional details of megger, which is similar to the permanent magnet type moving coil instrument. It has a horseshoe shaped permanent magnet.
The moving system consists of two coils, pressure coil PC and current coil CC. The two coils are fitted to the spindle at a fixed angle (90). A pointer attached to the spindle can move over an ohmic scale graduated from 0 to infinity.
The necessary voltage for resistance measurement (usually 250 V or 500 V ) is generated by means of a small permanent magnet DC generator provided within the instrument and is hand driven. The pressure coil or control coil is connected across the armature through a current limiting resistor Rp. The current coil or deflecting coil is connected to the armature through a current limiting resistor Rc and test probe terminals.
Operation
With the test probes in open position, if the generator handle is rotated at full speed, the pressure or controlling coil alone gets the current. The current carrying pressure coil sets itself perpendicular to the magnetic field provided by the permanent magnet and the pointer shows the reading as infinity marked on one end of the scale.
When the generator handle is rotated slowly with test probes short circuited, a high current flowing through the current or deflecting coil causes a large deflection and the pointer moves to the other end of the scale with zero as the reading.
With sufficient insulation resistance available across the test probes, if the generator handle is rotated at full speed, the moving system gets a deflection proportional to the ratio of voltage to current in insulation resistance.