Question

Being a technical person identify the portable instrument which is used to measure resistance of the electrical machinery or system that gives the output directly. It can also be operated either by battery or mechanically. The testing voltage used by the instrument is normally in the order of 500, 1000 or 2500 V. Explain the principle and operation of instrument with clear sketch.

Answer 1

This portable instrument is called Megaohmmeter. It is more popularly known as Megger, named after the popular brand which made it in earlier days. It is generally used to measure the resistances of the insulations in electrical machineries or systems. The measurement verifies the electrical insulation level or quality of the equipments such as Motors, Generators, Transformer windings and bushings, cables etc.

It is of two types:

(1). Hand Driven or Mechanical Type

(2). Battery Operated or Electronic Type

Circuit Diagram :

Earth Control Coil Permanent Magnet Permanent Magnet Rc Unknown Resistance Control Ckt Resistance Scale Hand Driven Generator Line Pointer Hand Rotor Ro Deflecting Ckt Resistance 0 Permanent Magnet Permanent Magnet Guard Deflecting Coil Mega Ohm Meter

Working Principle : The unknown resistance to be measured is connected between the Line and Earth terminals. This resistance after connection becomes in series with the Control coil and Deflecting Coil.

The voltage for testing is produced by rotating the hand rotor connected to the Permanent Magnet Hand Driven Generator in case of mechanical Megger and by Battery in case of electronic Megger.

500V DC is sufficient for 440V ranged equipments. 1000V, 2500V and 5000V DC is used for higher voltage rating equipments. Now a days upto 12kV Meggers are available.

The Control Ckt Resistance R_C and Deflecting Ckt Resistance R_D are provided to protect the circuit against low values of external unknown resistances connected by mistake or for shorting (continuity checking) purpose.

The deflecting torque produced is directly proportional to the external ckt voltage and inversely proportional to the current.

When no resistance or very high resistance is connected, Zero curent or very less currents flows and the resulting torque due to deflecting coil is maximum and the pointer stays at infinity position.

In case of short circuit the deflection pointer comes to Zero.

In case of an intermediate resistance the deflection pointer shows some value in the middle.