The megohmmeter or megger is a portable deflection instrument widely used to check the insulation resistance of electrical cables and equipment. In other words, a megohmmeter can be described as a high resistance measurement instrument. The instrument has a constant high-voltage source, normally produced by a hand-cranked generator. The voltage may range anywhere between 100 V and 5000 V. The diagram below illustrates the circuit diagram of a megger:
In reference to the figure above, the pointer on a hand-cranked megger is deflected by a permanent magnet moving coil (PMMC) system with two coils. There is no mechanical controlling force; instead, the coils are connected to oppose each other. One coil is identified as a control coil and the other as a deflection coil.
Voltage is applied to the control coil via a standard resistor R1, so that the controlling force is proportional to the generator voltage divided by R1. The deflection coil is supplied via Rx, the resistance to be measured, and R2, the internal deflection circuit resistance. The deflecting force is proportional to the generator voltage divided by Rx + R2. Deflection is proportional to the difference between R1 and Rx + R2, and the instrument scale can be calibrated to directly indicate Rx.
When the megger is measuring an open circuit, no current flows in the deflecting coil. As a result, the force from the control coil causes the pointer to be deflected to one end of the scale. This end is marked infinity (โ). When measuring a short circuit, the pointer is deflected to the opposite end of the scale from infinity, and this end is marked 0 ๐ฎ. When the pointer is stationary at the center of the scale, the deflecting and control forces are equal and Rx + R2 = R1 or Rx = R1 โ R2. The scale is marked equal to (R1 โ R2) ๐ฎ at this point and is proportionately marked at other points. Range changing can be realized by switching to different values of R2.
Related articles:
- Types of Resistors
- Basic Features of a DC Potentiometer
- How to calibrate a dc voltmeter
- Standards Used in Measurements & Their Classifications
- Power Measurement in AC Circuits (Single-phase & Polyphase Systems)
- AC Circuits with Resistors, Inductors and Capacitors
- Power Measurements in DC Circuits
- Null vs. Deflection Type Instruments
