Page 861 - Basic Electrical Engineering

6
resistance measure is of the order of mega ohms (× 10 Ω). The basic
constructional details of a megger is explained below.
A megger consists of two coils placed at 90° with each other as shown in

Fig. 11.32. One is called the current coil CC and the other is called the
pressure coil PC. A magnetic needle is placed in the magnetic field created by

the two coils. A pointer is connected to the magnetic needle. The pointer
moves over a scale. DC supply is provided across the terminals 1 and 2. A

megger is a portable instrument, and hence the dc Supply is made available
from a small dc generator which forms a part of the instrument. DC voltage is

generated by rotating the generator by hand with the help of a handle
provided of the purpose.
R is the resistance of the insulation which is to be measured. The hand-
x
driven dc generator provides a constant dc voltage. When dc supply is

available, current flows through the pressure coil. The magnetic needle along
with the pointer gets aligned with the axis of the pressure coil PC indicating
infinite resistance ∞, as shown. On the other hand, when resistance

measurement terminals X and Y are closed, current will flow through the
current coil CC. The needle and pointer will move in the clockwise direction

and align with the magnetic field created by the current coil. The pointer will
move to O position on the seak. When R is connected for measurement and
x
its value is comparatively low (current high) the torque produced due to coil
CC is higher than torque produced by coil PC. The position of the pointer

will be towords 0Ωside on the scale. When the value of R is high, very low
x
current will flow through the coil CC, and hence the needle will align
towords the coil PC and indicate resistance towards infinity.

11.14 MULTIMETER AND MEASUREMENT OF RESISTANCE

A single instrument is used to measure three quantities, viz current (i.e.,
Amperes), voltage (i.e., Volts), and resistance (i.e., Ohms). Multimeters are,

therefore, also called AVO meters. Multimeters are available as analog type,
electronic type, or digital type.