PART B

The permanent magnet moving coil instruments are most accurate type for d.c.
measurements. The action of these instruments is based on the motoring principle. When a
current carrying coil is placed in the magnetic field produced by permanent magnet, the
coil experiences a force and
bearing > moves. As the coil is moving and

Pointer the magnet is permanent, the
instrument 15 cailed permanent
magnet moving coil instrument.
This basic principle is called
[ऋ Mirror D'Arsonval principle. The

N amount of force experienced by

the coil is proportional to the

Permanent ⋅ ∙
magnet ‏ام‎ and current passing through the coil.
98 The PMMC instrument is
Fig. 5.8 Construction of PMMC instrument shown in the Fig. 5.8.

The moving coil ಇ either
rectangular or circular in shape. It has number of turns of fine wire. The coil is suspended
so that it is free to turn about its vertical axis. The coil is placed in uniform, horizontal
and radial magnetic field of a permanent magnet in the shave of a horse-shoe. The iron
core is spherical if coil is circular and is cylindrical if the coil is rectangular. Due to iron
core, the deflecting torque increases, increasing the sensitivity of the instrument.

The controlling torque is provided by two phosphor bronze hair springs.

The damping torque is provided by eddy current damping. It is obtained by
movement of the aluminium former, moving in the magnetic field of the permanent
magnet.

The pointer is carried by the spindle and it moves over a graduated scale. The pointer
has light weight so that it can deflect rapidly. The mirror is placed below the pointer to
get the accurate reading by removing the parallax. The weight of the instrument is
normally counter balanced by the weights situated diametrically opposite and rigidly
connected to it. The scale markings of the basic ‏ا‎ PMMC instruments are usually
linearly spaced as the deflecting torque and hence the pointer deflection are directly
proportional to the current passing through the coil.

The top view of PMMC instrument is shown in the Fig. 5.9.

In a practical PMMC instrument, a Y shaped member is attached to the fixed end of
the front control spring. An eccentric pin through the instrument case engages the Y
shaped member so that the zero position of the pointer can be adjusted from outside.

Gel ‏وس‎ Balanang worwgnt

Fig. 5.9 PMMC Instrument