The document provides an overview of Permanent Magnet Moving Coil (PMMC) instruments, which are used for accurate direct current measurements based on the D'Arsonval principle. It details the construction components including the permanent magnet, movable coil, and pointer mechanism, as well as the working principle and applications such as ammeters and voltmeters. PMMC instruments offer high accuracy and low power consumption but are only suitable for DC measurements and can be costly and delicate.

1. PMMC
INSTRUMENT
Dr. Shweta Singh
Assistant Professor
MUIT, Lucknow

2. Introduction
 The permanent magnet moving coil instruments
are most accurate type for direct current
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 moves.
 As the coil is moving and the magnet is permanent,
the instrument is called permanent magnet moving
coil instrument.
 This basic principle is called D’Arsonval principle.
 The amount of force experienced by the coil is
proportional to the current passing through the coil.

3. Construction of PMMC
Instrument

4. Construction
 PMMC Instrument consists of the following given component for its proper working.
(1.)Permanent Magnet
 it is a horseshoe permanent magnet that provides magnetic flux. The soft iron pole
piece is used to get uniform flux.
(2.)Magnetic Coil
 it is a movable coil made of thin winding wire and has a few numbers of turns placed
on the rectangular aluminum frame. this frame along with the coil is attached to the
spindle.
(3.)Iron Core
 it is a soft iron core placed in the magnetic field and its function is to minimize the
reluctance effect.
(4)Pointer and Scale
 A light pointer is attached to the spindle which moves over the scale and gives the
reading. Sometimes a mirror is also provided in the instrument for an accurate
reading.
(5.)Spring
 Two Flat spiral spring of phosphor bronze is attached to both sides of the spindle.
The function of the spring is to provide controlling torque and supply electric current
to the coil.
(6.)Jewel

5. Working Principle of PMMC
 D’Arsonval Movement Principle :
 • An action caused by electromagnetic
deflection, using a coil of wire and a
magnetized field. When current passes through
the coil, a needle is deflected.
 • Whenever electrons flow through a conductor,
a magnetic field proportional to the current is
created. This effect is useful for measuring
current and is employed in many practical
meters.
 • The basic dc meter movement is known as
the D’Arsonval meter movement because it
was first employed by the French scientist,
Jacques Arsene d'Arsonval, in making
electrical measurement.

6. Why PMMC instrument cannot
be used for AC
 These types of instruments are only used
for measuring the DC quantities as if we
apply AC current to these type of
instruments the direction of current will
be reversed during negative half cycle
and hence the direction of torque will
also be reversed which gives average
value of torque zero. The pointer will not
deflect due to high frequency from its
mean position showing zero reading.
However it can measure the direct current
very accurately.

7. Applications
 Ammeter:
 Voltmeter:
 Galvanometer:
 Ohm Meter:

8. Advantages of PMMC Instrument
 The scale of the PMMC instruments is
correctly divided.
 Its range of either voltage or current can be
easily extended by using a suitable shunt.
 Damping force is very effective.
 The PMMC instruments have high accuracy
because of the high torque weight ratio.
 There is no hysteresis loss.
 Power consumption is low.
 The PMMC instruments use shelf shielding
magnet which is useful for the aerospace
applications.

9. Disadvantage of PMMC
Instrument
 The moving coil instrument can only
be used on D.C supply as the current
reversal produces a torque on the coil.
 It’s very delicate and sometimes uses
an AC circuit with a rectifier.
 It’s costly as compared to moving coil
iron instruments.
 It may show an error due to the loss of
magnetism of the permanent magnet.

10. Thank You