This document describes the construction and working of a permanent magnet moving coil (PMMC) instrument. It consists of a fixed magnet and a movable coil mounted on a frame. Current passing through the coil experiences a torque due to the magnetic field, causing it to deflect. The coil is connected to a pointer and scale to display measurements. PMMC instruments can be used as ammeters or voltmeters and have advantages like high accuracy, uniform scale, and efficient damping. Their main disadvantage is that they can only be used for DC circuits.

1. PMMC INSTRUMENTS by S. BHARGAV is licensed under a Creative Commons Attribution-
ShareAlike 4.0 International License.

2. ▶Permanent magnet moving coil is the basic measuring
instrument which can be used for measuring current
and voltages
▶They can be converted into either voltmeter and
ammeter

3. ▶ It consists of fixed and moving parts
▶ Fixed part mrans magnet and moving part
contains coil
.
▶ The moving coil is wound with many
turns of enameled or silk covered copper
wire.
▶ The coil is mounted on rectangular
aluminum former, which is pivoted on
jeweled bearings.
▶ The coils move freely in the field of a
permanent magnet.
▶ Most voltmeter coils are wound on metal
frames to provide the required electro-
magnetic damping.
▶ Most ammeter coils, however, are wound
on non-magnetic formers, because coil
turns are effectively shorted by the
ammeter shunt.
▶ The coil itself, therefore, provides electro
magnetic damping.
CONSTRUCTION

4. ▶ PERMANENT MAGNET
▶ RECTANGULAR COIL
▶ CONTROLLED SPRINGS
▶ ALLUMINIUM
CYLINDRICAL CORE
▶ POINTER
▶ PIVOTS
▶ SCALE
▶ DUST PROOF CASE
PARTS:

5. MAGNET SYSTEMS
It consists of permanent horse shoe magnet such
rthat the flux is maximum in the centre

6. CONTROL
Control is provided by spring mechanism.
Spring system is also used to lead the current into the
system

7. ▶ RECTANGULAR IN SHAPE
WOUND ON ALUMINIUM
FORMER WITH LARGE NO.OF
TURNS.
▶ WIDTH OF RECTANGLE IS
LESS THAN DISTANCE b/w
POLES OF PM WITH AN AIR
GAP.
DEFLECTING COILS

8. ▶ A LIGHT ALUMINIUM
CYLINDER WITH PIVOT
AT TOP AND BOTTOM IS
MADE TO FIT OVER THE
AXLE OF MOVING COIL
AND ACTS AS DAMPING
MECHANISMS.
ALUMINIUM CORE

9. ▶ The pointer is carried by the spindle
and moves over a graduated scale.
▶ The pointer is of lightweight
construction and, apart from those
used in some inexpensive
instruments has the section over the
scale twisted to form a fine blade.
▶ This helps to reduce parallax errors
in the reading of the scale. When the
coil is supported between two jewel
bearings two phosphor bronze
hairsprings provide the control
torque.
▶ These springs also serve to lead
current in and out of the coil.
POINTER AND SCALE:

10. ▶ THE WHOLE
INSTRUMENT IS
ENCLOSED IN A DUST
PROOF CASE
▶ THE SHAPE AND SIZE
OF THE CASE DEPENDS
UPON THE CAPACITY OF
THE INSTRUMENT
DUST PROOF CASE

11. ▶ IT WORKS ON THE
PRINCIPLE OF DC
MOTOR
▶ WHEN CURRENT
PASSES THROUGH THE
COIL,IT PRODUCES FLUX
OF THE CORE
WORKING

12. ▶ PRINCIPLE OF OPERATION IS BASED ON
FLEMINGS LEFT HAND RULE
▶ i.e WHEN A CONDUCTOR IS PLACED IN A
MAGNETIC FIELD IT EXPERIENCES A FORCE
BECAUSE OF WHICH IT MOVES AWAY
WORKING

13. TORQUE
>Torque, moment or moment of force is the tendency of
a force to rotate an object about an axis, fulcrum, or pivot.
>DEFLECTING TORQUE=TOTAL FORCE*DISTANCE
=>Td=NABI
N=NO.OF TURNS
B=FLUX DENSITY
A=AREA OF CROSS SECTION
I=CURRENT
>AT FINAL DEFLECTION Td=Tc
=>Tc PROPORTIONAL TO I
HERE DAMPING IS EDDY CURRENT DAMPING

14. ▶ - The PMMC consumes less power and has great
accuracy.
▶ - It has uniformly divided scale and can cover arc of
270 degree.
▶ - The PMMC has a high torque to weight ratio.
▶ - It can be modified as ammeter or voltmeter with
suitable resistance.
▶ - It has efficient damping characteristics and is not
affected by stray magnetic field.
▶ - It produces no losses due to hysteresis.
ADVANTAGES:

15. ▶ -The moving coil instrument can only be used on D.C
supply as the reversal of current produces reversal of
torque on the coil.
▶ - It’s very delicate and sometimes uses ac circuit with
a rectifier.
▶ - It’s costly as compared to moving coil iron
instruments.
▶ - It may show error due to loss of magnetism of
permanent magnet.
▶ You may also read Minimize the risk of electrical
shock on ship.
DISADVANTAGES: