WORKING OF

PMMC

INSTRUMENT

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

▶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

▶ 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

▶PERMANENT MAGNET▶RECTANGULAR COIL▶CONTROLLED SPRINGS▶ALLUMINIUM

CYLINDRICAL CORE▶POINTER▶PIVOTS▶SCALE▶DUST PROOF CASE

PARTS:

MAGNET SYSTEMSIt consists of permanent horse shoe magnet such rthat the flux is maximum in the centre

CONTROLControl is provided by spring mechanism.Spring system is also used to lead the current into the system

▶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

▶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

▶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:

▶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

▶IT WORKS ON THE PRINCIPLE OF DC MOTOR▶WHEN CURRENT

PASSES THROUGH THE COIL,IT PRODUCES FLUX OF THE CORE

WORKING

▶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

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=NABIN=NO.OF TURNSB=FLUX DENSITYA=AREA OF CROSS SECTIONI=CURRENT>AT FINAL DEFLECTION Td=Tc=>Tc PROPORTIONAL TO IHERE DAMPING IS EDDY CURRENT DAMPING

▶- 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:

▶-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: