working of pmmc instruments
TRANSCRIPT
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WORKING OF
PMMC
INSTRUMENT
PMMC INSTRUMENTS by S. BHARGAV is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
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▶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
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▶ 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
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▶PERMANENT MAGNET▶RECTANGULAR COIL▶CONTROLLED SPRINGS▶ALLUMINIUM
CYLINDRICAL CORE▶POINTER▶PIVOTS▶SCALE▶DUST PROOF CASE
PARTS:
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MAGNET SYSTEMSIt consists of permanent horse shoe magnet such rthat the flux is maximum in the centre
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CONTROLControl is provided by spring mechanism.Spring system is also used to lead the current into the system
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▶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
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▶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
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▶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:
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▶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
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▶IT WORKS ON THE PRINCIPLE OF DC MOTOR▶WHEN CURRENT
PASSES THROUGH THE COIL,IT PRODUCES FLUX OF THE CORE
WORKING
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▶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
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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
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▶- 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:
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▶-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: