WORKING OF PMMC INSTRUMENTS

working of pmmc instruments

1. WORKING OF PMMC INSTRUMENT 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: