PMMC INSTRUMENT

Construction of PMMC Instruments

The constructional features of this instrument are shown in Fig.

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 SYSTEMSOld style magnet system consisted of relatively long U shaped permanent magnets having soft iron pole pieces.Owing to development of materials like Alcomax and Alnico, which have a high co-ercive force, it is possible to use smaller magnet lengths and high field intensities.The flux densities used in PMIMC instruments vary from 0.1 Wb/m to 1 Wb/m.

CONTROLWhen 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. The control torque is provided by the ribbon suspension as shown.This method is comparatively new and is claimed to be advantageous as it eliminates bearing friction.

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

THE FLUX DENSITY AT ONE SIDE INCREASES WHILE OTHER SIDE DECREASES

THIS IMBALANCE EXERTS A FORCE ON THE CONDUCTOR IN THE DIRECTION OF LEAST FLUX DENSITY

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.

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DISADVANTAGES:

The basic sources of error in PMMC instruments are friction, temperature and aging of various parts. To reduce the frictional errors ratio of torque to weight is made very high.

The most serious errors are produced by the heat generated or by changes in the temperature. This changes the resistance of the working coil, causing large errors. In case of voltmeters, a large series resistance of very low temperature coefficient is used. This reduces the temperature errors.

Errors in PMMC Instrument

The aging of permanent magnet and control springs also cause errors. Opposite errors in PMMC is caused by weakening of magnet and spring cause. The weakening of magnet causes less deflection while weakening of control springs cause large deflection, for a particular value of current. The proper use of material and pre-ageing during manufacturing can reduce the errors due to weakening of control springs.