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Permanent Magnet Moving Coil (PMMC) Instrument

In this type of instrument, deflection occurs due to the movement of a coil therefore it is called moving coil instrument or simply MI instrument. This type of instrument has a limitation that it works only on DC supply.

Working Principle of PMMC Instrument

The working principle of this instrument is based on the magnetic effect of electric current, which states that when a current-carrying conductor is kept in the influence of the magnetic field then this conductor experiences a force and tries to move out of the magnetic field.

Construction of PMMC

The figure given below show the construction of a PMMC instrument.

PMMC Instrument consists of the following given component for its proper working.

(1.)Permanent Magnet

it is a horseshoe permanent magnet that provides magnetic flux. The soft iron pole piece is used to get uniform flux.

(2.)Magnetic Coil

it is a movable coil made of thin winding wire and having a few numbers of turns placed on the rectangular aluminum frame. this frame along with the coil is attached to the spindle.

(3.)Iron Core

it is a soft iron core placed in the magnetic field and its function is to minimize the reluctance effect.

(4)Pointer and Scale

A light pointer is attached to the spindle which moves over the scale and gives the reading. Some times mirror is also provided in the instrument for an accurate reading.

(5.)Spring

Two Flat spiral spring of phosphor bronze is attached to both sides of the spindle. The function of spring is to provide controlling torque and supply electric current to the coil.

(6.)Jewel

it is placed at each side of the spindle which acts as a bearing to minimize the friction effect.

Working Of PMMC Instrument

When a direct current(DC) is passed through the coil then the coil will produce magnetic flux. Due to the reaction of horseshoe magnet magnetic field and coil magnetic field a deflecting torque is developed which is proportional to the product of the current and the magnetic field strength in the air gap.

In other words, the pointer's deflection is proportional to the current. The aluminum former used to house the coil and work as a damper also because eddy currents are produced in it which produces the damping torque.

Advantage of PMMC Instrument

PMMC instrument has some advantages over another measuring instrument which are given below:-
  • Its range of either voltage or current can be easily extended by using a suitable shunt.
  • Damping force is very effective.
  • there is no hysteresis loss.
  • power consumption is low.
  • its scale has uniform marking.

The disadvantage of PMMC Instrument

  • The main disadvantage of the PMMC instrument is that it works only in DC.
  • it is costly.
  • Friction and thermal losses can develop in it.
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