Permanent magnet moving coil

The permanent magnet moving coil instruments (PMMC)

The permanent magnet moving coil instrument or in short PMMC instruments. These PMMC instruments are very accurate and useful in DC measurements. The permanent magnet moving coil instruments are also known as D’alvanometer.

Permanent magnet moving coil instruments

Construction of PMMC instruments

The permanent magnet moving coil instruments consist of several parts. These parts are explained below :

  • Permanent magnets

PMMC instruments use the high-intensity permanent magnet for magnetic field production. These magnets are made up of the material Alcomax and Alnico because these materials have high coercive force.

  • Moving coil

Moving coil is one of the most essential parts of the permanent magnet moving coil instruments. Moving coil is nothing but a copper wire placed on a rectangular block. The moving coil is placed in between the permanent magnets.

  • Control

Control of produced torque is possible because of springs. These springs are used to control the torque as well as to provide the path to lead the current in and out. These springs are made up of phosphorus bronze.

  • Damping

Damping torque is generated by the aluminum core’s movement within the magnetic field to keep the movement of the coil at rest.

  • Pointer

The pointer displays the value of current or voltage on the scale. The pointer is connected to the moving coil.

Working of PMMC instruments

The permanent magnet moving coil instruments works on the principle that when current flows through the moving coil it produces a magnetic field that interacts with the magnetic fields produced by the permanent magnet.

This interaction of magnetic fields results in the exertion of deflection torque on moving coils. the direction of deflection torque can be determined by Fleming’s left-hand rule. This deflection torque is balanced by the restoring torque produced by the spring which is attached to the stationary part of the meter.

The scale shows the value of the current flowing through the coil, which is the equilibrium position between the deflecting torque and restoring torque. The deflection torque is directly proportional to the current flowing through the coil.

Advantages of PMMC instruments:

  • PMMC instruments have a uniform scale
  • Low power consumption
  • High torque to weight ratio which results in high accuracy
  • These instruments consume very low power
  • These instruments don’t get affected by stray magnetic fields. It is because of the use of unidirectional fields.

Disadvantages of PMMC instruments:

  • PMMC instruments are costlier than Moving iron instruments.
  • PMMC instruments Can only be used for DC measurements. Because for AC measurements the deflecting torque reverses the direction too fast to permit the movement of the needle.

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