Permanent Magnet Moving Coil Instrument (PMMC)


This is a variation of moving coil instruments. There are two types of moving coil instrument. One is permanent magnet moving coil instrument and another is dynamometer type instrument. We will discuss in this article about the permanent magnet moving coil instrument only.

Working Principle of Permanent Magnet Moving Coil Instrument

In this type of instrument, we allow one coil to move inside a magnetic field. Whenever we place a current carrying conductor inside a magnetic field there is a mechanical force acting on it. Depending on this principle we construct the permanent magnet moving coil instrument.

Construction of Permanent Magnet Moving Coil Instrument

As the name suggests, there must be a permanent magnet in the instrument. We allow rotating of one very lightweight coil of fine wire of many turns on a lightweight aluminium frame inside the magnetic field created by two poles of the permanent magnet. When the current flows through this coil, there will be a mechanical force acting on the coil. This is due to the interaction of the current with the magnetic field. This mechanical force acts as the deflecting force of the instrument. We use an alnico made U shaped permanent magnet for the purpose.  The curved pole faces provide a cylindrical space between them as shown.

magnet
Magnet

We place an iron core surrounded by a lightweight aluminium frame and pivoted at the center of the space between two pole faces. We provide the only coil of the instrument on that aluminium frame.

Moving Coil
Moving Coil

We attach two phosphor bronze hairsprings in the moving system. One is above the coil and other is below the coil. We connect the terminals of these two springs to two leads of the coil. These connections provide the path of the current to the coil through the springs. These springs also provide the controlling torque to the moving system. The springs are spiralled in opposite direction to neutralize temperature effects in the performance of the instrument.

Hair Spring
Hair Spring

We attach the pointer of the instrument to the shaft of the moving system. Due to the movement of the coil in the magnetic field the pointer rotates on the scale of the instrument.

Permanent Magnet Moving Coil Instrument
Permanent Magnet Moving Coil Instrument

Advantages of Permanent Magnet Moving Coil Instrument

  • The instrument consumes very low power because of its light coil.
  • The angle of deflection of the pointer is directly proportional to the current, hence, the scale is uniform.
  • The instrument produces very high torque compared to the weight of the moving system.
  • As there is no magnetization or demagnetization of iron during operation of the instrument, the instrument is free from hysteresis.
  • The aluminum frame used for holding the coil provides eddy current damping. So, a permanent magnet moving coil instrument does not require any additional damping mechanism.
  • As two strong magnetic poles confine the operating field of the instrument in between them. Hence, the stray magnetic field cannot much affect the performance of the instrument.

Disadvantages of Permanent Magnet Moving Coil Instrument

  • Because of the sophisticated construction, this instrument costs more than a moving iron instrument.
  • Because of the presence of a permanent magnet and springs in the instrument, errors in reading may arise in old instrument due to aging effects in the magnet and spring.

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