Permanent Magnet Moving Coil Type Instruments | Freely Electrons

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Permanent Magnet Moving Coil Instruments

Permanent Magnet moving coil instruments or PMMC Instruments are used to create the stationery magnetic field between the moving coil. The working principle of these instrument is “When a current carrying conductor is placed in a magnetic field, a force is produced which tends to move it out of a magnetic field in a one direction.”

Construction

PMMC instruments consists of a permanent magnet and a rectangular shaped coil (consists of current carrying conductors). It has many turns and the its core consists of either aluminum or copper. A U-shaped permanent magnet is fixed on a soft iron cylinder between the magnetic poles.
Working of PMMC Instruments
A U-shaped permanent magnet makes a magnetic field uniform and to decrease the reluctance of the air between the poles and increases the magnetic flux. The rectangular coil is supported by bearings and a light pointer is attached to it. The rectangular core provides damping by eddy currents induced in it. The coil sides can move freely between the air gaps and between the poles. The two springs are attached to the core in opposite direction to neutralize the effects of temperature.As a result, the pointer deflects on a scale.

Deflecting Torque OF PMMC Instruments

PMMC Instruments acts upon deflecting torque to move the pointer on a scale. Deflecting torque is a force which is produced when the electric current flows through the coil by acting upon the both sides of the coil and deflecting torque is produced. Here are the parameters of the deflecting torque:


Advantages of PMMC

  • These PMMC instruments consume low power.
  • It has uniform scale and it extends up to 1700.
  • These instruments have high weight ratio.
  • These are upgraded with the help of shunts and resistances to measure high range of current and voltage.
  • Hysteresis losses cannot occur in these instruments.
  • These consists of very effective eddy current damping.

Disadvantages of PMMC

  • These PMMC instruments are expensive.
  • Some errors occurred due to getting old of control springs and permanent magnets.