1. Attraction type MI Instrument-

The current to be measured is passed to the fixed coil. The spindle is fixed to the moving iron is kept near the hollow fixed coil. When the d.c. current flows through the terminal, a magnetic field is produced by the fixed coil. By the magnetic induction, the moving iron gets magnetized. The south pole of moving iron is attracted by the north pole of fixed coil. Thus the deflecting force is produced due to the force of attraction. Deflection depends upon the force of attraction. The force of attraction depends upon the magnetic field produced by the fixed coil. The magnetic field depends upon the current flowing through the coil.

 When the instrument is used on A.C., the polarities of the magnetic field of the fixed coil and moving iron reverses simultaneously. Therefore, the force is provided in the same direction during the +ve and -ve half cycles. Thus, MI instruments are used for both A.C and D.C. The springs provided, produce the required controlling torque. The air friction damping system is used to provide the required damping torque.

  2. Repulsion type MI Instrument-

The repulsion type of instrument has a hollow fixed coil with a fixed iron attached to it. The moving iron is connected to the spindle. When current flows through the coil, a magnetic field is produced by it. Both the fixed iron and moving iron are magnetized with same polarity. Since they are kept in the same magnetic field, similar poles of fixed and moving vanes get repelled. Due to magnetic repulsion, the deflecting torque is produced. The moving iron moves in such a way that reluctance offered is minimum for the flux path.

Spring control method of controlling system is provided using two springs. Air friction damping is used to reduce oscillations.

Merits:

(1) It can be used by both A.C. and D.C.

(2) It is a cheaper instrument.

(3) It is robust.

(4) Springs are not used as terminals.

Demerits:

  1. Scale is not uniform.
  2. Power consumption is high.
  3. Errors due to iron piece.

Errors in the instrument:

  • Frictional error -The spindle is supported by jewel bearings at both the ends. So when spindle rotates, the friction between the tips of the spindle and bearings results error. Therefore, the tips of the spindle is made semicircular to reduce error.
  • Temperature error -The resistance of the coil changes with the variation in the temperature. This affects the deflection of instruments. The coil should be made up off magainin, so that the resistance is almost constant.
  • Error due to stray field -The external magnetic field should not affect the operation of meter. The meters are shielded so that, the external field does not enter the instrument.
  • Hysteresis error -The value of flux are different when the current is increased and decreased due to hysteresis effect. This error can be reduced by using small iron parts with narrow hysteresis loops.

Also Read- Dynamometer type single-phase power factor meter


Sachidananda Dhar

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