What is Moving Iron Instrument? Principal & Construction | Advantage & Disadvantage

Moving Iron Instrument

What is Moving Iron Instrument?

These instruments are sensibly exact, less expensive and simple in construction. These instruments are generally utilized in research laboratories and on electric boards. Moving iron instruments are normally utilized either as ammeters or voltmeters. Moving iron instruments are of two types:

  1. Attraction type
  2. Repulsion type
Moving Iron Instrument

Attraction Type Moving Iron Instruments

Principle - When an unmagnetized soft iron piece is placed in the magnetic field of a coil, the piece is attracted to the coil. The moving system of the instrument is attached to a soft iron piece and the operating current is passed through a coil placed adjacent to it. The operating current sets up a magnetic field that attracts the iron piece and thus creates deflecting torque in the pointer to move over the scale.

Construction - It consists of a hollow cylindrical coil (or solenoid) that is kept fixed. An oval-shaped soft iron piece is attached to the spindle in such a way that it can move in or out of the coil. The pointer is attached to the spindle so that it is deflected with the motion of the soft iron piece. The controlling torque on the moving system is usually provided by the spring control method while the damping is provided by air friction.

Working Principle - When the instrument is connected in the circuit, the operating current flows through the coil. This current sets up a magnetic field in the coil. The coil then behaves like a magnet and it attracts the soft iron piece towards it. The pointer attached to the moving system moves from zero position across the dial.
Attraction type Moving Iron Instruments

If the current in the coil is reversed, the direction of the magnetic field also reverses and so does the magnetism produced in the soft iron piece. Hence the direction of deflecting torque remains unchanged. Therefore, such instruments can be used both for dc as well as ac measurement of current and voltage.
The force F pulling the soft iron piece towards the coil depends upon
(i) The field strength H produced by the coil.
(ii) The pole strength M developed by the iron piece

Repulsion type Moving Iron Instruments

Principle - These instruments are based on the principle of repulsion between the two iron pieces magnetized with the same polarity.

Construction - Any Repulsion Instrument consists of a fixed cylindrical hollow coil that consists of the operating current. Inside the coil, there are two soft iron pieces of vanes, one of which is fixed and the other is movable. The fixed iron vane is attached to the coil whereas the movable vane is attached to the spindle. Under the action of deflection torque, the pointer attached to the spindle moves over the scale.

The controlling torque is produced by the spring control method and damping torque is provided by air friction damping in repulsion type instruments.

Working Principle - When the instrument is connected in a circuit and current is flowing through the circuit, the current sets up a magnetic field in the coil within the instrument. The magnetic field magnetizes both the iron vanes in the same direction (i.e. both pieces become magnets with the same polarity) they repel each other. Due to this force of repulsion, only movable iron vane can move as the other piece is fixed and cannot move. The result is that the pointer attached to the spindle moves from zero position.

If the current in the coil is reversed, the direction of deflection torque remains unchanged. This is because both iron vanes are in the same magnetic field and so they will be magnetized similarly and consequently repel each other irrespective of the direction of the magnetic field. Hence, such instruments can be used both for ac and dc measurements. The deflection torque is generated due to the repulsion between the similarly charged iron pieces.

Advantage and Disadvantage of Moving Iron Instruments

The moving iron instruments have the following advantages:
  • They are cheap. robust and simple in construction.
  • The instruments can be used for both ac as well as dc circuits.
  • These instruments have a high operating torque.
  • These instruments are reasonably accurate.

The following are the disadvantages of moving iron instruments:
  • These instruments have a non-uniform scale.
  • These instruments are less sensitive to changes in operating variables.
  • Errors are introduced due to the change in frequency in case of ac measurement.
  • The power consumption of these instruments is relatively higher.

Error in Moving Iron Instruments

(1) Errors due to Hysteresis
Since the iron parts move in the magnetic field, hysteresis loss occurs in them The effect of this error will result in higher readings when current increases than when it decreases. The hysteresis error can be eliminated by using "mumetal" or "permalloy" which have negligible hysteresis loss.

(ii) Error due to Stray Fields
Since the operating magnetic field is comparatively weak, therefore such instruments are susceptible to stray fields. This may give rise to wrong readings. This error is eliminated by shielding the instrument with an iron enclosure.

(iii) Error due to Temperature
Changes in temperature affect the circuit resistance of the coil and stiffness of the control springs.

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