Attraction Type Moving Iron (MI) Instrument - Construction, Working & Torque Equation

The moving iron type instruments are one of the types of measuring instruments used for measuring voltage or current. These instruments use a movable piece of iron placed in the magnetic field that deflects the pointer over the scale and hence named moving iron instrument.

There are two types of moving iron (MI) instruments. They are, attraction type and repulsion type moving iron instruments. In this article let us learn about attraction type moving iron instrument.

The working principle of attraction type moving iron instrument is based on magnetic attraction, which attracts an iron piece when placed near a magnet field. Here, the magnet field will be produced by an electromagnet.

Construction of Attraction Type Moving Iron Instrument :

It consists of a fixed coil that is flat with a narrow opening in it. A moving iron that is made of soft iron is mounted on a spindle. The coils are wound with a number of turns that depend upon the range of the instrument. The pointer is mounted on a spindle which consists of a graduated scale for showing the deflection. The construction of attraction type moving iron is shown below.

Attraction Type Moving Iron (MI) Instrument

The controlling torque is provided by the springs or if the instrument is vertically operated gravity control can also be employed. This instrument uses air friction damping to damp out oscillations which consist of a movable piston made of aluminum placed in an air chamber.

Since the operating magnetic field produced by the coil winding is not much strong, the eddy current damping which uses permanent magnets can distort the main field. Thus eddy current damping cannot be used and fluid friction damping is not much preferred. The moving iron is made of sheet metal for obtaining a uniform scale.

Working of Attraction Type Moving Iron Instrument :

Whenever coil winding is connected across the supply to be measured, it setups a magnetic field. The intensity of the magnetic field is higher inside the coil compared to the intensity of the outside, and hence low reluctance exists inside the coil.

As the moving iron tries to occupy the low reluctance position, it is moved and gets attracted to the fixed coil. As the iron piece moves, the pointer also moves to show the deflection. The instrument attains the equilibrium position when controlling torque balances the deflecting torque.

Attraction Type Moving Iron (MI) Instrument

Torque Equation of Moving Iron Instruments :

  • Td = Deflection torque in N-m
  • θ = Deflection in radians
  • L = Inductance in Henry
  • I = Initial current
  • dI = Change in initial current
  • dL = Change in inductance
  • dθ = Change in deflection
The voltage drop across the ammeter is given by,
Attraction Type Moving Iron (MI) Instrument
The energy consumed by the meter,
For a small increment in current (dI), the deflection is increased by dθ. We know that the energy stored by the inductance of the meter = 1/2 LI2. Therefore, new energy storage due to increment in current,
= 1/2 (L + dL)(I + dI)2
The change in stored energy is,
Attraction Type Moving Iron (MI) Instrument
Neglecting the second and third order differential terms, we get change in stored energy as,
= LIdI + 1/2 I2 dL ...(2)
We have work done = Td × dθ = Td. From the law of conservation of energy, we have,
Energy consumed = Work done + Change in stored energy
Substituting equations 1, 2, and 3 in 4, we get,
Attraction Type Moving Iron (MI) Instrument
     From the above, we can say deflecting torque produced is proportional to the square of the current. For ac measurement, the readings obtained will be the RMS value of the current or voltage. Due to square law response of the instrument, the scale of moving iron instruments is not uniform.

Advantages of Attraction Type MI Instruments :

  • The instruments can be used for measuring both dc and ac quantities.
  • Simple in contruction.
  • Since the winding coil is kept stationary, these instruments are robust and reliable.
  • As attraction type instruments have lower inductance, the measurement can be done over a wide range of frequencies.
  • A shunt can be connected in parallel with the basic instrument in order to measure heavy currents.

Disadvantages of Attraction Type MI Instruments :

  • These are not suitable for economical production in manufacturing.
  • The power consumption is higher for a low voltage range.
  • Accuracy in the readings cannot be obtained due to the non-uniform scale.

Applications of Attraction Type MI Instruments :

  • Heavy current moving iron instruments.
  • Moving iron voltmeters.
  • Moving iron power factor meters.
  • Moving iron synchroscope.

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