The most common ammeters and voltmeters for laboratory or switch board use at power frequencies are moving iron instruments. These instruments are quite cheap in cost, simple in construction and reasonably accurate at fixed power supply frequency. These instruments can be used on a.c. as well as on d.c.Moving iron instruments are use either as voltmeter or ammeter only.

Classification of Moving Iron Instruments

These instruments are divided into two type; (i) Attraction type and (ii) Repulsion Type

Attraction Type Moving Iron Instruments

Working Principle

When a soft iron piece is placed in the magnetic field of a current carrying coil, it is attracted towards the centre of the coil. This is because the piece tries to occupy a position of minimum reluctance. This results, a force of attraction is exerted on the soft iron piece and deflection in the needle takes place.

Construction

The coil is flat and has a narrow slot like opening. The moving iron is a flat disc or a sector eccentrically mounted. The controlling torque is provided by springs but gravity control can be used for panel type of instruments which are vertically mounted. Damping torque is provided by air friction with the help of light aluminium piston which moves in a fixed chamber.

attraction type moving iron instruments

Working

When the instrument is connected in the circuit, the operating current flows through the stationary coil. A magnetic field is set up and the soft iron piece is magnetized which attracted towards the centre of the coil. Thus, the pointer attached to the spindle is deflected over the calibrated scale.

Note:

If current in the coil is reversed the direction of magnetic field produced by the coil will reverse, also magnetism produced in the soft iron piece will reverse. Hence the direction of deflecting torque remains unchanged. Thus, Attraction type MI instruments can be used on d.c. as well as on a.c.

Deflecting torque

Td = I2

The controlling torque is provided by the spring

Tc = θ

Where θ is angle of deflection

Repulsion Type Moving Iron Instruments

Working Principle

The basic principle of a repulsion type repulsion type moving iron is that the repulsive forces will act when two similarly magnetized iron pieces are placed near to each other.

Construction

There are two different designed are in common;

  1. Radial vane type: In this type, the vanes are radial strips of iron. The strips are placed within the coil. This fixed vane is attached to the coil and the movable one to the spindle of instrument.
  2. Co-axial vane type: In this type of instrument, the fixed and moving vanes are section of co-axial cylinders.

repulsion type moving iron instrumentsThe controlling torque is provided by spring control method while damping torque is provided by air friction.

Working

When the instrument is connected in the circuit, the operating current flows through the coil. A magnetic field is set up along the axis of the coil. This field magnetises both pieces similarly i.e. both the pieces attain similar polarities. A force of repulsion acts between the two, therefore, movable piece moves away from the fixed piece. Thus, the pointer attached to the spindle deflects over the calibrated scale.

Note:

If the current in the coil is reversed, the direction of the magnetic field produced by the coil is reversed. Although the polarity of the magnetized soft iron pieces is reversed but still they are magnetized similarly and repel each other. Hence, the direction of defecting torque remains unchanged. Thus, these instruments can be used on d.c. as well as a.c.

Deflecting torque

Td = I2

The controlling torque is provided by the spring

Tc = θ

Where θ is angle of deflection

Advantages of MI Instruments

  • These instruments are cheap and robust in construction.
  • They can be used on both a.c. and d.c.
  • They are reasonably accurate.
  • They posses high starting torque.

Disadvantages of MI Instruments

  • They cannot be calibrated with a high degree of precision with d.c. on account of the effect of hysteresis in the iron rods.
  • They are not very sensitive.
  • They have non-uniform scale.

Errors in MI instruments

  • Error due to hysteresis
  • Error due to stray magnetic field
  • Error due to temperature
  • Error due to change in frequency

Range

Ammeters : From about 0-20mA to 0-800mA maximum with current transformer.

Voltmeters: From about 0-1V to 0-800V maximum without potential transformer.