What are Various Magnetic Electromechanical Instruments

These are instruments that utilize the magnetic effects produced by current, voltage etc. to produce a mechanical deflection which can be calibrated for the measurement of the same aforementioned quantities. These instruments can broadly be classified in to three types:-

Permanent Magnet Moving Coil (PMMC) Instruments Moving Iron Instruments Dynamometer type Instruments

PMMC instruments are constructed by placing a moving coil in the magnetic field of a Permanent magnet. When the current flows through the coil, it produces its own magnetic field. The interaction of the two magnetic fields causes a deflection in the coil which can be calibrated to measure the value of the current.

Moving Iron Instruments basically consists of a ferromagnetic soft iron piece that is mounted near a coil on a spindle between two bearings. When the current passes through the coil, it produces a magnetic field. This field magnetizes the soft iron core in its direction and therefore causes the iron piece to swing towards the coil. A moving iron instrument thus constructed is known as ‘attraction type of moving iron instruments’.In another configuration, two identical soft iron pieces are mounted near a coil. When current flows through the coil, it magnetizes both the pieces in the same direction. This causes a force of repulsion to develop between the pieces that cause a swing in them. This type is known as ‘repulsion type of moving iron instruments’.

Dynamometer type instruments utilize two coils, one fixed and one movable to produce an interaction between their magnetic fields and therefore cause a deflection in the movable coil. This deflection is a measure of the current flowing through the movable coil. It is also very important in this case to know the strength of the magnetic field produced by the fixed coil in order to calibrate the instrument.

Thus all these type of instruments uses the magnetic effects of current to produce a mechanical deflection in some way or another. The controlling force in all these instruments is usually provided by a spring system. Being the simplest in construction, moving iron instruments are the cheapest among the three types but PMMC instruments surpass the other two in terms of accuracy. The choice of the type thus depends on the individual needs of the user.

A few instruments that utilize the above discussed magnetic effects are Ammeters, Voltmeters, Wattmeter and integrating meters.

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