MEASURING INSTRUMENTS

Moving Coil And Moving Iron Instruments

                                  Moving Coil

Moving Coil Instruments are used for measuring DC quantities. They can be used on AC systems when fed through bridge rectifiers. Center magnet system is incorporated in our moving coil instruments which completely shields the movement from the effect of external magnetic fields. The movement is pivoted between synthetic sapphire jewel bearings for frictionless operation.The permanent magnet moving coil instrument is the most accurate type DC measurements.

CONSTRUCTION

The flat rectangular coil of thin enameled insulated wire of suitable number of turns wound on a light     nonmetallic or aluminum frame is suspended between the cylindrically concave poles of magnet by a     thin phosphor bronze strip. One end of the wire of the coil is soldered to strip. The other end of the strip     fixed to the frame of the galvanometer and connected to an external terminal. It serves as one leas     current lead through which the current enters or leaves the coil. The other end of the wire of the coil is     soldered to a loose and soft spiral of wire connected to another external terminal. The soft spiral of a     wire serves as the other current lead. A soft-iron cylinder, coaxial with the pole pieces, is placed within     the frame of the coil and is fixed to the body of the galvanometer. In the space between it and the pole     pieces, where the coil moves freely, the soft iron cylinder makes the magnetic field stronger and radial     such that into whatever position the coil rotates, the magnetic field is always parallel to its plane.

Working

When a current passes through the coil, it experiences a magnetic deflecting torque,which tends to rotate it from its rest position. As the coil rotates it produces a twist in the suspension   strip. The twist in the strip produces an electric restoring torque. The coil rotates until the elastic  restoring torque due to the strip does not equal and cancels the deflecting magnetic torque, then it   attains equilibrium and stops rotating any further movements.

Moving Iron instruments

Moving Iron Instruments are generally used for measuring AC Voltage and Currents. A feature of the moving element is that it is fitted with synthetic sapphire jewels. The movement is light, quick acting, but extremely robust. An efficient system of fluid damping is employed. The movement is efficiently shielded against the effect of external magnetic fields. These instrument is used to measure current and voltage with higher accuracy with any other instrument used in this field.

Working

Moving iron instrument consists of iron plate and a magnetic field .This iron plate is placed in magnetic field produced by stationary metal coil.This coil is excited by the current or volage under measurement use.when coil is excited it becomes an electromagnet  and iron plate moves with increasing rate of electromagnetic field.At that timeiron plate  inductance is high and reluctance is low(inductance and reluctance are inversely proportional).Thus force experienced to moving iron plate is always in such direction so as to increase the inductance of coil.