Purpose of Shunt

Purpose of Shunt Points : Purpose of Shunt, Ammeter Shunts The basic movement of a d.c. ammeter is a PMMCD. Arsonval galvanometer. The coil winding of a basic movement is small and light, it can carry small currents since the construction of an accurate instrument with a moving coil to carry currents greater than100mA is impracticable due to the bulk and weight of the coil that would be required.

When heavy currents are to be measured, the major part of the current is bypassed through a low resistance called “Shunt”. The basic movement (meter) and its shunt to produce an am meter. Material of Shunt The general requirements for shunts are:
(i) The temperature coefficient of shunt and instrument should be as nearly as possible the same.
(ii) The resistance of the shunts should not vary with time.
(iii) They should carry the current without excessive temperature rise.
(iv) They should have a low thermal electromotive force.

“Manganin” is usually used for shunts of d.c. instruments it gives low value of thermal emf with copper although it is liable to corrosion and is difficult to solder.

“Constantan” is a useful material for a.c. circuits since its comparatively high thermal emf, being unidirectional, is ineffective on these circuits. The construction of shunts for low currents is enclosed in the meter casing but for currents above 200A, they are mounted separately. Shunts for heavy currents are mounted externally. The meters using external shunts are usually designed to operate at a full scale voltage rating. These are usually 50, 75 or 10 mV. This is the voltage across the potential terminals of the shunt when full scale current flows through the load.