Electrical Engineering ⇒ Topic : Extension of Range of PMMC Ammeter

Extension of Range of PMMC Ammeter

The range of a permanent-magnet moving coil ammeter can be extended by connecting a low resistance, called *shunt in parallel with the moving coil of the instrument as shown in Fig. (a).The shunt bypasses most of the line current and allows a small current through the meter which it can handle without burning.

Let    R_m, = meter resistance

             S = shunt resistance

           I_m = full-scale deflection (f.s.d.) current

              I = full range current of the meter

Voltage across shunt = Voltage across the meter

                                                                   figure (a)

Note that multiplying power of a shunt is the ratio of circuit current to be measured to the meter current. The multiplying power of a shunt is constant and indicates the factor by which the meter current must be multiplied to obtain the circuit current. Suppose the meter has a resistance of 5 Q and requires 15 mA for full-scale deflection. In order that the meter may read 1A, the value of shunt is given by ;

                     Multiplying power of shunt       

Note that multiplying power of a shunt plays an important role in the extension of ammeter range.

Extension of Range of PMMC Ammeter

The range of a permanent-magnet moving coil ammeter can be extended by connecting a low resistance, called *shunt in parallel with the moving coil of the instrument as shown in Fig. (a).The shunt bypasses most of the line current and allows a small current through the meter which it can handle without burning.

Let    R_m, = meter resistance

             S = shunt resistance

           I_m = full-scale deflection (f.s.d.) current

              I = full range current of the meter

Voltage across shunt = Voltage across the meter

                                                                   figure (a)

Note that multiplying power of a shunt is the ratio of circuit current to be measured to the meter current. The multiplying power of a shunt is constant and indicates the factor by which the meter current must be multiplied to obtain the circuit current. Suppose the meter has a resistance of 5 Q and requires 15 mA for full-scale deflection. In order that the meter may read 1A, the value of shunt is given by ;

                     Multiplying power of shunt       

Note that multiplying power of a shunt plays an important role in the extension of ammeter range.