Electrical Engineering ⇒ Topic : Extending Range of Moving-Iron Instruments

Extending Range of Moving-Iron Instruments

As explained above, moving-iron instruments are used mainly on a.c. circuits. Therefore, range extension shall be discussed with reference to a.c. measurements.

(1)  Ammeter. Shunts are not used to extend the range of moving-iron a.c. ammeters. It is because the division of current between the operating coil and the shunt varies with frequency (since reactance of the coil depends upon frequency). In practice, the range of moving-iron a. c. ammeter is extended by one of the following two methods

(i) By changing the number of turns of the operating coil. For example, suppose that full scale deflection is obtained with 400 ampere-turns. For full-scale reading with 100 A, the number of turns required would be = 400/100 = 4.

Similarly, for full-scale reading with 50 A, the number of turns required is = 400/50 = 8. Thus the ammeter can be arranged to have different ranges by merely having different number of turns on the coil. Since the coil carries the whole of the current to be measured , it has a few turns of thick wire. The usual ranges obtained by this method are 0-250 A.

For ranges above 0-250 A, a current transformer is used in conjunction with 0-5 A a.c. ammeter as shown in Fig. (a). The current transformer is a step-up transformer i.e., number of secondary turns is more than the **primary turns. The primary of this transformer is connected inseries with the load and carries the load current. The a.c. ammeter is connected across the secondary of the transformer. Since in Fig. (a), the current transformer ratio is 10 : 1, it means that the line (or load) current is equal to 10 times the reading on the a.c. meter.

                                                       Load current, I_L = 3 x 10 = 30 A

                                                       figure (a)

2. Voltmeter. The range of a moving-iron a.c. voltmeter is extended by connecting a high resistance (multiplier) in series with it. For ranges higher than 0-750 V, where power wasted in the multiplier would be excessive, a 0-110 V a.c. voltmeter is used in conjunction with a potential transformer as shown in Fig. (a). The potential transformer is a step-down transformer i.e., number of primary turns is more than the secondary turns. The primary of the transformer is connected across the load across which voltage is to be measured. The a.c. voltmeter is connected across the secondary. Since in Fig. (a), the potential transformer ratio is 20 : 1, the load voltage is equal to 20 times the reading on the a.c. voltmeter.

                                          Load voltage, V_L = 100 x 20 = 2000 V

Note that both secondaries of the instrument transformers are grounded as a safety measure