Measurement of three phase power by two wattmeter

Measurement of three phase power by two wattmeter method 1

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Slide Wire DC Potentiometer

Principle of Operation • A basic potentiometer circuit consists of a slide wire AB of uniform cross section and Unit length. Generally slide wire is made up of manganin. Let r be the resistance per unit Length of slide wire. • The battery supplies a current through the slide wire which is limited with the help of regulating resistance i. e. rheostat. • The battery B 1 Whose e. m. f. is to be measured is connected in series with a galvanometer G and switch K. • When the switch K is opened, the current through slide wire is i. If the sliding contact is at position C, let the length AC be l Units, then the voltage drop across AC is given by ir 1.

• Consider that switch K is closed which puts the battery B 1 in the circuit. • The battery B 1 whose e. m. f. is to be measured is connected such that the voltage drop along the slide wire and e. m. f. of B 1 oppose each other. • The deflection in the galvanometer G depends on of magnitude of voltage drop across the slide wire portion AC and e. m. f. of B 1. • If the voltage drop across length l of the slide wire is greater than e. m. f. of battery B 1, then the current will flow in the direction A to C through the galvanometer. Similarly if the e. m. f. of battery B 1 is greater than the voltage drop across the length l of the slide wire, then the current will flow in the direction C to A through the galvanometer. • The most important condition exhibiting the basic principle of the potentiometer is that no current flows through the galvanometer when the two e. m. f’s are equal.

• Generally a scale is provided along with the slide wire which enables to measure the length of portion AC. • Hence to measure e. m. f. of a battery, first adjust a current through slide wire with switch K open. • Then insert battery whose e. m. f. is to be measured. • By closing switch K, adjust sliding contact such that the galvanometer shows zero deflection. Measure the length of the portion of the slide wire with the help of scale provided. Then the unknown e. m. f. E of battery is given by, • E = i(r. l) • where r 1 is the resistance per unit length, i is the working current adjusted using rheostat R.