Electrical Engineering ⇒ Topic : Use of Single-Phase Wattmeter

Use of Single-Phase Wattmeter

Before discussing the special techniques employed to measure power in a 3-phase circuit, it is desirable to consider how a single-phase wattmeter is used to measure power in a single-phase circuit.

A single-phase wattmeter consists of two coils, one fixed and one movable carrying a pointer which moves over a scale. The fixed coil (called current coil) is of low resistance and is inserted in series with the line so that it carries the line current. The movable coil (called potential coil) is of high resistance and is connected like a voltmeter across the line. The small current in the potential coil is thus equal to the input voltage divided by the resistance of the potential coil. The wattmeter *deflection is, therefore, proportional to the average power

                                                       figure (a)

(i.e. VI cos Φ) delivered to the circuit. Fig. (a) shows connections of the wattmeter to measure power in a single phase circuit.

It is clear that a wattmeter has four terminals ; two for current coil and two for potential coil. When connected in the circuit to measure power, sometimes it gives backward deflection i.e. it reads downscale. This is due to the improper connections. As shown in Fig.(a), one current coil and one potential coil terminals are marked, usually ± on an actual instrument. To obtain upscale reading on the meter, ± terminal of current coil is connected to the line side of the circuit and ± terminal of potenital coil is connected to the same line lead as the current coil (See Fig. (a). A reversal of either coil connection will result in a backward deflection.