Electrical Engineering ⇒ Topic : Three-phase Wattmeter

Three-phase Wattmeter

Two single phase wattmeters are required to measure the total power taken by a 3-phase circuit (See Art.). Two such wattmeters may be combined into one so that total 3-phase power is read from a single scale. Such an an-angement gives rise to a 3-phase wattmeter. Fig.(a) shows a 3-phase dynamometer type wattmeter which is most commonly used in polyphase circuits. The arrangement consists of two similar dynamometer wattmeters; the two voltage coils being mounted on the same shaft and rotate in their respective cun-ent coils. The wattmeter elements are connected according to two-wattmeter method and the resulting deflection of the instrument pointer is a function of the algebraic summation of the torques produced by the individual element. The deflection of the instrument pointer is therefore a function of the total power.

In the design of such a 3-phase wattmeter, care must be taken that the two elements have no mutual action i.e. field from one element must not produce torque in the other. This may be checked by exciting the current circuit of one element and the voltage circuit of the other. There should be no deflection for this connection. Also the two elements must be matched in characteristics. This may be checked by connecting the elements with voltage circuits in parallel and the current circuits in series opposing. Again there should be no deflection 

                                                                        figure (a)

Fig. (b) shows the terminals of a 3-phase wattmeter. It has two current coils (L₁S₁ and L₃S₃) and two voltage coils (V₁ V₂ and V₂V₃). The letters L and S respectively stand for load and supply.

                                                  figure (b)

Note that current coils and voltage coils are connected in the two supply lines (S₁ and S₃ in the present case) of the 3-phase circuit. Observing *polarity is essential in a 3-phase wattmeter because if the instrument is not connected correctly, it may indicate upscale but incorrectly. A convenient rule to follow is to connect the instrument so that, as the current flows from the supply, it enters both the current and voltage circuit: of the wattmeter at the marked (0 or ±) terminals. Not the arrowheads in the diagram. The arrowhead between S₁ and L₁ indicates that positive direction of current from S₁ to L₁. Similarly positive direction of voltage from V₁ to V₂ and from V₂ to V_3.