Electrical Engineering ⇒ Topic : Proof for TwoWattmeter Method

David
 
Proof for TwoWattmeter Method In twowattmeter method for the measurement of 3phase power, the current coils of the two wattmeters are connected in any two lines and the potential coil of each joined to the third line as shown in Fig.(a). We now prove from first principles that the algebraic sum of the readings of the two wattmeters gives the total power drawn by the 3phase load (Y or Δ, balanced or unbalanced). Fig. (a) shows the Yconnected load.
figure (a) Note. With single phase loads, a wattmeter will deflect backward only if it is improperly connected. This is also true when the threewattmeter method is used to measure power in a 3phase circuit. However, in twowattmeter method, the reverse deflection may be due to improper connections of the wattmeter or low power factor of the load. As we shall see in Art. , if the pf. of 3phase balanced load is less than 0.5, then one of the wattmeters (wattmeter connections being correct) will read downscale. In order to obtain upsacle reading, reverse either potential or current coil of this wattmeter. The reading obtained after reversal of coil connection should be taken as negative. This explains the significance of the term "algebraic sum of wattmeter readings" in the twowattmeter method.  
 
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