112 ELEMENTS OF ELECTRICAL ENGINEERING. 



If E is the electromotive force between mains of a three-wire 

 three-phase system, and if / is the current in each main, then 

 the total power delivered by the mains is : 



P=\/~$EIcQsO (b) 



in which cos 6 is the power factor of each receiving circuit. 

 This form of the equation is confusing for the reason that the 

 voltage E, the current /, and the power factor do not all 

 refer to a given circuit. The form (a) of the equation is there- 

 fore to be preferred. 



58. Measurement of power in polyphase systems. When a 

 polyphase system is unbalanced, the power delivered by each 

 phase must be measured exactly as in the case of an ordinary 

 single-phase system (see Arts. 18, 19 and 20). When a poly- 

 phase system is balanced, the measured power delivered by one 

 phase may be multiplied by the number of phases to give the 

 total power, but exact similarity of the various receiving circuits 

 is never realized in practice and therefore a considerable error 

 may be made if the total power is assumed to be nP, where P 

 is the measured power delivered by one phase and n is the 

 number of phases. Therefore it is usually considered necessary 

 to measure the actual total power. 



Best method for measuring power approximately by means of a 

 single wattmeter. It is usually very inconvenient to disconnect 

 the current coil of a wattmeter and transfer it quickly from one 

 main to another, but it is always very easy to change the con- 

 nections of the voltage coil. The best method therefore for 

 using a single wattmeter for measuring the power delivered to 

 approximately balanced receiving circuits by a three-wire system 

 (three-phase) is to connect the current coil in a given main and 

 take the wattmeter readings with the voltage coil connected from 

 the given main first to one and then to the other of the two 

 other mains. The sum of these two wattmeter readings gives a 

 fairly accurate value for the total power delivered. 



