THE POLYPHASE CIRCUIT.. 



There are several methods of connecting the wattmeter 

 'to the circuit, the most usual of which are now .to b.e 

 described. 



Wattmeter Measurements in Three-phase Circuit. Method I. One 

 obvious method of measuring the power given to or [taken 

 from a three-phase circuit is to connect the wattmeter in one 

 of the three-branch circuits, as indicated in Fig. 152. If the 



Ai A 



FIG. 152. CONNECTION OF WATTMETER IN BRANCH CIRCUIT. 



power given to the circuit is to be measured, the watt- 

 meter would be so connected as to indicate the output of 

 one phase of the generator. If the power measured is 

 that given to a motor, transformer, or other set of branch 

 circuits, the wattmeter would measure the power in 

 one branch circuit. As explained in the preceding section, 

 the total power of the system = 3 c e where c and e are 

 the current and voltage in one branch of the star or mesh. 

 Thus the wattmeter will read one-third of the total power, 

 and the reading must be multiplied by three, unless this 

 use of it has been already foreseen and allowed for in the 

 calibration of the instrument. 



It is frequently more convenient to insert the watt- 

 meter in the line circuit, as the windings of the generator 

 or motors may not be easy to disconnect. This can be 

 done by employing the next method of measurement. 



Method II. What would at first sight appear an 

 equally simple method is to connect the wattmeter so as 

 to read the product of current and voltage of the line, in 

 the manner shown in Fig. 155. In this case it must, 

 however, be remembered that the current C and voltage E 

 are not in phase with each other, even in a non-inductive 

 circuit. Thus in the line supplied by a star-connected 

 generator, the line current C is the same as the current c 



