304 



THE POLYPHASE CIRCUIT. 



The diagrams sufficiently indicate the connections to 

 be made. It is to be noted that in the case of the star 

 connection a single ammeter measures both the current in 

 the line and in the branch circuit. With the mesh con- 

 nection a single voltmeter measures both line and branch 

 voltage. 



For each of the connections indicated in the diagram, 

 take readings of the line current and voltage, and the 

 current and voltage of one of the branch circuits. This 

 should, if possible, be repeated for two or three values of 

 the resistances. In each case the values of all these 

 resistances should be equal. Enter the results in the 

 form shown below, and note that the two values of the 

 power entered in the last two columns should be the same. 



COMPARISON OF POWER IN MAIN AND BRANCH CIRCUITS. 



Power in Inductive Circuits. If the load is partly inductive, 

 the current and voltage will not be in phase with each 

 other. In this case the power in each branch circuit will 

 be c e cos tf> instead of ce watts. If the angle of lag in all 

 the branch circuits is the same ( = <f>) the expression for 

 the power in the line becomes 



power = 3 c e cos <f> or ^3 C E cos tf>, 



which is the general expression for the power transmitted 

 by a three phase line with equal loading on all phases. 



In general, the measurement of the power of a 3-phase 

 system necessitates the employment of a wattmeter when 

 the load is partly inductive, as the methods in which only 

 ammeters and voltmeters are employed become very 

 complicated. 



