6-B] 



MEASUREMENT OF POWER. 



223 



also be required, each current coil carrying the delta current and 

 each potential coil being subjected to the delta (or line) voltage. 



3. This method of measuring the separate phase loads is 

 simple in principle and is commonly used on a 2-phase circuit 

 (6), but it is not capable of general application inasmuch as 

 phase loads are not always separable. On a 3-phase circuit 

 in testing, for example, a 3-phase induction motor it may be 

 impossible to measure delta current or star voltage, so that some 

 method not requiring either of these measurements becomes 

 necessary; furthermore, the method is open to objection on 

 account of the number of measurements required, unless the 

 assumption is made that all phases are alike, so that measure- 

 ments are necessary on one phase only. 



4. Polyphase Power Factor. A polyphase system is a com- 

 bination of single-phase elements. If E, I and W are, respectively, 

 the voltage, current and power for any separate element, the 

 power factor for that element is W-^r-EI, by definition. When 

 the separate elements or phases of a polyphase system have the 

 same power factor, this is the power factor for the whole system. 



5. When, however, the separate elements have different power 

 factors, there is no one power factor that has a definite value or 

 physical significance for the whole system. It is convenient, 

 however, to obtain a kind of average power factor for the system, 

 the value of which will depend upon the method used in its 

 determination.* An average power factor may be satisfactorily 

 determined when the separate phases are nearly alike, but has 

 little meaning when they are widely different. 



6. Two-phase Load. Two-phase power is usually measured 

 by two wattmeters, one on each phase, as just described. 



7. When the phases are independent, as in a, Fig. i, Exp. 

 6-A, the measurements differ in no respect from measurements 

 made on single-phase circuits. 



*(Sa). See A. S. McAlliser, Alternating Current Motors, p. 12; A. 

 Burt, Three-phase Power Factor, A. I. E. E., p. 613, Vol. XXVII., 1908. 



