224 POLYPHASE CURRENTS. [Exp. 



8. On a 3-wire, 2-phase circuit, as in b, Fig. i, Exp. 6-A, the 

 same method may also be used, the two wattmeter current coils 

 being located in the two " outer " conductors, A and B, respectively. 

 With the wattmeters thus located, the sum of their two readings 

 will give the true power (23) for any load whatsoever, even 

 when part of the load is between A and B. (These connections 

 are seen in Fig. I, in which X and Y are the outer conductors 

 and Z is the common conductor or return.) 



9. When the load in a 3-wire 2-phase system is balanced and 

 there is no load between the two outer conductors A and B, one 

 wattmeter may be conveniently used by connecting the current 

 coil in the common conductor; one end of the potential coil is 

 connected to the common conductor and the other end connected 

 first to one and then to the other outer conductor. A reading* 

 is taken in each position and the algebraicf sum gives the total 

 power. (The connections are seen in Fig. 7, in which Z is the 

 common conductor.) A 3-wire 2-phase circuit is likely not to 

 be balanced ( 14, Exp. 6-A) and the method should be used 

 with caution. 



10. On a 4-wire, quarter-phase, 2-phase system, as in c and 

 d, Fig. i, Exp. 6-A, two wattmeters, one on each phase, will give 

 the correct power only when the load is balanced. The method 

 may be used for testing a single machine, but not for measuring 

 the power of a circuit when the character of its load is unknown. 



* (Qa). For a balanced load, power can be determined from a single 

 reading of the wattmeter by connecting the current coil in the common 

 conductor and connecting the potential circuit from the common conductor 

 to the middle point of two approximately equal non-inductive resistances, 

 Ri R a , connected across the two outer conductors as in Fig. 5. A single 

 reading of the wattmeter gives one half the total power, if the wattmeter 

 is calibrated as a single-phase instrument with R l and R 3 connected in 

 parallel with each other and in series with the potential circuit (36a). 

 See also 333. 



t (9b). For low power factors, when 6 exceeds 45, the reading of the 

 wattmeter in one position is negative. The similar case for a 3-phase 

 circuit is fully discussed later. 



