222 POLYPHASE CURRENTS. [!-XP. 



EXPERIMENT 6-B. Measurement of Power and Power Fac- 

 tor in Polyphase Circuits. 



PART I. GENERAL DISCUSSION. 



I. Preliminary. For measuring power in any 3-wire sys- 

 tem, the best method is the two-wattmeter method 23 ; for the 

 particular case of a balanced 3-phase load, some one-wattmeter 

 method, 32-9, may be used. 



For measuring power in systems with more than three wires, 

 the n i wattmeter method of 16 is correct for all cases; for 

 the particular case of a balanced 2-phase load, on a 4-wire sys- 

 tem, the method of 10, employing two wattmeters, may be used. 



An unknown load should not be assumed to be balanced. 



It will be understood that, in cases where several wattmeters 

 are described as being required, a single instrument may be 

 used and shifted by suitable switches from circuit to circuit, 

 readings being taken successively in the different positions. 



2. Separate Phase Loads. In any single-phase system 

 power is measured by means of a wattmeter, the current coil 

 being connected in series and the potential coil in parallel with 

 the circuit, as discussed in Appendix III., Exp. 5~A. An exten- 

 sion of this method can be applied to a polyphase system, if the 

 phases are separately accessible so that the load of each phase 

 can be separately measured. A wattmeter is then used for racli 

 phase load, with current coil in series and potential coil in parallel 

 with the particular load being measured, the total power being 

 the arithmetical sum of the several wattmeter readings. 



For example, to measure the power in three star-connected re- 

 sistances on a 3-phase circuit by this method, three watlnuurs 

 would be required, each current coil carrying the star (or line) 

 current and each potential coil being subjected to the star voltage. 

 With three resistances delta connected, three wattmeters would 



