PHASE METERS 



531 



The component / sin 6 is sometimes called the idle or wattless 

 current. It will be seen that operating a circuit at unity power 

 factor is equivalent to so operating it that the idle current is 

 zero. 



A two-circuit electrodynamometer, with the movable circuit 

 placed across the line and the fixed coils traversed by the line 

 current, will give a deflection 



D = KI F I M cos 0, 



where 6 is the phase difference of Ip and IM- If the circuit of 

 the movable coil is non-inductive the instrument is an ordinary 

 wattmeter, but if this circuit could be made perfectly reactive 

 the phase of IM would be shifted 90 with respect to the line 

 voltage and 



D = KI F I M cos (90 - 6) = KI F I M sin 6. 



At a constant voltage the deflection would be proportional to the 

 idle current, 7 sin 0, or at any voltage, to the idle volt-amperes. 



ab 



C' 



66 



FIG. 315. Connections for measuring idle volt-amperes in balanced three- 

 phase circuit. 



is 



On account of the energy dissipated in the reactor, it 

 impossible to shift the phase of the potential-coil current by 

 exactly 90, and the phase shift will depend on the frequency. 



Reference to the theory of the induction wattmeter will show 

 that this instrument would be converted into an idle current 

 meter if its potential circuit were made perfectly non-inductive. 



The idle volt-amperes in a balanced three-phase circuit may 

 be measured by the use of wattmeters if the coils be connected 

 in circuit as shown in Fig. 315. 



