70 



ALTERNATING CURRENTS 



the plane of coil F as there is no restraining torque. Hence, at 

 unity power-factor, the entire moving element takes such a 

 position that the coil M is in the plane of coil F. 



If the power-factor of the load is zero, the current and the 

 voltage differ in phase by 90. Hence the current in coil M and 

 the flux due to coil F have a time-phase difference of 90, and 

 coil M exerts no turning moment. However, the current in coil 

 M' is now in time-phase with the flux due to coil F, and therefore 

 coil M' will move into the plane of coil F. The moving system will 

 then have a position of 90 from its position at unity power- 

 factor. That is, when the current changes its time-phase by 90, 

 the moving element of the indicator changes its space-position 

 by 90. The direction in which the element turns depends on 



Load 



FIG. 67. Three-phase power-factor indicator. 



whether the current lags or leads the voltage. For intermediate 

 power-factors, it can be shown that the angle of the moving sys- 

 tem corresponds to the circuit power-factor angle. If the scale 

 is calibrated in degrees, the pointer can be made to indicate the 

 power-factor angle of the circuit. To make the indicator read 

 power-factor, it is necessary merely to make the scale divisions 

 proportional to the cosine of the power-factor angle. In prac- 

 tice the current is led into the moving system through strips of 

 annealed silver foil which exert no appreciable control on the 

 moving system. 



As it is impossible to obtain either a pure resistance or a pure 

 inductance, the currents in coils M and M ' will not differ ex- 

 actly by 90 in time-phase. It can be shown that if the space 

 angle between coils M and M' be made equal to the angle 



