542 



ELECTRICAL MEASUREMENTS 



as shown is perpendicular to the plane of the paper. To increase 

 the turning moment acting on the movable system and thus 

 reduce the effect of the brush friction, both the field and movable 

 coils are wound on laminated iron cores. The similarity of the 

 arrangement to the Tuma phase meter is at once apparent. 



The index tries to point to the angle of phase difference between 

 the machines, and its rate of movement is dependent on the 

 difference of the machine speeds. It will move forward or back- 

 ward or come to rest depending on whether the incoming machine 

 is running faster, slower or at the same speed as the other machine. 

 The pointer may come to rest in any position on the dial. This 

 merely means that both machines are running at the same speed, 

 though not necessarily in phase. The speed of the incoming 

 machine must be altered very gradually and the main switch 

 closed as the pointer slowly drifts across the index mark. 



FIG. 326. Weston synchroscope. 



Weston Synchroscope. This instrument is an electrodyna- 

 mometer with a spring control. The fixed coils are connected 

 across the station bus-bars, in series with a suitable resistor, while 

 the movable coil, in series with a condenser, is placed across the 

 incoming machine. 



If the machines are in synchronism or 180 out of synchronism, 

 the currents in the two coils will be in quadrature and the index 



