544 ELECTRICAL EQUIPMENT 



ically at the frequency for which they are designed, which makes 

 the use of the frequency indicators evident. 



(2) They are valuable when synchronizing machines, since 

 they can be connected on the incoming machine and indicate its 

 speed, showing whether it is too high or too low. However, 

 where a synchronism indicator is installed they are not required 

 for this purpose, since this instrument shows whether the speed of 

 the incoming machine is high or low. 



Synchronism Indicator. (1) The synchronism indicator affords 

 the quickest and safest means for paralleling machines, since it 

 shows when the machines are in step and in phase, indicating by 

 the position of the needle the difference in the phase relations 

 between the machines, and telling whether the incoming machine 

 is running too fast or too slow. It is superior to synchronizing 

 with lamps, because the latter give no indication of the relative 

 speed of the incoming machine. The lamps will indicate when the 

 machines are of the same frequency, but the phase relations can 

 be judged only by the brilliancy of the light. 



When synchronizing with lamps dark, the phase relation of the 

 machines will be shown by the brilliancy of the light to a point 

 where the machines are approximately 45 out of phase, below 

 which point there will not be sufficient voltage across the lamp to 

 make it glow. Again, in case there is an inopportune failure of 

 the lamp, the operator might be misled and throw the machines 

 together when out of phase with possible disastrous results. 



When synchronizing with lamps bright, it is difficult to deter- 

 mine, after watching the lamps for some time, at just what instant 

 they are burning at full brilliancy, and, therefore, at just what 

 instant the machines are in synchronism. 



Synchronizing on high-tension lines, while often desirable, 

 has been out of the question because of the excessive cost and 

 space required for installing the necessary potential transformers 

 for a secondary synchronism indicator. A glow synchronism 

 indicator is now available for this purpose on circuits of 13,200 

 volts and above. The new indicator depends for its operation 

 upon the principle of electrostatic discharge in a vacuum. 



The instrument case resembles the ordinary round pattern 

 switchboard instrument. Inside the case are receptacles for hold- 

 ing the special glowers which project through holes in the cover. 

 Connections from the line to the device are made through con- 



