ALTERNATOR RECi: I..\r 1< >\ A\D Ol'KHATION 171 



ment shows very accurately the position of synchronism. The 

 synchroscope is connected across but one phase. It is possible 

 that one phase of each machine may be in synchronism, but the 

 other two out of phase due to wrong phase rotation. The cor- 

 rect phase rotation must be determined by lamps or by other 

 means before depending entirely upon the synchroscope. Syn- 

 chronizing lamps are often used in conjunction with a synchro- 

 scope so that the operator has a check on the instrument. 



75. Hunting of Alternators. The driving torque of a recipro- 

 cating engine, or of a gas engine, is not uniform during a revolution 

 of the fly-wheel, but varie's from zero at the dead centers to a 

 maximum at some intermediate position. Even with a heavy 

 fly-wheel, this variation of torque may impart impulses to the 

 induced emf., causing it to be ahead of its proper position at some 

 instants and behind it at other instants. This causes heavy syn- 

 ch roni /ing currents to flow between machines in parallel and 

 often causes their rotating members to "oscillate" as they are 

 rot at ing. The angular effect of the crank position can be appre- 

 ciated when it is realized that in a 60-pole alternator a dis- 

 placement of one mechanical or space-degree in the rotating 

 member makes a difference of 30 electrical degrees in the phase 

 angle of the electromotive force. The above impulses are often 

 communicated to the system, causing synchronous motors and 

 converters to oscillate. These oscillations are called " hunting. 1 ' 

 Hunting may become serious if the engine governors have a 

 natural frequency of oscillation nearly the same as that of the 

 machine rotors. The oscillations may (hen become cumulative 

 and may even cause the machines to go out of synchronism. 



Remedies for hunting are to use heavy fly-wheels, to put dash- 

 pots on the engine governors, and t<> use amort isseur or squirrel- 

 cage windings around the field, such as is shown in Fig. 29&, 

 page .819. Where several engine-driven units are used, they are 

 often paralleled when their cranks occupy different annular 

 positions. This minimi/es the effect of the engine impub 



vtem, although their effect is increased in the local inter- 

 ehange riim-nts between generators. 



