SYNCHRONOUS MACHINERY 285 



but have different wave shapes, then, since, due to the presence 

 of the higher harmonics, the voltages are not equal at every in- 

 stant, wattless cross currents will flow to correct these inequalities 

 in voltage. These currents will usually be very small since the 

 voltages producing them are small and they are of high frequency 

 and thus the path through the two machines offers a high im- 

 pedance to them. The impedance is, however, only the true 

 impedance and not the synchronous impedance. 



Thus three kinds of cross currents may exist in parallel opera- 

 tion of alternators, (1) wattless currents transferring magnetiza- 

 tion between the machines due to a difference of terminal voltage, 

 (2) currents transferring power between the machines due to 

 phase displacements between their voltages, and (3) higher fre- 

 quency wattless currents due to differences of wave form. 



174. Hunting. If two alternators are operating in parallel 

 and one drops behind the other in phase due to a sudden decrease 

 in the speed of its prime mover, the second machine supplies power 

 to pull it into synchronism again. The impulse received causes it 

 to swing past its mean position and it oscillates a few times before 

 falling into step. The period of the oscillation depends on the 

 weight of the rotating mass and on the strength of the magnetic 

 field, that is, on the pull between the field poles and the induced 

 armature poles. The greater the weight of the machine the lower 

 will be the frequency, and the stronger the field excitation the 

 greater will be the magnetic pull and the higher will be the fre- 

 quency. 



If the action producing the speed pulsation is repeated periodi- 

 cally and coincides with the natural period of the machine the 

 oscillation instead of dying out will increase in amplitude until it 

 is limited by the friction losses produced or until the machines fall 

 out of step. When the oscillations tend to become cumulative 

 the machines are said to be hunting. 



Hunting may occur in a similar way in the case of a synchronous 

 motor supplied by an alternator. If the load on the motor sud- 

 denly increases it falls back in phase to receive the extra power 

 required and oscillates about its final phase position before running 

 again in synchronism. This oscillation may become continuous as 

 in the case of alternators. 



Hunting is reduced by putting short-circuited grids in the pole 

 faces or between the poles or in some cases by placing a complete 



