DAMPING COILS 



137 



the oscillations. The time during which a series of reinforcing 

 disturbances is received may then become so considerable, and the 

 amplitude of oscillation of 8 may increase to such an extent, as to 

 ultimately carry it beyond the top of the power curve (Fig. 99), 

 into the region of instability, when the motor will " topple over," 

 dropping out of synchronism. 



One of the most troublesome consequences of the pendular motion 

 we have just considered, which is variously known as " hunting," 

 " pumping," " surging," or " phase-swinging," are the comparatively 

 large fluctuations in the generator p.d. caused by the current 

 fluctuations. If the generator supplying the motor is also used for 

 feeding incandescent lamps, the hunting of the motor will cause the 

 brightness of the lamps to vary periodically, rendering the lighting 

 extremely unsatisfactory. 



74. Prevention of Hunting 



One method of reducing the amplitude of the oscillations is 

 immediately suggested by an inspection of the power curves of Fig. 

 99. From these power curves it is evident that a given change 

 of torque will be obtained with a smaller change of when the field 

 is strong than when it is weak. Thus the use of strong fields is 

 favourable in checking hunting. 



A highly effective and largely used method of preventing hunting 



FIELD 

 POLES 



COPPER 

 / DAMPER.S 



FIG. 102. Damping Grids fitted to Alternator Poles. 



and its troublesome accompaniments is that in which special devices, 

 known as " damping coils " or " dampers," are employed to dissipate 

 the energy of the oscillations. 



One of the earliest forms of damper consisted of a solid band of 

 copper closely surrounding the top of the pole-piece. This device 



