318 ALTERNATING CURKKXTS 



normal value of the line or armature current /i for the given 

 input PI. The power represented by curve AB is obviously 



Pi = V3F/! 



for a three-phase motor having a line voltage V. 



CD, Fig. 293, is a V-curve taken for a value of powerP 2 , which 

 is obviously greater than Pi. EF is a third curve taken for a 

 still greater value . of power, P 3 . A curve drawn through the 

 lowest points of the V-curves is a unity power-factor curve. 

 Curves XX and XY, drawn through the V-curves at the proper 

 points are 0.8 power-factor curves, XX being for lagging current 

 and XY for leading current. Curves for other power-factors 

 may also be found in a similar manner. These curves are called 

 compounding curves. 



It should be noted that the normal field current varies with 

 the value of power input to the motor. 



129. Amortisseur or Damper Windings. Figure 295 shows 

 the rotating field structure of a synchronous motor, around which 

 a squirrel-cage winding is built. The conductors of the squirrel 

 cage are embedded in the pole faces of the rotor. This winding 

 serves two purposes. 



It assists the motor in starting and it damps out any tendency 

 of the rotor to oscillate or "hunt." Such windings are called 

 amortisseur or damper windings or simply dampers. If the motor 

 is connected to a system which receives its power from a recipro- 

 cating engine unit, there may be pulsations in the supply fre- 

 quency caused by the variable driving torque of the engine. The 

 synchronous motor is very sensitive to phase changes, as has 

 already been shown, and small changes in the phase of the supply 

 voltage may produce considerable changes in the energy current 

 which the motor takes from the line. This produces pulsations 

 in the motor torque. If these pulsations have a frequency nearly 

 equal to the natural frequency of oscillation of the rotor, they 

 may cause it to oscillate periodically about its normal position. 

 That is, the rotor alternately accelerates and retards, although 

 the average speed does not change. This is called hunting (see 

 Par. 75, page 171). These oscillations may become so great as 

 to cause the motor to fall out of synchronism. 



Hunting may also be caused by system disturbances, such as 



