138 



ALTERNATING CURRENTS 



was not very effective, as it is merely capable of resisting a change in 

 the total flux i.e. fluctuations in the demagnetizing effect of the 

 armature current but offers no resistance to the periodic distortion 

 of the field, or the swaying of the flux inside the pole-piece 

 (corresponding to fluctuations in the cross-magnetizing effect of the 

 armature current). 



A more effective form of damper is that illustrated in Fig. 102. 

 It consists of a regular grid of copper embedded in the pole-piece, the 

 outer bars forming a closed band around the pole-piece. 



Leblanc's damper (" amortisseur ") is shown in Fig. 103. A series 

 of thick rods of copper is embedded in each pole-shoe, and these rods 



COPPER. RINGS 



COPPER RODS, 



LAMINATED FIELD POLE 



FIG. 103. Leblanc's " Amortisseur.' 



are connected at each end by heavy rings of copper. The system 

 practically forms a squirrel-cage rotor winding like that used in 

 induction motors, and any oscillations of the flux relatively to the 

 field-poles are damped so powerfully that the arrangement is capable 

 of preventing hunting even in the most troublesome cases. 



75. Starting of Synchronous Motor 



In starting a single-phase synchronous motor, it is necessary to 

 run the machine up to the speed of synchronism i.e. up to the 

 speed at which the frequency of the motor e.m.f. equals that of the 

 supply p.d. and to adjust the phase of the motor e.m.f. so that it is 

 in opposition to the p.d. The excitation is conveniently adjusted to 

 such a value as to make the e.m.f. equal to the p.d. At the instant 

 of closing the armature switch, no current will in that case pass 

 through the armature. If the supply of power to the auxiliary 

 motor used in starting up the synchronous motor be now cut off, 

 the motor will begin to undergo retardation, and receive power 

 as already explained ( 69). The load may now be put 011, and 

 the excitation adjusted to correspond to minimum armature current 

 i.e. to maximum power factor. 



