lo-A] SYNCHRONOUS MOTOR. 309 



10. Damping. Hunting may be effectively reduced by damp- 

 ing, commonly obtained by heavy copper grids imbedded in or 

 surrounding the pole faces of the motor or by a complete squirrel 

 cage structure. Any oscillations of the armature and armature 

 flux induce currents in these copper structures and these cur- 

 rents, on account of RI 2 losses, tend to check or damp the oscil- 

 lations. 



ii. Starting a Poy phase Motor with Alternating Current. 

 If a polyphase synchronous motor with its field unexcited is con- 

 nected to the line, the polyphase currents in the armature will 

 set up a revolving field which, on account of the losses in the 

 pole faces and damping coils, will bring the motor nearly to speed 

 as an induction motor. (That the starting of an induction motor 

 is produced by losses is shown in 50, Exp. 8-A. The damping 

 coils act as a short-circuited secondary and when they have the 

 complete squirrel cage structure they are particularly effective in 

 starting.) When the machine is nearly in synchronism as an 

 induction motor, it will usually come into complete synchronism 

 on account of the magnetic attraction between the separate poles 

 of the field and the magnetic poles of the armature which finally 

 stay together so that there is no slip. This attraction is due to 

 the tendency of the lines of force in the air gap to shorten. 



When the motor is in synchronism or nearly in synchronism, 

 the field may be excited and the motor will lock in step ; the arma- 

 ture current then decreases. 



The disadvantages of alternating current starting are: (i) the 

 large current drawn from the line; (2) the high induced voltage 

 in the field winding. The starting current can be cut down by 

 starting at a lower voltage than normal by means of auto-trans- 

 formers ( 12, Exp. 8-A). The danger of damage to insulation 

 on account of the induced voltage is reduced by separating the 

 field spools from each other on starting by a " break-up switch " ; 

 when synchronism is reached or nearly reached, the field is 

 excited by closing this switch. 



