312 ELECTRICAL MACHINERY 



the period of acceleration the field of the synchronous motor 

 is left without excitation; after synchronous speed has 

 been reached it is gradually excited and, if the armature 

 current decreases, the excitation is increased until the normal 

 value is obtained. It sometimes happens that the armature 

 pulls into synchronism with improper polarity, in which 

 case the armature current mil increase as the field current 

 is increased. Some method must then be used to pull the 

 armature " into step " (correct polarity), such as re- 

 starting. 



High Starting Current and Low Torque. The current 

 taken from the line for starting a synchronous motor in 

 this manner is generally two or three tunes the full-load 

 current, but in spite of this large current the starting torque 

 is not high unless the machine has been properly designed. 

 It was said that this method of starting depended upon the 

 reaction of the eddy currents in the pole faces, but with a 

 laminated pole we know these currents cannot be high 

 because of the subdivision of the path for the eddy 

 currents. To give a fair starting torque in this case 

 it is necessary to put in the pole faces damping grids or 

 amortisseurs. 



Damping Grids in Pole Faces. These consist of heavy 

 bars of copper imbedded in slots in the pole face (the 

 direction of the slots is parallel to the armature shaft) and 

 short circuited at their ends by a copper band surrounding 

 the pole. In fact, these grids, cross-bars and band, are 

 sometimes made in one piece, a copper casting. These 

 grids form low-resistance paths for the eddy currents and 

 so help to produce a good starting torque. They also 

 tend to damp out oscillations of the armature (called 

 " hunting ")* and from this action they derive their name. 

 In Fig. 201 are shown some poles of a synchronous motor 

 on which the damping grids may be seen. 



* See page 327 for explanation of this term. 



