AIR-GAP FLUX DISTRIBUTION 279 



position relatively to the curve of field m.m.f . ; and the resultant 

 m.m.f. over the armature surface can be obtained exactly as for 

 the direct-current machine (see Fig. 53, page 137). An approxi- 

 mate method of predetermining the displacement angle /5 for 

 any load and power factor will be explained in Art. 98. 



Armature M.M.F. Curve of Single-phase Alternator. When 

 single-phase currents are taken from an armature winding, the 

 m.m.f. due to this winding as a whole must necessarily be of zero 

 value at the instant of time when the current is changing from its 

 positive to its negative direction. This suggests that the mag- 

 netizing effect of the loaded armature will be pulsating; that is 

 to say, it cannot be of constant strength at any given point con- 

 sidered relatively to the poles, whatever may be the phase dis- 

 placement of the current relatively to the developed voltage. If 

 the change of current in any given conductor be considered over 

 a complete cycle, and if at the same time the position of this con- 

 ductor relatively to the poles be noted, it will be seen that, rela- 

 tively to the field magnet system, the armature windings produce 

 a pulsating field of double the normal frequency. The actual 

 flux component due to the armature currents will not, however, 

 pulsate to any appreciable extent, because the tendency to vary 

 in strength at comparatively high frequencies is checked by the 

 dampening effect of the field coils, even if the pole shoes and 

 poles are laminated. 



No modern single-phase alternator, unless of very small size, 

 should be built without amortisseur windings, or damping grids. 

 These consist of copper conductors in holes or slots, running 

 parallel to the shaft, in the faces of the field poles. They are 

 joined together at both ends by heavy copper connections, and 

 form a "squirrel cage 7 ' of short-circuited bars which damp out 

 the flux pulsations, and also prevent the sweeping back and 

 forth, or "swinging," of the armature flux due to "hunting" 

 when synchronous alternating-current machines are coupled in 

 parallel. 



Returning to the magnetizing effect of the single-phase arma- 

 ture, it is, therefore, the average or resultant armature m.m.f. 

 considered relatively to the poles with which we are mainly con- 

 cerned. The most satisfactory way of studying an effect of this 

 kind is to draw the actual m.m.f. curves at definite intervals of 

 time, and then average the values so obtained for different points 



