DESIGN OF ALTERNATORS 239 



Referring again to the type of generator to operate at moderate 

 speeds, it is not necessary for the field to rotate, and small units, 

 especially when the voltage is low, may be built generally on the 

 same lines as D.C. dynamos, i.e., with rotating armatures and 

 an external crown of poles. In this case the commutator is re- 

 placed by two or more slip rings connected to the proper points 

 on the armature winding. For a three-phase generator three 

 slip rings are required, and since two rings only are necessary if 

 the field rotates, the design with stationary armature is the more 

 common. It should also be observed that the insulation of the 

 slip rings for the comparatively low voltage of the exciting circuit 

 offers no difficulty, whereas the insulation of the alternating- 

 current circuit may have to withstand high pressures. 



The field magnet windings of salient pole alternators are gen- 

 erally similar to those of D.C. dynamos, that is to say, all the 

 poles are provided with exciting coils. Machines have been 

 built in the past with windings on alternate poles only; with a 

 single exciting coil (as in the "MORDEY" flat-coil alternator); 

 and, again, without any windings on the rotating part. The 

 latter type is known as the inductor alternator, and the field 

 winding is then put on the stationary armature core, thus dis- 

 pensing with slip rings for the collection of either alternating or 

 continuous current. Iron projections on the rotating part so 

 modify the reluctance of the magnetic circuit through the arma- 

 ture coils that alternating e.m.fs. are generated therein; but these 

 machines, together with those having single exciting coils, have 

 the disadvantage that the magnetic leakage is very great and 

 the design therefore uneconomical. 



67. Number of Phases. Whether a machine is to supply 

 single-phase, two-phase, or three-phase, currents does not ap- 

 preciably affect the design. The calculations on a machine for 

 a large number of phases are not more difficult than when the 

 number of phases is small. The theory of the single-phase gen- 

 erator is, in fact, somewhat less simple than that of the polyphase 

 machine. In the succeeding articles it is the three-phase gen- 

 erator that we shall mainly have in mind, because it is the most 

 commonly met with, but the points of difference in the electrical 

 design of single-phase and polyphase generators will be pointed 

 out as they arise. 



Whatever may be the type of machine, or number of poles, 

 we may consider the armature conductors to be cut by the mag- 



