230 ELECTRICAL MACHINERY 



Reasons for a Stationary Armature. There are several 

 reasons why the field of an alternator generally rotates 

 instead of the armature. The armature winding ordinarily 

 generates quite a high voltage, perhaps 2300 volts or even 

 11,000 volts. Such voltages require that the armature 

 winding be very carefully insulated and it is much easier 

 to insulate a stationary winding than it is to insulate one 

 designed to run at a high speed. In the latter case the 

 winding has to be designed to stand properly the mechanical 

 stresses due to centrifugal forces and, as a result, the insula- 

 tion is likely to be inferior. 



As the armature of an alternator is not equipped with 

 a commutator, it is not at all necessary to have the armature 

 rotate and so they are nearly always stationary. 



Special Field Structure for Turbo-alternators. The steam 

 turbine is used very often as the driver for alternators and 

 a turbine must necessarily run at high speed to use the steam 

 economically. Now the field of an alternator can be designed 

 to stand the mechanical stresses due to this high rotative 

 speed much better than the armature can. The field 

 structure may be made of blocks of machine steel, while 

 an armature must be made of laminated iron; also the field 

 winding requires but very little insulation, while the wind- 

 ing of an ordinary armature demands insulation perhaps 

 one-eighth of an inch or more in thickness. This amount 

 of insulation makes the armature mechanically weak and 

 unable to stand high speeds of rotation. 



Typical Forms of Alternators. Fig. 137 shows a small 

 alternator having a rotating armature, with slip rings, 

 etc., and Fig. 6 shows a revolving-field alternator which 

 generates 2300 volts and is for use in a large a-c. power 

 plant, The field structure is very .strongly made and the 

 coils are strap wound as described on page 72 and shown 

 in Fig. 37. The multi-polar machine illustrated in Fig. 6 

 was designed to be run at a low speed by a reciprocating 

 engine. Alternators designed for turbine drive are much 



