32 



ELECTRICAL MACHINERY 



Material for Yoke. The material of which the field 

 frame is made depends upon whether it is to be of the 

 revolving or stationary type. When the field frame is 

 stationary, cast steel is used at present; in the earlier 

 types of machine, cast iron was used in building the field 

 frame. Cast steel has much higher permeability than 

 cast iron and so a higher flux density may be used with 

 steel than with iron; because of this fact the field frame 

 of steel, to carry a certain magnetic flux, is much lighter 

 than would be the case if iron were used. 



Material for Poles, The poles themselves are not cast 

 as a part of the yoke except in the case of small, cheap 



machines. In discussing the de- 

 sign of field coils we shall show 

 that it is advantageous to keep 

 the cross-section of the pole 

 as small as possible, and this 

 makes necessary the use of 

 laminated iron, which has higher 

 permeability than any other 

 kind. Most field poles are, 

 therefore, built up of lamina- 

 tions, each about one-sixteenth 

 of an inch thick, and are 

 bolted to the yoke. Fig. 9 

 shows this construction. 



Field Frame for Alternators. An alternating current 

 generator generally has a revolving field; the field frame 

 of such a machine designed for reciprocating-engine drive 

 is made of a cast steel yoke and laminated steel poles, 

 the same as a continuous current machine. The field frame 

 for a turbo-alternator, however, is of special construction, 

 being generally made from machine steel and having no 

 projecting pole pieces. Because of the high speeds at which 

 the revolving fields run, enormous mechanical strains occur 

 and the field must be designed with this idea in mind. 



FIG. 9. A Laminated Field 

 Pole. 



