PARTS OF A DYNAMO-ELECTRIC MACHINE 29 



the magnetic field. Fig. 5 shows a continuous current 

 machine with a stationary field and Fig. 6 shows an alter- 

 nator with a rotating field. 



When the field frame has only two pole pieces (there 

 must always be at least two), it is called a bipolar machine; 

 practically all small motors and generators are of this 

 form. Machines of 5 h.p. capacity and less are generally 



FIG. 5. General View of a c-c. Machine, Showing Stationary Field 

 and Revolving Armature. 



bipolar; special high speed machines (driven by steam 

 turbines) have been built bipolar in very large sizes. 

 For generators or motors of ordinary speeds, above 10 

 h.p. in capacity, the multipolar form of field frame is gen- 

 erally used. 



Number of Poles. The number of poles depends upon 

 the capacity of the machine and the speed at which it is 

 designed to operate. Of course the number of poles is 



