CHAPTER IV 

 DIRECT-CURRENT MACHINERY 



88. The Direct-current Dynamo. A direct-current dynamo 

 consists of an electric circuit, connected to a commutator and 

 tapped by brushes, revolving in a magnetic field which is produced 

 by stationary electric circuits. 



Such a machine is illustrated in Fig. 109 and comprises the fol- 

 lowing parts: 



1. Yoke ^ 



2. Pole pieces V Magnetic circuit 



3. Armature core J 



4. Armature winding ) ~> , . . . . 



> Revolving electric circuit. 



5. Commutator ) 



6. Brushes and brush holders. Collecting apparatus. 



7. Field winding. Stationary exciting circuit. 



89. Yoke. The yoke serves mechanically as the frame of the 

 machine and magnetically to carry the flux from pole to pole; it 

 is usually made of cast iron but in machines where great weight 

 is objectionable it is sometimes made of cast steel which has greater 

 strength and permeability. 



90. Pole Pieces. The pole pieces or pole cores are usually 

 made of cast steel or sheet steel and are bolted to the yoke. For 

 small machines the yoke and poles are sometimes cast in one piece. 

 All solid poles must have laminated pole faces bolted to them in 

 order to reduce the eddy current loss due to local variations of the 

 magnetic density in the pole faces as the armature teeth move 

 across them. 



The pole cores carry the field windings of the machine. Solid 

 poles are made circular and so have the greatest section for a given 

 perimeter and require the smallest length of field copper. Lam- 

 inated poles must, however, be made rectangular. 



The section of the pole face is made much greater than that of 

 the pole core in order to reduce the flux density in the air gap. 



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