482 



FARM MOTORS 



lines of force to induce a small current into the circuit 

 around the field coils. This current more highly excites 

 the field magnets until the dynamo soon picks up or 

 establishes its rated E.M.F. 



645. Shunt dynamo. In the so-called shunt-wound 

 dynamo a small portion of the current is led off from 

 the brushes bb (Fig. 363), and through a great many 

 turns of very fine Avire which encircle the core of the 

 magnet. In such a dynamo, as the load increases the 

 E.M.F. slightly decreases, and as the load decreases the 



FIG. 363 SHUNT-WOUND DYNAMO FIG. 364 SERIES-WOUND DYNAMO 



E.M.F. increases. Hence, if the current fluctuations are 

 great and quite frequent it would keep an attendant oc- 

 cupied to keep the field resistance regulated for the load. 

 (See Fig. 368.) 



646. Series-wound dynamo. In the so-called series- 

 wound machines the whole of the current is carried through 

 a few turns of very coarse wire which encircles the field mag- 

 nets (Fig. 364). Since every change of current alters the 

 field magnetizing current, consequently in the current in- 

 duced in the armature the E.M.F. at the brushes will vary 

 with every change of resistance in the external circuit. 



647. Compound-wound dynamo. In the compound- 



