384 ELEMENTS OF ELECTRICAL ENGINEERING. 



A motor may be supplied with constant current in which case 

 the speed and terminal voltage of the motor vary in a definite 

 way with the motor load, and the characteristic curve may in this 

 case show : (i) The relation between speed and torque : (2) The 

 relation between terminal voltage and speed ; (3) The relation 

 between terminal voltage and torque. Constant current motor 

 driving, however, seldom or never occurs in practice. 



25. The magnetization curve of a dynamo. A clear under- 

 standing of the characteristic curves of a dynamo when used as 

 a generator or as a motor depends upon precise knowledge of the 

 relation between the field current and the flux 4> which enters the 

 armature from a north pole of the field ; or, since the electro- 

 motive force, QZ'n y induced in the armature when the armature 

 is driven at a prescribed constant speed, is proportional to the 

 flux <!>, it is sufficient to know the relation between the induced 

 electromotive QZ'n and the field current. The curve which 

 shows this relation is called the magnetization curve of the 

 dynamo. 



The magnetization curve of a dynamo may be determined by 

 driving the machine at the prescribed speed, and observing the 

 electromotive force between its brushes * for different observed 

 values of current sent through its field windings from some out- 

 side source. Figs. 18 and 19 are typical magnetization curves 

 of a dynamo which is rated as a 3O-kilowatt generator; Fig. 18 

 shows the magnetization curve of the machine when its field is 

 wound with many turns of fine wire (shunt dynamo), and Fig. 1 9 

 shows the magnetization curve of the same machine when its field 

 is wound with few turns of coarse wire (series dynamo). 



Dependence of magnetization curve on speed. When the mag- 

 netization curve of a dynamo has been determined for a given 

 speed ;z, the curve for any other speed n f may be found by multi- 



* When the current in the armature is very small, the electromotive force between 

 the brushes of a dynamo is very nearly equal to the total electromotive force $Z'n 

 induced in the armature. 



