APPENDIX B. 



CHARACTERISTIC CURVES. 



24. Definition of the characteristic curve of a generator and of 

 a motor. (a) Of a generator. When a dynamo is used as a 

 generator it is usually driven at a definite constant speed, and the 

 current output of the machine may be large or small, according 

 to the resistance of the receiving circuit. When the current out- 

 put of a generator varies, the electromotive force between its ter- 

 minals generally changes, and the curve showing the relation 

 between terminal voltage of the generator as ordinates and cur- 

 rent output as abscissas, is called its (external) characteristic curve. 

 Sometimes it is desirable to plot the curve showing the relation 

 between current output of generator as ordinates and resistance 

 of receiving circuit as abscissas. This curve is sometimes called 

 the ampere-ohm characteristic of the generator to distinguish it 

 from the volt-ampere characteristic above described. 



(b) Of a motor. When a dynamo is used as a motor it is 

 usually supplied with current from constant voltage mains, and 

 the load on the motor may be large or small according to the 

 amount of power required to drive the machinery to which the 

 motor is connected. When the load on the motor varies, its 

 speed, its current intake, and the torque which it develops, all 

 change in a definite way, and a curve showing the relation 

 between any two of these elements may be called a characteristic 

 curve of the motor. Thus the characteristic curve of a motor 

 may show (i) the relation between speed as ordinates and torque 

 as abscissas, (2) the relation between speed as ordinates and cur- 

 rent intake as abscissas, (3) the relation between torque (or, in 

 case of a street car motor, tractive effort) as ordinates and current 

 intake as abscissas. 



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