CHARACTERISTIC CURVES. 409 



only when certain of the mechanical conditions of driving are 

 constant. Thus we might have a characteristic relation between 

 the current output and electromotive force of a generator driven 

 by an invariable torque, or of a generator driven at invariable 

 speed. The latter condition nearly always occurs in practice ; 

 that is to say, electric generators are usually driven at an approx- 

 imately constant speed. Therefore, the only important type of gen- 

 erator characteristic is that which corresponds to constant speed. 



On the other hand, the relation between the speed of a motor 

 and the torque developed by it is definite only when certain of 

 the electrical conditions of driving are constant. Thus we may 

 have a characteristic relation between the speed and torque of a 

 motor which is supplied either with an invariable current, or with 

 current from constant voltage mains. The latter condition, or an 

 approximation to it, usually occurs in practice, and, therefore, the 

 most important type of motor characteristic is that which corre- 

 sponds to constant impressed electromotive force. 



The speed-torque characteristic of a motor may be determined 

 experimentally as follows : Provision is made for supplying the 

 motor current from constant voltage mains. The motor is loaded 

 by means of a Prony brake. The load is varied through a wide 

 range by adjusting the brake, and corresponding values of speed 

 and torque are observed. The observed speeds are then plotted 

 as ordinates, and the corresponding values of the observed torques 

 plotted as abscissas. 



An interesting characteristic curve of a motor is the torque- 

 current curve at stand-still. This curve may be determined ex- 

 perimentally by fixing a lever to the motor pulley and observing, 

 by means of a spring dynamometer, the pull at the end of this 

 lever for various values of armature current. The product of 

 this observed pull into the length of the lever arm gives the value 

 of the torque and these values are to be plotted as ordinates and 

 the corresponding currents as abscissas. This stationary torque 

 exceeds the running torque, for a given value of field excitation 

 and given value of the armature current, by the amount of torque 



