POWER LOSSES IN GENERATORS AND MOTORS. 123 



the rheostats and controlling devices. The brushes are not sup- 

 ported in a rocker arm but in rigid holders attached to the inside 

 of the case, and when the brushes are badly worn and need 

 to be replaced, they may be reached through a hand hole which 

 is covered by a metal lid one end of which shows in Fig. 91. 



62. Characteristic curves of motors. The behavior of a motor 

 under given electrical conditions of driving is most clearly repre- 

 sented by a curve showing the relationship between any two of 

 the following variable quantities : speed, torque, and mechanical 

 power output. When the current is supplied from constant vol- 



1000 



800 



600 



4oo 



200 



"-^4 



\ 



Power output in % of full rated load 



75 



Fig. 92. 



1Z5 



tage mains, it is often convenient to show by a curve the relation- 

 ship between the current and any one of the three mechanical 

 factors : speed, torque, and power output. Such a curve is called, 

 in general, a characteristic curve of the motor. In speaking of a 

 characteristic curve it is customary to designate it in terms of the 

 quantities which are represented by its coordinates, always nam- 

 ing its ordinates first, and its abscissas second. For example, 

 a speed-torque characteristic would be a curve of which the 

 ordinates represent speeds, and the abscissas represent torques. 



(a) Shunt and compound motor characteristics. The curve 

 A, Fig. 92, is a speed-power characteristic of a simple shunt 



