THE CONTINUOUS CURRENT GENERATOR 139 



from the room and is therefore at the same temperature 

 as the room.) 



The safe temperature rise in ordinary machines has 

 been fixed by the A.I.E.E. at 50 C. above room temperature, 

 hence the line KM is erected (in Fig. 89) at this value. 

 With good ventilation the rate of radiation at this tem- 

 perature is equal to the rate of heat production by the 

 current OG and so we say the safe current, in so far as 

 heating is concerned, is OG. If the ventilation is medium 

 the safe current is OH and if the ventilation is poor the 



Temperature rise 

 Armature curreut 



FIG. 89. Curves of Heat Generation and Heat Radiation. 



safe current is 01. This diagram shows the enormous 

 advantage of forced ventilation gained by equipping an 

 enclosed machine with a ventilating fan and proper air 

 ducts. By this means it is possible to increase its safe 

 current capacity by 200% or 300%. A certain railway 

 motor for example, might be rated as 75 h.p. with ordinary 

 ventilation and by suitable ventilation its rating might be 

 increased to perhaps 200 h.p. Practically all enclosed 

 machines are now designed with the idea of getting the 

 best ventilation possible. 



Commutation Limits Capacity. The other limit to the 



