48 



ELECTRICAL MACHINERY 



The question of ventilation of electric machinery will 

 be taken up here because there are some features of con- 

 struction of the armature core which are incorporated in 

 the design with the idea of keeping the armature well 

 ventilated. 



Capacity Limited by Temperature Rise. The capacity 

 or rating of any dynamo-electric machine is fixed by the 

 safe permissible temperature rise. Suppose we have a 

 generator which the manufacturers have given a rating 

 of 50, kw. and upon being tested it is found that the 

 armature of this machine becomes so hot, when carrying 

 a load of 50 kw., that there is likelihood of the insulation 



FIG. 23. Semi-closed Slot Gives Greater Effective Area of Air Gap 

 Than Open Slot. 



on the armature becoming damaged. It is evident that 

 50 kw. is more load than the machine can safely carry 

 continuously or we say the manufacturer has over-rated 

 the generator. As we know the heat generated in the 

 armature windings will be less if the load is less, we now 

 test it at 40 kw. load and after running at this load for 

 a sufficient length of time to have reached constant tem- 

 perature, the armature is fairly cool; say not more than 

 30 C. above room temperature. 



It has been determined by the standardization committee 

 of the American Institute of Electrical Engineers that for 

 ordinary machines the safe temperature rise in the arma- 

 ture is 50 C. above the temperature of the room in which 



