LOSSfiS IN ARMATURES 105 



33. Ventilation of Armatures. Recent improvements in 

 dynamo-electric machinery have been mainly along the lines 

 of providing adequate means by which the heat due to power 

 losses in the machine can be carried away at a rapid rate, 

 thus increasing the maximum output from a given size of 

 frame. 



Still air is a very poor conductor of heat; but when a large 

 volume of cool air is passed over a heated surface, it will effectu- 

 ally reduce the temperature which may, by this means, be kept 

 within safe limits. 



The rotation of the armature of an electric generator will pro- 

 duce a draught of air which may be sufficient to carry away the 

 heat due to PR and hysteresis losses without the aid of a blower 

 or fan. Self-ventilating machines are less common at the present 

 time than they were a few years ago; but, by providing a suffi- 

 cient number of suitably proportioned air ducts in the body 

 of the armature, machines of moderate size may still be built 

 economically without forced ventilation. Radial ducts are 

 provided by inserting special ventilating plates at intervals 

 of 2 to 4 in., and so dividing the armature core into sections 

 around which the air can circulate. The width of these venti- 

 lating spaces (measured in a direction parallel to the axis of 

 rotation) is rarely less than % in. or more than J^ in. in machines 

 without forced ventilation. A narrower opening is liable to 

 become choked up with dust or dirt, while the gain due to a 

 wider opening is very small, and does not compensate for the 

 necessary increase in gross length of armature. The ventilating 

 plates usually consist of iron stampings similar in shape to the 

 armature stampings, but thicker. Radial spacers of no great 

 width, but of sufficient strength to resist crushing or bending, 

 are riveted to the flat plates; they are so spaced as to coincide 

 with the center of each tooth, and allow the air to pass outward 

 by providing a number of small openings on the cylindrical 

 surface of the armature. 



Openings must also be provided between the shaft and the 

 inside bore of the armature through which the cool air may be 

 drawn to the radial ventilating ducts. The radial spacers on 

 the ventilating plates assist the passage of the air through the 

 ducts, their function being similar to that of the vanes in a 

 centrifugal fan. Apart from the ventilating ducts, the outer 

 cylindrical surface of the armature is effective in getting rid of a 



