LOSSES IN ARMATURES 107 



sources of information. A very good treatment of the subject 

 will be found in Chaps. IX and Xof PROFESSOR MILES WALKER'S 

 recently published book on dynamo-electric machinery. 1 



A good practical rule for estimating the quantity of air nec- 

 essary to carry away the heat when forced ventilation is used 

 is based on the fact that a flow of 1 cu. ft. of air per minute will 

 carry heat away at the rate of 0.536 T watts, where T is the 

 number of degrees Centigrade by which the temperature of the 

 air has been increased while passing over the heated surfaces. 

 Thus, if the difference in temperature between the outgoing 

 and incoming air is not to exceed 19C., it will be necessary to 

 provide at least 100 cu. ft. of air per minute for each kilowatt 

 lost in the machine. 



The power required to drive the ventilating fan is not very 

 easily estimated as it depends upon the velocity of the air 

 through the passages. The velocity of the air through the ducts 

 and over the cooling surfaces is usually from 2,000 to 4,000 ft. 

 per minute and should preferably not exceed 5,000 ft. per minute; 

 with higher velocities the friction loss might be excessive. 



As a very rough guide to the power required to drive the 

 ventilating fans, the following figures may be useful: 



For 50-kw. dynamo, 300 watts. 

 For 400-kw. dynamo, 1,500 watts. 

 For 1,000-kw. dynamo, 3,000 watts. 



34. Cooling Surfaces and Temperature Rise of Armature. 



Specifications for electrical machinery usually state that the 

 temperature rise of any accessible part shall not exceed a given 

 amount after a full-load run of about 6 hr. duration. The 

 permissible rise of temperature over that of the surrounding air 

 will depend upon the room temperature. It usually lies between 

 40 and 50C. The surface temperature is actually of little 

 importance and is no indication of the efficiency of a machine, 

 but, by keeping the surface temperature below a specified limit, 

 the internal temperatures are not likely to be excessive, and 

 the durability of the insulation upon which the life of the ma- 

 chine is largely dependent will thereby be ensured. The de- 

 signer must, however, see that ventilating ducts or surfaces are 

 provided at sufficiently frequent intervals to allow of the heat 



1 MILES WALKER: "Specification and Design of Dynamo-electric Ma- 

 chinery," LONGMANS, GREEN & Co. 



