LOSSES IN ARMATURES 



109 



not exceed 40C., and the watts to be dissipated by the cooling 

 surfaces of the armature core will consist of: 



1. The hysteresis and eddy-current losses in the teeth. 



2. The hysteresis and eddy-current losses in the core below 

 the teeth. 



3. The PR losses in the " active" portion of the armature 

 winding. 



All these losses can be calculated in the manner previously 

 explained. The copper loss to be taken into account is not 

 the total PR loss in the armature winding, but is this total loss 



21 

 multiplied by the ratio ^i _T / ' 



where l a is the gross length of 



the armature core, and l is the length of the end connections 

 of one coil, as calculated by formula (52) of Art. 29. 



The various cooling surfaces may be considered separately, 

 and the watts carried away from each surface computed 

 independently. The total cooling surface may conveniently 

 be divided into: 



1. The outside cylindrical surface of the (revolving) armature. 



2. The inside cylindrical surface over which the air passes 

 before entering the radial cooling ducts. 



3. The entire surface of the radial ventilating ducts, and 

 the two ends of the armature core. 



The cooling effect of the external surface at the two ends of 

 the armature core is generally similar to that of the radial venti- 

 lating spaces, and it is convenient to think of the two end rings 

 as being equivalent to an extra duct. Thus, in Fig. 35, the 

 number of ducts is shown as five, and the cooling surface of each 



duct (both sides) is ~ (D 2 d 2 ). The calculations would be 



made on the assumption that there are six ducts. If the number 

 of radial vent ducts provided is n, the total cooling surface of 

 the ducts and the two ends of the armature will be 



+ 1). 



The outside cylindrical surface of the armature will be taken 

 as irDl a , where l a is the gross length, no deduction being made 

 for the space taken up by the vent ducts. The cooling surface 

 of the end connections beyond the core is not taken into account. 



The area of the inside cylindrical surface is irdl a . 



