138 ELECTRICAL MACHINERY 



Current in the Neutral. When the load on the two sides 

 of the system is exactly balanced the neutral carries no 

 current at all. When the load is unbalanced the neutral 

 wire carries a current equal to the amount by which the 

 load is unbalanced. For example if there is a load of 100 

 amperes on one side of a three-wire system and only 70 

 amperes on the other the neutral will carry the difference, 

 30 amperes. 



27. Capacity of a Dynamo-electric Machine. The 

 capacity of a generator or motor is limited by either one 

 or the other of two effects, namely, heating or commutation. 



Features Limiting the Temperature Rise. As the load on 

 a machine increases, more current must flow through its 

 armature (and series field if the machine has one) and the 

 heat produced in these circuits is proportional to the square 

 of the current. The temperature to which the windings will 

 rise depends upon two factors; the rate at which heat is 

 produced and the rate at which it can be radiated, or sent off 

 into the surrounding medium. The rate at which heat is 

 radiated from any body (e.g., an armature) depends upon the 

 difference in temperature between it and the surrounding air 

 and upon the amount of air that is carried over the radiating 

 surface. This second condition is really involved in the first 

 because if but little air is supplied it soon gets hot and so 

 reduces the temperature difference whereas if a lot of air is 

 supplied, as in forced ventilation, the air is carried away 

 from the radiating surface before it has time to get hot. 



Effect of Ventilation Upon Capacity. In Fig. 89 are 

 shown two sets of curves, those for heat radiation and 

 that for heat production. The curve OAB gives the rate 

 of heat production plotted against the armature current 

 as abscissae. The three lines OC, OD, and OE, show the 

 rate of radiation for good, medium and poor ventilation; 

 the abscissae for these curves are the difference between 

 the armature temperature and room temperature. (It 

 is supposed that the air for ventilation is taken directly 



