308 ELECTRICAL EQUIPMENT 



The rating of a generator is usually determined by its permis- 

 sible temperature rise caused by the current. This rise neces- 

 sarily increases with increasing load and also with decreasing 

 power-factor. Thus, for a given Kv.A. output, the total heat 

 losses are larger for low than for high power factors, the differ- 

 ence being due to the heat generated by the increased field cur- 

 rent which is required to overcome the armature reaction and 

 maintain the given current and terminal voltage. 



Alternating-current generators are generally designed to oper- 

 ate a normal load and 80 per cent power factor without ex- 

 ceeding a specified temperature rise; and should such a machine 

 have to be operated with a load having a lower power factor, its 

 rating will be reduced when based on the same temperature guar- 

 antee. The true operating power factor should, therefore, be 

 carefully considered in selecting the capacity of the generating 

 units. The power factor depends not only on the type of appa- 

 ratus comprising the load, but also on the load factor at which 

 they are operated. 



To obtain the total Kv.A. capacity of a system, the sum of 

 the wattless components of the different loads should be calcu- 

 lated, the efficiency, power factor and load factor being duly 

 considered. The total capacity is then equal in Kv.A. to 



V (Total Kw. energy) 2 + (Total Kv.A. wattless) 2 , 

 and the combined power factor of the load 



_ Total Kw. energy 

 Total Kv.A. ' 



It is obvious that a generator must not be permitted to be 

 operated under such conditions that it will attain such excessive 

 temperatures which will cause the insulation employed in their 

 construction to deteriorate, and the A.I.E.E. Standardization 

 Rules contains the following table, giving the highest tempera- 

 tures and temperature rises to which various classes of insulating 

 materials may be subjected. While it was recognized that the 

 manufacturers could successfully employ class B insulation at 

 150 C. and even higher, it was not felt that sufficient data was 

 available to recommend this and the institute adopted 125 C. 

 as a conservative limit for this class of insulation, any increase 

 above this figure being considered a special guarantee. The ambient 



