312 ELECTRICAL EQUIPMENT 



reading. In single-layer windings, with detectors be- 

 tween coil side and core and between coil side and wedge, 

 add to the highest reading 10 C. plus 1 C. per 1000 volts 

 above 5000 volts of terminal pressure. 



Thus, for a three-phase machine with an 11, 000- volt single- 

 layer winding, the correction to be added to the maximum " ob- 

 servable " temperature in estimating the " hottest-spot " tem- 

 perature, is 16 C., and the permissible temperature rise is, 

 therefore, 49 C. For double-layer windings the permissible 

 rise is 60 C. and for single-layer windings for 5000 volts or less 

 55 C. 



Increased altitude has the effect of increasing the temperature 

 rise of some types of machinery. In the absence of information 

 in regard to the height above sea level at which the machine is 

 intended to work in ordinary service, this height is assumed not to 

 exceed 1000 meters (3300 feet). For machinery operating at an 

 altitude of 1000 meters or less, a test at any altitude less than 

 1000 meters is satisfactory, and no correction shall be applied to 

 the observed temperature. Machines intended for operation 

 at higher altitudes shall be regarded as special, and when a ma- 

 chine is intended for service at altitudes above 1000 mecers (3300 

 feet) the permissible temperature rise at sea level shall be reduced 

 by 1 per cent for each 100 meters (330 feet) by which the altitude 

 exceeds 1000 meters. 



Efficiency. The efficiency of a generator is the ratio of the 

 kilowatt output to the kilowatt input at the rated Kv.A. and 

 power factor. The difference between these two quantities is 

 equal to the losses. The method commonly and most readily 

 used for obtaining the efficiency is to determine these losses and 

 then compute the efficiency by dividing the power output by the 

 sum of the power output plus the losses. 



The guaranteed efficiency should always refer to the energy 

 load and it is most important that the power factor of the load is 

 also given. In certain cases the guaranteed efficiency is based on a 

 Kv.A. output, but the inconsistency of such a method is apparent, 

 as the following example will illustrate : 



Assume a generator rated 100 Kv.A. (100 Kw. 1.0 P.F.) or 

 100 Kv.A. (80 Kw. .8 P.F.), and that the losses at unity and 80 

 per cent power factors are 10 and 11 Kw. respectively, the effi- 

 ciency is then: 



