SYNCHRONOUS GENERATORS 313 



Based on 100 Kw. 1.0 P.F. 



mo 



= 91 per cent. 



Based on 80 Kw. .8 P.F. 



80 



Based on 100 Kv.A. .8 P.F. 



Eff.=^=90 per cent. 



From the last two values it is seen that for 80 per cent power 

 factor if based on the Kv.A., a 2 per cent greater efficiency guar- 

 antee can be made, although this value has no meaning, as it is 

 based on apparent power. 



It is, of course, equally important that all the losses are included 

 and that they are figured on the same basis, in order that a fair 

 comparison may be made of the efficiencies guaranteed by differ- 

 ent manufacturers. 



The A.I.E.E. Standardization rules require that for syn- 

 chronous generators the following losses are included in deter- 

 mining the efficiency: (1) core losses, (2) PR loss in all windings 

 based upon rated Kv.A. and power factor, (3) stray load losses, 

 (4) friction of bearings and windage, (5) rheostat losses corre- 

 sponding to rated Kv.A. and power factor. 



Bearing Friction and Windage may be determined as follows: 

 Drive the machine from an independent motor, the output of 

 which shall be suitably determined. The machine under test 

 shall not be excited. This output represents the bearing friction 

 and windage of the machine under test. 



Core Loss. Follow the above test with an additional reading 

 taken with the machine separately excited so as to produce at 

 the terminals a voltage corresponding to the calculated internal 

 voltage for the load under .consideration. The difference between 

 the output obtained by this test and that obtained by the 

 previous one shall be taken as the core loss, neglecting the brush 

 friction. The internal voltage shall be determined by correcting 

 the terminal voltage for the resistance drop only. 



PR Loss may be calculated directly from the resistance meas- 



