80 COMMERCIAL ELECTRICAL TESTING 



STANDARD EFFICIENCY TESTS are made by the method 

 of losses. 



Employing the same nomenclature as that used in 

 calculating the standard efficiency of direct current 

 generators, a motor efficiency is calculated as follows : 



C 4 = C L ~Ce 



Watts input W A = C L V L 



W\ = Core loss taken from the core loss curve corres- 



ponding to VLCR 

 Then ZW= 



as before. 



Watts output W b = W a -2W and = ^7 



W a 



Since motors are always rated according to horse- 



Wh 

 power output H.P. = 



If, as in the case of direct current generators, only 

 a running light is taken and it is desired to combine 

 the resistances of the series and commutating pole 

 fields with their respective shunts and to combine the 

 losses in the shunt field and rheostats, then 



2W = Running light + C\R 4 +C\R b +C<>V +C* L Rs F 



In the case of shunt motors 



2jy = Running light +O>tRi 



The remarks n~ade under the subject of direct current 

 generators r in reference to the calculation of brush 

 friction, brush contact resistance and hot resistances, 

 as well as to all other efficiency calculations, apply in 

 the case of "mo tors. 



It will be seen from Fig. 23 that motor efficiencies 

 are plotted with amperes line as abscissae and per cent 

 efficiency and horse power output as ordinates. The 

 horse power curve should be produced to intersect the 

 axis of X at running light amperes line. 



For NORMAL LOAD HEAT RUN the machine is run 

 under load until it has reached constant temperatures, 

 and these are then recorded. All series field shunt 



