X.  Efficiency  and  Losses 

22.  Besides  the  above  described  curves  the  efficiency  curves 
are  of  interest.  The  efficiency  of  alternators  and  synchronous 
motors  is  usually  so  high  that  a  direct  determination  by  measuring 
the  mechanical  power  and  the  electric  power  is  less  reliable  than 


10      20     30     4ff     50      60      TO     80     90     100  UO    120    130  140    150   160    170    ISO    150    200  KW. 

FIG.  70. — Synchronous  generator,  efficiency  and  losses. 

the  method  of  adding  the  losses,  and  the  latter  is  therefore  com- 
monly used. 

The  losses  consist  of  the  following:  the  resistance  loss  in  the 
armature;  the  resistance  loss  in  the  field  circuit;  the  hysteresis 
and  eddy  current  losses  in  the  magnetic  circuit;  the  friction  and 
windage  losses,  and  eventually  load  losses,  that  is,  losses  due  to 
eddy  currents  and  hysteresis  produced  by  the  load  current  in  the 
armature. 

The  resistance  loss  in  the  armature  is  proportional  to  the 
square  of  the  current,  I. 

The  resistance  loss  in  the  field  circuit  is  proportional  to  the 
square  of  the  field  excitation  current,  that  is,  the  square  of  the 

nominal  generated  or  counter-generated  e.m.f.,  EQ. 
10 


150     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

The  hysteresis  loss  is  proportional  to  the  1.6th  power  of  the 
real  generated  e.m.f.,  El  =  E  ±  Ir. 

The  eddy  current  loss  is  usually  proportional  to  the  square  of 
the  generated  e.m.f.,  E\. 

The  friction  and  windage  loss  is  assumed  as  constant. 

The  load  losses  vary  more  or  less  proportionally  to  the  square 
of  the  current  in  the  armature,  and  should  be  small  with  proper 
design.  They  can  often  be  represented  by  an  "effective"  arma- 
ture resistance. 

Assuming  in  the  preceding  example  a  friction  loss  of  2000  watts; 
an  iron  loss  of  3000  watts,  at  the  generated  e.m.f.  EI  =  1000;  a 


10  30  30  40   50  60  70 


1.00  UO  120  130  140  150  100  170  ISO  190  200  K.W, 


FIG.  71. — Synchronous  motor  efficiency  and  losses. 

resistance  loss  in  the  field  circuit  of  800  watts,  at  EQ  =  1000, 
and  a  load  loss  at  full  load  of  600  watts. 

The  loss  curves  and  efficiency  curves  are  plotted  in  Fig.  70 
for  the  generator,  with  the  current  output  at  non-inductive  load 
or  0  =  0  as  abscissas,  and  in  Fig.  71  for  the  synchronous  motor, 
with  the  mechanical  power  output  as  abscissas.