XII.  Efficiency  and  Losses 

61.  The  losses  in  a  commutating  machine  which  have  to  be 
considered  when  deriving  the  efficiency  by  adding  the  individual 
losses  are: 

1.  Loss  in  the  resistance  of  the  armature,  the  commutator 
leads,  brush  contacts  and  brushes,  in  the  shunt  field  and  the  series 
field  with  their  rheostats. 

2.  Hysteresis  and  eddy  currents  in  the  iron  at  a  voltage  equal 
to  the  terminal  voltage,  plus  resistance  drop  in  a  generator,  or 
minus  resistance  drop  in  a  motor. 

3.  Eddy  currents  in  the  armature  conductors  when  large  and 
not  protected,  and  in  pole  faces  when  solid  and  the  air  gap 
is  small. 

4.  Friction  of  bearings,  of  brushes  on  the  commutator,  and 
windage. 

5.  Load  losses,  due  to  the  increase  of  hysteresis  and  of  eddy 
currents  under  load,  caused  by  the  change  of  the  magnetic  dis- 
tribution, as  local  increase  of  magnetic  density  and  of  stray  field. 

The  friction  of  the  brushes  and  the  loss  in  the  contact  resist- 
ance of  the  brushes  are  frequently  quite  considerable,  especially 
with  low-voltage  machines. 

Constant  or  approximately  constant  losses  are:  friction  of 
bearings  and  of  commutator  brushes,  and  windage;  hysteresis 
and  eddy  current  losses;  and  shunt  field  excitation.  Losses 


D.'C.  COMMUTATING  MACHINES  199 

increasing  with  the  load,  and  proportional  or  approximately 
proportional  to  the  square  of  the  current:  armature  resistance 
losses;  series  field  resistance  losses;  brush  contact  resistance 
losses;  and  the  so-called  "load  losses."