XIV.  Division  of  Load  in  Parallel  Operation 

26.  Much  more  important  than  equality  of  terminal  voltage 
before  synchronizing  is  equality  of  frequency.  Inequality  of 
frequency,  or  rather  a  tendency  to  inequality  of  frequency  (since 
by  necessity  the  machines  hold  each  other  in  step  or  at  equal 
frequency),  causes  cross  currents  which  transfer'energy  from  the 
machine  whose  driving  power  tends  to  accelerate  to  the  machine 
whose  driving  power  tends  to  slow  down,  and  thus  relieves  the 
latter  by  increasing  the  load  on  the  former.  Thus  these  cross 
currents  are  power  currents,  and  cause  at  no  load  or  light  load 
the  one  machine  to  drive  the  other  as  synchronous  motor,  while 
under  load  the  result  is  that  the  machines  do  not  share  the  load  in 
proportion  to  their  respective  capacities. 

The  speed  of  the  prime  mover,  as  steam  engine  or  turbine, 
changes  with  the  load.  The  frequency  of  alternators  driven 
thereby  must  be  the  same  when  in  parallel.  Thus  their  respect- 
ive loads  are  such  as  to  give  the  same  speed  of  the  prime  mover 
(or  rather  a  speed  corresponding  to  the  same  frequency) .  Hence 
the  division  of  load  between  alternators  connected  to  independent 
prime  movers  depends  almost  exclusively  upon  the  speed  regula- 
tion of  the  prime  movers.  To  make  alternators  divide  the  load 
in  proportion  to  their  capacities,  the  speed  regulation  of  their 
prime  movers  must  be  the  same;  that  is,  the  engines  or  turbines 
must  drop  in  speed  from  no  load  to  full  load  by  the  same  percent- 
age and  in  the  same  manner. 

If  the  regulation  of  the  prime  movers  is  not  the  same,  the  load 
is  not  divided  proportionally  between  the  alternators,  but  the 
alternator  connected  to  the  prime  mover  of  closer  speed  regula- 
tion takes  more  than  its  share  of  the  load  under  heavy  loads,  and 


SYNCHRONOUS  MACHINES  155 

less  under  light  loads.     Thus,  too  close  speed  regulation  of  prime 
movers  is  not  desirable  in  parallel  operation  of  alternators.