VI.  Characteristic  Curves  of  Alternating-current  Generator 

15.  In  Fig.  59  are  shown,  at  constant  terminal  voltage  E,  the 
values  of  nominal  generated  e.m.f.  E0,  and  thus  of  field  excitation 
FQ,  with  the  current  7  as  abscissas  and  for  the  three  conditions, 

1.  Non-inductive  load,  p  =  1,  q  =  0. 

2.  Inductive  load  of  0  =  60  degrees  lag,  p  =  0.5,  q  =  0.866. 

3.  Anti-inductive  load  of  —  6  =  60  degrees  lead,  p  =  0.5, 

q  =  -0.866. 


SYNCHRONOUS  MACHINES 


139 


The  values  r  =  0.1,  XQ  =  5,  E  =  1000,  are  assumed.  These 
curves  are  called  the  compounding  curves  of  the  synchronous 
generator. 

In  Fig.  60  are  shown,  at  constant  nominal  generated  e.m.f.  EQ, 
that  is,  at  constant  field  excitation  F0,  the  values  of  terminal  vol- 


E  = 


000 
£=5, 


'•*& 


z 


EO.F, 


500 


20     10     00     80    100     120    UO     160    180    200AMP. 

FIG.  59. — Synchronous  generator  compounding  curves. 

tage  E  with  the  current  I  as  abscissas  and  for  the  same  resistance 
and  synchronous  reactance  r  =  0.1,  XQ  =  5,  for  the  three  different 
conditions, 

1.  Non-inductive  load,  p  =  1,  q  =  0,  EQ  =  1127. 

2.  Inductive  load  of  60  degrees  lag, 

p  =  Q.5,  q  =  0.866,  E0  =  1458. 


140     ELEMENTS  OF  ELECTRICAL  ENGINEERING 

3.  Anti-inductive  load  of  60  degrees  lead, 

p  =  0.5,  q  =  -0.866,  E0  =  628. 

The  values  of  EQ  (and  thus  of  FQ)  are  assumed  so  as  to  give 
E  =  1000  at  I  =  100.  These  curves  are  called  the  regulation 
curves  of  the  alternator,  or  the  field  characteristics  of  the  syn- 
chronous generator. 

In  Fig.  61  are  shown  the  load  curves  of  the  machine,  with  the 


40   60   80   100   120   140   160   180   200   220   240   260   280  AMP. 

FIG.  60. — Synchronous  generator  regulation  curves. 

current  I  as  abscissas  and  the  watts  output  as  ordinates  corre- 
sponding to  the  same  three  conditions  as  Fig.  60.  From  the 
field  characteristics  of  the  alternator  are  derived  the  open-cir- 
cuit voltage  of  1127  at  full  non-inductive  load  excitation,  which 
is  1.127  times  full-load  voltage;  the  short-circuit  current  225  at 
full  non-inductive  load  excitation,  which  is  2.25  times  full-load 
current;  and  the  maximum  output,  124  kw.,  at  full  non-induct- 
ive load  excitation,  which  is  1.24  times  rated  output,  at  775 
volts  and  160  amp.  It  depends  upon  the  point  on  the  field 


SYNCHRONOUS  MACHINES 


141 


characteristic  at  which  the  alternator  works,  whether  it  tends 
to  regulate  for,  that  is,  maintains,  constant  voltage,  or  constant 
current,  or  constant  power,  approximately. 


z 


L 


7 


\ 


H 


20    40    60    80   100   120   140   160   180   200   220   240   260   280  AMP. 

FIG.  61. — Synchronous  generator  load  curves.