B.  LIGHTING  ONLY 


I 

II 

Time 

Load 

=  Percent. 

Time  X 
load 

Losses 

Time  X 
losses 

Losses 

Time  X 
losses 

2hr. 

IK 

125 

250 

4.10 

8.20 

3.54 

7.08 

2hr. 

H 

75 

150 

2.11 

4.22 

2.55 

5.10 

20  hr. 

Mo 

5 

100 

1.00 

20.00 

2.00 

40.00 

S  = 

500 

32.42 

52.18 

Input                            532.42                     552.18 

Per  cent,  loss                   6.11                        9  .  45 

Per  cent,  efficiency       93  .89                       90  .  55 

As  seen,  while  I  and  II  have  the  same  full-load  efficiency, 
97.1  per  cent.,  I,  the  low  core-loss  type,  gives  a  much  higher 
all-day  efficiency,  the  more  so  the  shorter  the  time  of  heavy  load, 
that  is,  is  far  preferable  for  general  distribution,  as  "lighting 
transformer." 

Inversely,  in  large  power  transformers  in  transmission  systems, 
the  high  partial  load  efficiency  of  the  low  core-loss  type  is  of  less 
importance,  as  such  transformers  are  usually  not  run  at  partial 
load,  but  with  a  decrease  of  load  on  the  system,  transformers  and 
generators  are  cut  out  and  the  remaining  ones  kept  loaded.  Of 


ALTERNATING-CURRENT  TRANSFORMER       285 

importance,  however,  is  low  i2r  loss.  Under  emergency  conditions 
requiring  overloading  of  some  transformer,  the  increased  loss 
is  all  in  the  copper,  and  the  less  therefore  the  i2ry  the  less  is 
the  danger  of  destruction  by  overheating  in  a  case  of  a  temporary 
overload.  Thus  the  low  izr  loss  type  of  transformer  is  preferable 
for  large  power  units.