VI.  Heating  and  Ventilation 

122.  As  the  transformer  is  a  stationary  apparatus,  it  does  not 
have  the  advantage  of  dissipating  the  heat  produced  by  the 
internal  losses,  by  the  natural  ventilation  of  the  air  currents  pro- 
duced by  the  centrifugal  forces  in  rotating  apparatus,  and  it  is 
therefore  fortunate  that  the  transformer  is  the  most  efficient 
apparatus  (except  perhaps  the  electrostatic  condenser)  and  thus 
has  to  dissipate  less  heat  than  any  other  apparatus  of  the  same 
output.  Thus  in  smaller  transformers  radiation  and  the  natural 
convection  from  the  surface  are  often  sufficient  to  keep  the  tem- 
perature within  safe  limits. 

Smaller  distribution  transformers  usually  are  installed  out- 
doors, on  poles,  and  then  require  protection  by  enclosure  in  an 
iron  case  or  tank.  This  still  further  reduces  the  heat  radiation, 
and  therefore  such  transformer  cases  are  now  almost  always  filled 
with  oil,  the  oil  serving  to  carry  the  heat  from  the  transformer 
iron  and  windings  to  the  case.  Incidentally,  the  oil  filling  also 
protects  the  transformer  from  the  failure  of  insulation  by  con- 


ALTERNATING-CURRENT  TRANSFORMER       295 

densation  of  moisture  during  the  variation  of  atmospheric  tem- 
perature and  humidity. 

In  larger  oil-cooled  transformers,  the  tank  is  made  corrugated, 
even  with  large  double  corrugations,  to  give  a  very  large  external 
surface  to  dissipate  the  heat. 

Much  more  effectively,  however,  the  heat  can  be  carried 
away  by  mechanical  ventilation,  and  size  and  cost  of  the  trans- 
former thereby  materially  reduced.  Therefore  practically  all 
larger  transformers  have  forced  ventilation.  Various  methods 
of  forced  ventilation  are: 

(a)  Oil  circulation.  The  warm  oil  is  pumped  from  the  top 
of  the  transformer  tank,  through  some  cooling  device.  Often 
also  a  drying  device  to  take  out  any  trace  of  moisture — and  then 
fed  back  into  the  bottom  of  the  tank. 

(6)  Water  circulation.  Cooling  water  is  pumped  through 
a  system  of  pipes  located  under  the  oil  at  the  top  of  the  trans- 
former tank.  This  is  the  most  common  design  of  large  trans- 
formers. 

(c)  Air  blast.  Coils  and  iron  are  subdivided  by  ventilating 
ducts,  and  a  low-pressure  air  blast  forced  through  the  ventilating 
ducts.  This  is  the  cleanest  method,  as  no  oil  is  used.  However, 
it  is  limited  to  low  and  moderate  voltages — up  to  about  33,000; 
at  higher  voltage,  the  mechanical  and  chemical  action  of  corona 
appearing  at  the  coils  reduces  their  life,  and  the  oil  becomes 
necessary  for  insulation. 

Numerous  modifications  of  the  various  types  have  been 
built  and  are  in  use,  as  water-cooled  oil  transformers  with 
natural  circulation  of  the  water  through  outside  radiating 
pipes,  etc.