Problems

Transformer Core Losses

The abi l i ty  of   i ron  or s teel   to carry  m agnet i c  f l ux   i s   m uch greater  than  i t   i s   i n  ai r,  and  thi s  abi l i ty   to al l ow m agnet i c  f l ux   to  f l ow  i s

cal l ed  permeabi l ity .   M os t   trans form er  cores   are  cons tructed  from   l ow  carbon  s teel s   whi ch  can  hav e  perm eabi l i t i es   i n  the

order  of   1500  com pared  wi th  j us t   1.0  for  ai r.   Thi s   m eans   that   a  s teel   l am i nated  core  can  carry   a  m agnet i c  f l ux   1500  t im es

bet ter  than  that   of   ai r.   Howev er,   when  a  m agnet i c  f l ux   f l ows   i n  a  trans form ers   s teel   core,   two  ty pes   of   l os s es   occur  i n  the

s teel .  One  term ed "eddy  current   l os s es " and  the other  term ed "hy s teres i s   l os s es ".

Hys te re s i s   Los s e s

Trans form er Hy s teres i s  Los s es   are caus ed becaus e of   the  fri ct i on of   the m ol ecul es   agai ns t   the  f l ow of   the  m agnet i c  l i nes  of

force  requi red  to  m agnet i s e  the  core,  whi ch  are cons tant l y   changi ng  i n  v al ue  and di rect i on  f i rs t   i n  one di rect i on  and  then  the

other  due  to  the  i nf l uence  of   the  s i nus oi dal   s uppl y   v ol tage.   Thi s   m ol ecul ar  fri ct i on  caus es   heat   to  be  dev el oped  whi ch

repres ents   an  energy   l os s   to  the  trans form er.   Ex ces s i v e  heat   l os s   can  ov ert im e  s horten  the  l i fe  of   the  i ns ul at i ng  m ateri al s

us ed  i n  the m anufacture of   the wi ndi ngs  and s tructures .  Therefore,  cool i ng of  a  trans form er  i s   im portant .

A l s o,   trans form ers  are des i gned  to operate at  a part i cul ar s uppl y   frequency .  Loweri ng  the  frequency  of   the s uppl y  wi l l  res ul t   i n

i ncreas ed hy s teres i s  and  hi gher  tem perature  i n  the  i ron  core.  So reduci ng  the s uppl y   frequency   from  60 Hertz   to  50 Hertz  wi l l

rai s e  the am ount  of  hy s teres i s  pres ent ,  decreas ed  the V A  capaci ty  of   the  trans form er.

Eddy Curre nt  Los s e s

Trans form er  Eddy   Current   Los s es   on  the  other  hand  are  caus ed  by   the  f l ow  of   ci rcul at i ng  currents   i nduced  i nto  the  s teel

caus ed  by   the  f l ow of   the  m agnet i c  f l ux   around  the core.   Thes e  ci rcul at i ng  currents   are  generated becaus e  to  the  m agnet i c

caus ed  by   the  f l ow of   the  m agnet i c  f l ux   around  the core.   Thes e  ci rcul at i ng  currents   are  generated becaus e  to  the  m agnet i c

f l ux   the  core  i s   act i ng  l i ke a  s i ngl e  l oop  of  wi re.  Si nce  the  i ron  core  i s  a  good conductor,   the eddy  currents   i nduced by   a s ol i d

i ron  core  wi l l   be  l arge.   Eddy   currents   do  not   contri bute any thi ng  towards   the  us eful nes s   of   the  trans form er  but   i ns tead  they

oppos e  the  f l ow  of   the  i nduced  current  by   act i ng  l i ke  a  negat i v e  force  generat i ng res i s t i v e  heat i ng  and power  l os s  wi thi n  the

core.

La mi na ti ng  the   I ron Core

Eddy   current   l os s es   wi thi n  a  trans form er  core  can  not   be  el im i nated  com pl etel y ,   but   they   can  be  great l y   reduced  and

control l ed by  reduci ng  the  thi cknes s  of   the s teel   core.   Ins tead of  hav i ng one bi g  s ol i d  i ron core as   the  m agnet i c core m ateri al

of   the  trans form er or coi l ,   the m agnet i c path  i s  s pl i t  up  i nto m any   thi n pres s ed s teel  s hapes  cal l ed "l am i nat i ons ".

The  l am i nat i ons   us ed  i n  a  trans form er  cons truct i on  are  v ery   thi n  s tri ps

of   i ns ul ated  m etal   j oi ned  together  to  produce a  s ol i d but   l am i nated core

as  we s aw abov e.  Thes e  l am i nat i ons  are  i ns ul ated  from  each other by  a

coat   of   v arni s h  or  paper  to  i ncreas e  the  ef fect i v e  res i s t i v i ty   of   the  core

thereby   i ncreas i ng  the  ov eral l   res i s tance  to  l im i t   the  f l ow  of   the  eddy

currents .   The  res ul t   of   al l   thi s   i ns ul at i on  i s   that   the  unwanted  i nduced

eddy   current   power-l os s   i n  the  core  i s   great l y   reduced,   and  i t   i s   for  thi s

reas on  why   the  m agnet i c  i ron  ci rcui t   of   ev ery   trans form er  and  other

el ectro-m agnet i c  m achi nes   are  al l   l am i nated.   Us i ng  l am i nat i ons   i n  a

trans fom er cons truct i on reduces  eddy  current   l os s es .

The  l os s es   of   energy ,   whi ch  appears   as   heat   due  both  to  hy s teres i s   and  to  eddy   currents   i n  the

m agnet i c  path,   i s   known  com m onl y   as   "trans form er  core  l os s es ".   Si nce  thes e  l os s es   occur  i n  al l

m agnet i c  m ateri al s   as   a  res ul t   of   al ternat i ng m agnet i c  f i el ds .   Trans form er  core  l os s es   are  al way s

pres ent   i n  a  trans form er  whenev er  the  prim ary   i s   energi z ed,   ev en  i f   no  l oad  i s   connected  to  the

s econdary   wi ndi ng.   A l s o  thes e  hy s teres i s   and  the  eddy   current   l os s es   are  s om et im es   referred  to

as  "trans form er  i ron  l os s es ",  as   the m agnet i c  f l ux  caus i ng  thes e  l os s es   i s  cons tant  at  al l   l oads .

Eddy Curre nt  Los s e s

But   there  i s   al s o  another  ty pe  of  energy   l os s   as s oci ated  wi th  trans form ers   cal l ed  "copper  l os s es ".

Trans form er  Copper  Losses  are  m ai nl y   due  to  the  el ectri cal   res i s tance  of   the  prim ary   and

s econdary   wi ndi ngs .   M os t   trans form er  coi l s   are  m ade  from   copper  wi re  whi ch  has   res i s tance  i n

Ohm s ,  ( Ω  ).  Thi s  res i s tance oppos es   the m agnet i s i ng currents   f l owi ng  through  them .

When  a  l oad  i s   connected  to  the  trans form ers   s econdary   wi ndi ng,   l arge  el ectri cal   currents   f l ow  i n

both  the  prim ary   and  the  s econdary   wi ndi ngs ,   el ectri cal   energy   and  power  (  or  the  I   R  )  l os s es

occur as   heat .  General l y  copper  l os s es   v ary  wi th  the  l oad  current ,  bei ng alm os t  z ero  at  no-l oad,  and

at   a  m ax im um   at   ful l -l oad  when  current   f l ow  i s   at   m ax im um .   Trans form ers   wi th  hi gh  v ol tage  and  current   rat i ngs   requi re

conductors  of   l arge cros s -s ect i on  to hel p m i nim i s e  thei r copper  l os s es .

Then we can def i ne an  i deal   trans form er as  hav i ng:

1.  No Hy s teres i s   l oops  or Hy s teres i s   l os s es    →  0

2.   Inf i ni te Res i s t i v i ty  of  core m ateri al  gi v i ng z ero Eddy  current   l os s es    →  0

3.  Zero wi ndi ng res i s tance gi v i ng z ero  I R copper  l os s es    →  0

In  the  nex t   tutori al   about   T ransf ormers  we  wi l l   l ook  at   Tr a n s f o r m e r   L o a d i n g   of   the  s econdary   wi ndi ng  wi th  res pect   to  an

el ectri cal   l oad and s ee  the ef fect  a "NO-l oad" and a "ON-l oad" connected  trans form er has  on  the prim ary  wi ndi ng current