Download eq16

TRANSFORMERS / TRANSMISSION LINES
POWER LOSSES
Vp

Vs
np
ns
ns
Vp
np
Vs 
Ps  Pp
Is 
np
ns
Ip
Vs I s  V p I p
Ploss  I 2 R
Vp
ac potential difference across primary windings of transformer [volt V]
Vs
ac potential difference across secondary windings of transformer [volt V]
np
number of turns in primary windings of transformer
np
number of turns in secondary windings of transformer
Ip
ac current through primary windings of transformer [ampere A]
Is
ac current through secondary windings of transformer [ampere A]
Pp
power - primary windings of transformer [watt W]
Ps
power - secondary windings of transformer [watt W]
R
resistance [ohms ]
Ploss
power loss – rate of energy dissipation as heat due to a current I through a
resistance R [watt W]
Secondary
Primary
~VP
Np
NS
~VS
output
input
laminated iron core
Equation Mindmap eq16:
Doing Physics on Line
1
Changing magnetic flux in primary = Changing magnetic flux in secondary
Step-up transformer : increase in secondary voltage (decrease in secondary current)
nS > nP VS > VP IS < IP
Step-down transformer: decrease in secondary voltage (increase in secondary
current)
nS < nP VS < VP IS > IP
Ps = Pp
Ideal transformer
Non-ideal transformer
Ps < Pp
Ps =  Pp
efficiency
0<1
power losses in transformer core
The ferromagnetic core used in transformers is laminated to reduce ohmic heating
caused by induced eddy currents. Laminations reduce the magnitude of eddy currents.
This reduces the ohmic heating of the metal core.
induced eddy currents
induced eddy currents
Binduced
Bcoil
I
I
current increasing
current
increasing
eddy currents cause a heating effect
insulating layers
Laminations reduce magnitude of eddy currents
 less ohmic heating ( I 2 R )
Power losses in transmission lines Ploss = I 2 R
Energy transferred in transmission lines at high voltages and hence smaller currents
to reduce power losses due to ohmic heating.
Equation Mindmap eq16:
Doing Physics on Line
2