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Transcript Transformer

Transformer
AC Source
Alternating current comes from generators, not batteries.
• Ideal sinusoidal source
The symbol for an AC source uses a sine curve.
R
DV
D V AC V 0 sin t
I AC
V0
R
sin t
Internal Flux
Faraday’s law links flux to
voltage.
DV N
• Change in voltage to
change in flux
• Change in flux to change in
voltage
D
M
Dt
The flux can link one
conductor to another.
• Iron for better flux link
• Minimize losses
DVA
R
N
A
NB
Primary Coil
D
M
Dt
An AC voltage source
produces a changing voltage.
DVA
R
N
NB
A
D
Dt
M
DV A
NA
The changing voltage creates
an opposing magnetic flux in
the iron.
Secondary Coil
Faraday’s law describes the
induced voltage in the
second coil.
• Assume all field lines from
primary go through
secondary
The induced voltage directly
depends on the primary
voltage.
DVA
R
N
NB
A
DVB N B
D
Dt
M
NB
DV A
NA
Turns Ratio
The output voltage depends on
the ratio of the turns in the
coils.
• Symbol represents linked coils
If NB > NA then the voltage
increases.
• Step-up transformer
If NB < NA then the voltage
decreases.
• Step-down transformer
DVB
NB
NA
DV A
Transformers
Commercial transformers wind
two coils around a linkng
medium.
• Single cylinder of air or iron
• Connecting bar of iron
The schematic symbol
represents two coils.
AC Adapter
An AC adaptor is an example
of a step-down transformer.
• Convert 120 V AC to 18 V DC
Power transmission uses step-
up up and step-down
transformers.
• Power plant: 10 kV to 345 kV
• Substation: 345 kV to 7200 V
• Power pole: 7200 V to 120 V
Power Law
Transformers do not create
power.
• Energy conserved
• Losses small compared to
resistors
For no power loss, current and
voltage must maintain same
power.
• Current decrease for step-up
• Current increase for stepdown
Pout Pin
V out I out V in I in
I out
V in I in
V out
V in I in
N out
N in
I out
N in
N out
V in
I in
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