Download The Transformer Explained Using Faraday`s Law File

BACK EMF
When the coil L is connected in series with the cell V it produces an increasing
magnetic field as the current through the coil rises. This induces a “back emf” in
the reverse direction to the emf produced by the cell.
The magnetic field stores energy transferred from V
When S is moved so that L is in series with R only, the back emf
drives a current through R dissipating the energy stored.
Transformers
• An alternating current through a coil
around a soft iron core sets up an
alternating magnetic flux.
North Pole
South Pole
South Pole
North Pole
North Pole
South Pole
The simple transformer
A.C.
Supply
Primary Coil
Secondary
Coil
The changing magnetic flux in the core produced by the primary coil,
induces an alternating emf in the secondary coil.
A step down transformer
Fewer turns on the secondary coil, compared to the primary steps down the
voltage
Higher
voltage
in
Lower
voltage
out
Iron
core
2:1 results in halving the voltage
3:1 gives 1/3 of the voltage etc
A step up transformer
Here more turns on the secondary than the primary steps up the output
voltage.
1:2 doubles the voltage
1:3 gives 3x the voltage etc.
Explaining the effect
A.C.
Supply
Secondary
Coil
Primary Coil
The flux linkage through the
secondary coil at any point in
time :is Φs
The e.m.f produce in the secondary coil
is equal to the rate of change of flux
linkage by faraday’s law
Εs  N
 s
s t
The changing flux is also producing a back e.m.f. in the primary coil. This is
effectively equal to the voltage applied to the primary coil
Εp  N
 p
p t
The transformer formula
Εs  N
 s
s t
Εp  N
 p
p t
Now Φ =AB
A is the coil cross sectional area
B is the flux density
These have the same value for both the primary and secondary
coils so
ΔΦs
Δt

ΔΦp ΔΦ

Δt
Δt
Ε s 
Ε

 P
t
N
N
s
P
ΕS
Ε
 P
N
N
s
P
This relationship is often written in terms of turns ratio.
N
N
p
s
VP

VS
There is little resistance in the coils so that the emf generated in
the secondary coil is effectively the same as the voltage it supplies
to the secondary circuit.
Secondary voltage
Primary voltage
=
secondary turns
primary turns