Teaching Notes for Unit 9.3 Motors and Generators Topic of NSW HSC Physics Course

1. Topic 9.3
3.3.2 Transformers

2. Transformers
● A transformer is an electrical device that can convert an
alternating electrical supply at one voltage to a different
(higher or lower) voltage.
● Transformers that increase voltage are said to be step-up
transformers.
● Transformers that decrease voltage are said to be step-down
transformers.

3. Transformers
● A transformer has two
windings or coils wound on
to an iron core.
● The core is laminated to
reduce eddy currents and
energy losses.
● The iron core acts to
transmit and trap the
magnetic flux generated by
the primary winding.

4. Transformers
● An alternating current
generates a magnetic field
in the primary winding.
B=μ0μr
N p I (t)p
l p
● The magnetic flux is
transmitted through the iron
core.
Φ=μ0μr
N p Ap I0p sin(ωt)
l p
● This changing magnetic flux
induces an emf in the
secondary winding.
εs=−N s
Δ Φ
Δ t

5. Transformers
● It can be shown that:
V p
V s
=
N p
Ns
● If 100% efficiency is
assumed:
Pp=Ps
V p I p=V s I s
V p
N V =
p
s
Ns
=
I s
I p

6. Transformers and Energy
● A transformer works using the principle of
conservation of energy.
● The energy flowing into the primary coil flows
must flow out of the transformer
● This is mostly in the form of electrical energy from
the secondary coil.

7. Transformer Losses
● A real transformer is not 100% efficient.
● Eddy currents are formed in the iron core due to
induction. This is minimised by laminating the iron
core to reduce the area of induction and hence the
size of the induced current.
● Winding resistance. The windings of the
transformer are usually made of thick copper to
reduce heating and loses to a minimum
● Hysteresis losses are caused when magnetising
and demagnetising the iron core. This causes
heating and energy loss.

8. Laminating
● Transformers are made out
of laminated iron rather
than solid iron.
● This decreases the space
available for eddy currents
and hence increase the
resistance of the core.
● The increased resistance
causes a lower eddy
current and hence less
heating.

9. Transformer Problems
● A step-up transformer converts 10kV into
400kV for transmission. If the primary coil has
200Turns, how may turns are on the secondary
coil?
● A transformer converts 240V into 5V to charge
an iPod. The secondary coil has 400T, how
many turns are on the primary coil?

10. Transformer Problems
● A particular transformer is 95% efficient and is
designed to turn 240V into 12V and a maximum
current of 5A. The primary winding has 1000T.
● How many turns are on the secondary coil?
● What current flows in the primary coil when the
transformer is running at maximum power?

11. Using Transformers
● Transformers are
everywhere:
● They are used in
substations to step-up or
down the voltage to or
from the grid.
● These transformers are so
big and generate so much
heat that they usually need
to be cooled with oil.

12. Using Transformers
● Small transformers are
found in plug tops.
● These step down the
mains voltage from 240V
to a more usable level for a
device to use.
● They often also have a
device in them called a
rectifier that converts AC to
DC.

13. Rectifier
● Electrical components
called diodes allow
electricity to flow in one
direction only.
● Using 4 diodes allows the
negative half of AC to be
flipped positive.
● A capacitor stores energy
and releases is slowly,
smoothing out the bumps in
the AC.

14. Why are Transformers useful for
Society?
● Transformers allow for voltage to be changed at
will to suit a particular application.
● Transformers increase efficiency for power
transmission.
● The use of transformers allows more efficient
AC generators to be used.
● The output of a transformer can be converted to
DC as required using only 5 components.