Download Transformer - Hafiz Physics

Transformer
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A transformer transforms (change) an alternating p.d. (voltage) from one
value to another of greater or smaller value.
It has primary and secondary coils wound on a complete soft iron core
Primary coil (the input coil) – the incoming
voltage, Vp
Iron core – link the two coils
Secondary coil (the output coil) – provide the
voltage Vs to the external circuit
How does the transformer induce an alternating voltage in the secondary coil?
When the alternating current flows through the primary coil, it sets up an
alternating magnetic field in the core and in the secondary coil.
This changing field induces an alternating voltage in the secondary coil.
Why don’t we use direct current (d.c.) instead of alternating current (a.c.) in the
primary coil?
Unless there is a changing current in the primary coil, no voltage is induced in the
secondary coil.
Connecting a d.c. supply to the transformer can damage the transformer. High
current flows through the primary coil which can make it overheat.
A simple transformer.
AC voltages can be increased or decreased using a transformer.
Suppose that the field lines pass through both coils, and the coils waste no
energy because of the heating effects, therefore the equation applies:
secondary voltage secondary turns

primary voltage
primary turns
Vs N s

Vp N p
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STEP-UP AND STEP-DOWN TRANSFORMERS
(a) Step-up Transformer
A ‘step-up’ transformer has more turns on the secondary than the primary and Vs
is greater than Vp.
(b) Step-down transformer
A ‘step-down’ transformer has fewer turns on the secondary than the primary and
Vs is less than Vp.
DESIGNS OF
TRANSFORMER
Both methods are design
to trap the magnetic field
in the core so that all the
field lines from one coil
pass through the other.
All transformer waste some energy because of the heating effects.
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Why there is a heating effect and how to reduce this heating effect?
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The coils are not perfect electrical conductors and heat up because of
resistance. To keep the resistance low, thick copper wire is used where
possible.
The core is itself a conductor, so the changing magnetic field induces currents
in it. These circulating currents have heating effect. To reduce them, the core
is laminated (layered): it is made from thin, insulated sheets of iron rather
than a solid block.
Large, well designed transformers can have efficiencies as high as 99%. In other
words, their useful power output is 99% of their power input.
Power
Assume that all electrical energy given to the primary is conserved and transform
to secondary, then
Power in primary = power in secondary
VpIp= VsIs
Exercise:
1. How does a step-up transformer differ from the step-down transformer?
2. Explain each of the following:
(a) the transformer will not work on DC
(b) the core of the transformer need to be laminated
(c) If the transformer increase voltage, it reduces current.
Prepared by Cikgu Muhd Hafizuddin@ SMSB2006
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