Transformer
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Application
• A lamp is connected to a mains supply of 240
V. However, its light bulb is supplied by a
voltage of only 12 V.
• The mains voltage has been reduced by a
device called a transformer.
• The transformer is situated in the base of the
lamp.
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What is transformer ?
• A transformer is a device used for changing
the voltage and size of an alternating current.
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The structure of a transformer
• It consists of a core made of ferromagnetic material
and two coils – a primary and a secondary coil.
• The primary coil is connected to the voltage whose
magnitude we want to change.
• The altered voltage appears at the ends of the
secondary coil.
coil
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The core of a transformer
• The operation of a
transformer is based on
the phenomenon of
mutual induction.
• The alternating current,
which flows through the
primary coil, induces an
alternating current in the
secondary coil.
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Higher voltage but lower current
• When a transformer
increases the voltage it
simultaneously
decreases the current,
and vice versa.
• This implies that the
power of the current
flowing through the
secondary coil can at
best be equal to the
power of the current
flowing through the
primary coil.
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• In reality it is usually a few percent lower.
• Assuming that the efficiency of a transformer
is almost 100%, we can denote it as:
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The efficiency of a transformer
• The efficiency of a transformer is defined as
the ratio of the power obtained from the
secondary coil to the power supplied to the
primary coil:
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Increase the efficiency of transformer
• In order to minimise the losses that could
cause eddy currents, the core is made of thin
plates, which are isolated from one another
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• In the coil of the transformer through which a
higher current is flowing, the wire used is
thicker.
• This makes it possible to prevent excessive
heating of the coil.
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Power in a Parallel Circuit
• the total power consumed by the parallel
circuit is:
Since the total current and source voltage
are known, the total power can also be
computed by: yschow@smkbpj(a)
Power in a Series Circuit
• In a series circuit the total power is equal to
the SUM of the power dissipated by the
individual resistors.
• PT = P1 + P 2 + P3 . . .Pn
To check the answer, the total power delivered by the
source can be calculated:
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Generating electricity
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Energy
• Renewable energy • Non-renewable
sources : an energy sources: an energy
resource that is resource that cannot be
continually replaced replaced once it has
and will not run out.
out been used.
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Fossil fuel power stations
• Most of the world's electricity supplies are
generated by largepower stations which burn
fossil fuels: coal, oil and gas.
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The National Grid system
• National grid= A network of power stations, transformer
stations and transmission lines.
Power station high voltage station Extra high voltage line
Step-down transformer station high voltage transmission line Step-up transformer station
Midium voltage transmission line Low- voltage pylon transformer high voltage station
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• Electrical energy is transmitted at a high
voltage and use alternating current.
• A step-up transformer which increases the
voltage to as high as 110,000 or 132, 000 V.
• Step-down transformers are used to decrease
the voltage before being delivered to the
consumers.
• The long tick cables used as transmission lines
are made of copper or aluminium
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Power lost in cable
• The heating effect due to the resistance of the
cables causes loss of electrical energy as heat
energy.
• When a current flows in a cable, the power
loss, through heating is where R is the
resistance of the cable.
• The power loss can be reduced by
• (a) reducing the resistance of the cables
• (b) reducing the current in the cables
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• In practice, the energy that is supplied by a
power station to the town increases in voltage
and thus the magnitude of the current in the
conductors decreases.
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