# Transformers PPT

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Current and magnetism
Every electric current produces a magnetic field. The shape
and strength of the magnetic field depends on the shape of
the wire carrying the current. A single straight wire carrying a
direct current is surrounded by a circular magnetic field:
Every point on an infinite wire is
equivalent to every other, so the
magnetic field must be the same at
every point – it is made up of
concentric circles.
A much stronger magnetic field can be
made by twisting a wire into a tight coil,
or solenoid. This creates a magnetic
field like that of a bar magnet.
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Field around a wire
The direction of the magnetic
field around a straight wire can
be worked out by using the
right hand grip rule.
–
Grip a wire so that your thumb
points in the direction of the
conventional current (from the
positive to the negative terminal
of a battery).
Your fingers will curl around the
wire in the direction of the
magnetic field (from north to
south pole).
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+
Field around a solenoid
The right hand grip rule can
also be used to find the
orientation of the magnetic
field around a solenoid:
–
N
Grip the solenoid so that your
fingers follow the direction of the
conventional current.
Your thumb will now point towards
the north pole of the electromagnet
created by the solenoid.
+
S
The electromagnet can be made stronger by increasing the
number of coils, or by adding an iron core.
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Inducing current in a coil
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Electromagnetic induction
What do we know so far about the relationship between current
and magnetism?
 All currents have a magnetic field associated with them.
 A wire in a changing magnetic field will experience an
induced current.
The first of these effects is the basis of the electromagnet.
The second effect is called
electromagnetic induction, or the
dynamo effect. It converts
movement into electrical energy.
This is the basis of the generator.
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Electricity and magnetism
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Linking circuits with magnetism – results
In the experiment, a current was induced in the second circuit
when the first circuit was switching on or off. In order for power
to be transferred continuously between two circuits, the current
in the first circuit must be changing continuously.
This can be achieved by using an alternating current.
In order for as much power to be transferred as possible, the
two circuits must be as closely magnetically linked as possible.
This can be achieved by
winding the two circuits into
tight coils around an iron
core. This is a transformer.
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Primary side – how it works
A transformer links two circuits together. To understand how it
works, it is important to look at each side separately.
The primary side is simply an electromagnet. By passing an
electric current through a coil of wire, we make a magnetic
field, just like the field around a bar magnet.
Direct current makes
one end of the iron
north, and the other
end south.
–
+
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N
S
Secondary side – how it works
The secondary side is not connected directly to any power
supply. It is just a piece of iron with some wire wrapped
around it.
The secondary side works using electromagnetic induction.
To make a current flow, a magnetic field needs to be
changing perpendicular to the coil.
When there is an alternating
current in the primary side,
the direction of the magnetic
field around the transformer
alternates. This induces a
second alternating current in
the secondary side.
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How a transformer works – summary
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Parts of a transformer
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Properties of transformers
Transformers transfer power between circuits.
The design of a transformer determines the characteristics of
the electricity flowing in its secondary circuit. The frequency
of the alternating current in the secondary circuit will always
match the primary circuit, but what about current and voltage?
The voltage in each circuit is related to the number of coils
on each side of a transformer by the following equation:
primary voltage
secondary voltage
Vp
Vs
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=
primary turns
secondary turns
=
Np
Ns
Step-up transformers
A step-up transformer is used to increase voltage. It has
more turns on its secondary side than on its primary side.
But the power in the
secondary circuit cannot be
greater than the power in the
primary circuit, or the
transformer would be more
than 100% efficient!
What is the relationship between power, voltage and current?
P=V×I
A step-up transformer increases voltage, but reduces current.
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Step-up transformer calculations
A transformer has 100 turns on its primary coil. It has an input
voltage of 35 V and an output voltage of 175 V.
How many turns are on the secondary coil?
Vp
Vs
Vs
Vp
Ns
Np
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=
=
=
Np
Ns
Ns
Np
Vs
Ns
Ns
=
Vs
× Np
Vp
=
175
× 100
35
=
500 turns
Vp
Step-up transformer uses
Step-up transformers are used in the following applications:
 power transmission
Step-up transformers are
used to increase the voltage
generated in power stations,
so that it can be transported
around the country at
extremely high voltages.
 using European appliances in the USA
The USA mains runs at 110 V, while the UK uses 230 V.
Goods made for the UK, but used in the USA, need a
transformer to increase their supply voltage.
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Step-down transformers
A step-down transformer is used to decrease voltage. It has
fewer turns on its secondary side than on its primary side.
This kind of transformer can
be found in many places
around the home, as a lot of
appliances use lower
voltages than the 230 V
provided by the National Grid.
A mobile charger, for
instance, contains a stepdown transformer, which is
why it is larger than a
normal plug.
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Step-down transformers calculations
A transformer has 200 turns on its primary coil and 50 turns
on its secondary coil. The input voltage is 920 V.
What is the output voltage?
Vp
Vs
Vs
Vp
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=
=
Vs
=
Vs
=
Np
Ns
Ns
Np
Ns
× Vp
Np
50
× 920
200
= 230 V
Isolating transformers
An isolating transformer has the same number of coils on
its primary and secondary sides.
A transformer has 100 turns
on the primary side, and
100 turns on the secondary
side. If the primary voltage
is 230 V, what is the
secondary voltage?
Np = Ns
Np
Ns
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=1
Vp
Vs
=
Np
Ns
=1
Vs = Vp = 230 V
Why use an isolating transformer?
Isolating transformers do not change the voltage of a power
supply. So what are they used for?
Isolating transformers are used in
devices such as electric shaver
sockets, to isolate an appliance from
the mains.
By separating a device, such as a
shaver, from its mains supply, the risk of
shock is much reduced. This is important
in a bathroom where electrical items are
at risk of getting wet.
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Transformers around the home
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Step-down transformer uses
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Transformers around the home
How many transformers can you find in this house?
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What is the National Grid?
The National Grid is a network of power lines designed to
carry mains electricity around the country, from the power
stations where it is generated to the homes and factories
where it is used.
Transformers are an important part of the National Grid,
because electricity must be transported at a much higher
voltage than it is generated at or used at in homes.
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Power loss in cables
When electrical energy is carried in wires, a current must flow.
There is a power loss in cables which is related to the
amount of current flowing:
power loss = current2 × resistance
P = I2 × R
 Power is measured in watts (W).
 Current is measured in amps (A).
 Resistance is measured in ohms (Ω).
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Power loss in cables – example
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Transformer power
A step-up transformer may
increase voltage but it cannot
create energy!
In a perfect transformer the
power in is equal to the
power out. As power = V × I,
if voltage goes up, then
current must go down.
primary
Vp
Ip
secondary
Vs
Is
power in = power out
Pp = Ps
Vp × Ip = Vs × Is
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Transformer power example
A transformer has a primary voltage of 1000 V and a primary
current of 0.5 A.
If the secondary circuit has a
primary
current of 0.01A flowing, what is
the secondary voltage?
Vp × Ip = Vs × Is
Vs = Vp × Ip
Is
= 1000 ×
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0.5
0.01
Vp
Ip
secondary
Vs
Is
= 50000 V
Step-up transformers in the National Grid
A step-up transformer is
positioned near a power
station.
This raises the voltage of
the generated electricity,
around the country.
High voltages are used because a high voltage results
in a low current flowing, for a fixed power.
A low current means the wires lose less energy as heat
over long distances.
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Step-down transformers in the National Grid
Step-down transformers are
positioned close to homes and
factories.
They are used to reduce the
voltage from the very high voltages
used for transmission.
High voltages are useful for saving
energy, but are very dangerous.
Household appliances need much
lower voltages, so the voltage is
reduced while the current increases,
for a fixed amount of power.
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The National Grid
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Glossary
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Anagrams
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Multiple-choice quiz
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