Transcript Transformers and their Role in Transmission

Science A
Physics – P1
Topic 2b :
Transmission of Electricity
Transformers
Michael Faraday
In 1831, Michael Faraday wrapped two
insulated coils of wire around a large iron
ring, attached to a chair. When he passed a
current through one coil, a momentary
current was induced in the other coil.
In this experiment, Faraday had developed
the first transformer.
Michael Faraday
1791 - 1867
Transformers
Transformers are made by wrapping two coils of wire around a
soft iron core. An alternating current can then be passed through
the primary coil. Transformers do not work with direct current.
The secondary coil is positioned near to the primary coil, so that
it can pick up the changing magnetic field in the iron core.
Transformers transfer energy continuously from the primary coil
to the secondary coil.
Transformers
As the changing field cuts through the wires of the secondary
coil, a voltage is induced across the secondary coil.
Both the size and direction of this induced voltage changes, as the
voltage applied to the primary coil changes.
This means that when the secondary coil is connected to a complete
circuit, an alternating current flows through the secondary coil,
and a similar current flows through the primary coil.
Step-up Transformers
A step-up transformer changes a low voltage to a high voltage.
Step-up transformers have a greater number of turns on the
secondary coil than on the primary coil.
Step-up transformers increase the voltage across the secondary
coil, compared to that across the primary coil.
Step-Up Transformers
Step-up transformers
All types of power stations use step-up transformers to substantially
increase the voltage before it is distributed.
Step-down Transformers
A step-down transformer changes a high voltage to a low voltage.
Step-down transformers have fewer turns on the secondary coil
than on the primary coil.
Step-down transformers decrease the voltage across the
secondary coil, compared to that across the primary coil.
Two Types of Transformer
It is the relative number of turns on the two coils that determines
whether the voltage induced in the second coil is greater, or less
than, the voltage in the primary coil.
A step-up transformer
A step-down transformer
Transformers - Uses
A transformer, such as this one, is used as part of the National
Grid, which delivers electricity to homes and industries.
These transformers work using the alternating current produced
by power stations.
Why Does the National Grid use High Voltages?
On the National Grid, turn up the voltage to see what happens to the
energy lost as heat. The power remains constant throughout.
Why Does the National Grid use High Voltages?
On the National Grid, turn up the voltage to see what happens to the
energy lost as heat. The power remains constant throughout.
Why Does the National Grid use High Voltages?
There are three factors to consider:
 Resistance (R)
 Voltage (V)
 Current (I)
The greater the resistance to the
flow of electricity, the more it is
converted into heat.
Heat is the form of energy by
which power is lost.
The formula for the power (P) loss
due to resistance is:
P = I2 x R
Power, Current & Resistance
If you know any two of Power (P), Current (I) or Resistance
(R), you can use the formula triangle to find the remaining
value...
P =
I2
x R
R = P
I2
I2 = P
R
Why Does the National Grid use High Voltages?
To transmit large amounts of electrical power around the National
Grid, you either use a high voltage or a high current.
In both cases the power output from the power station remains
constant - this is explained by the power equation:
P=IxV
For a constant power...
 When the current is high, the voltage is low.
 When the voltage is high, the current is low.
If electricity was transmitted from the power station with a voltage
of 25,000 V the current would be 8,000A.
As power loss due to resistance is calculated by P = I2 x R, this
would cause the cables in the National Grid to overheat.
Power, Current & Voltage
If you know any two of Power (P), Current (I) or Voltage (V),
you can use the formula triangle to find the remaining value...
P = I x V
V = P
I
I = P
V
Why Does the National Grid use High Voltages?
When the transmission voltage from the power station is increased
from 25,000 V to 400,000 V (increased by 16 times), the current
is reduced by the same factor.
This makes the current in the National Grid:
8,000 A = 500 A
16
Although this is still a relatively high current, the use of thick wires
reduces the resistance, so the heating effect is reduced.
The use of a high voltage and a low current allows electricity to be
transmitted around the National Grid with minimal power loss.
Two 100W light bulbs from Europe and America
are compared. What happens to the current if we
connect these lamps to the correct power source?
(i) a 230V supply, (ii) a 120V supply?
• i) 100W using 230V
• ii) 100W using 120V
power
current 
voltage
100 W

230 V
 0.43 A
power
current 
voltage
100 W

120 V
 0.83 A
A typical lighting circuit in a British house is
designed to take up to 5 A.
How many 100 W light bulbs could you run from it?
• 1 lamp take 0.43A
5A
Number of lamps 
 11.63
0.43A
 11, 100W lamps could be run on a 5A fuse
Safety and the National Grid
Safety and the National Grid
For safety reasons, the high voltage cables on electricity
pylons are hung from porcelain insulators.
Safety and the National Grid
At power stations and substations, remote switchgear is
used to switch on and off the flow of electricity. This helps
to protect the workers from the danger of electrocution.
Overhead Power Lines
Overhead power lines are very
dangerous. You do not have to
touch them to be electrocuted,
as the electricity can jump
through the air.
For your safety and that of
others, here are some simple
rules for you to follow...
Overhead Power Lines
 DO NOT climb a pylon or
touch power lines.
 DO NOT climb trees which
have power lines passing close
by them.
 DO NOT go fishing or fly kites
near power lines.
 DO NOT try to retrieve
anything stuck in power lines.