Modern society is geared to using electricity.
Electricity has characteristics that have made it uniquely suited for
powering a highly technological society. There
are many energy sources that can be readily converted into electricity.
In Australia, most power plants burn a fuel such as coal or use the
energy of falling water to generate electricity on a large scale.
Electricity is also relatively easy to distribute.
Electricity authorities use high voltage transmission lines and
transformers to distribute electricity to homes and industries around each
state. Voltages from power stations
can be as high as 500 000 volts but by the time this reaches homes, the
electricity has been transformed to 240 volts.
While it is relatively economical to generate electric power at a steady
rate, there are both financial and environmental issues that should be
considered when assessing the long-term impact of supplying commercial and
The design of a motor for an electrical appliance requires
consideration of whether it will run at a set speed, how much power it must
supply, whether it will be powered by AC or DC and what reliability is required.
The essentials of an electric motor are the supply of electrical energy
to a coil in a magnetic field causing it to rotate.
The generation of electrical power requires relative motion
between a magnetic field and a conductor. In
a generator, mechanical energy is converted into electrical energy while the
opposite occurs in the electric motor.
The electricity produced by most generators is in the form
of alternating current. In general,
AC generators, motors and other electrical equipment are simpler, cheaper and
more reliable than their DC counterparts. AC
electricity can be easily transformed into higher or lower voltages making it
more versatile than DC electricity.
increases students’ understanding of the applications and uses of Physics and
the implications for society and the environment.
Some internet browsers (eg Firefox) do not
accurately display text symbols such as Greek letters used to represent
quantities in Physics. For example, capital delta is displayed as D and lower
case phi as f in Firefox. This is just something to be aware of in case you do
come across such issues. The square root sign is another one not displayed
properly by some browsers. Any symbols used in equations produced by equation
editors will of course display properly.
Magnetic Flux Density Vector
measure of the strength of a magnetic field is the Magnetic Flux
Density Vector, B. This
is also called the Magnetic Induction Vector. The higher the value of B, the
stronger the magnetic field.
The direction of the B vector at a point in space is the direction
of the magnetic field at that point.
The SI Unit for B is called the tesla (T). Most magnetic fields are much
smaller than 1T. A
mathematical definition of B will be given later.
Moving Charges In A Magnetic Field
A moving electric charge carries with it an associated
magnetic field. Thus, an
electric charge moving through a magnetic field experiences a force, due
to the interaction of the two magnetic fields present. The size of this force is given
F = q
Where q = size of charge, v = velocity of charge
perpendicular to the field and B = magnetic flux density vector.
If the charge enters the field at an angle q to the field direction, instead
of perpendicular to it, we must use the component of v that is at
right angles to the field direction.
Thus the formula becomes:
F = q
v B sin q
The direction of the force on a charge in a magnetic
field may be determined by using Fleming’s Left Hand Rule. Hold the thumb, first finger and
second finger of the LEFT hand mutually at right angles. Point the first finger in the
direction of the magnetic field.
Point the second finger in the direction of conventional current
flow (ie in the direction of flow of positive charge). The thumb then points in the
direction of the force on the charged particle.
Although the theory above on "moving charges
in magnetic fields" is not actually in the Syllabus for this topic -
it appears in the "From Ideas To Implementation" topic - I have
chosen to do it here since it makes good sense as an introduction to the
following section on "forces on current-carrying conductors".
Magnetic Force On Current-Carrying Conductors
Consider a conductor of length L, sitting in a magnetic
field of flux density B and carrying a current I, as shown below:
Clearly, a current is simply a flow of charge
and since all moving charges experience a force when travelling through a
magnetic field, a conductor carrying a current through a magnetic field
will also experience a force.
The size of this force can be shown mathematically to be:
F = B
I L sin q
is the angle made by the conductor with the magnetic field. See below.
Note that the magnitude of the force on a current-carrying
conductor depends on:
The strength of the magnetic field in which it is located
(indicated by the size of the magnetic flux density vector);
The magnitude of the current in the conductor;
The length of the conductor sitting in the external magnetic
The angle between the direction of the external magnetic field and
the direction of the length of the conductor.
The direction of the force on a current-carrying
conductor sitting in a magnetic field is found by Fleming’s Left Hand
Rule, as previously described.
It should also be clear that neither a charge travelling parallel
to a magnetic field nor a current-carrying conductor lying parallel to a
magnetic field will experience any force due to the field, since for both,
sinq = 0.
Try this java demonstration:
Use Fleming's LHR to predict the direction of the force
on the conductor in the demo.
Parallel Current-Carrying Conductors
Consider two very long, straight, parallel,
current-carrying conductors as shown below:
The magnetic field produced by the current flowing
through conductor 1 will pass through conductor 2. Thus, all of the charges flowing
through conductor 2 will be flowing through a magnetic field and will thus
experience a force. The same
argument can be applied to deduce that conductor 1 will also experience a
The size of the force F acting on each of the
conductors is given by:
where k = m0/2p = 2 x 10-7 SI Units,
I1 & I2 are the currents in conductors 1 & 2
respectively, d is the distance between the conductors and l is the common
length of the conductors. (m0 is the
permeability of free space and is a measure of the ability of free space
to support a magnetic field.)
Fleming’s LHR can be used to show that:
When the currents in the conductors are in the SAME direction, the
force between the conductors is ATTRACTIVE.
When the currents in the conductors are in OPPOSITE directions,
the force between the conductors is REPULSIVE.
Torque in Current Loops in a Magnetic Field
A torque is defined as the
turning moment of a force.
The torque about an axis of rotation is the product of the
perpendicular distance of the axis from the line of action of the force
and the component of the force in the plane perpendicular to the axis. See
the diagram below.
For the situation above, the
torque t on the bar about the
t = F d
Consider a rectangular coil
of wire carrying a current I and sitting in a magnetic field of flux
density B, with its plane parallel to the field direction, as shown
Therefore, the coil turns
under the action of an applied net torque, with CD coming up out of
the page and AB going down into the page. Once the coil has passed through
the position where its plane is perpendicular to the field direction, the
direction of the net torque is reversed. (Use Fleming’s LHR to verify this
for yourself – remember the current direction in the coil stays the same
throughout.) Thus, the coil
will eventually stop and then turn in the opposite direction. And so the motion will
This tendency of a
current-carrying loop to turn whenever it sits in a magnetic field is
called the “motor effect”.
The size of the torque on a
coil of n turns of wire may be shown to be:
t = B I A n
B = magnetic flux density of the field, I = current flowing in coil, A =
area of coil, n = number of turns of wire in coil and q
= initial angle made by plane of coil and the B field direction.
that when q
= 90o (coil perpendicular to field direction), no torque exits
since then F and d are both in the same
The DC Electric Motor
One simple application of the motor effect is the DC
electric motor. A simple
electric motor consists of a current-carrying loop situated in a magnetic
field, with its plane initially parallel to the field direction. Clearly, for the loop to continue
to rotate in one direction, the current running through the loop must
reverse direction just as the loop reaches the position where it is
perpendicular to the field direction. A split ring commutator is
used to achieve this reversal of the current direction.
The split ring commutator is attached to the loop and
conducts current into the loop by rubbing against the brushes. The brushes are usually carbon
rods that carry current from the external power source to the
commutator. See the diagram
below (note that it has not been drawn to scale – commutator has been
drawn larger than is actually the case):
The split ring is arranged so that each half of the
commutator changes brushes just as the loop reaches the position where its
plane is perpendicular to the field direction. Changing brushes reverses the
current in the loop. As a
result, the direction of the force on each side of the loop is reversed
and the loop continues to rotate in the same direction. This process is repeated each
half-turn. Thus, the loop
spins in the magnetic field.
In practice, electric motors have several rotating
loops. Together they make up
the armature (or rotor) of the motor. The magnetic field in which the
armature sits is called the field structure (or stator) of the
motor. This can be produced either by permanent magnets as in
the simple case shown above or more usually by current-carrying
coils called field coils wound around iron cores called pole
pieces. These sit
opposite one another inside the motor frame.
Try this java demonstration:
The above demo gives a very good view of a split-ring
commutator in action. It clearly shows how the commutator reverses
the current in the coil every half cycle.
Also, have a look at this link on
how to build a simple working DC
The entire group of field lines that flow out of the
N pole of a magnet constitute the flux of the magnet, represented by f (phi). The SI Unit of magnetic flux is
the weber (Wb). Most
magnets have flux values in the microweber range (mWb).
Two magnets are of equal strength if they have the same
flux (the same total number of lines emerging from their N poles). But if the area of the pole face
of one magnet is half that of the other, then the concentration of lines
of force must be twice as great in the magnet with the smaller pole
face. This degree of
concentration of flux is what we called earlier the flux density,
So clearly B = f /A.
Thus, if a uniform magnetic flux density B, extends
over an area A, the magnetic flux is given by:
f = B A
The Discovery of Electromagnetic
term “electromagnetic induction” refers to the creation of an
electromotive force (voltage) in a conductor moving relative to a
magnetic field. The effect
was discovered by the British scientist Michael Faraday
1831, Faraday discovered that moving a magnet near a wire induces an
electric current in that wire. In one experiment he showed that
when a permanent magnet moved towards a coil of wire connected to a
sensitive galvanometer, a current was induced in one direction in the
coil. When the magnet was
stationary or inside the coil, no current flowed through the coil. When the magnet was removed from
the coil, another current was induced in the coil, this time in the
opposite direction to the original induced current. Faraday reasoned that the presence
of an induced current implied the presence of an induced electromotive
force (emf) that caused the current.
further experiments Faraday showed that an induced emf and a corresponding
induced current was produced whenever there was relative motion between
the magnet and the coil. He also showed that this induced
emf & current were proportional to:
The relative velocity of the magnet and
The strength of the magnet;
The number of turns of wire per unit
the same year Faraday demonstrated the induction of one electric current
Faraday’s Law of Electromagnetic
eventually deduced from his experiments that an emf was induced in the
coil, only when magnetic field lines were being cut by the coil. Faraday’s Law of
Electromagnetic Induction states that: An emf is induced whenever a coil
or circuit experiences a change of magnetic flux with time and the
magnitude of the emf depends on the rate of change of the magnetic flux
through the coil or circuit.
for a conductor of N turns of wire, cutting through a magnetic flux of
in a time of Dt,
the emf e,
induced across the ends of the conductor is:
(Although this equation is not
required by the syllabus, it is useful in understanding Faraday’s Law and
of Induced emf
Let us now consider how an induced emf originates. The
diagram below shows a magnetic field directed down into the plane of the page.
A copper wire is being moved to the right through this magnetic field at
a constant velocity v.
Since the copper wire contains many free electrons, these
electrons are literally moving to the right through the magnetic field.
Therefore we have a situation where charged particles, electrons, are
moving through a magnetic field. We
know from our earlier work that whenever this happens the charged particles
experience a force. The size of this force is F
Fleming’s Left Hand Rule gives the direction of this
force. Applying this rule to the
motion of the electrons we have: field direction (index finger) down into the
page, conventional current direction (2nd finger) to the left of the
page (since the electrons are moving to the right) and therefore the direction
of the force on the electrons (indicated by the thumb) is down towards the
bottom of the page, as indicated by the arrow inside the copper wire in the
Thus, in the above situation, electrons will move down to
the bottom end of the wire making that end negatively charged and leaving the
top end of the wire positively charged. It
is this charge separation between the ends of the wire that creates the emf or
potential difference between the ends of the wire.
If the wire were moved through the field whilst being attached to an
external circuit, it would act as a battery for the circuit, supplying current
that would flow around the circuit.
Direction of Induced emf – Lenz’s Law
Lenz’s Law states that the
direction of an induced current is always such that the changes causing
the induction are opposed. In
other words, an induced emf always opposes the changes that caused it.
remind us of this fact, a minus sign is included in the equation for
Lenz’s Law is really a consequence of the
conservation of energy law, since if the induced emf did not oppose
the changes that caused it, then it would be possible to create a
self-perpetuating energy supply.
The Second Law of Thermodynamics proves that such an energy
supply is impossible.
Back emf in
Law can be used to explain an interesting effect in electric motors. In an electric motor, a current
supplied to a coil sitting in a magnetic field causes it to turn. However, while the coil of the
motor is rotating, it experiences a change in magnetic flux with
time and by Faraday’s Law an emf is induced in the coil. By Lenz’s Law this induced emf
must oppose the supplied emf driving the coil. Thus, the induced emf is called a
back emf. As the coil
rotates faster, the back emf increases and the difference between the
constant supplied emf and the back emf gets smaller. Clearly, this difference between
the two emf’s is equal to the potential difference across the motor coil
and hence determines the actual current in the coil.
It is interesting to note that when the motor is first
turned on and the coil begins to rotate, the back-emf is very small, since
the rate of cutting flux is small.
This means that the current passing through the coil in the forward
direction is very large and could possibly burn out the motor. To ensure that this does not
happen, adjustable starting resistors in series with the motor are often
used, especially with large motors.
Once the motor has reached its normal operating speed, these
starting resistors can be switched out, since by then the back emf has
reached a maximum and has thereby minimised the current in the coil.
Note also that if the load on the motor is increased at
some time, the motor will slow down, reducing the back-emf and allowing a
larger current to flow in the coil.
Since torque is proportional to current, an automatic increase in
torque will follow an increase in load on the motor.
When a solid conductor is placed in a
region of changing magnetic flux, circular eddy currents are induced
in the conductor. Lenz’s Law can
be used to explain the direction of flow of eddy currents in particular cases.
Consider the case below. A
uniform magnetic field is set up by placing a magnetic north pole above the
plane of this page and a south pole below the plane of the page.
The magnetic field direction is therefore down into the page as shown.
Next a copper sheet originally sitting stationary in the magnetic field
is pulled out of the field in the plane of the page as shown below.
By Faraday’s Law, eddy currents will form where there is
a change of flux with time, that is along the right hand edge of the
field where the metal sheet is leaving the field.
By Lenz’s Law, the induced eddy
currents must oppose the change that caused them.
That is they must oppose the relative motion between the conductor and
the magnetic field.
The easiest way to determine the
direction of the currents is to ask which direction of the current around the
loop would produce a magnetic field that will oppose the motion of the
conductor. Clearly, the clockwise
direction of the eddy currents as shown in the diagram would produce a south
pole inside the loop on this side of the metal sheet and a north pole inside the
loop on the other side. The south
pole on this side of the metal sheet would be attracted to the north pole of the
magnet (on this side of the sheet) producing the original field.
Likewise the north pole created by the eddy currents on the other side of
the sheet will be attracted towards the south pole of the magnet on the other
side of the sheet. The net effect is that there is an attractive force that
acts on the copper sheet and opposes the motion of the sheet to the right.
As usual in Physics there are many
different ways to reach the same conclusion.
A variation on the above is to realize that as the conductor is pulled
out of the field, the magnetic flux passing through the conductor at the right
hand edge of the field changes from a particular value to zero.
Ultimately then, the change at work here is the disappearance of field
lines along the right hand edge of the field as the conductor is pulled out of
the field. So, the eddy currents
will be in such a direction as to create a magnetic field that strengthens the
original field by putting new field lines in to replace those that are
disappearing. Thus, the
direction of the eddy currents must be clockwise as shown above.
We can easily verify the direction of the
eddy currents in the above example by using Fleming’s LHR.
Clearly, if the eddy current in the magnetic field is moving up towards
the top of the page and the field direction is down into the page, by
Fleming’s LHR, the force on the metal sheet is towards the left, opposing its
motion to the right. Thus, the
eddy currents are in a direction such that the changes causing
them are opposed.
The current Syllabus asks you to “explain
the production of eddy currents in terms of Lenz’s Law”.
note that to explain the production of eddy currents you need both Faraday’s
Law and Lenz’s Law. Note
also that while the methods used in the example above can be applied to solve
most cases, there are several other ways to determine the direction of eddy
currents. Another one using
Lenz’s Law as a starting point is as follows.
Pick a point on the metal surface in the
magnetic field but close to where the field ends. Since the eddy current must oppose the motion of the metal
sheet (by Lenz’s Law), the eddy current will cause a force on the sheet in
the opposite direction to the motion of the sheet.
So, the force is back towards the left, the field is down into the page
& therefore by Fleming’s LHR, the eddy current must move up towards the
top of the page. Since the eddy
current forms at the boundary of the magnetic field (ie where the magnetic flux
changes from a particular value to zero), the eddy current will form in a
clockwise direction in this case, as shown.
In other words, the circle must come out of the field, not go further
back into the field.
Note that since eddy currents oppose the
motion of the conductor in which they flow, they can be used for electromagnetic
braking purposes. Examples
include the EM braking used in the manufacture of electronic balances and in
Theme Park rides such as The Giant
Drop at Dreamworld on the Gold Coast. Eddy
currents can also be used to produce heat in induction cooktops and induction
important application of electromagnetic induction is in the generation of
electric current. AC
generators produce an electric current via the motion of coils in a
magnetic field or by rotating a magnet within a stationary coil. The term alternator is also
often used interchangeably with the term electric generator. Strictly speaking, an alternator
refers to an electromagnet rotating inside a fixed coil, such as is the
case in most power stations. Consider the diagram of a simple AC generator
Clearly, the main
components of a generator are:
The armature – a coil wound around a metal core and
mounted between the poles of an electromagnet.
The electromagnet consisting of an iron core surrounded by
a set of coils called the field windings.
A steady current flows through these coils to produce the required
The slip rings – each end of the armature coil is
connected to a metal ring. These
rings are mounted on the armature shaft but are insulated from it and from each
The graphite brushes – these connect the slip rings to an
external circuit and conduct the current induced in the armature coil to the
The armature is mechanically driven by a steam
turbine or a belt & pulley system or by hydroelectric means. As the armature turns, one side
moves up through the magnetic field and the other side moves
downwards. The coil thus
experiences a change of magnetic flux with time. The result is that an emf is
induced in one direction in one side of the coil and in the other
direction in the other side of the coil. Thus, these emf’s act in the same
sense around the coil. The
ends of the coil are connected to slip rings against which rest graphite
brushes. When these brushes
are connected across an external circuit, the induced emf produces an
Each time the coil passes through the position where
its plane is perpendicular to the magnetic field lines, the direction of
the emf in the coil is reversed.
Hence an alternating current is produced at a frequency
equal to the number of revolutions per second of the armature.
An alternating current generator may be
converted to a direct current generator in a couple of
By using a split ring commutator instead of slip rings.
The split ring commutator is mounted on the armature shaft but is
insulated from it. The commutator
reverses the connections of the coil to the external circuit each time the
current in the coil reverses. Thus, a DC output is achieved from the AC generator.
By using a bridge rectifier circuit.
This is an arrangement of electronic components (diodes) that
converts the AC output from the generator to a DC output.
that in an electric current generator, mechanical energy is transformed
into electrical energy.
In an electric current motor, electrical energy is transformed
into mechanical energy.
Try these java demonstrations:
- may take a few seconds for the java applet to load
- may take a few seconds for the java applet to load
Effects of AC Generators
development of AC generators has had both positive and negative
effects on society and the environment. Firstly,
from a social point of view, electricity generation has allowed the
development of the highly mechanized and electronic lifestyles to which
people in the developed world have become accustomed. Our lives are made easier every
day by the use of vast numbers of electrical gadgets. Electricity runs our lighting, our
heaters & air conditioners, our computers and communications
equipment, our refrigerators, toasters, electric fry pans & stoves,
washing machines, vacuum cleaners, stereos, TV’s, garden equipment,
industrial equipment and so on – the list is almost endless. Unfortunately, because electricity
has greatly reduced the amount of physical labour necessary to live our
everyday lives, it has also resulted in negative social effects such as a
reduction in unskilled jobs and a reduction in the size of work-forces
needed to perform certain jobs, which have led to increased
in terms of the environment, the development of AC generators has had mainly negative effects. Most power generation stations
around the world still use fossil fuels as their energy source. Fossil fuel power stations produce
thermal pollution, acid rain and air pollution due to the release of
particulate matter and oxides of nitrogen and sulfur. Fossil fuel power stations release
huge amounts of carbon dioxide into the atmosphere, which adds to
the Greenhouse Effect, which is believed to be raising Earth’s
temperature. Fossil fuel
power stations also indirectly cause the land desecration and pollution
associated with the coal mining, necessary to maintain supply of fossil
Physiological Effects of HV Power
Lines - Not Examinable
Some epidemiological studies
have suggested that a link may exist between exposure to power-frequency
electric and magnetic fields (EMFs) and certain types of cancer, primarily
leukaemia and brain cancer. Other studies have found no such link.
Laboratory researchers are studying how such an association is
biologically possible. At this point, there is no conclusive scientific
evidence of a link between the electromagnetic radiation produced by high
voltage power lines and physiological conditions such as leukaemia or
cancer. Scientists agree
that better information is needed.
The AC Electric Motor
An AC electric motor consists of two main parts:
The armature or rotor – usually cylindrical that rotates
about the axis of the motor’s shaft. The
rotor usually completes one revolution for each cycle of the AC electricity
The field structure or stator – this is the
stationary part of the motor usually connected to the frame of the machine. It supplies the external magnetic field in which the rotor
sits and which produces the torque on the rotor.
Both the rotor and
stator have a core of ferromagnetic material to enhance the magnetic
field. This core usually
consists of thin laminations of steel separated by thin insulating layers
to reduce the size of induced eddy currents that would reduce the
efficiency of the motor.
In some AC motors slip rings are used to conduct
electricity to and from the motor.
Note that AC motors (with a few exceptions) do not use split
AC motors can be classified as either
single-phase or polyphase motors. Single-phase motors run on only
one of the three phases of current produced by power stations and can
therefore operate on the domestic electricity supply. Polyphase motors run on two or
three of the phases of current produced by power stations and are mainly
used for high power applications including heavy industry.
We will now consider single-phase AC induction
motors as an example of AC motors.
The Single-Phase AC Induction
The single-phase AC induction motor is the most
common AC motor in use today.
A changing magnetic field in the stator induces an AC
current in the rotor. The
current in the rotor produces its own magnetic field, which then interacts
with the magnetic field of the stator, causing the rotor to turn. Clearly, the name induction motor
comes from the fact that no current is fed directly to the rotor from the
mains supply. Current is
induced in the rotor by the changing magnetic field of the
The rotor of an induction motor consists of a
cylindrical arrangement of copper or aluminium conducting bars attached to
two end rings at either end of the bars. These end rings short circuit the
bars and allow current to flow from one side of the cylinder to the
other. This type of rotor is
usually referred to as a squirrel cage, owing to its resemblance to
the cage or wheel that people use to exercise pet squirrels or mice. See diagram below.
The squirrel cage fits into a laminated iron
core or armature, which is mounted on the shaft of the motor.
The stator consists of a number of coils of wire
wrapped on laminated iron cores.
The stator surrounds the rotor. Single-phase alternating current
flowing through the stator coils produces a changing magnetic field that
threads through the rotor.
This changing magnetic field induces an alternating current in the
rotor, which in turn sets up its own changing magnetic field. Various special techniques beyond
the scope of this course are used to ensure that the changing magnetic
field produced by the stator actually rotates and drags the magnetic field
of the rotor around with it.
Thus, the rotor rotates in the same direction as the rotating field
of the stator.
remaining notes on AC Induction motors are not required by the current
Syllabus. They have been left on this site as interesting extension
Single-phase AC induction motors tend to be low
power motors. There are a
couple of reasons for this.
Firstly, as with all induction motors, current is not fed directly
to the rotor from the mains supply, it is induced by the changing magnetic
field of the stator. Less
energy reaches the rotor than would be the case if the mains supply was
connected directly to the rotor.
Secondly, the rotating stator field in a single-phase induction
motor is irregular and is not of constant strength as in a polyphase
induction motor. Therefore
the torque developed by a single-phase induction motor is irregular or
pulsating, which results in more energy being wasted as vibration and
sound than occurs in a polyphase induction motor.
Due to their
inherent low power, AC induction motors are used in applications such as
power tools (drills, saws, sanders etc), electric kitchen implements
(beaters, food processors etc) and other household appliances (hair
dryers, fan heaters etc).
Though not in the syllabus, it is worth noting that not
all induction motors are low power motors. Polyphase induction motors
can be much more powerful than their single-phase counterparts and are
often used in high power applications including heavy industry. Part of their power advantage
comes from the fact that it is much easier to establish stable,
constant-strength, rotating stator fields in polyphase (especially
three-phase) induction motors than it is in single-phase motors.
AC induction motors have several advantages over
other types of motor.
- Simplicity of design – simple & cheap to construct;
- Reliability – they have no brushes or commutators and there
is little friction to wear parts away;
can be built to suit almost any industrial requirement;
are economical and efficient to run for most purposes;
- Polyphase induction motors are self-starting – that is, when
you turn the AC source on, the motor starts to work. (Single-phase induction motors are
not self-starting and therefore require some special starting means.)
AC induction motors have some disadvantages as
only work on AC;
maximum speed is limited by the supply frequency (a 50 Hz supply limits
the motor to about 3000 rpm);
- Starting torque is low – they do not get heavy loads moving
are not as efficient as some other AC motors when used in heavy industrial
AC induction motors form one of three general
classes of single-phase AC motor.
For the sake of completeness, the other two classes of
single-phase AC motors are:
Commutator motors – the AC series motor, the
repulsion motor and the repulsion-induction motor. The universal motor,
capable of working on either AC or DC electricity, is an example of an AC
series motor. Universal
motors are used in vacuum cleaners, sewing machines, food mixers and
Synchronous motors – when using fixed frequency AC
these motors maintain constant speed and are therefore used in clocks and
other devices requiring a constant rate of rotation.
A transformer is a device
in which an input alternating current produces an output alternating
current of different voltage.
A step-up transformer results in an increased voltage. A step-down transformer
results in a decreased voltage.
The transformer itself
consists of two separate coils, a primary and a secondary,
usually wound around a soft iron core, to intensify the magnetic
field in the primary. The
transformer works on the principle of mutual induction. An alternating current flows in
the primary, thus creating a changing magnetic field that threads through
the secondary. Thus, there is
a changing magnetic flux in the secondary coil, which produces a current
in that coil. Since the
magnetic field is changing at a given frequency, the current induced in
the secondary coil is also an alternating one.
If the secondary coil
contains more turns of wire than the primary, the transformer will be a
step-up one. If the
secondary coil contains less turns of wire than the primary, the
transformer will be a step-down one. A step-up transformer is
where VP = primary
voltage, VS = secondary voltage, NP = number of
turns of wire in primary coil and NS = number of turns of wire
in secondary coil.
Derive this last
By the principle of self-induction, the changing B field in the primary
also gives rise to a back-emf and a back current in the primary that by
Lenz’s Law opposes the original voltage. The induced current in the
secondary coil has an alternating magnetic field associated with it, which
strengthens the back-emf in the primary coil.
In an ideal
transformer no energy is lost and so the energy input is the same as
the energy output per unit time.
We can write this as:
Power in = Power out
IP = VS IS
From which we
This formula implies that in
a step-up transformer, although the output voltage is higher than the
input voltage, the output current is lower than the input current. This is a direct consequence of
the conservation of energy law. Clearly, if the output current did
not decrease compared to the input current, VS IS
would be greater than VP
IP and energy would have been created from nothing.
Similarly for step-down transformers, the output voltage is lower than the
input voltage but the output current is higher than the input current.
From a practical point of
view, transformers need to be designed carefully. There are many sources of
potential energy losses in transformers, heat losses in the coils, heat
losses in the core due to eddy currents and possible magnetic
leakage. The eddy currents in
the core are induced by the changing magnetic fields threading through the
transformer. To reduce the
effect of eddy currents, the core is made of layers called laminations
separated by thin insulating layers, rather than being one solid block of
Modern transformers operate
with around 99% efficiency and have many practical applications. Perhaps the most important
application is in the transfer of electrical energy from a power station
to its point of use.
Energy losses in power
transmission lines can be shown to be proportional to I2R. Clearly, the lower the current,
the lower the energy loss in the line. For this reason, the 240 V power
produced by the generators at a power station is stepped up by
transformers to very high voltages (220 kV to 500 kV) before being
transmitted from the power station. In this way the current flowing in
the transmission lines is very low and energy loss in the lines is
minimised. (Also, lower
currents mean that smaller diameter transmission lines can be used, which
leads to savings in materials and construction costs.)
In order to be used by the
various consumers, the voltage needs to be stepped down to the required
value. This happens at
electricity sub-stations, where transformers step the high voltage down to
240 V for domestic use or other particular values for industry and public
transport (eg electric trains).
Transformers are also used in
certain electrical appliances in the home that are connected to the mains
domestic power supply. For
example, transformers in
the 1 to 100 watt power level are often used as step-down transformers to
couple electronic circuits to loudspeakers in radios, television sets, and
Hi-Fi equipment. In
many electronic devices a number of different voltages are required for
Transformers are used to convert the 240 V mains supply voltage to
the required voltage. This
can be achieved by having several secondary coils wrapped around the
primary or by having one secondary coil and tapping into it after the
appropriate number of turns of wire.
IMPACT OF TRANSFORMERS
Transformers have had a
major positive impact on society. Transference of electrical energy
with low levels of loss, over huge distances between power stations and
consumers is only possible due to the existence of transformers. If electrical energy could not be
transmitted efficiently over large distances many more power stations with
their associated pollution would be necessary to supply power to their
Transformers enable power
from the one power station to be used in many different applications. Sub-stations can step the power
down in stages to the various levels required by households, industry,
public transport and so on.
Without transformers, different industries requiring different
voltages would have to build generators to produce those specific
The existence of transformers
has enabled the construction of many of the electronic labour-saving and
entertainment devices we take for granted. TV’s, computers, mobile
phones, stereos, radios, electronic clocks, many kitchen appliances and
countless other electronic devices require transformers for their
Plan, chose equipment or resources for and perform a first-hand
investigation to demonstrate the production of an alternating
Gather secondary information to compare advantages and disadvantages of
AC and DC generators and relate these to their use.
Analyse secondary information on the competition between Westinghouse and
Edison to supply electricity to cities. (Homework
- see my Useful Links page for several
Gather and analyse information to identify how transmission lines are:
Insulated from supporting structures; and
Protected from lightning strikes.
(Homework - see my Useful Links page for several
Perform an investigation to model the structure of a transformer to
demonstrate how secondary voltage is produced.
Gather, analyse and use available evidence to discuss how difficulties of
heating caused by eddy currents in transformers may be overcome.
Gather and analyse secondary information to discuss the need for
transformers in the transfer of electrical energy from a power station to its
point of use. (Homework)
Perform an investigation to demonstrate the principle of an AC induction
motor. (In class)
Gather, process and analyse information to identify some of the energy
transfers and transformations involving the conversion of electrical energy into
more useful forms in the home and industry.
A step up transformer has a primary coil of 600 turns and a secondary
coil of 1200 turns. If the voltage
across the primary coil is 12 V, determine the voltage across the secondary.
An electrician uses a transformer to convert the 240 V mains supply to 12
V. State the ratio of the number of
turns in the primary to the number of turns in the secondary of the transformer
that will do the job. (20:1)