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Physics 1
Motors and Generators
Motors and Generators
Part 1: Motors and Magnetic Forces
The Motor Effect
The motor effect is the action of a force experienced by a current-carrying conductor in an
external magnetic field. The magnitude of the force on the current-carrying conductor is
affected by:
o The strength of the magnetic field in which it is located, with the force proportional
to magnetic field strength
o The magnitude of the current in the conductor, with the force proportional to
current
o The length of the conductor in the field, with the force proportional to length
o The angle between the conductor and the external magnetic field
 The force is at a maximum when the conductor is at right angles to the field
and zero when it is parallel to the field
Forces Between Two Parallel Conductors
A force between the conductors exists because the magnetic field due to the current in each
conductor interacts with that of the other conductor. If the parallel conductors carry current
in the same direction, they experience an attractive force. If they carry current in opposite
directions, they experience a repulsive force.
The magnitude of the force between the 2 conductors varies directly with the magnitude of
the currents in the wire and lengths of the wire, and inversely with the distance of
𝐹
𝐼 𝐼
separation between the conductors. Mathematically, this is represented as 𝑙 = 𝑘 1𝑑 2 .
Torque
Torque is the turning effect of a force. It is the product of the tangential component of the
force and the distance the force is applied from the axis of rotation. Mathematically, this is
represented as 𝜏 = 𝐹𝑑.
Forces Experienced by a Current-Carrying Loop
In position (a), side LK experiences a force that is vertically
upwards, while side MN experiences a force of equal
magnitude that is vertically downwards, causing the coil to
turn in a clockwise direction. As the coil is parallel to the
magnetic field, torque is at its maximum.
When the coil is perpendicular to the magnetic field as in
position (b), the forces are approximately in line, so they
approximately cancel out and the torque is zero, but the
coil’s momentum keeps it rotating.
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In position (c), the direction of the forces have been inverted, as the direction of the current
has changed, allowing the coil to keep rotating in the same direction. In position (d), torque
is at a maximum again, but side MN has the upward force and side LK has the downwards
force.
Features of a DC Electric Motor
Feature
Curved permanent
magnets or
electromagnet (stator)
Armature
Rotor coils
Split ring commutator
Brushes
Axel
Function
Provides the external magnetic field for the current carrying wire
to experience a force. A radial magnetic field allows the torque to
be a maximum for longer.
Frame around which the coil of wire is wound and rotates in the
magnetic field. It is usually made of ferromagnetic material to
concentrate the magnetic field.
Wound onto armature and ends connected to bars on the
commutator. The coils provide the torque as the current passing
through the coils interact with the magnetic field
Split ring of metal that reverses the direction of current through
the rotor coils every half turn to ensure that the torque is always
acting in the same direction. Provides points of contact between
the coils and the external electric circuit
Conductors that make electrical contact with the moving
commutator from the external circuit and prevent the tangling of
wires. Usually made of graphite and spring loaded.
Cylindrical bar of hardened steel passing through the centre of the
armature and commutator, which provides a centre of rotation
and is a point where energy can be extracted from the motor.
Production of the Magnetic Field
The required magnetic fields in DC motors can be produced by permanent magnetic shaped
for fit around the armature. Alternatively, it can be provided by electromagnets wound so
that pairs of coils facing the rotor coils have the same magnetic fields as would be produced
by permanent magnets.
Galvanometer and Loudspeaker
A galvanometer is a very sensitive device that can
measure small amounts of current and works on the
principle of the motor effect. It consists of a fine coil
wound many times around a soft iron core placed
inside a radial magnetic field from permanent
magnets. A needle is attached to the centre of the
iron core.
When current flows the coil experiences a force due
to the motor effect, which stretches the spring. The
needle is rotated until the magnetic force on the coil
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is matched by a counter balancing spring. The scale on the galvanometer is linear as the
amount of deflection is proportional to the current flowing through the coil.
A loudspeaker is a device that transforms electrical energy into sound energy. It consists of
a circular magnet that has one pole on the outside and the other on the inside. A coil of wire
(voice coil) is wound on the centre pole piece and is connected to the output of the
amplifier and attached to the speaker cone.
The amplifier provides an AC current that changes direction at the same frequency as the
sound to be produced. The current also changes magnitude in proportion to the amplitude
of the sound. The coil is caused to move in and out very rapidly by the motor effect, which
causes the paper speaker cone to create sound waves as it vibrates. The nature of the sound
waves is purely dependent on the AC signal input. (Pitch – Frequency, Volume – Amplitude)
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Part 2: The Generator
Michael Faraday’s Discoveries
In 1831, Faraday discovered that a current-carrying conductor in a magnetic field
experiences a force after the discovery by Oersted in 1820 that an electric current produces
a magnetic field. In 1831, Faraday discovered electromagnetic induction, which is the
generation of an EMF or electric current through the use of a magnetic field.
In his first successful experiment, Faraday sought to produce and detect a current in a coil of
wire by a magnetic field. He wound copper wire connected to DC power source around
wood and a second length of wire connected to a galvanometer. When switched on, the
galvanometer shows a spike of current, then returned to zero, and when switched off, it
exhibited a spike in the reverse direction before returning to zero.
Faraday also showed that moving a magnet near a coil could generate an electric current in
the coil, with the magnitude of the induced current depending on the speed at which the
magnet is moving towards or away from the coil. These discoveries led to Faraday’s
conclusion that a current can be induced in a conductor from the relative movement
between it and a magnetic field.
Magnetic Flux
Magnetic flux is the amount of magnetic field passing through a given area and is measured
in weber (Wb). The strength of a magnetic field, B, is also known as magnetic flux density,
which is measured in tesla (T) or weber per square metre (Wb ms-2)
The stronger the magnetic field at a point, the higher the magnetic flux density at that point
and the more magnetic flux lines there are cutting through at given area
Mathematically, magnetic flux density, B, is represented as 𝜑 = 𝐵𝐴.
Generating a Potential Difference
For an EMF to be generated, there had to be a change in the amount of magnetic flux
threading the coil. The potential difference increases as the rate of change of flux in the
circuit increases.
The EMF (𝜀) induced in a conductor is equal to the amount of magnetic flux through the
Δ𝜑
circuit that is changing with time, That is, 𝜀 = −𝑛 Δ𝑡 , which expresses Faraday’s Law of
Electromagnetic Induction.
Lenz’s Law
Lenz law states that:
When a conductor cuts flux, the induced EMF always gives rise to a current that creates a
magnetic field that opposes the original change in flux through the circuit.
If the induced current did not oppose the cause of induction, then the wire would speed up,
which increases the change in up and the wire would speed up indefinitely, which would
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oppose the Law of Conservation of Energy. Thus, the current must flow to oppose the cause
of induction, so Lenz’s law is an application of the Law of Conservation of Energy.
Back EMF
Back EMF is an electromagnetic force that opposes the main current flow in a circuit. When
the coil of a motor rotates, it is cutting magnetic flux, which will induce an EMF that opposes
the cause of induction. Thus, the induced current will flow in the opposite direction of the
input current that limits the size of the input current. The induced EMF works against the
supply EMF and is referred to as back EMF.
This decreases the torque, slowing motor’s rotation and giving it a maximum speed. When a
greater load is applied to the motor, the armature rotates more slowly, reducing the back
EMF, so a greater current flows through the coils, resulting in an increased torque. It also
prevents high currents from damaging the rotor coil. At low speeds, the back EMF is small,
so a resistor protects the motor coils from the large currents that could flow and burn out
the motor, but is replaced at higher speeds as back EMF fulfils this role.
Eddy Currents
An eddy current is a circular or whirling current induced in a conductor that experiences a
change in magnetic flux due to relative movement between the conductor and the magnetic
field. Eddy currents are an application of Lenz’s law as the magnetic fields set up by the
eddy currents oppose the changes in the magnetic field acting in the regions of the
conductor that induced the eddy currents.
Induction Cooktops
1) When AC in the induction coil under the cooktop produces an oscillating magnetic field,
the field lines pass through the ceramic cooktop and cut the metal of the saucepan
2) Eddy currents are generated in the saucepan
3) These eddy currents are very large as the currents in the coil are very large
4) These eddy currents generate heat energy, so the saucepan needs to be made of the
metal with significant resistance
5) The heat energy in the saucepan transfers energy into the food
Eddy Current Braking
1) When a flat conductor cuts magnetic flux
2) Eddy currents are generated in the conductor
3) These eddy currents have their own magnetic field
4) The eddy currents’ magnetic field oppose the original magnetic field (Lenz’s law)
5) This results in the motion being opposed and it will slow down
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Motors and Generators
Part 3: Generators for Large Scale Power Production
Components of a Generator
Changing magnetic flux is essential for a generator to function, so a magnetic field must
exist, which can be provided by of permanent magnets or electromagnets. The changing flux
is created by relative motion between coils and the magnetic field from the magnets.
The armature is an iron frame around which the rotor coils are wound and is mounted on
the axle, which rotates in the magnetic field. The coils usually consist of many turns of
copper wire wound on the armature. Torque is applied to the axle to make the rotor spin.
The brushes maintain contact with the ends of the coils through the slip ring commutators
for AC generators or the split ring commutators for DC motors and conduct electric current
from the coils to the external circuit.
Comparison of Motors and Generators
Structurally, generators and electric motors are quite similar as they both have a stator that
provides a magnetic field and a rotor that rotates within the magnetic field that can be
provided by permanent magnets or electromagnets.
The rotor in both an electric motor and a generator consist of coils of wire wound onto the
armature and connected to an external circuit through the commutator and brushes.
However, the function of an electric motor is the reverse of a generator. An electric motor
converts electrical energy into mechanical energy, while a generator converts mechanical
energy into electrical energy.
A motor operates when a current-carrying wire in an external magnetic field experiences a
force, resulting in torque that causes it to rotate. In contrast, a generator operates when a
coil is rotated in a magnetic field, resulting in changing magnetic flux that induces an EMF.
AC and DC Generators
The essential difference between AC and DC generators
is the nature of the connection between the rotor coils
and the external circuit.
In an AC generator, the brushes run on slip rings which
maintain a constant connection between the rotating
coil and the external circuit, meaning that the induced
EMF changes polarity with every half turn of the coil,
which is reversed from its previous position. The
voltage in the external circuit varies like a sine wave
and the current varies in direction.
In a DC generator, the brushes run on a split-ring
commutator which reverses the connection between
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Motors and Generators
the coil and the external circuit for every half-turn of the coil, but the induced EMF changes
polarity also with every half-turn of the coil. Thus, the current produced flows in one
constant direction and the voltage in the external circuit fluctuates between zero and a
maximum.
Energy Losses in Transmission Lines
Heat is generated in transmission lines because of the resistance of the wires, which
becomes significant over long distances. The power loss in transmission lines is given by 𝑃 =
𝐼 2 𝑅. As the resistance of the conductor is relatively constant, power loss is affected most by
the size of the current. Energy losses are kept to a minimum by transmitting the electricity
at the highest practicable voltage with the lowest practicable current.
The type of electricity transmitted over long distances is mostly AC as it can be changed
easily to high voltages with correspondingly low current by using step up transformers.
Energy losses can be minimised by carefully choosing materials and the design of
conductors. Transmission lines are usually made of copper or aluminium as these metals
have low resistance. The thicker the conductor, the lower the heat losses, but heavier
conductors require more expensive support structures.
Energy is also lost through the induction of eddy currents in the iron core of transformers.
The circulation of eddy currents in the transformer core generates heat because of the
resistance of the iron, which constitutes as an energy loss from the electrical system.
Impact of AC generators on Society and the Environment
The development of AC generators has led to the widespread application of some of the
useful features of AC electricity. As AC electricity can easily be transformed and transmitted
cheaply over great distances, it has enabled the widespread use of AC electricity and
allowed the development of extensive, reliable AC electricity networks for domestic and
industrial use throughout the world.
Positive impacts include:
 The affordability of electricity has promoted the development of a wide range of
machines, processes and appliances that improve our standard of living
 Many traditionally manually done tasks have been made easier to are being done by
specially made appliances
 Allowed for electronic communication
 AC power generating plants can be located far away from urban areas, shifting
pollution away from homes and workplaces, improving the environment of cities
 Many people can enjoy increased convenience and leisure and many new industries
prosper with the introduction of new technologies made possible by electricity.
Negative impacts include:
 However, as power transmission lines having a negative visual impact on the
environment and often requiring the clearing of environmentally sensitive areas.
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Air pollution from power stations burning fossil fuels may be a cause of acid rain and
contributes to atmospheric carbon dioxide that is linked with climate change
Reduced demand for unskilled labour and an increase in unemployment
The availability of electricity has created a dependence on it, so a disruption to the
supply of electricity compromises safety and causes widespread inconvenience and
loss of production, with a major failure possibly leading the an economic crisis.
The use of AC electricity has led to accidents including electric shocks and fires
associated with electricity.
Excessive development fuelled by the availability of AC power has adverse effects on
both society and the environment, such as in waste management
Entrench inequality for those who cannot assess the internet or acquire electronic
appliances.
Advantages and Disadvantages of AC and DC Generators
AC
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Advantages
Easy to change voltage using
transformers
Energy losses minimised by high voltage
transmission
Induction motors have fewer moving
parts, so are more reliable and easier to
maintain (similar with slip rings)
Three-phase AC currents made possible,
with a variety of applications including
heavy machinery
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Disadvantages
Constant back EMF that lowers power
Emitted electromagnetic radiation that
interferes with electronic equipment
Requires thicker insulation to minimise
interference from other cables
High towers due to high voltages
Frequency at 50Hz can readily cause
heart fibrillation
Co-ordination of frequency and phase
with the national grid
DC
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Advantages
Converse of disadvantages of AC
DC is more powerful than AC for a given
voltages and preferred in heavy duty
tools
May be more convenient and efficient
for devices that run solely on DC e.g. CRT
and battery rechargers
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Disadvantages
Converse of advantages of AC
Split ring commutator complicates
design, more expensive construction
and maintenance
Gap in the split ring can cause sparks
High energy losses at transmission
Complex to transform using rectifier
Westinghouse and Edison
Thomas Edison was a wealthy, famous inventor and was pioneering electricity supply in
around 1879 using DC. However, DC could only be generated and distributed at the
voltages used by consumers, which meant that currents were large, so there were
expensive energy losses over distances of more than 1-2 kilometres. Thus, to supply a large
city, numerous power stations would be required throughout and an unattractive amount of
wires to carry the required current.
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George Westinghouse was a wealthy businessman and inventor, who bought the patent of
the AC system from Nikola Tesla. He opened his own electric company in 1886 to compete
with Edison. The main advantage of AC was that transformers could be used to step the
voltage up or down as required. As such, AC could be generated at moderately low voltages,
stepped up to high voltages for transmission over long distances and stepped down to lower
voltages for consumers.
AC could therefore be transmitted over long distances with less energy loss than DC, which
meant that generators could be located at a distance from consumption centres. The
hydroelectric development of Niagara Falls transmitted large amounts of power to Buffalo,
New York about 30km away, which demonstrated the superiority of the efficiency of AC
transmission. Also, Tesla’s invention of the induction motor, which proved to be reliable
and economical in industry and homes, only runs on AC, further increasing its popularity.
Edison attempted to prove that AC was very dangerous by electrocuting animals in public
experiments and convinced authorities to use AC electricity for the electric chair. However,
the advantages of AC were obvious and it eventually became the dominant form in which
electricity is generated around the world.
Protection of Transmission Lines
In dry air, static electricity can jump about 1cm for every 10 000V. As such, transmission
lines need to be well insulated and a fair distance from the metal towers that carry them. .
Otherwise metal towers could become live or wires could short circuit and disrupt the
electricity distribution.
Ceramic insulating stacks/chains are used for this purpose and are chosen for its strength
and insulating properties, even at high voltages. The disc shape minimises the chance of a
spark jumping the gap and the smooth surface ensures that things like rain, water and dust
do not accumulate and allow the wire to spark.
When transmission wires are struck by lightning, there is a risk of the system being
damaged, overloaded ad shutting as well as damage to infrastructure (e.g. transformers,
poles and wires).
They are also protected from lightning by shield conductors at the top of the tower that are
non-current carrying, which will be hit first and earth the current through the earth wire.
The metal tower itself acts as an earth protection against lightning strikes as it is made of
metal. The towers are also kept at least 150m apart so that each tower is protected from
adjacent towers.
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Motors and Generators
Part 4: Transformers
Purpose of Transformers
A transformer is a device that increases or decreases the size of AC voltages as it passes
through them. Transformers consist of a primary coil that is connected to an AC power
source, a secondary coil that is connected to the load, which are both wound onto the same
soft iron core.
When AC power is fed into the primary coil, a constantly changing magnetic field threads
the secondary coil, which induces an EMF in the second coil with the same frequency. By
varying the number of turns of the coil, the size of the magnetic flux produced and the
voltage induced can be manipulated to be larger or smaller than the primary coil.
The domestic supply voltage in Australia is 240V single-phase AC. Industrial and commercial
supply is usually 415V three-phase AC. However, many appliances require voltages other
than the supply voltage. Thus, transformers are placed in the circuit between the AC supply
and the component to alter the supply voltage, so it is common for the step-up or stepdown transformer to be built into the appliance as part of its power supply.
Step-Up and Step-Down Transformers
Step-up transformer
Consists of 2 inductively coupled coils wound
on a laminated iron core
Turns in secondary coil > primary coil
Output voltage > input voltage
Output current < input current
Used at power stations to increase voltage
and reduce current for long-distance
transmission
Used in television sets to increase voltage to
operate the cathode ray tube
Step-down transformer
Consists of 2 inductively coupled coils wound
on a laminated iron core
Turns in secondary coil < primary coil
Output voltage < input voltage
Output current > input current
Used a substations and in towns to reduce
transmission line voltage for domestic and
industrial use
Used in computers and appliances to reduce
household electricity to low voltages for
electronic components
The ratio of primary to secondary voltage in a transformer is the same as the ratio of the
number of turns in the primary and secondary coils. Mathematically, this relationship is
expressed as
𝑉𝑝
𝑉𝑠
=
𝑛𝑝
𝑛𝑠
=
𝐼𝑠
𝐼𝑝
.
Conservation of Energy in a Transformer
The law of conservation of energy states that:
Energy cannot be created or destroyed, but can be transformed from one form to another.
In an ideal transformer, the input power is equal to the output power. However, real
transformers produce heat because of the resistance of the iron core from eddy currents,
representing an energy loss to the system. Thus, the electrical power output is less than the
input by the amount of power lost through heating in the transformer.
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In addition, when AC electricity is stepped up or down by transformers, the increase or
decrease in voltages is accompanied by a corresponding respective decrease or increase in
the current so that the power transmitted remains the same, in line with the Law of
Conservation of Energy.
Role of Transformers in Electricity Sub-Stations
Electricity from power stations is transmitted through the national grid at very high voltages
to minimise energy loss due to resistance in the conducting transmission wires as the energy
is carried over great distances. A typical generator has an output of 23kV (each coil has an
output of 220MW), with the three-phase power entering a transmission substation, where
transformers step up the voltage to 330kV.
The transmission lines end at a terminal station where the voltage is stepped down to 66kV
for transmission to local substations where it is stepped down again to 11kV. Pole
transformers then step the voltage down to 415V for industry and 240V for domestic
consumption.
Thus, transformers in electricity substations progressively reduce the voltage as it comes
closer to the consumer. At each stage, the output voltage is chosen to match demand for
power and the distances over which supply is needed.
Transformers in the Home
Electricity supplied to homes is typically 240V AC, but many household appliances function
at voltages other the mains domestic power supply. Some appliances require step-up
transformers, such as televisions as the cathode ray tubes, as they require large voltages.
Other appliances require step-down transformers, such as electronic devices (e.g.
recharging mobile phones and music players), to ensure correct operation as well as for
safety reasons. In addition, some ovens and cooktops step down the voltage to increase the
current which increases the heating effect of such devices.
Impact of the Development of Transformers on Society
The development of transformers made the long distance transmission of AC electricity
more economical than DC system, eventually establishing Westinghouse’s AC system as the
standard in modern society. Power loss during transmission is significantly reduced due to
the development of transformers, increasing the efficiency of electricity transmission over
long distances.
As such, the generation of electricity can be centralised in one location that is distant from
metropolitan areas to reduce the level of pollution, visual impacts and electrical hazards.
Also, industries can be decentralised and located away from residential areas and power
stations as transformers allow the distribution of electricity to remote locations, which has
also had the effect of improving living standards in rural communities through the provision
of grid-supplied, high-voltage electricity.
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Transformers have allowed electricity to be readily accessible as an energy resource
allowing large cities to spread, but had led to social dislocation in urban areas as people
move further away from friends, family and workplaces. Thus, the development of the
transformer has changed people’s lifestyle as electricity has become a necessity to every
home.
Cooling Transformers
As the soft iron core is subject to changing magnetic flux from the primary coil, eddy
currents will be generated in it which generates heat which dissipates into the surroundings.
The heat lost by the core can be minimised by lamination, which means that the core is
constructed using stacks of thin iron sheets coated with insulation materials. This increases
the resistance of the core to the flow of eddy currents, restricting the circulation off large
eddy currents.
Other solutions include heat sinks and fins, ventilated cases to remove heat by convection,
pumping oil around the transformer to transfer heat produced outside and using cooling
mechanisms (e.g. fans). These will help improve the overall energy efficiency of the
transformer.
Transformers to Transfer Electrical Energy
As current passes through conductors, energy (mainly heat) is lost to surroundings. The
amount of energy lost is related to size of current and resistance of conductor (𝑃 = 𝐼 2 𝑅).
Since resistance is proportional to length, a long transmission wire inevitably has high
resistance, therefore energy lost as heat.
By raising the voltage, the size of the current through the wire is decreases without
changing the power being transmitted. Since energy lost is proportional to square of size of
current, smaller current reduces the energy lost during transmission dramatically, thus
making transmission more efficient. Making wires from materials that have low resistance,
such as aluminium or copper, will also minimise energy losses.
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Motors and Generators
Part 5: AC Motors
Features of an AC Electric Motor
Essentially, a normal AC motor is the same as a DC motor except that it has slip rings instead
of a split ring commutator that simply conducts electricity from the power source. The
nature of AC allows the current to change direction every half turn and consequently, a
constant direction of rotation results. The speed of the AC motor depends on the torque of
the coil and the frequency of the AC power.
AC Induction Motors
An induction motor is an AC machine in which torque is produced by the interaction of a
rotating magnetic field produced by the stator and currents induced in the rotor. The stator
of a three-phase induction motor consists of a series of three sets of coils wound on soft
iron cores that surround the rotor. These are connected to the frame of the motor in a way
that it produces a magnetic field that rotates at the same frequency as the mains supply at
50 Hz.
The rotor of the AC motor consists of a number of conducting aluminium or copper bars
that are attached to the end rings at either end. This forms a squirrel-cage rotor, with the
end rings short-circuiting the bars and allowing a current to flow from one side of the cage
to the other. This is encased in a laminated iron armature that intensifies the magnetic field
passing through the conductors of the rotor cage and the laminations decrease heating
losses due to eddy currents.
When AC in the field coils of the stator produces a rotating magnetic field, the expanding
and contracting magnetic field lines cut the bars of the squirrel cage rotor. Eddy currents are
generated I the squirrel cage which have their own magnetic field that opposes the original
magnetic field. This results in the squirrel cage rotor chasing the rotating magnetic field of
the stator.
However, for an induction motor to do work, there must be relative movement between the
bars and the magnetic field to induce a current and a force. Thus, when a load is applied to
an induction motor, the rotor slows down, so the slip speed increases. As such, the relative
movement between the bars and the field and induced current and force increase as well.
AC induction motors are considered unsuitable for use in heavy industry because their low
power rating, from the magnetising of working parts of the motor and creating induction
currents in the rotor, would make them too expensive to run.
Energy Transfers and Transformations
Electrical energy is transferred from the primary coil to the secondary coil in a transformer.
Electrical energy is transferred by induction from the stator to the rotor in an induction
motor, both in the home and in industry.
Electrical energy is transformed into a range of other useful types of energy both in the
home and in industry:
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In the home
In industry
Electrical energy to radiant energy
light in light globes
 X-rays in medical imaging
heat in toaster and kettle
 light in laser circuit printing
microwaves in microwave oven
 heat in induction ovens
radio waves in cordless phone
 microwaves in wood curing
 radio waves in communication
Electrical energy to mechanical
rotation in food blender motor
 rotation in industrial motors
vibration in television speaker
 vibration in television speaker
 kinetic energy and gravitational potential
energy in fun park rides
Electrical energy to chemical potential
recharging batteries
 process of electroplating