Motors and Generators€¦ · Web viewelectromagnetic induction, which is the generation of an EMF or electric current through the use of a magnetic field. In his first successful

PhysicsMotors and Generators

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Motors and GeneratorsPart 1: Motors and Magnetic Forces

The Motor EffectThe 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 lengtho 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 ConductorsA 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

separation between the conductors. Mathematically, this is represented as Fl=k

I1 I2d

.

TorqueTorque 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 asτ=Fd.

Forces Experienced by a Current-Carrying LoopIn 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 FunctionCurved permanent magnets or electromagnet (stator)

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.

Armature 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.

Rotor coils 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 commutator 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

Brushes 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.

Axel 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 FieldThe 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 LoudspeakerA 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 DiscoveriesIn 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 FluxMagnetic 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 areaMathematically, magnetic flux density, B, is represented asφ=BA.

Generating a Potential DifferenceFor 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

circuit that is changing with time, That is,ε=−n ΔφΔt , which expresses Faraday’s Law of

Electromagnetic Induction.

Lenz’s LawLenz 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 EMFBack 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 CurrentsAn 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 Cooktops1) 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 saucepan2) Eddy currents are generated in the saucepan3) These eddy currents are very large as the currents in the coil are very large4) These eddy currents generate heat energy, so the saucepan needs to be made of the

metal with significant resistance5) The heat energy in the saucepan transfers energy into the food

Eddy Current Braking1) When a flat conductor cuts magnetic flux2) Eddy currents are generated in the conductor3) These eddy currents have their own magnetic field4) 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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Part 3: Generators for Large Scale Power Production

Components of a GeneratorChanging 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 GeneratorsStructurally, 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 GeneratorsThe 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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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 LinesHeat 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 2R. 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 EnvironmentThe 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. 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


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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

ACAdvantages Disadvantages

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

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 gridDC

Advantages Disadvantages 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

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 EdisonThomas 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.

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


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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 LinesIn 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.

Part 4: Transformers


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Purpose of TransformersA 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 step-down transformer to be built into the appliance as part of its power supply.

Step-Up and Step-Down TransformersStep-up transformer Step-down transformer

Consists of 2 inductively coupled coils wound on a laminated iron core

Consists of 2 inductively coupled coils wound on a laminated iron core

Turns in secondary coil > primary coil Turns in secondary coil < primary coilOutput voltage > input voltage Output voltage < input voltageOutput current < input current Output current > input currentUsed at power stations to increase voltage and reduce current for long-distance transmission

Used a substations and in towns to reduce transmission line voltage for domestic and industrial use

Used in television sets to increase voltage to operate the cathode ray tube

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 asV p

V s=n pns

=I sI p

.

Conservation of Energy in a TransformerThe 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-StationsElectricity 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 HomeElectricity 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 SocietyThe 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 TransformersAs 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 EnergyAs 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 2R). 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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Part 5: AC Motors

Features of an AC Electric MotorEssentially, 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 MotorsAn 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 TransformationsElectrical 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 industryElectrical energy to radiant energy

light in light globes heat in toaster and kettle microwaves in microwave oven radio waves in cordless phone

X-rays in medical imaging light in laser circuit printing heat in induction ovens microwaves in wood curing radio waves in communication

Electrical energy to mechanical rotation in food blender motor vibration in television speaker

rotation in industrial motors vibration in television speaker kinetic energy and gravitational potential

energy in fun park ridesElectrical energy to chemical potential

recharging batteries process of electroplating

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Motors and Generators€¦ · Web viewelectromagnetic induction, which is the generation of an EMF or electric current through the use of a magnetic field. In his first successful