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One application of magnetic fields in household appliances Essay

One thing that uses magnetic fields is the electric motor, which is used in many household appliances, such as electric fans, microwave ovens, and other small appliances. In this instance the electric motor has an electric current, giving it also this magnetic field.

An electric motor converts electricity into mechanical motion.

Most electric motors work by electromagnetism, but motors based on electrostatic forces also exist. The overarching concept is that a force is generated when a current-carrying element is subjected to a magnetic field. In a cylindrical motor, the rotor rotates because a torque is developed when this force is applied at a given distance from the axis of the rotor.

Most electromagnetic motors are rotary, but linear types also exist. In a rotary motor, the rotating part (usually on the inside) is called the rotor, and the stationary part is called the stator. The motor contains electromagnets that are wound on a frame. Though this frame is often called the armature. The electromagnetic field works as shown in the diagrams below.

DC motors.

A simple DC electric motor. When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation.

The armature continues to rotate.

When the armature becomes horizontally aligned, the commutator reverses the direction of current through the coil, reversing the magnetic field. The process then repeats.

Single-phase AC induction motors.

Electric motors have no electrical connection between the rotor and the outside world. They operate due to a moving magnetic field induces a current to flow in the rotor. This current flow in the rotor creates the second magnetic field required to produce a torque. The motor was introduced in 1888 and it initiated what is known as the second industrial revolution, making possible the efficient generation and long distance distribution of electrical energy using the alternating current transmission system.

A common single-phase motor is the shaded pole motor, which is used in devices requiring lower torque, such as electric fans, microwave ovens, and other small household appliances. In this motor, small single-turn copper “shading coils” create the moving magnetic field.

Another common single-phase AC motor is the split-phase induction motor, commonly used in major appliances such as washing machines and clothes dryers. Compared to the shaded pole motor, these motors can generally provide much greater starting torque by using a special startup winding along with a centrifugal switch and a starting capacitor.

Three-phase AC motors.

For higher-power applications where a polyphase electrical supply is available, the three phase AC induction motor is used. The phase differences between the three phases of the polyphase electrical supply create a rotating electromagnetic field in the motor. There are two types of rotors in use.

Through electromagnetic induction, the rotating magnetic field creates a current in these conductors, which in turn sets up a counterbalancing magnetic field that causes the rotor to turn in the direction that the field is rotating.

This type of motor is becoming more common in appliances such as locomotives, where it is known as the asynchronous traction motor. If the rotor coils are fed a separate field current to create a continuous magnetic field, the result is a called a synchronous motor because the motor rotates in synchronism with the rotating magnetic field produced by the three phase electrical supply.

Bibliography.

An electric motor converts electrical energy into mechanical motion. The reverse task, that of converting mechanical motion into electrical energy, is accomplished by a generator or dynamo. In many cases the two devices differ only in their application and minor construction details. Electrical energy or Electromagnetic energy is a form of energy present in any electric field or magnetic field, or in any volume containing electromagnetic radiation. …

Most electric motors work by electromagnetism, but motors based on other electromechanical phenomena, such as electrostatic forces and the piezoelectric effect, also exist. The fundamental principle upon which electromagnetic motors are based is that there is a mechanical force on any wire when it is conducting electricity while contained within a magnetic field. The force is described by the Lorentz force law and is perpendicular to both the wire and the magnetic field. In a rotary motor, there is a rotating element, the rotor. The rotor rotates because the wires and magnetic field are arranged so that a torque is developed about the rotor’s axis. In physics, magnetism is a phenomenon by which materials exert an attractive or repulsive force on other materials. … An electrostatic motor or capacitor motor is a type of electric motor based on the attraction and repulsion of electric charge. … Piezoelectricity is the ability of certain crystals to produce a voltage when subjected to mechanical stress. … In physics, the Lorentz force is the force exerted on a charged particle in an electromagnetic field. …

Most magnetic motors are rotary, but linear types also exist. In a rotary motor, the rotating part (usually on the inside) is called the rotor, and the stationary part is called the stator. The motor contains electromagnets that are wound on a frame. Though this frame is often called the armature, that term is often erroneously applied. Correctly, the armature is that part of the motor across which the input voltage is supplied or that part of the generator across which the output voltage is generated. Depending upon the design of the machine, either the rotor or the stator can serve as the armature. r0t0r > * http://www. … The stator is the fixed part of a rotating machine. … In the physical sciences, potential difference is the difference in potential between two points in a conservative vector field. …

DC motors.

One of the first electromagnetic rotary motors, if not the first, was invented by Michael Faraday in 1821, and consisted of a free-hanging wire dipping into a pool of mercury. A permanent magnet was placed in the middle of the pool. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire. This motor is often demonstrated in school physics classes, but brine is sometimes used in place of the toxic mercury. This is the simplest form of a class of electric motors called homopolar motors. Michael Faraday Michael Faraday (September 22, 1791 – August 25, 1867) was a British scientist (a physicist and chemist) who contributed significantly to the fields of electromagnetism and electrochemistry.

The classic DC motor has a rotating armature in the form of an electromagnet with two poles. A rotary switch called a commutator reverses the direction of the electric current twice every cycle, to flow through the armature so that the poles of the electromagnet push and pull against the permanent magnets on the outside of the motor. As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet. During that instant of switching polarity, inertia keeps the classical motor going in the proper direction. (See the diagrams below.) Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. … A commutator is an electrical switch that periodically reverses the current in an electric motor or electrical generator. … Inertia is the tendency of any state of affairs to persist in the absence of external influences. …

However, there are a number of limitations in the classic design, many due to the need for brushes to rub against the commutator. The rubbing creates friction, and the higher the speed, the harder the brushes have to press to maintain good contact. Not only does this friction make the motor noisy, but it also creates an upper limit on the speed and causes the brushes eventually to wear out and to require replacement. The imperfect electric contact also causes electrical noise in the attached circuit.

These problems vanish when you turn the motor inside out, putting the permanent magnets on the inside and the coils on the outside thus designing out the need for brushes in a brushless design. However such designs need electronic cuircuits to control the switching of the electromagnets (the function that is performed in conventional motors by the commutator). This article is about the resistive force. … … A brushless DC motor is an electric motor that operates like a DC motor, but with the roles of the rotor and stator reversed. …

AC motors.

Induction motors operate because a moving magnetic field induces a current to flow in the rotor. This current in the rotor creates the second magnetic field required (along with the field from the stator windings) to produce a torque. Induction motors are simple and therefore relatively cheap to construct. They do not rely on brushes like the DC motor, and usually have a longer life. They are by far the most common type of motor for applications above 1 kW. Typically the rotor has no electrical connection to the outside world, except in the case of wound-rotor induction motors. Electromagnetic induction is the production of an electrical potential difference (or voltage) across a conductor situated in a changing magnetic flux. …

All induction motors are characterized by the fact that when no load is applied to the motor, the rotor rotates at a slightly slower rate than the mains frequency (or an integer submultiple of the mains frequency). This is because the rotor must “slip” backwards against the moving magnetic field in order to induce any current in the rotor. The slip increases (and the motor speed decreases) as the load on the motor increases.

The rotating magnetic field principle was conceived by Nikola Tesla in 1882 and he employed it to invent a two-phase induction motor in 1883. Michael von Dolivo-Dobrowlsky invented the first modern three-phase “cage-rotor” in 1890. Introduction of the motor from 1888 onwards initiated what is known as the second industrial revolution, making possible the efficient generation and long distance distribution of electrical energy using the alternating current transmission system, also of Tesla’s invention (1888)[1] (http://www.tfcbooks.com/tesla/system.htm). A notable recent invention by Anadish K. Pal (U.S Patent 6717300) is to use an induction motor to sense its own rotation in the absence of the driving electric power. This invention has applications in alarm systems and early failure prediction of the induction motors. Nikola Tesla was an inventor, and electrical engineer. …

AC motors generally come in two types: single phase and three phase. An alternating current (AC) is an electrical current where the magnitude and direction of the current varies cyclically, as opposed to direct current, where the direction of the current stays constant. … The generation of AC electric power is commonly three phase, in which the waveforms of three supply conductors are offset from one another by 120°. These three conductors are commonly housed in a single conduit (e. … Three phase voltages Three phase systems have 3 waveforms (usually carrying power) that are 2/3 960; radians (120 degrees,1/3 of a cycle) offset in time. …

Single-phase AC induction motors.

A polyphase induction motor will continue to rotate even if one phase is disconnected, at reduced torque. However, a polyphase motor at standstill will not generate any net starting torque if connected only to a single-phase supply. The key to the design of single-phase motors, then, is to provide a rotating magnetic field to produce starting torque.

A common single-phase motor is the shaded pole motor, which is used in devices requiring lower torque, such as electric fans, microwave ovens, and other small household appliances. In this motor, small single-turn copper “shading coils” create the moving magnetic field. Part of each pole is encircled by a copper coil or strap; the induced current in the strap opposes the change of flux through the coil (Lenz’s Law), so that the maximum field intensity moves across the pole face on each cycle. small shaded-pole motor shading-coils A Shaded pole motor is a type of AC single phase induction motor. … The concept of torque in physics, also called moment or couple, originated with the work of Archimedes on levers. … Household Electric Fan A fan has two purposes. … Microwave oven A microwave oven is a kitchen appliance employing microwave radiation primarily to cook or heat food. …

Another common single-phase AC motor is the split-phase induction motor, commonly used in major appliances such as washing machines and clothes dryers. Compared to the shaded pole motor, these motors can generally provide much greater starting torque by using a special startup winding in conjunction with a centrifugal switch. A major appliance is a large machine which accomplishes some routine housekeeping task, which includes purposes such as cooking, food preservation, or cleaning, whether in a household, institutional, commercial or industrial setting. …

Front-loading washing machine. … A dryer or drier most commonly means: a clothes dryer / tumble-dryer a hair dryer There are also various industrial dryers for everything from wet paper to candy. … A startup winding, also known as the auxiliary winding, is used to create the torque needed to start a single phase induction motor. … A centrifugal switch is an electric switch that operates using the centrifugal force created from a rotating shaft, most commonly that of an electric motor or gasoline engine. …

Single-phase AC synchronous motors

Small single-phase AC motors can also be designed with magnetized rotors (or several variations on that idea). The rotors in these motors do not require any induced current so they do not slip backward against the mains frequency. Instead, they rotate synchronously with the mains frequency. Because of their highly accurate speed, such motors are usually used to power mechanical clocks, audio turntables, and tape drives; formerly they were also much used in accurate timing instruments such as strip-chart recorders or telescope drive mechanisms.

The shaded-pole synchronous motor is one version. A turntable is any rotating platform: a phonograph (or the platter of), for playing phonograph records (often utilised by hip hop DJs to play and mix or scratch vinyl records, such as a 45-RPM EP, see turntablism) a device used at some railroad facilities to turn a locomotive or… A tape drive, also known as a streamer, is a peripheral device that reads and writes data stored on a magnetic tape or a punched tape. … shading-coils within the magnetic circuit of the field coil Shaded-pole synchronous motors are a class of AC motor. …

Because inertia makes it difficult to instantly accelerate the rotor from stopped to synchronous speed, these motors normally require some sort of special feature to get started. Various designs use a small induction motor (which may share the same field coils and rotor as the synchronous motor) or a very light rotor with a one-way mechanism (to ensure that the rotor starts in the “forward” direction). Inertia is the tendency of any state of affairs to persist in the absence of external influences. …

Three-phase AC induction motors

For higher-power applications where a polyphase electrical supply is available, the three phase (or polyphase) AC induction motor is used. The phase differences between the three phases of the polyphase electrical supply create a rotating electromagnetic field in the motor. Three phase voltages Three phase systems have 3 waveforms (usually carrying power) that are 2/3 960; radians (120 degrees,1/3 of a cycle) offset in time. … Polyphase electrical systems supply alternating current electrical power in overlapping phases. …

There are two types of rotors in use. Most motors use the squirrel cage rotor discussed above. An alternate design, called the wound rotor, is used when variable speed is required. In this case, the rotor has the same number of poles as the stator and the windings are made of wire, connected to slip rings on the shaft. Carbon brushes connect the slip rings to an external controller such as a variable resistor that allows changing the motor’s slip rate. In certain high-power variable speed wound-rotor drives, the slip-frequency energy is captured, rectified and returned to the power supply through an inverter. A squirrel cage rotor is the rotating part commonly used in an AC induction motor. …

Several methods of starting a polyphase motor are used. Where the large inrush current and high starting torque can be permitted, the motor can be started across the line, by applying full line voltage to the terminals. Where it is necessary to limit the starting inrush current (where the motor is large compared with the short-circuit capacity of the supply), reduced voltage starting using either series inductors, an autotransformer, thyristors, or other devices are used. A technique sometimes used is wye-delta starting, where the motor coils are initially connected in wye for acceleration of the load, then switched to delta when the load is up to speed. Transistorized drives can directly vary the applied voltage as required by the starting characteristics of the motor and load. Transformers – Typical electrical configurations. … The thyristor is a solid-state semiconductor device similar to a diode, with an extra terminal which is used to turn it on. …

As in the single-phase motor, through electromagnetic induction, the rotating magnetic field induces a current in the conductors in the rotor, which in turn sets up a counterbalancing magnetic field that causes the rotor to turn in the direction the field is rotating. And just as with the single-phase motor, the rotor must always rotate slower than the rotating magnetic field produced by the polyphase electrical supply; otherwise, no counterbalancing field will be produced in the rotor. Electromagnetic induction is the production of an electrical potential difference (or voltage) across a conductor situated in a changing magnetic flux. …

This type of motor is becoming more common in traction applications such as locomotives, where it is known as the asynchronous traction motor. Traction motor typically refers to those motors that are used to power the driving wheels of a railroad locomotive, electrical multi-unit train (such as a subway or light rail vehicle train), or a tram. …

The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:

RPM = 2 * F * 60/p

where

RPM = (Synchronous) Revolutions per minute

F = AC power frequency

p = Number of poles, usually an even number but always a multiple of the number of phases

The torque is a function of the amount of slip, or difference in rotation, between the rotor and stator fields. Standard motors have between 2-3% slip, special motors may have up to 7% slip, and a class of motors known as torque motors are rated to operate at 100% slip (0 RPM/full stall).

The torque is determined by the amount of slip, or difference in rotation, between the rotor and stator fields.

The speed in this type of motor has traditionally been altered by having additional sets of coils or poles in the motor that can be switched on and off to change the speed of magnetic field rotation. However, developments in power electronics mean that the frequency of the power supply can also now be varied to provide a smoother control of the motor speed. Power electronics is the technology associated with the efficient conversion, control and conditioning of electric power by static means from its available input form into the desired electrical output form. …

Three-phase AC synchronous motors

As with single-phase motors, if the rotor coils of a three-phase motor are fed a separate field current to create a continuous magnetic field (or if the rotor consists of a permanent magnet), the result is a called a synchronous motor because the rotor will rotate in synchronism with the rotating magnetic field produced by the polyphase electrical supply.

A synchronous motor can also be used as an alternator. An alternator is a generator that produces alternating current by converting mechanical energy to electrical energy. …

Nowadays, synchronous motors are frequently driven by transistorized variable-frequency drives. This greatly eases the problem of starting the massive rotor of a large synchronous motor. They may also be started as induction motors using a squirrel-cage winding that shares the common rotor: once the motor reaches synchronous speed, no current is induced in the squirrel-cage winding so it has little effect on the synchronous operation of the motor.

Synchronous motors are occasionally used as traction motors; the TGV may be the best-known example of such use. The TGV is Frances train à grande vitesse; literally high-speed train. Developed by Alstom and SNCF, and operated by SNCF, the French national railway company, it connects cities in France, especially Paris, and in some other neighbouring countries, such as Belgium and Switzerland. …

Induction motors are the workhorses of industry and motors up to about 500 kW in output are produced in highly standardized frame sizes, making them nearly completely interchangeable between manufacturers (although European and North American standard dimensions are of course different).


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