Motor AC Simple

7/29/2019 Motor AC Simple

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Motor AC Simple

Principio de Funcionamiento
http://www.tuveras.com/maquinaasincrona/motorasincrono1.htm#comienzo


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Constitucin de la Mquina Asncrona Trifsica. Tipos de Motores


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Ver una animacin del motor asncrono

Motor con Rotor Bobinado
http://www.tuveras.com/maquinaasincrona/animacionasincrono/motorasincrono.htmlhttp://www.tuveras.com/maquinaasincrona/animacionasincrono/motorasincrono.html


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Motor con Rotor en Jaula de Ardilla


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Motor con Rotor en Doble Jaula de Ardilla


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Motor con Rotor de Ranuras Profundas


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Par en los Motores de Jaula de Ardilla


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VER IMGENES DE:Rotor en Jaula Rotor Bobinado Despiece Dos motores
http://cargarfoto%28%27rotorjaula.jpg%27%2C%27800%27%2C%27500%27%29/http://cargarfoto%28%27rotorbobinado.jpg%27%2C%27600%27%2C%27570%27%29/http://cargarfoto%28%27despiece.jpg%27%2C%27800%27%2C%27481%27%29/http://cargarfoto%28%27dosmotores.jpg%27%2C%27500%27%2C%27735%27%29/http://cargarfoto%28%27rotorjaula.jpg%27%2C%27800%27%2C%27500%27%29/http://cargarfoto%28%27rotorbobinado.jpg%27%2C%27600%27%2C%27570%27%29/http://cargarfoto%28%27despiece.jpg%27%2C%27800%27%2C%27481%27%29/http://cargarfoto%28%27dosmotores.jpg%27%2C%27500%27%2C%27735%27%29/


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Campo Magntico GiratorioEl Campo magntico giratorio se obtiene con tres devanados desfasados 120 (acoplados en estrella otringulo) y conectados a un sistema trifsico de c. a.

( Imagen animada. El punto rojo es una marca de referencia para ver que Nr


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Conexin de los Devanados


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Induction motorFrom Wikipedia, the free encyclopedia

Jump to:

Three-phase induction motors
http://en.wikipedia.org/wiki/File:Silniki_by_Zureks.jpghttp://en.wikipedia.org/wiki/File:Silniki_by_Zureks.jpg


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Animation of a squirrel-cage AC motor

An induction motor (orasynchronous motor) is a type ofalternating current motorwherepower is supplied to the rotorby means ofelectromagnetic induction.

An electric motorconverts electrical power to mechanical power in its rotor(rotating part).

There are several ways to supply power to the rotor. In a DC motorthis power is supplied

to the armature directly from a DC source, while in an induction motor this power is

induced in the rotating device. An induction motor is sometimes called a rotating

transformerbecause the stator(stationary part) is essentially the primary side of the

transformerand the rotor (rotating part) is the secondary side. The primary side's currents

evokes a magnetic field which interacts with the secondary sides mmf to produce aresultant torque, henceforth serving the purpose of producing mechanical energy. Induction

motors are widely used, especiallypolyphaseinduction motors, which are frequently used

in industrial drives.

Induction motors are now the preferred choice for industrial motors due to their ruggedconstruction, absence of brushes (which are required in most DC motors) and thanks to

modern power electronics the ability to control the speed of the motor.

History

The induction motor with a wrapped rotor was invented byNikola Tesla in 1882 in Francebut the initial patent was issued in 1888 after Tesla had moved to the United States. In his

scientific work, Tesla laid the foundations for understanding the way the motor operates.

The induction motor with a cage was invented by Mikhail Dolivo-Dobrovolskyabout ayear later inEurope. Technological development in the field has improved to where a 100

hp (74.6 kW) motor from 1976 takes the same volume as a 7.5 hp (5.5 kW) motor did in

1897. Currently, the most common induction motor is the cage rotor motor.

[edit] Principle of operation and comparison tosynchronous motors
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A 3-phase power supply provides a rotating magnetic field in an induction motor.

The basic difference between an induction motor and a synchronous AC motoris that in the

latter a current is supplied onto the rotor. This then creates a magnetic field which, through

magnetic interaction, links to the rotating magnetic field in the stator which in turn causesthe rotor to turn. It is called synchronous because at steady state the speed of the rotor is the

same as the speed of the rotating magnetic field in the stator.

By way of contrast, the induction motor does not have any direct supply onto the rotor;

instead, a secondary current is induced in the rotor. To achieve this, stator windings arearranged around the rotor so that when energised with a polyphase supply they create a

rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field

pattern induces current in the rotor conductors. These currents interact with the rotatingmagnetic field created by the stator and in effect causes a rotational motion on the rotor.

However, for these currents to be induced, the speed of the physical rotor must be less than

the speed of the rotating magnetic field in the stator, or else the magnetic field will not be

moving relative to the rotor conductors and no currents will be induced. If by some chancethis happens, the rotor typically slows slightly until a current is re-induced and then the

rotor continues as before. This difference between the speed of the rotor and speed of the

rotating magnetic field in the stator is calledslip. It is unitless and is the ratio between the

relative speed of the magnetic field as seen by the rotor (theslip speed) to the speed of therotating stator field. Due to this an induction motor is sometimes referred to as an

asynchronous machine.

Formula

The relationship between the supply frequency,f, the number of poles,p, and the

synchronous speed (speed of rotating field), ns is given by:

From this relationship:
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where

n = Revolutions per minute (rpm)

f = AC power frequency (hertz)

p = Number of poles (an even number)

The rotor speed is:

wheres is theslip.

Slip is calculated using:

A synchronous motor always runs at synchronous speed with 0% slip.

Note on the use ofp: Some texts refer to number of pole pairs instead ofnumber of poles.For example a 6 pole motor would have 3 pole pairs. The equation of synchronous speed

then becomes:

wherep is the number ofpole pairs.

Construction

The stator consists of wound 'poles' that carry the supply current to induce a magnetic field

that penetrates the rotor. In a very simple motor, there would be a single projecting piece of

the stator (asalient pole) for each pole, with windings around it; in fact, to optimize the

distribution of the magnetic field, the windings are distributed in many slots located aroundthe stator, but the magnetic field still has the same number of north-south alternations. The

number of 'poles' can vary between motor types but the poles are always in pairs (i.e. 2, 4,6, etc.).

Induction motors are most commonly built to run on single-phase orthree-phasepower, but

two-phase motors also exist. In theory, two-phase and more than three phase induction

motors are possible; many single-phase motors having two windings and requiring a
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capacitor can actually be viewed as two-phase motors, since the capacitor generates a

second power phase 90 degrees from the single-phase supply and feeds it to a separate

motor winding. Single-phase power is more widely available in residential buildings, butcannot produce a rotating field in the motor (the field merely oscillates back and forth), so

single-phase induction motors must incorporate some kind of starting mechanism to

produce a rotating field. They would, using the simplified analogy of salient poles, haveone salient pole per pole number; a four-pole motor would have four salient poles. Three-

phase motors have three salient poles per pole number, so a four-pole motor would have

twelve salient poles. This allows the motor to produce a rotating field, allowing the motorto start with no extra equipment and run more efficiently than a similar single-phase motor.

There are three types of rotor:

Squirrel-cage rotor

The most common rotor is a squirrel-cage rotor. It is made up of bars of either solid copper

(most common) or aluminum that span the length of the rotor, and are connected through aring at each end. The rotor bars in squirrel-cage induction motors are not straight, but have

some skew to reduce noise and harmonics.

Slip ring rotor

A slip ring rotor replaces the bars of the squirrel-cage rotor with windings that areconnected to slip rings. When these slip rings are shorted, the rotor behaves similarly to a

squirrel-cage rotor; they can also be connected to resistors to produce a high-resistance

rotor circuit, which can be beneficial in starting

Solid core rotor

A rotor can be made from a solid mild steel. The induced current causes the rotation.

Speed control

The synchronous rotational speed of the rotor (i.e. the theoretical unloaded speed with no

slip) is controlled by the number of pole pairs (number of windings in the stator) and by the

frequency of the supply voltage. Before the development of cheap power electronics, it wasdifficult to vary the frequency to the motor and therefore the uses for the induction motor

were limited.

The general term for a power electronic device that controls the speed of a motor as well as

other parameters is inverter. A typical unit will take the mains AC supply, rectify andsmooth it into a "link" DC voltage, and, then convert it into the desired AC waveform. In

general, a DC-to-AC converter is called an inverter, which is probably where the motor-

control invertergets its name.

Main article: Variable-frequency drive
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Because the induction motor has no brushes and is easy to control, many older DC motors

are being replaced with induction motors and accompanying inverters in industrial

applications.

Starting of induction motors

Three Phase

Direct-on-line starting

The simplest way to start a three-phase induction motor is to connect its terminals to the

line. This method is often called "direct on line" and abbreviated DOL.

In an induction motor, the magnitude of the induced emfin the rotor circuit is proportional

to the stator field and the slip speed (the difference between synchronous and rotor speeds)of the motor, and the rotor current depends on this emf. When the motor is started, the rotor

speed is zero. The synchronous speed is constant, based on the frequency of the suppliedAC voltage. So the slip speed is equal to the synchronous speed, the slip ratio is 1, and theinduced emf in the rotor is large. As a result, a very high current flows through the rotor.

This is similar to a transformer with the secondary coil short circuited, which causes the

primary coil to draw a high current from the mains.

When an induction motor starts DOL, a very high current is drawn by the stator, in theorder of 5 to 9 times the full load current. This high current can, in some motors, damage

the windings; in addition, because it causes heavy line voltage drop, other appliances

connected to the same line may be affected by the voltage fluctuation. To avoid sucheffects, several other strategies are employed for starting motors.

Star-delta starters

An induction motor's windings can be connected to a 3-phase AC line in two different

ways:

wye (starin Europe), where the windings are connected from phases of the supplyto the neutral;

delta (sometimes mesh in Europe), where the windings are connected between

phases of the supply.

A delta connection results in a higher voltage to the windings than a wye connection (the

voltage is multiplied by ). A star-delta starter initially connects the motor in wye, which

produces a lower starting current than delta, then switches to delta when the motor has

reached a set speed. Disadvantages of this method overDOL starting are:

Lower starting torque, which may be a serious issue with pumps or any devices withsignificant breakaway torque
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Increased complexty, as more contactors and some sort of speed switch or timers

are needed

Two shocks to the motor (one for the initial start and another when the motorswitches from wye to delta)

Variable-frequency drivesVariable-frequency drives (VFD) can be of considerable use in starting as well as running

motors. A VFD can easily start a motor at a lower frequency than the AC line, as well as alower voltage, so that the motor starts with full rated torque and with no inrush of current.

The rotor circuit's impedance increases with slip frequency, which is equal to supply

frequency for a stationary rotor, so running at a lower frequency actually increases torque.

Resistance starters

This method is used with slip ring motors where the rotor poles can be accessed byway of

the slip rings. Using brushes, variable power resistors are connected in series with thepoles. During start-up the resistance is large and then reduced to zero at full speed.

At start-up the resistance results in the stator's field strength being weakened less. As aresult, the inrush current is reduced. Another important advantage is higher start-up torque.

As well, the resistors generate a phase shift in the field resulting in the magnetic force

acting on the rotor having a favorable angle.

Autotransformer starters

such starters are called as auto starters or compensators, consists of an auto-transformer.

Series Reactor starters

In series reactor starter technology, an impedance in the form of a reactor is introduced in

series with the motor terminals, which as a result reduces the motor terminal voltage

resulting in a reduction of the starting current; the impedance of the reactor, a function ofthe current passing through it, gradually reduces as the motor accelerates, and at 95 %

speed the reactors are bypassed by a suitable bypass method which enables the motor to run

at full voltage and full speed. Air core series reactor starters or a series reactor soft starter isthe most common and recommended method for fixed speed motor starting. The applicable

standards are [IEC 289] AND [IS 5553 (PART 3) ]

Single Phase

In a single phase induction motor, it is necessary to provide a starting circuit to start

rotation of the rotor. If this is not done, rotation may be commenced by manually giving a

slight turn to the rotor. The single phase induction motor may rotate in either direction andit is only the starting circuit which determines rotational direction.
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For small motors of a few watts the start rotation is done by means of a single turn of heavy

copper wire around one corner of the pole. The current induced in the single turn is out of

phase with the supply current and so causes an out-of-phase component in the magneticfield, which imparts to the field sufficient rotational character to start the motor. Starting

torque is very low and efficiency is also reduced. Such shaded-pole motors are typically

used in low-power applications with low or zero starting torque requirements, such as deskfans and record players.

Larger motors are provided with a second stator winding which is fed with an out-of-phase

current to create a rotating magnetic field. The out-of-phase current may be derived by

feeding the winding through a capacitor, or it may derive from the winding having differentvalues of inductance and resistance from the main winding.

In some designs the second winding is disconnected once the motor is up to speed, usually

either by means of a switch operated by centrifugal force acting on weights on the motor

shaft, or by a positive temperature coefficient thermistor which after a few seconds of

operation heats up and increases its resistance to a high value, reducing the current throughthe second winding to an insignificant level. Other designs keep the second winding

continuously energised during running, which improves torque.

Squirrel-cage rotorFrom Wikipedia, the free encyclopedia

Jump to: navigation,search

Figure 1. Squirrel cage rotor

Figure 2. Stator and rotor laminations
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Figure 3. Diagram of the squirrel-cage (showing only three laminations)

A squirrel cage rotor is the rotating part often used in an AC induction motor. An electricmotorwith a squirrel cage rotor is sometimes called a squirrel cage motor.

Contents

[hide]

1 Structure

2 Theory

3 Practical Demonstration

4 Use in synchronous motors

5 Induction generators

6 See also

Structure

In overall shape, it is a cylinder mounted on a shaft. Internally it contains longitudinal

conductive bars (usually made of aluminum or copper) set into grooves and connected

together at both ends by shorting rings forming a cage-like shape. The name is derived fromthe similarity between this rings-and-bars winding and a squirrel cage (or, as it is

commonly known, a hamster wheel).

The core of the rotor is built with stacks of electrical steel laminations. Figure 3 shows

three of many laminations used.

Theory

The field windings in the statorof an induction motor set up a rotating magnetic field

around the rotor. The relative motion between this field and the rotation of the rotor induces

electric current in the conductive bars. In turn these currents lengthwise in the conductorsreact with the magnetic field of the motor to produce forceacting at atangentorthogonal to

the rotor, resulting in torque to turn the shaft. In effect the rotor is carried around with the

magnetic field but at a slightly slower rate of rotation. The difference in speed is calledslip

and increases with load.
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The conductors are often skewed slightly along the length of the rotor to reduce noise and

smooth out torque fluctuations that might result at some speeds due to interactions with the

pole pieces of the stator. The number of bars on the squirrel cage determines to what extentthe induced currents are fed back to the stator coils and hence the current through them.

The constructions that offer the least feedback employ prime numbers of bars.

The iron core serves to carry the magnetic field across the motor. In structure and materialit is designed to minimize losses. The thin laminations, separated by varnish insulation,

reduce stray circulating currents that would result in eddy current loss. The material is a

low carbon but highsilicon iron with several times the resistivity of pure iron, further

reducing eddy-current loss. The low carbon content makes it a magnetically soft materialwith lowhysteresis loss.

The same basic design is used for both single-phase and three-phase motors over a wide

range of sizes. Rotors for three-phase will have variations in the depth and shape of bars to

suit the design classification.

Practical Demonstration

To demonstrate how the cage rotor works, the stator of a single-phase motor and a copper

pipe (as rotor) may be used. If adequate ac power is applied to the stator, an alternatingmagnetic field revolves around the stator. If the copper pipe is inserted inside the stator,

there will be an induced current in the pipe, and this current produces another magnetic

field. The interaction of the stator revolving field and rotor induced field produce a torque

and thus rotation.

Use in synchronous motors

Synchronous motors must use other types of rotors although they may employ a squirrel

cage winding to allow them to reach near-synchronous speed while starting. Once operating

at synchronous speed, the magnetic field is rotating at the same speed as the rotor, so nocurrent will be induced into the squirrel cage windings and they will have no further effect

on the operation of the synchronous motor.

Induction generators

Three phase squirrel cage induction motors can also be used as generators. For this to work

the motor must either be connected to a grid supply or an arrangement of capacitors. If themotor is run as a self exciting induction generator (SEIG) the capacitors can either be

connected in a delta or c2c arrangement. The c2c method is for producing a single phaseoutput and the delta method is for a three phase output. For the motor to work as a

generator instead of a motor the rotor must be spun just faster than its nameplate speed, this

will cause the motor to generate power after building up its residual magnetism.
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Rotating magnetic fieldFrom Wikipedia, the free encyclopedia

Jump to: navigation,search

A rotating magnetic field is a magnetic field which changes direction at (ideally) aconstant angular rate. This is a key principle in the operation of thealternating-current

motor. In 1882,Nikola Tesla identified the concept of the rotating magnetic field. In 1885,

Galileo Ferraris independently researched the concept. In 1888, Tesla gained U.S. Patent0,381,968 for his work. Also in 1888, Ferraris published his research in a paper to the

Royal Academy of Sciences in Turin.

Contents

[hide]

1 Description

2 See also

3 Further reading

4 Patents

5 External articles

Description

A symmetric rotating magnetic field can be produced with as few as three coils. The threecoils will have to be driven by a symmetric 3-phase AC sine current system, thus eachphase will be shifted 120 degrees in phasefrom the others. For the purpose of this example,

the magnetic field is taken to be the linear functionof the coil's current.
http://en.wikipedia.org/wiki/Rotating_magnetic_field#column-onehttp://en.wikipedia.org/wiki/Rotating_magnetic_field#searchInputhttp://en.wikipedia.org/wiki/Rotating_magnetic_field#searchInputhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Alternatorhttp://en.wikipedia.org/wiki/Alternatorhttp://en.wikipedia.org/wiki/Alternatorhttp://en.wikipedia.org/wiki/Nikola_Teslahttp://en.wikipedia.org/wiki/Galileo_Ferrarishttp://www.google.com/patents?vid=381968http://www.google.com/patents?vid=381968http://en.wikipedia.org/wiki/Turinhttp://toggletoc%28%29/http://en.wikipedia.org/wiki/Rotating_magnetic_field#Descriptionhttp://en.wikipedia.org/wiki/Rotating_magnetic_field#See_alsohttp://en.wikipedia.org/wiki/Rotating_magnetic_field#Further_readinghttp://en.wikipedia.org/wiki/Rotating_magnetic_field#Patentshttp://en.wikipedia.org/wiki/Rotating_magnetic_field#External_articleshttp://en.wikipedia.org/wiki/Symmetrichttp://en.wikipedia.org/wiki/In_phasehttp://en.wikipedia.org/wiki/In_phasehttp://en.wikipedia.org/wiki/Linear_functionhttp://en.wikipedia.org/wiki/Linear_functionhttp://en.wikipedia.org/wiki/File:3phase-rmf-noadd-60f-airopt.gifhttp://en.wikipedia.org/wiki/File:3phase-rmf-noadd-60f-airopt.gifhttp://en.wikipedia.org/wiki/Rotating_magnetic_field#column-onehttp://en.wikipedia.org/wiki/Rotating_magnetic_field#searchInputhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Alternatorhttp://en.wikipedia.org/wiki/Alternatorhttp://en.wikipedia.org/wiki/Nikola_Teslahttp://en.wikipedia.org/wiki/Galileo_Ferrarishttp://www.google.com/patents?vid=381968http://www.google.com/patents?vid=381968http://en.wikipedia.org/wiki/Turinhttp://toggletoc%28%29/http://en.wikipedia.org/wiki/Rotating_magnetic_field#Descriptionhttp://en.wikipedia.org/wiki/Rotating_magnetic_field#See_alsohttp://en.wikipedia.org/wiki/Rotating_magnetic_field#Further_readinghttp://en.wikipedia.org/wiki/Rotating_magnetic_field#Patentshttp://en.wikipedia.org/wiki/Rotating_magnetic_field#External_articleshttp://en.wikipedia.org/wiki/Symmetrichttp://en.wikipedia.org/wiki/In_phasehttp://en.wikipedia.org/wiki/Linear_function


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Sine wave current in each of the coils produces sine varying magnetic field on the rotation

axis. Magnetic fields add as vectors.

Vector sum of the magnetic field vectors of the stator coils produces a single rotating vector

of resulting rotating magnetic field.

The result of adding three 120-degrees phased sine waves on the axis of the motor is asingle rotating vector. The rotor has a constant magnetic field. The N pole of the rotor will

move toward the S pole of the magnetic field of the stator, and vice versa. This magneto-

mechanical attraction creates a force which will drive rotor to follow the rotating magnetic

field in a synchronous manner.
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U.S. Patent 381968: Mode and plan of operating electric motors by progressive shifting;

Field Magnet; Armature; Electrical conversion; Economical; Transmission of energy;Simple construction; Easier construction; Rotating magnetic field principles.

A permanent magnet in such a field will rotate so as to maintain its alignment with the

external field. This effect was utilized in early alternating currentelectric motors. A

rotating magnetic field can be constructed using two orthogonal coils with a 90 degreephase difference in their AC currents. However, in practice such a system would be

supplied through a three-wire arrangement with unequal currents. This inequality would
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cause serious problems in the standardization of the conductor size. In order to overcome

this, three-phase systems are used where the three currents are equal in magnitude and have

a 120 degree phase difference. Three similar coils having mutual geometrical angles of 120degrees will create the rotating magnetic field in this case. The ability of the three phase

system to create the rotating field utilized in electric motors is one of the main reasons why

three phase systems dominate in the world electric power supply systems.

Rotating magnetic fields are also used in induction motors. Because magnets degrade with

time, induction motors use short-circuited rotors (instead of a magnet) which follow the

rotating magnetic field of a multicoiled stator. In these motors, the short circuited turns of

the rotor develop eddy currents in the rotating field of stator which in turn move the rotorby Lorentz force. These types of motors are not usually synchronous, but instead

necessarily involve a degree of 'slip' in order that the current may be produced due to the

relative movement of the field and the rotor.

NOTE: The rotating magnetic field can actually be produced by two coils, with phases

shifted about 90 degrees, but such a field would not be symmetric due to the differencebetween the magnetic susceptibility of the ferromagnetic materials of pole and of air. In the

case where only two phases of sine current are available, four poles are commonly used.
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