MASINI ELECTRICE ROTATIVE DE CURENT ALTERNATIV-MASINA ASINCRONA

1. Generalitati privind masina asincrona2. Elemente constructive de baza ale masinii asincrone3. Functionarea masinii asincrone4. Actionari cu motoare asincrone1. Generalitati privind masina asincrona

1.1 Definitie

Se numeste masina asincrona acea masina de curent alternativ care, la frecventa data a retelei, functioneaza cu o turatie variabila cu sarcina

Masinile electrice asincrone sunt caracterizate prin faptul ca au viteza de rotatie putin diferita de viteza campului inductor, de unde si numele deasincrone.

Ele pot functiona in regim de motor, in regim de generator sau in regim de frana. In practica, cea mai larga utilizare o au ca motoare electrice.

Dupa modul de realizare a infasurarii indusului, exista doua tipuri principale de masini asincrone:

masini asincrone cu rotorul bobinat si cu inele colectoare (pe scurt masini asincrone cu inele).

masini asincrone cu rotorul in scurtcircuit (sau cu rotorul in colivie).

1.2 Semne conventionale

In figura de mai jos sunt reprezentate o parte din semnele conventionale pentru masinile asincrone.

a)- motorul asincron trifazat cu rotorul in scurtcircuitb)- motorul cu rotorul bobinatc)- motor asincron monofazatd)- motor asincron monofazat cu faza auxiliaraIn cazul masinilor cu inele, capetele infasurarii statorului sunt legate la o placa de borne ; aceasta infasurare (trifazata), poate fi legata in stea sau in triunghi.

1.3 Domenii de utilizare ale masinii asincroneMotoarele asincrone trifazate formeaza cea mai mare categorie de consumatori de energie electrica din sistemul energetic, fiind utilizate in toate domeniile de activitate: masini-unelte (strunguri, raboteze, freze, polizoare, masini de gaurit, ferastraie mecanice etc.), poduri rulante, macarale, pompe, ventilatoare etc.

Pana de curand, motoarele asincrone erau utilizate ca motoare de antrenare in actionarile cu turatie constanta; prin dezvoltarea electronicii de putere, actionarile reglabile cu motoare asincrone au capatat o extindere remarcabila, datorita fiabilitatii lor net superioare, in compartie cu motoarele de curent continuu.

FlexFORM - Program de formare profesionala flexibila pe platforme MECATRONICE POSDRU/87/1.3/64069.

Realizat prof. Jana Popa, Colegiul Tehnic Dorin Pavel, Alba Iulia, jud. Alba

http://www.cursuri.flexform.ro/courses/L2/document/Cluj-Napoca/grupa9/Popa_Jana/site/pagina1.html

2. Elemente constructive de baza ale masinii asincrone

2.1 Sectiune longitudinala printr-o masina asincrona

La masinile asincrone, statorul este inductor si rotorul este indus. Elementele componente ale acestora se pot observa in sectiunea longitudinala din figura alaturata

1. miezul magnetic statoric

2. miezul magnetic rotoric

3. infasurare statorica

4. infasurare rotorica

5. arbore

6. rulmenti

7. carcasa

8. ventilator

2.2 StatorulStatorul masinii asincrone joaca rolul de inductor. In stator se obtine un camp magnetic invartitor, pe cale electrica, cu ajutorul unei infasurari trifazate parcurse de curenti alternativi trifazati, infasurare asezata in crestaturi.

Din punct de vedere constructiv, statorul are forma unui cilindru gol realizat din tole de otel. Crestaturile pot fi semiinchise (a, b)sau deschise (c). Crestaturile semiinchise se utilizeaza la masini de puteri mici, in timp ce crestaturile deschise (care permit realizarea infasurarii afara, pe, sablon) sunt utilizate la masini de puteri mari.

Statorul sau inductorul, cuprinde: carcasa, pachetul de tole (miezul) statoric cu infasurarile si scuturile (palierele laterale)

Miezul statoric-reprezentat in imaginea alaturata- este realizat din tole de otel electrotehnic de 0,5 mm grosime, izolate cu lac sau oxizi

Carcasa si scuturile port lagar - se realizeaza prin turnare din fonta, din otel sau din aliaje ale aluminiului (pentru puteri mici).

2.3 RotorulRotorul cuprinde:miezul magnetic rotoric(pachetul de tole) ,infasurarile, arborele (axul), inelele de contact (daca rotorul este bobinat) siventilatorul.MIEZUL MAGNETIC ROTORICMiezul rotoriceste realizat din tole de otel electrotehnic de 0,5 mm grosime, izolate sau neizolate, tole obtinute prin stantare. La periferia miezului sunt distribuite crestaturile rotorice in care sunt plasate conductoarele infasurarii induse.

INFASURAREA ROTORICA

INCLUDEPICTURE "http://www.cursuri.flexform.ro/courses/L2/document/Cluj-Napoca/grupa9/Popa_Jana/site/rotor%20dubla%20colivie.jpg" \* MERGEFORMATINET Daca masina asincrona este cu rotorul bobinat, atunci infasurarea rotorica este de tipul infasurarilor de c.a. trifazate, cu pas diametral, intr-un strat sau in doua straturi.

Crestaturile in acest caz sunt semiinchise avand de obicei forma de para.

Daca masina este cu rotorul in scurtcircuit, atunci infasurarea rotorica este de tipul colivie realizata prin turnare din bare de Cu sau Al scurtcircuitate la capete de doua inele din acelasi material. Turnarea coliviei se face prin injectie direct in crestaturile rotorice (inchise sau semiinchise).

VENTILATORULVentilatia infasurarii statorice se realizeaza, de obicei la puteri mici si medii, cu ajutorul ventilatorului axial montat pe axul masinii iar ventilatia infasurarii rotorice se realizeaza cu ajutorul aripioarelor de pe inelele de scurtcircuitare care se toarna odata cu colivia.

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3. Functionarea masinii asincrone

3.1 Principiul de functionare al masinii asincrone

Statorul are amplasate trei infasurari identice in crestaturile sale. Aceste infasurari sunt decalate, intre ele, cu 120. Prin bobine va circula curentii i1, i2si i3. Acesti curenti formeaza un sistem trifazat simetric (curenti egali).

Trecerea curentului prin bobine, conformlegii inductiei electromagnetice, duce la producerea unui camp magnetic alternativ cu repartitie sinusoidala.

In consecinta, infasurarea statorica parcursa de curentul alternativ trifazat da nastere unui camp magnetic invartitor care se roteste in sensul succesiunii fazelor cu turatia de sincronism n1unde:

n 1 = turatia campului magnetic invartitor statoric (turatie sincrona);

f = frecventa curentilor din stator (50 Hz);

p = numarul de perechi de poli.

Daca rotorul este in repaus, acest camp va induce in fazele infasurarii rotorice, conform legii inductiei electromagnetice, tensiuni electromotoare. In cazul in care infasurarea rotorica este scurtcircuitata sau se racordeaza pe o impedanta trifazata simetrica, aceste tensiuni electromotoare vor determina aparitia unor curenti indusi. Prin interactiunea campului magnetic statoric cu acesti curenti indusi, vor lua nastere forte electromagnetice [F] care se vor exercita asupra fiecarui conductor rotoric. Acestor forte le corespunde un cuplu M, obtinut prin insumarea tuturor cuplurilor determinate de fortele ce actioneaza asupra conductoarelor rotorice, care determina punerea in miscare a rotorului, cu turatian, in sensul campului invartitor statoric.

INCLUDEPICTURE "http://www.cursuri.flexform.ro/courses/L2/document/Cluj-Napoca/grupa9/Popa_Jana/site/turatia%202.jpg" \* MERGEFORMATINET In miscare, rotorul induce, in infasurarea rotorica, curenti de frecventa f2, curenti care produc un camp invartitor de turatie n2:

Unda invartitoare a campului magnetic rotoric este sincrona cu cea statorica deoarece rotorul se roteste cu turatian, sin2+ n = n1.

Daca rotorul se va roti din exterior cu turatian = n1, in infasurarea rotorica nu se vor mai induce curenti, in masina nu va mai aparea cuplu electromagnetic.

Daca rotorul va fi lasat liber, datorita fortelor de frecare si a cuplului rezistent de la ax, turatia rotorului va descreste. Rotorul aluneca fata de campul statoric ramanand in urma.

Alunecarea, marimea caracteristica motoarelor asincrone, se defineste astfel:

3.2 Functionarea in regim de motor asincronIn regim de motor masina absoarbe putere electrica din retea, pe la bornele infasurarii statorice, si furnizeaza, la arbore, putere mecanica.

Acesta este cel mai utilizat regim de functionare al masinii asincrone. Bilantul de puteri este redat in figura alaturata.

Unde:

P1- puterea electrica absorbita pe la bornele infasurarii statorice;

PM- puterea electromagnetica (transferata in rotor prin intermediul campului electromagnetic);

Pmec- puterea mecanica;

P2- puterea utila la arbore;

pJ1- pierderile prin efect Joule, din infasurarea statorului;

pFe- pierderile in miezul feromagnetic;

pJ2- pierderile prin efect Joule, din infasurarea rotorului;

pmecv- pierderile mecanice si de ventilatie.

Viteza rotorului, in acest caz, este mai mica decat viteza de sincronism (0< n < n1, 0 < s < 1).

3.3 Functionarea in regim de generator asincron

Daca masina este antrenata, cu ajutorul unui motor auxiliar, in sensul de miscare, cu o viteza n > n1(s < 0), se schimba sensul de deplasare al rotorului fata de campul inductor statoric. Prin urmare se va schimba si sensul tensiunii electromotoare induse, respectiv al curentului indus, si, implicit, al cuplului.

In aceasta situatie masina primeste putere mecanica pe la arbore (de la motorul auxiliar) si cedeaza putere electrica pe la bornele infasurarii statorice. Se spune ca masina functioneaza in regim de generator. Bilantul de puteri este redat in figura.

3.4 Functionarea in regim de frana

In cazul regimului defrana electromagnetica, masina este antrenata, din exterior, in sens contrar campului statoric (W < 0, s > 1).

Ea primeste astfel putere mecanica pe la arbore, putere electrica pe la bornele infasurarii statorice, intreaga putere rezultata, dupa acoperirea pierderilor, fiind disipata pe infasurari.

FlexFORM - Program de formare profesionala flexibila pe platforme MECATRONICE POSDRU/87/1.3/64069.

Realizat prof. Jana Popa, Colegiul Tehnic Dorin Pavel, Alba Iulia, jud. Alba

4. Actionari cu motoare asincrone

A. Pornirea motoarelor asincrone

Pentru pornirea motoarelor asincrone trebuiesc asigurate urmatoarele conditii: cuplul de pornire sa fie suficient;curentul de pornire sa nu depasasca valoarea admisibila pentru reteaua de alimentare a motorului; durata procesului sa fie cat mai scurta.

Motoarele asincrone cu rotorul in scurtcircuit pot fi pornite utilizand urmatoarele metode:

a) pornirea prin cuplare directa la retea;

b) pornirea stea-triunghi;

c) pornirea cu bobine de reactanta;

d) pornirea cu autotransformator.

B. Reglarea vitezei la motoarele asincroneModificarea turatiei motoarelor asincrone, in timpul functionarii, se poate realiza, prin modificarea parametrilor de care depinde caracteristica mecanica si anume:

- modificarea, in sensul cresterii, a rezistentei circuituluirotoric prin introducerea unei rezistente interne reglabile, in circuitul rotoric(metoda utilizata la motoarele asincrone cu rotorul bobinat),metoda de regula neeconomica;

- schimbarea numarului de perechi de poli ai infasurarii statorice obtinandu-se o turatie variabila in trepte;

- modificarea frecventei tensiunii de alimentare;

- modificarea tensiunii de alimentare si a frecventei in acelasi timp astfel incat raportul U/f=constant, metoda cea mai utilizata in prezent.

C.Schimbarea sensului de rotatiePentru schimbarea sensului de rotatie al motoarelor asincrone trifazate este suficient a se schimba doua faze intre ele la alimentare. Comanda poate fi facuta manual sau automat.

The Motor / GeneratorsBoth motor/generators in the Prius are very similar in design. They are synchronous, AC, permanent magnet motors, which are very flexible and efficient. They do require complicated control electronics. The rotor (the part that spins with the shaft) contains powerful permanent magnets. If you put these magnets on your refrigerator, you'd need tools to get them off again! There are no coils of wire in the rotor and no electrical connections to it. This increases reliability over DC motors which have windings in the rotors. All windings are in the stator (the part of the motor/generator that stays still and is fixed to the frame of the car). The control electronics passes an alternating current through these windings to turn the rotor. This current must be "synchronous" with the rotor's movement. This means that the current must pass first in one direction and then in the other at the precise time that each magnet embedded in the rotor passes the winding. A position sensor on the shaft tells the control electronics where the rotor is and how fast it is spinning.

Operation of a Synchronous Motor

The animation at right shows a hypothetical synchronous, AC, permanent magnet motor. I used to think that the Prius motor/generators were like this, but someone has begun taking a wrecked drive train apart and it is now clear they are somewhat different. Still, this animation took me a while to create and I'm not about to delete it! It does illustrate the general principle well enough.

It may take a little study to figure this animation out, so let me direct your attention to several things in sequence. First stare at the pole of the top-most (orange) winding. Observe that it pulses alternately red and blue, indicating that the current in the winding generates alternately a north and south magnetic pole here. This is, of course, due to the current in the winding passing first in one direction and then the other. Now look at all the other orange windings. Their poles pulse in the same pattern as the top one and at the same time. The orange winding are all connected together and carry the same current. Next, look at the green windings. Their poles pulse in the same way, but not at the same time. This is also true of the magenta windings. To see how the different windings generate magnetic poles moving in a circular manner, wait for a red (or blue, if you prefer) pulse at one pole and then move your eyes to the next counter clockwise winding and wait for your color there. It should take about a second. With a little practice, you will be able to follow a pulse around the stator. Now for the really tricky part. As you follow a red pulse around the stator poles, glance at the rotor and note the positions of the magnetic poles there. You will see a red rotor pole being "pushed" around in front of the red pulse you're following by magnetic repulsion. You will also see a blue rotor pole being "pulled" by magnetic attraction behind the red pulse you're following. That's it. That's how it works. The control electronics senses the rotor position and keeps the stator poles magnetized so as to both push and pull the rotor around.

If you watch a rotor pole pass by a stator pole, you will see that this is the time at which the winding current changes direction and the stator pole briefly has no magnetization. It changes from having opposite magnetization to the approaching pole, pulling it closer, to having the same magnetization, thus pushing it away. If you watch for the time at which a winding reaches peak current and the stator pole reaches its most intense magnetization, you will see that a rotor "magnet" is then exactly adjacent to it. The like pole of this magnet is toward the counter clockwise direction, and is pushed away. The unlike pole is toward the clockwise direction, and is pulled onward. This is why the motor, as I have drawn it, runs counter-clockwise. To run the same motor clockwise, it would only be necessary to change the timing of the current pulses so that at the peaks of stator magnetization the like pole of the adjacent stator magnet is in the clockwise direction and the unlike pole is in the counter clockwise direction.

It is my belief that the timing of current in the windings relative to the rotor position will always be as I have shown for operation as a motor. That is, current will peak in a winding when exactly between rotor poles. If this did not happen, the motor would operate less efficiently. If the computers require less torque from the motor, they tell the control electronics to pass less current though the windings. The rotor poles are never allowed to "catch up" to the rotating magnetic field when motor operation is required.

The Prius Motor/Generators

Watch this space! So far, we know that the stator of MG2 has 48 "teeth", that is 48 metal protrusions towards the stator. Each winding passes around several teeth, in the "slots" between them. The windings for each of the three phases are overlapped. They probably produce 8 magnetic poles which rotate more smoothly than in my animation (which only has six). The rotor probably has eight poles too, with eight permanent magnets embedded in it.

Motor / Generator Control Electronics

Motor / Generators from eCycle, Incorporated

Although motor/generators they make are not quite the same as the Prius ones,eCycle, Incorporatedare more forthcoming than Toyota in their technical information. The information they publish about themotor / generatorused in their hybrid motorcycle is worth a look. It is a synchronous, AC, permanent magnet motor, like the Prius, and eCycle add "brushless" to the description. It is also described as "12-pole". The implication is that the stator and rotor both have 12 poles. The rotor is described as having "12 neodymium iron boron (NdFeB) magnets mounted on its circumference". The stator appears (from the photorgraph) to have 12 overlapping windings on 36 armature poles. This is where I think it differs from the Prius MGs. I have shown the Prius MGs with nine stator armature poles and non-overlapping windings. Even so, in my animation you can see that the currents in the windings cause only six magnetic poles to rotate around the stator. The number of effective magnetic poles can be different from the number of physical poles around which the wire is wound.

One-stage vs Two-stage power conversion:explanation

Example of motor / generator and control electronicssystem efficiency.

Torque curveTorque curve of an electric motor

The functionality of the electric motors described at the previous chapters is based on the interaction of magnetic fields. The generated forces are acting on the rotor as well as on the stator of the motor. Decisive is the force component pointing tangential to the rotor:

Force components inside of an electric motor:Attracting forces are acting between the magnetic poles of the permanent magnets of the stator and the electromagnets of the rotor. The resulting force is pointing into the direction of the orange vector. This force can be split into two components: One of them points tangential to the arc of the rotor movement (blue arrow) the second points perpendicular to the first one (magenta colored arrow). Because of the fact that the motor construction is symmetrically, the forces acting along the axis of the rotor coil (magenta colored arrows) have the same value but point in opposite directions. The sum of those forces gives zero. The remaining forces pointing tangentially to the rotor try to rotate the armature anticlockwise.Like explained at the chaptertorque, we need to know the length of the lever and the absolute value of the force acting perpendicular to the lever to calculate it's value. The length of the lever (lR) is given by the radius of the rotor and the tangential force acting on the rotor is given by (FTR). So we get:M = 2 * FTR* lRAccording to "action and reaction" a torque with the same value but contrary direction (clockwise) is acting on the stator. There is:M = 2 * FTS* lS= 2 * FTR* lRThe distance between the center of rotation and the stator (lR) is greater than the radius of the armature, hence the absolute value of the force acting on the stator is lower than those acting on the armature.Because of the fact that the torque acting on the stator equals those acting on the rotor (except the direction of rotation), the function of rotor and stator can be swapped. At a so calledoutrunnerthe stator coils form the center of the motor (=rotor at the drawing above) while the permanent magnets spin within an overhanging rotor which surrounds the core (=stator at the drawing above). Motors like that (sometimes calledexternal-rotor configuration) are not commutated mechanically but use electric controllers (=brushless). Conventional configured motors are calledinrunners.Torque and angle of rotation

Let's have a look at the torque in relation to the angle of rotation at a conventionally constructed inrunner. To simplify the mathematics we consider the magnetic field of the permanent magnets to be homogeneous. For this reason the absolute value of force acting on the rotor is constant - simply it's direction is changing. Furthermore there is no commutation of the rotor coils and just one of the electromagnets is connected to the input voltage. Therewith we get two special positions of the rotor:At the upper drawing the acting forces point into the axis of the enabled electromagnet, consequently the resulting torque is zero.At the lower drawing the rotor is arranged perpendicular to the lines of the magnetic field generated by the permanent magnets by what the resulting forces point tangential to the rotor and the point of maximal torque is reached.

The rotation angle of the rotor equals the angle between the force F and it's tangential component FT.With the correlation:

we get:

For the correlation between the angle of rotation and the torque we get:[4.4]

Where is:M - Torque, F - force acting on the magnetic poles of the enabled electromagnet, RR- radius of the rotor, - angle of rotation

Because of the fact that F and RRare constant values we get a cosine function:

Torque in correlation to the angle of rotation:The blue plot demonstrates the torque of the (not commutated) rotor. At = 0 the maximum torque is given, decreasing to zero at an angle of 90. At the angle of 180 the highest negative value is reached. In reality it means that the rotor is pulled contrarily to it's original direction of rotation by now. To make the motor work, the polarity of the electromagnet is swapped by the commutator by what the original rotating direction is acting. The green plot shows the torque of the second electromagnet and is lagging for 90. The commutator is enabling / disabling the electromagnets each 90 degrees and it is swapping the polarity each 180 degrees. To make the maximum torque act on the rotor, the switching points should be chosen in a way that the maximum torque is inside of those 90 degrees. Therefrom the first switching point is reached at 45 degrees.Torque curve of a commutated rotor:The drawing besides shows the torque curve of an electric motor with two permanent magnets at the stator and two electromagnets at the rotor. The commutator is switching the electromagnets each 90, by what the torque is altered in between M.

As a conclusion we can see that the torque of an electric motor is increasing the more electromagnets the rotor has, because the range of values for M is getting closer to the maximum value.In praxis the magnetic field of the stator isn't homogeneous as we assumed to simplify the mathematics. From there the force acting on an electromagnet of the rotor increases the closer it gets to a magnetic pole of the stator. Hence the torque is increasing with the number of magnetic poles of the stator, too.

Switching time

Besides the angle of rotation the torque of an electric motor is dependent from the strength of the acting magnetic fields. Until now we assumed that the maximal possible magnetic field of an electromagnet is reached as soon as it is enabled by the commutator. As we saw at the chapterswitching operation, the correlation given by[3.36]shows the progression of the current when an inductor is connected to the input voltage:

Current through an electromagnetic coil after switching on.

If the electromagnetic coil is enabled by the commutator for the time span t3, the (almost) maximal current is running through it's loops and the (almost) maximal magnetic field strength is created. The (almost) maximal torque is acting on the rotor. If the time span is shortened to t2by increasing rotation speeds, a just slightly lower torque is acting, because the current through the coil is decreasing slightly. When reducing the time span to t1, the coil gets disconnected from the input voltage even though just 3/4 of the maximal current is reached. Accordingly the torque decreases significantly:

Torque in relation to the revolution speed.The number of switching operations is increasing with the revolution speed and the number of electromagnets at the rotor. The current running through the loops of the rotor and therefore the torque is decreasing.Correlation[3.36]shows that the current through an inductor is increasing faster, the lower its inductance becomes. Low inductance means a lower magnetic field strength with the same value for the maximal current. As we will see some later, that provokes a worse efficiency.

Motoare de curent continuu

Motorul electric de curent continuu (cu perii) este un motor electric cu comutaie intern care este alimentat de la o surs de curent continuu.

Motor de curent continuu bipolar - Reprezentare de principiu al motorului

Cnd nfurarea rotorului este alimentat, n jurul lui se genereaz un cmp magnetic (poziie relativ a polilor magnetici, de la stnga spre dreapta: N-NS-S). Polul nord al rotorului e respins de polul nord al statorului spre dreapta i e atras de polul sud al statorului (din dreapta), producnd un cuplu mecanic motor care ntreine micarea de rotaie.

Rotorul continu rotaia.

Cnd rotorul este (ajunge) n poziie orizontal (poziie relativ a polilor N-SN-S), colectorul electric de comutare al sensului curentului continuu inverseaz sensul curentului prin nfurarea rotorului, inversnd polii cmpului magnetic produs de rotor, se ajunge astfel la poziia relativ a polilor magnetici "N-NS-S" i procesul continu conform figurii (i explicaiei de sub figur) din stnga paginii.

Procesul se reia.

Colectoarele cu lamelele deformate, sparte sau cu perii de contact necorespunztoare pot provoca vibraii excesive la motoarele de curent continuu cu comutaie intern. Frecvena de defect va fi frecvena natural a lamelei, care este egal cu numrul de lamele nmulite cu turaia de lucru, RPM.

Dac vrful de la de 360 Hz din spectrul de vibraie se ridic n mod semnificativ, cauzele cele mai probabile sunt: un circuit deschis la nfurri, precum i eventuale conexiuni electrice slbite.

Defectele motoarelor continue se pot recunoate ca fiind vibraii de mare amplitudine aprute la 6FLi armonici ale acesteia. Aceste defecte includ nfurri i conexiuni ntrerupte. Alte defecte cum ar fi siguranele arse pot determina vrfuri de mare amplitudine n intervalul 1 - 5FL.Stepper Motors (rotary) Information

INCLUDEPICTURE "http://partimages.globalspec.com/04/1304/46304_large.png" \* MERGEFORMATINET

INCLUDEPICTURE "http://partimages.globalspec.com/27/2827/47827_large.png" \* MERGEFORMATINET Image Credit:Digi-Key Corporation|ElectroCraft|Numatics, Inc.

Stepper motors areDC (direct current) electricmotors designed for precise motion control. They consist of multiple sets of coils and magnets which are designed to allow rotor movement in angular incrementscalled steps.Stepping can be done in full step, half step or other fractionalstepsinboth forward and reverse.Stepper motors are rugged, reliable, cheap, and easy to control devices that producehigh torque at slow speeds. They are used in a variety of applications and equipment, includingmachine tools, process control systems, and tape and disk drive systems.

Stepper motor systemsconsist of two basic components:an electricmotor anda controller system.During operation, a controller supplies pulses to a driver, which interprets these signals to send proportional voltage to the motor.This voltage is applied to poles around the rotor whichenergizes the coils or changes theirpolarity. The resulting magnetic interaction between the poles and the rotor causes the rotor to moveand produce the torque required for the application. This movement is done in equal angular increments called "steps".

Stepper Motor vs. Servomotor

Servomotors and stepper motors are bothtypes of motors used for precise motion control applications. Unlike servomotors however, stepper motors do not require the use of encoders or other position feedback devices in order to function.Sinceawhole stepis a uniform and repeatabledistance (typically 1.8),the controller assumes the position of therotor based on the specified number of steps. Thisreduces control complexity and cost, but can be an issue if the motormisses a step due to overloading, since all subsequent movements will be off by one step. Because of this step mechanism, stepper motors areless suitablethan servomotors for high speed applications.

Types of Stepper Motors

The first step to selecting a stepper motor isunderstanding thedifferent types. Stepper motors can bedistinguished based on construction and polarity.

Construction

Stepper motors canbe distinguished by construction and design.Motor construction, along with driver configuration,dictates themotor's step angle, which is the angle of rotation of the shaft for each step, measured in degrees. The three main types of stepper motor technologyare permanent magnet, variable reluctance, and hybrids.

Permanent MagnetVariable ReluctanceHybrid

CostLowModerateHigh

DesignModerately complexSimpleComplex

Step angle (degrees)1.5 - 307.5 -300.5 - 15

Step modesFull, half, microTypically full step onlyFull, half, micro

NoiseQuietNoisyQuiet

Overview of the stepper motor types.Table Credit:MicroChip Technology Inc. Permanent magnet (PM)stepper motorsuse permanent magnets on the rotor. The rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft. These poles provide increased magnetic flux intensity, attributing to the PM motor's higher torque ratings compared to variable reluctance motors. The number of teeth on the rotor and stator determine the step angle that will occur each time the polarity of the winding is reversed. The greater the number of teeth, the smaller the step angle. Step angles in PM motors rangebetween 1.5 to 30 degrees, most commonlyfrom 7.5 to 15. Permanent magnet motors are the most common stepper motor, characterized byboth low cost and low resolution.

Components of a PM stepper motor.Image Credit:National Instruments Corporation Variable reluctance (VR)steppermotors have a free-moving, multi-toothedrotor composed of soft-iron along with a wound stator.When the stator windings are energized, the poles in the stator become magnetized. This magnetic force attracts the rotor teeth, resulting in rotation. No residual torque is produced in VR motors due to the lack of a permanent magnet. Step angles in these motors range from 7.5 to 30 degrees. Variable reluctance motors are the simplest stepper motors from a structural standpoint.

Simplified VR motor operation.Image Credit: Teravolt - Wikipedia user Hybrid motorsconsist of heavily toothedpermanent magnetrotors and toothed stators, plus prominent rotor poles like a VR rotor. The rotor teeth provide a better path to guide and utilize magnetic flux. These motors have very fine step angles: 0.5 to 15 degrees, most commonly between 0.9 and 3.6. Hybrid stepper motors are more expensive than other types of stepper motors, but provide the best performance in respect to step resolution, availabletorque, and speed. Their higher speed capability also means theyare less likelyto stall.

Polarity

All stepper motors are configured as either unipolar or bipolar.

Unipolarmotors have unidirectional current andrequire only one power source. Because the electronics are simpler, they are typically cheaper and easier to operate.They are typically used in low performance applications and arethe ideal choice for hobbyists seeking low cost precision motion control. VR motors are more commonly unipolar.

Bipolarmotors have bidirectional current,requiring two power sources and a switchable polarity power source.Because the windings are better utilized, bipolar motors are more powerful and produce higher torque than unipolar motors of the same weight. However, the electronics are more complex (requiring a bipolar switch), contributing to higher cost. Most stepper motors used in industry are bipolar motors.

This video by MicroChip Technology provides further explanation of unipolar and bipolar stepper motor configurations.

Video Credit:MicroChip Technology Inc.

Step Mode

Stepper motorsmay function in full step, half step, microstep, or other "step modes". The type of step mode output of any stepper motor is dependent on the design of the driver. For more information on stepper motor drivers, visit theStepper Motor Drivesinformation page on GlobalSpec.

Full step modeenergizes both phases constantly to achieve the full rated torque ateach motor position.In this mode, one pulse equals one step. Thus,a stepper motorwith 200 steps per revolution will rotate a complete 360 with 200 pulses.A unipolar driver in this mode energizes a single phase, while a bipolar driver energizes both coils for a full step.

Half step modealternates between energizing one coil and two coils. Motors operating in half step run at higher resolution (twice as many positions). However, torque also fluctuates in this mode, dropping when only a single coil is energized and rising when both are energized. Torque fluctuation is comprised in more advanced drivers by adjusting the applied current.

Microstep modemoves the rotor in fractions of a step by applying current to the windings in proportion to a mathematical function. Common step fractions are 1/4, 1/8, and 1/10. Some advanced drivers may be able to provide up to1/256 of a full step. Micro-stepping provides greater resolution and smoother motor operation, whichcan reduce the need for mechanical gearing. This step mode can, however,affect themotor's repeatability.

Specifications

Performance

The key performance specifications for sourcing a stepper motor are voltage, speed, torque, rotor inertia and step angle.

Terminal voltagerefers to the design voltage of the DC motor. Essentially the voltage determines the speed of a DC motor, and speed is controlled by raising or lowering the voltage supplied to the motor.

Speedorshaft speedis the rotational speed of the rotor shaft,expressed in rpm (revolutions per minute), rps (revolutions per second), orpps (pulses per second). Like all DC motors, stepper motor shaft speed isdirectly proportional to thesupplied voltage.Typically, the speed provided by the manufacturer is the no-load speed of the output shaft, or the speed at which the motor's output torque is zero. The complete shaft speed range can be found on a motor's speed/torque curve.

Torqueis a measure of rotational force produced by the motor, expressed in pound-feet (lbft), ounce-inches (ozin),or Newton-meters (Nm). It is proportional to the amount of current flowing through the motor windings, and varies also based on the drive design and step rate. There are often a number of torque specifications given by a manufacturer. The most important are holding torque, detent torque, and pull-out torque.

Holding torqueis the maximum torque the motorcan produceat its rated current while at rest.It is a measure of the stepper motor's "strength" to remain in a fixed position under load.

Detent torqueor residual torque is the torque required to rotate the motor's shaft while the windings are not energized.Itis based on the magnetic force developed between the rotor and stator during shutoff, andthusis only present in stepper motors with permanent magnets (PM and hybridtypes).Detent torqueis typically about one-tenth thestrength of the holding torque, and can be realized byturning the motor manually.

Pull-out torqueis the maximumtorque that can be puton a motor running at continuous speed without causing synchronization problems (causing the motor to misssteps).

Image Credit:Advanced Micro Controls, Inc.

Nearly allelectric motors have speed/torque curves associated with them, supplied by the manufacturer. These curves indicate the torque output of the motor at different rated speeds.It is important to understand these curves in order tosource a motor withperformance capabilities thatmatch the requirements of the application. It is also important to note thateachspeed/torque curve isunique to agiven motor and given driver. In other words, the same motor may have a very different speed/torque curve when used with a different driver. Output powerover the motor's operational range may also be indicated in the speed/torque performance curve (as in the example above).

Rotor inertiadefines the tendency of the rotor to continue its motion once moving. This specification is expressed in ounce-inches-seconds squared (oz-in-s2), and is largely dependent on motor weight. Stepper motors are designed with characteristically low rotor inertia to allow for precisespeed control.

Step angleis the angle of rotation of the shaft for each step, measured in degrees. Step angle is based on the construction (type) of the motor, as well as the motor drive configuration.

Other specifications that are important to consider include the current per phase, operating temperature, and output power.

Current/phase, expressed in amps per phase, refers to the maximum rated current per phase or winding fora steppermotor.

Operating temperatureis the maximum ambient temperature or ambient temperature rangefor safe and reliable motor operation. Operating above this range could decrease performance and efficiency or cause overheating and failure.

Output power, expressed in horsepower (hp) or watts (W), is the product of the motor torque and speed. It is usedas a relativegauge of the motor's output capabilities.

Power consumption, typically expressed in watts (W), is the product of the voltage and current supplied to the motor. This specification describes the power used by the motor. The power dissipation and thermal limits of the motor are not usually clearly defined by the manufacturer.

Sizing

Physical sizeis also important to consider when sourcing a stepper motor for a specific system or application.

Diameter/width- Diameter (cylindrical motors or width (square motors) is used to describe the size of the motor body, not including flanges.

Length- The length of the motor body or housing, not including the shaft.

NEMA frame size- NEMA frame sizes conform to a standard size and mounting configuration identified by the National Electrical Manufacturers Association (NEMA). Frame size numbers correspondto the diameter/widthof the motor body. For instance a size 11 stepper motor has a body diameter of approximately 1.1 inches.

Selection Tip:As a rule of thumb, the torque-inertia ratio in a motor is doubled with each decrease in frame size, regardless of other factors. For instance, an unloaded 34-size motor can accelerate twice as fast as a 42-size motor.

Shape- Stepper motors can be housed in a cylindrical or square shaped body.

Gearing

Stepper motors canincorporate gearing to adjust speed and torque output and reduce design complexity. Gearing is also used in stepper motors to increase resolution. There are a number of different gear assemblies that can be used:

Spur Planetary Worm HarmonicFor more information on these different types of gearboxes, visittheGearmotors Selection Guideon GlobalSpec.

Standards

Stepper motors may need to possess certain types of sealing or enclosure ratings, or may require compliance with certain standards.

Dust proofmotors are rated forprotection against dust infiltration with features such as total enclosure and labyrinth seals for shafts.

Drip-proofmotors contain ventilation openings that are designed so that drops of liquid or solid particles falling from any angle within 15 degrees of vertical cannot enter the motor.

ROHS compliantdevices comply with the Restriction of Hazardous Substances (ROHS) Directiveto restrict certain dangerous substances commonly used in electronic and electronic equipment. The RoHS directive tests for the presence of lead (Pb), cadmium (Cd), mercury (Hg), hexavalent chromium (Hex-Cr), polybrominated biphenyls (PBB), and polybrominated diphenyl ethers (PBDE).All compliant stepper systems must have none oracceptably low levels of these substances.

Sealingandwaterproofingstandards indicate various levels of protection from water, based on IP (Ingress Protection Rating) Code.

References

Guide to Stepper Motor Selection (pdf) - Astosyn

Introduction to Stepper Motors and Drives - Omega Engineering

Stepper Motor Basics - Solarbotics.net

Types of Stepper Motors - National InstrumentsRead user Insights about Stepper Motors (rotary)

Related Products & Services

AC MotorsAC motors include single, multiphase, universal, induction, synchronous, and gear motors. They also include servomotors.

AC ServomotorsAC servomotors are responsive, high-acceleration motors typicallyconstructed as permanent magnet synchronous motors.

DC MotorsDC motors are most commonly used in variable speed and torque applications. They include brushless and gear motors, as well as servomotors.

DC ServomotorsDC servomotors are generally smalland powerful for their size, and easy to control. Common types of DC servomotors include brushless or gear motors.

GearmotorsGearmotors consist of an AC or DC motor with an integral gearbox or gear head typically used to adjust the motor's output speed and torque.

Supplier DatasheetsPornirea motoarelor trifazate la tensiunea monofazat de 220V/50Hz

n principiu, dou faze snt nseriate i alimentate direct la 220V c.a., iar cea dea treia faz se alimenteaz printrun condensator de defazare teoretic de 90. Cf = condensator de funcionare Cp = condensator de pornire S=buton de start Condensatorul Cp nu este necesar la sarcini sczute.

Butonul de start se menine apsat doar pentru scurt timp, pn cnd motorul intr n turaie. Dac nu montm Cp, nu avem nevoie nici de S.Condensatoarele trebuie s aib tensiunea de strpungere de 400Vc.a.

Dac puterea motorului este cunoscut, curentul In se poate afla cu formulaIn = P/1,73Uncos n care: P = puterea motorului Un = tensiunea reelei n voli = randamentul motorului cos = factorul de putere. Toate aceste date se gsesc de obicei nscrise pe plcua cu datele motorulu.

Simbolizarea , marcarea ,clasificarea si modul de conectare al condensatoarelorTop of Form

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Tutoriale Electronica

Condensatoruleste o componenta de circuit care alaturi de rezistor este utilizata frecvent in circuitele electronice.Daca unui condensator ii este aplicata o tensiune continua U acesta se va incarca cu o sarcina Q ,raportul dintre cele doua marimi sarcina Q si tensiunea U este o marime constanta caracteristica pentru condensatorului ;acest raport se numeste capacitatea condensatorului.

C=Q/UUnitatea de masura pentru capacitate este faradul F utilizandu-se in mod frecvent submultiplii sai:1uf=10^-6 ;1nF=10^-9; 1pf=10^-12FUn condensator este alcatuit din doua armaturi metalice intre care se afla un dielectric.Parametrii condensatoarelorPrincipalii parametrii ai condensatoare sunt :

- capacitatea nominala Cn[F] reprezinta valoare capacitatii condensatorului care trebuie realizata prin procesul de fabricatiesi care este inscrisa pe corpul acestuia .- toleranta t este exprimata in procente si reprezinta abaterea maxima a valorii reale a capacitatii fata de valoarea nominala .- tensiunea nominala Un[V]este tensiunea continua maxima sau tensiunea alternativa eficace maxima care poate fi aplicata continuu la terminalele condensatorului in gama temperaturilor de lucru- rezistenta de izolatie Riz este raportul dintre tensiunea continua aplicata unui condensator si curentul care-l strabate la un minut dupa aplicare tensiunii- rigiditatea dielectrica reprezinta tensiunea maxima continua pe care trebuie sa o suporte condensatorul un timp minim (de obicei un minut ) fara sa apara strapungeri- intervalul temperaturilor de lucru (Tmin-Tmax)reprezinta limitele de temperatura intre care condensatorul functioneaza un timp indelungat.

Clasificarea condensatoarelor

Condensatoarele se pot clasifica dupa mai multe criterii :natura dielectricului,domeniul de frecventa,tipul constructiv,domeniul de utilizare.Din punct de vedere constructiv exista:- condensatoare fixe care isi mentin constanta valoarea capacitatii pe intreaga perioada de functionare- reglabile sunt mai numite semireglabile ,ajustabile sau trimere sae caracterizeaza prin faptul ca capacitatea lor poate fi modificata in limite reduse- variabile sunt condensatore ca caror capacitate poate si trebuie sa fie modificata frecvent intre anumite limite relativ largi (de ex. condensatoarele de acord pentru radio)In functie de natura dielectricului condensatoarele pot fi :- dielectric gazos (aer,vid)- dielectric lichid (ulei mineral sau de transformator,rar fabricate si utilizate)- dielectric solid organic sau anorganic au ca material dielectric sticla ,mica, materiale ceramice iar cu dielectric solid organic hartie ,pelicule sinrtetice nepolare (polistiren ,teflon, polipropilena,politetraflouretilena) si pelicule sintetice polare (poliepolieftlentereftalat, policarbonat,rasinapoliamidica)- dielectric pelicula de oxizi metalici au dielectricul dintr-o pelicula de oxid (Al2O3, Ta2O5, TiO2 ) cei mai utilizati fiind oxizii de aluminiu si tantal

Simbolizarea si marcarea condensatoarelor

a: condensator in general

b: condensator in general simbol tolerat

c: condensator de trecere

d: condensator de trecere simbol tolerat

e: condensator de trecere simbol nestandardizat

f: condensator electrolitic

g: condensator electrolitic simbol tolerat

h: condensator electrolitic simbol nestandardizat

i: condensator variabil

j: condensator variabil simbol tolerat

k: condensator semireglabil

l: condensator semireglabil simbol tolerat

Condensatoarele sunt marcate in clar sau prin culori(inele ,benzi ,puncte)prin simboluri alfanumerice sau cod literal.Indiferent care este sistemul de marcare carcacteristicile ce se inscriu pe corpul con sunt:In mod obligatoriu pe orice tip de condensator:Capacitatea nominalatoleranta valrii nominaleIn mod obligatoriu pe unele tipuri de cond :polaritatea bornelorterminalul conectal la armatura exttensiunea nominalacoeficient de temperatura al capacitatii

Codul numeric pentru marcarea capacitatii nominale este format din trei cifre. Primele doua reprezinta cifrele semnificative a capacitatii, iar a treia este factorul de multiplicare, conform tabelului de mai jos, in care sunt prezentate si citeva exemple.

Factor de multiplicare11010^210^310^4

Cod9R1234

Marcare109221102223474

Cn10pF220pF1 nF22 nF470

CuloareCifra semnFactor de multiplicareToleranteCoef de tempTensiune nominala

CuloareCond cerC cu hartC < 10pFC > 10pFC cu tantC cu stirof

Negru011220010630

Maro110100.11-331.6

Rosu210^210^20.252-754160

Orange310^310^3-2.5-15040-

Galben410^410^4-100-2206.363

Verde510^5-0.55-33016250

Albastru6-----470-25

Auriu

Exemple de condensatori

Conectarea condensatoarelor intr-un circuit se poate face in seria , paralel si mixt .

Pentru fiecare tip de conexiune a condensatorilor va rezulta un condensator ce va avea o capacitateechivalenta.

La conectarea in serie a mai multi condensatori condensatorul echivalent va avea o capacitate echivalenta data de formula :

1/Ce = 1/C1 +1/C2 +..1/Cn

In cazul in care condensatorii sunt conectati in parallel , capacitatea condensatorului echivalent va fi calculatautilizand formula:

Ce=C1+C2+C3++Cn

Calculul capacitatii condensatorilor conectati in conexiune mixta se face prin gruparea condensatoarelor in functie de cum sunt conectati ( serie , parallel ) si calcularea capacitatii acestora pana la reducerea la un singur condensator cu o capacitate echivalenta .

C1 , C1 , C3, Cn reprezinta condensatoarele valoarea capacitati condensatoarelor iar Ce reprezinta valoarea capacitatii echivalente .

Comments

1F=1*10^6 uFSat, 03/09/2013 - 23:42 Eltech1F=1*10^6 uF

reply1F= ?uFSat, 03/09/2013 - 20:50 Catalin (not verified)1F= ?uF

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Figura 1

Figura 2 O schem simpl de pornire a unui motor trifazat la 220V c.a. folosind doar un condensator de funcionare. Sensul de rotaie poate fi schimbat cu comutatorul basculant S. Schimbarea sensuluiNU se va facen timpul funcionrii motorului ci doar dup deconectarea de la reea i dup ce rotorul s-a oprit. Cf se alege din tabelul 1 n funcie de puterea motorului.

Figura 3 Tabelul 1

Maini electrice

Domeniu

ncepnd din anul 1922 atelierul de ntreinere i reparare maini electrice existente n Uzinele de Fier i Domeniile Reia - UDR - (denumirea din aceea perioad a uzinelor din Reia) a nceput s produc i maini electrice noi pentru teri. Etapa marcheaz apariia celei dinti fabrici de maini electrice din Romania.Aici s-au produs diferite motoare trifazate de curent alternativ, motoare i dinamuri de curent continuu, generatoare trifazate pentru centrale electrice, motoare de curent continuu pentru tramvaie, motoare trifazate pentru laminoare, grupuri convertizoare, transformatoare, etc.n anii 1930-1932 la UCM Reia se realizau primele maini electrice n construcie sudat, pionierat n tehnic, iar ncepnd cu 1952 respectiv 1960 producea i generatoare electrice pentru turbine cu abur, respectiv generatoare electrice pentru turbine hidraulice.UCM Reia a pstrat n profil mainile electrice de putere mare i speciale, n general cu caracter de unicat sau cel mult de serie mic.

Descrcai listele de referin cumainile electrice rotativeproduse la UCM Reia n format pdf.

n prezent UCM Reia proiecteaz i produce: motoare asincrone cu rotorul n scurtcircuit i cu rotor bobinat, verticale sau orizontale n domeniul: puteri 50010.000 kW; turaii 3003.000 rpm;

motoare sincrone i generatoare sincrone verticale sau orizontale n domeniul: puteri 50012.500 kW; turaii 1001.500 rpm;

motoare i generatoare de curent continuu n domeniul: puteri 5006.000 kW; turaii 401.000 rpm.

Date semnificative:

1922 - intr n funciune la Uzinele din Reia, fabrica de motoare electrice, prima de acest fel din Romania;

1924 - fabricarea primului generator electric de 1.800 kVA.

UCM Reia a livrat peste 1,5 mil. kW motoare electrice.

Maini electrice asincrone

Domenii de utilizare

Domeniul parametrilor

Descrcailista de referincu cele mai reprezentative motoare asincrone fabricate la UCM Reia, n format .pdfMaini electrice sincrone

Domeniul parametrilor

Descrcailista de referincu cele mai reprezentative motoare sincrone fabricate la UCM Reia, n format .pdfMaini electrice de curent continuu

Domeniul parametrilor

Descrcailista de referincu cele mai reprezentative motoare de curent continuu fabricate la UCM Reia, n format .pdf

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