Motor Effect

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  Electric Motors and Drives

  Fundamentals, types and applications

  • Austin Hughes(Author)
  • 2013(Publication Date)
  • Newnes

    (Publisher)

  We shall see that in order to make the most of the mechan-ism, we need to arrange for there to be a very strong magnetic field, and for it to interact with many conductors, each carry-ing as much current as possible. In some motors, it will be easy to differentiate between the parts of the motor which are responsible for setting up the magnetic field, and the parts carrying the conductors on which the forces act. In others, such as the induction motor, the physical distinction is not obvious, but we will find it helpful to think along the same lines to assist in understanding how such motors work. We will look first at what determines the magnitude and direction of the force, before turning to ways in which the mechanism is exploited to produce rotation. The concept of the magnetic circuit will have to be explored, since this is central to understanding why motors have the shapes they do. Magnetic flux and flux density will crop up continuously in the discussion, so a brief introduction to the terms is in-cluded for those who are not already familiar with the ideas involved. of it. In the course of the discussion, however, several very important general principles and guidelines emerge. These apply to all types of motor and are summarized in the con-cluding section. Experience shows that anyone who has a good grasp of these basic principles will be well equipped to weigh the pros and cons of the different types of motor, so all readers are urged to absorb them before tackling other parts of the book. Electric Motors 3 Electromagnetic force When a current-carrying conductor is placed in a magnetic field, it experiences a force. Experiment shows that the mag-nitude of the force depends directly on the current in the wire, and the strength of the magnetic field, and that the force is greatest when the magnetic field is perpendicular to the conductor. The direction of the force is shown in Figure 1.1, and is at right angles to both the current and the mag-netic field.

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  Practical Electric Motor Handbook

  • Irving Gottlieb(Author)
  • 1997(Publication Date)
  • Newnes

    (Publisher)

  In a Electric motor generalities 7 Fig. 1.6 Motor action exerted on current-carrying conductor in a magnetic field. Endowing magnetic lines of force with the elastic property of rubber- bands, enables one to visualize the motion imparted to a current-carrying conductor. The interaction of the magnetic fields as shown is found in vir- tually all electric motors. Downward motion of the conductor would occur if either (not both) the current direction or the magnetic poles were reversed. Note: Conventional current-flow is used in this book. general, but inviolate way, it tells us that 'any change in magnetic flux linkage is accompanied by effects opposing the change'. The electrlc motor as an energy converter At the very outset, we should concern ourselves with what electric motors do. A popular but erroneous notion is that electric motors create or produce mechanical energy. Mechanical energy is definitely not created; yes, it may be said to be produced at the shaft of the motor, but this is, at best, only a partial answer. We must point out that this mechanical energy comes at the expense of some other form of energy. The simple and true fact of the matter is that the electric motor (and the electric generator, as well) is an energy converter. More specifically, the motor converts electrical energy into mechanical energy. In so doing, it is never 100% efficient-in the overall budget of energy availability, there are always inevitable energy losses. These losses may manifest themselves as still other forms of energy, such as heat, light, sound, friction, radiation, etc. Energy, itself is the capability of doing work. In the practical world, it would be well to say that available energy represents the capability of doing usefulwork. Because of nature's previous activities, most of the useful energy 8 Practical Electric Motor Handbook ,, , . . . . sources stem from various chemical, gravitational, and nuclear arrangements of planetary matter.

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  Magnetic Fields

  A Comprehensive Theoretical Treatise for Practical Use

  • Heinz E. Knoepfel(Author)
  • 2008(Publication Date)
  • Wiley-VCH

    (Publisher)

  Typically, a dc motor is driven by the torque that results from the interaction ofcurrent-carrying loops on the rotor with a constant magnetic field fixed with the stator. Thechange in current direction (for the torque to always result in the same direction) is operated by the commutator that alternatively connects the loops with anouter dcvoltage source. Often dcmotors use permanent magnets for the stator magnetic field source6 393 5'. The linear motor (e) is basically a linear deployment ofthe rotating machine; for example, there are linear induction motors625 and linear dc motors640. For a simple arrangement ofthe magnetic pump (g) consider a conducting fluid in an insulating duct, where a transverse field B is applied anda transverse perpendicular current j is made to flow. Theresulting force F will force the fluid to flow along the duct. Forces within a conductor Atthis point it is instructive to find out how the magnetic Lorentz force (6.1-1) and, consequently, the volume force (6.1-10) act onaconductor. By its very nature, the volume 340 CHAPTER 6 MAGNETIC FORCES AND THEIR EFFECTS vd Figure 6.2-14 Electron drift and forces in a conducting rectangular slab. force acts primarily onthe current carrier-that is, the electrons in a metal. Fora thin-wire conductor, we canroughly saythat, since the electrons are not free toleave thewire through its surface, the force they experience in anouter field B is transmitted to the wire itself [Biot-Savart force (6.1-4)]. However, inreality the situation ismore complex: It relates to the Hull effect. Theelectrons flowing with drift velocity vd along the conductor (figure 6.2-14) experience the Lorentz force vd xB, thereby accumulating on oneside of the conductor; this establishes anelectric Hall field EH, which inturn acts on the electrons. For completeness' sake, we may even addanadditional transverse force FL deriving from various interaction processes ofthe electrons with the lattice' 2'.

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  Electric Motors and Drives

  Fundamentals, Types and Applications

  • Austin Hughes(Author)
  • 2013(Publication Date)
  • Newnes

    (Publisher)

  Our primitive set-up is simply a machine which is equally at home acting as motor or generator. A further important point to note is that the mechanical power (the first term on the right hand side of equation 1.20) is simply the motional e.m.f. multiplied by the current. This result is again universally applicable, and easily remembered. We may sometimes have to be a bit careful if the e.m.f. and the current are not simple d.c. quantities, but the basic idea will always hold good. Motoring condition Motoring implies that the conductor is moving in the same direction as the electromagnetic force (BIl), and at a speed such that the back e.m.f. (BLv) is less than the applied voltage V. In the discussion so far, we have assumed that the applied voltage is adjusted so that the current is kept constant. This was a helpful approach to take in order to derive the steady-state power relationships, but is seldom typical of normal operation. We therefore turn to how the moving conductor will behave under conditions where the applied voltage Vis constant, since this corresponds more closely with operation of a real motor. Behaviour with no mechanical load If we begin with the conductor stationary when the voltage Vis first applied, the current will immediately rise to a value of V/R, since there is no motional e.m.f. and the only thing which limits the current is the resistance. (Strictly we should allow for the effect of inductance in delaying the rise of current, but we choose to ignore it here in the interests of simplicity.) The current will be large, and a high force will therefore be 28 Electric Motors and Drives Figure 1.13 Dynamic behaviour of the elementary motor with no mechanical load We note that in this idealised situation (in which there is no load applied, and no friction forces), the conductor will continue to travel at a constant speed, because with no nett force acting on it there is no acceleration.

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  General Physics. Electromagnetism Optics

  • Pierluigi Zotto, Sergio Lo Russo, Paolo Sartori(Authors)
  • 2023(Publication Date)
  • Società Editrice Esculapio

    (Publisher)

  Mechanical Effects of a Magnetic Force 9.1 Introduction A magnetic field acts on charges in motion and it is defined through the effects that a magnetic force exerts on them. Understanding the mechanical action of a magnetic field on moving charges is therefore compulsory. 9.2 Motion of a Charge in a Uniform Magnetic Field The motion of a point-like charge q in a magnetic field is determined by Lorentz force  F = q  v ×  B . The velocity of a charge can always be expressed as the vector sum of a component  v  directed parallel to the field and a component  v ⊥ normal to it  v =  v  +  v ⊥ = v cos θ  u  + v sen θ  u ⊥ , where θ is the angle between vectors  v and  B. Then, the Lorentz force can be written as  F = q  v ×  B = q  v  +  v ⊥ ( ) ×  B = q  v ⊥ ×  B , being  v  ×  B = 0. † If a magnetic field is uniform and constant the motion in the two directions, parallel and normal to  B, can be separated and studied independently: in the direction parallel to  B the motion is rectilinear uniform, since no component of the force is present in this direction; in B v θ P + q v v – q v B + q v B F F 135 † Drawings in transverse section use symbol to represent vectors normal to the plane of the sheet and directed towards the observer and symbol to represent vectors normal to the plane of the sheet and directed away from the observer. 9 the direction normal to  B the motion is circular uniform, since the force, acting in the di- rection normal to the velocity, can change only its direction. The equation of motion on the plane normal to the magnetic field is therefore q  v ⊥ ×  B = m v ⊥ 2 R  u n , so the radius of the circumference covered during the circular motion is qBv sin θ = m v 2 sin 2 θ R ⇒ R = m v sin θ qB , proportional to the charge velocity and inversely proportional to the charge value and to the magnitude of the magnetic field.

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  Hall Effect Devices

  • R.S. Popovic(Author)
  • 2003(Publication Date)
  • CRC Press

    (Publisher)

  Chapter 3 Galvanomagnetic effects This chapter is devoted to the physical effects basic to the operation of Hall and magnetoresistance devices. These effects are called the Hall effect and the magnetoresistance effect. The Hall effect is the generation of a transverse electromotive force in a sample carrying an electric current and exposed to a perpendicular magnetic field. Depending on the sample geometry, this electromotive force may cause the appearance of a transverse voltage across the sample, or a current deflec-tion in the sample. The generation of this transverse voltage, called the Hall voltage, is the generally known way for the appearance of the Hall effect [1]. The magnetoresistance effect is an increase in sample resistance under the influence of a magnetic field. Both the Hall effect and the magnetoresistance effect belong to the more general class of phenomena called galvanomagnetic effects. Thence the title of this chapter. Galvanomagnetic effects are the manifestations of charge transport phenomena in condensed matter in the presence of a magnetic field. Taking the direction of the electric field as a reference, the Hall effect could be defined as a transverse isothermal galvanomagnetic effect. The magnetoresistance effect is a longitudinal isothermal galvanomagnetic effect. Galvanomagnetic effects also include non-isothermal galvano-magnetic effects, known as galvanothermomagnetic effects. A general treatment of galvanomagnetic effects can be found in several books, for example [2–8]. Our goal in this chapter is to understand the physical background of the Hall and the magnetoresistance effects. As we shall see, the two effects have a common origin and always coexist in a sample. We shall begin by discussing a simplified model of the Hall effect. This will give us a rough idea of the Hall effect and help to identify more detailed issues.

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  Science In The Making

  1850-1900

  • E. A. Davis(Author)
  • 1997(Publication Date)
  • CRC Press

    (Publisher)

  206 SCIENCE IN THE MAKING applied to Magnetic Phenomena. 17.1 coming more numerous towards the right. It may be shown that if the force increases towards the right, the lines of force will be curved towards the right. The effect of the magnetic tensions will then be to draw any body towards the right with a force depending on the excess of its inductive capacity over that of the surrounding medium. We may suppose that in this figure the lines of force are those surrounding an electric current perpendicular to the plane of the paper and on the right hand of the figure. These two illustrations will show the mechanical effect on a paramagnetic or diamagnetic body placed in a field of varying magnetic force, whether the increase of force takes place along the lines or transverse to them. The form of the second term of our equation indicates the general law, which is quite inde-pendent of the direction of the lines of force, and depends solely on the manner in which the force varies from one part of the field to another. We come now to the third term of the value of X, 1 (d{j da) -p,{j 47T dx -dy · Here p,{j is, as before, the quantity of magnetic induction through unit of area perpendicular to the axis of y, and f;-~= is a quantity which would disappear if adx+f3dy+rydz were a com-plete differential, that is, if the force acting on a unit north pole were subject to the condition that no work can be done upon the pole in passing round any closed curve. The quantity repre-sents the work done on a north pole in travelling round unit of area in the direction from +x to +y parallel to the plane of xy. Now if an electric current whose strength is r is traversing. the axis of z, which, we may suppose, points vertically upwards, then, if the axis of x is east and that of y north, a unit north pole will be urged round the axis of z in the direction from x to y, so that in one revolution the work done will be = 47Tr.

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