Motor Effect

6 Key excerpts on "Motor Effect"

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  eBook - ePub

  Practical Electricity for Aviation Maintenance Technicians

  • Dale Crane, Dennis Wilt(Authors)
  • 2017(Publication Date)
  • Aviation Supplies & Academics, Inc.

    (Publisher)

  8 Electric Motors and Generators

  Electric motors have become such a standard part of our lives that they are usually taken for granted. They are made in all sizes and power outputs, from the tiny motors that move the hands in analog wrist watches to the motors that drive ocean-going ships. Regardless of their size, all electric motors work on the same principle. One magnetic fields reacts with another magnetic field to produce a physical force.

  Figure 8-1 shows the basic way an electric motor works. The conductor (represented by the circle) in view A has no current flowing in it, and the lines of flux pass straight across the space from the north pole of the magnet to the south pole. But when current flows in the conductor as in view B, it produces a magnetic field, which surrounds the conductor.

  Figure 8-1 . When the magnetic field surrounding the conductor distorts the lines of flux between the poles of the magnet, a force is produced that tries to move the conductor out of the magnetic field.

  The lines of flux between the poles of the magnet try to remain as short as possible, and when they are distorted by the field surrounding the conductor, they produce a physical force that tries to move the conductor to the left, out of their field.

  The right-hand rule for motors helps understand this action. Hold the fingers of your right hand as shown in Figure 8-2, with the forefinger pointing in the direction of the lines of flux (from the north pole of the magnet to the south pole) and the second finger pointing in the direction of electron flow in the conductor (from negative to positive); the thumb will point in the direction the conductor will move. The amount of force that acts on the conductor is determined by the strength of the two magnetic fields.

  Figure 8-2

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  eBook - ePub

  Electric Motors and Drives

  Fundamentals, Types and Applications

  • Austin Hughes, Bill Drury(Authors)
  • 2019(Publication Date)
  • Newnes

    (Publisher)

  It is worth seeing what can be learned from these equations because, as noted earlier, this simple elementary ‘motor’ encapsulates all the essential features of real motors. Lessons which emerge at this stage will be invaluable later, when we look at the way actual motors behave.

  1.7.1 Motoring and generating

  If the e.m.f. E is less than the applied voltage V , the current will be positive, and electrical power will flow from the source, resulting in motoring action in which energy is converted from electrical to mechanical form. The first term on the right hand side of Eq. (1.22) , which is the product of the motional e.m.f. and the current, represents the mechanical output power developed by the primitive linear motor, but the same simple and elegant result applies to real motors. 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.

  Now let us imagine that we push the conductor along at a steady speed that makes the motional e.m.f. greater than the applied voltage. We can see from the equivalent circuit that the current will now be negative (i.e. anticlockwise), flowing back into the supply and thus returning energy to the supply. And if we look at Eq. (1.22) , we see that with a negative current, the first term (-VI ) represents the power being returned to the source, the second term (-EI ) corresponds to the mechanical power being supplied by us pushing the rod along, and the third term is the heat loss in the conductor.

  For readers who prefer to argue from the mechanical standpoint, rather than the equivalent circuit, we can say that when we are generating a negative current (− I ), the electromagnetic force on the conductor is (− BIl ), i.e. it is directed in the opposite direction to the motion. The mechanical power is given by the product of force and velocity, i.e. (− BIlv ), or − EI , as above.

  The fact that exactly the same kit has the inherent ability to switch from motoring to generating without any interference by the user is an extremely desirable property of all electromagnetic energy converters. Our primitive set-up is simply a machine which is equally at home acting as a motor or a generator.

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  eBook - ePub

  Low Frequency Electromagnetic Design

  • Perry(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  4 TORQUE AND BRAKING IN A MAGNETIC FIELD

  When an electrical conductor moves in the presence of a stationary magnetic field, “eddy” currents are induced within the conducting material. These currents tend to shield the conductor from the magnetic field lines and therefore alter the field inside and outside the conductor. Due to the induced currents, electromechanical “Lorentz” forces act on the conductor which impede its motion. These forces also create joulean heating as a result of Ohm’s law and conductor temperature tends to increase. This interaction is the physical basis for all types of devices such as motors, generators, actuators, and brakes.

  As we have noted in previous chapters, eddy currents are also induced in stationary conductors when ac excitation is employed. These currents also tend to reduce the magnetic field within the conductor and therefore, to increase the effective resistance of the conductor to current flow. This is the well known skin effect phenomenon which plays an important role in radio frequency and power systems.

  The fact that the skin effect principle applies in electromechanical as well as ac excitation problems is not a physical coincidence. When a stationary conductor is subjected to an alternating magnetic field, solutions to the diffusion equation dictate the magnetic field and current density distribution. These solutions are “evanescent” waves which decay in space and time with an exponential wave number which is reciprocally proportional to the “skin-depth.” It is therefore easy enough to understand how “skin effect” occurs in this case.

  It is superficially not as obvious why the skin effect principle applies in electromechanics also. The connection between the two cases can be qualitatively understood by considering a thought experiment by the observer of a conductor in uniform motion with respect to a stationary magnetic field. The observer changes reference frames by imagining himself at rest with respect to the conducting body. From this frame, the magnetic field is then perceived to be in uniform motion at the same relative speed with respect to the fixed conductor. The moving magnetic field can then be thought of as the sum of two alternating fields which are phase-displaced in space and time. Once decomposed, the total field solution in the conductor rest frame is a superposition of solutions of the diffusion equation

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  Discoveries and Inventions of the Twentieth Century

  • Edward Cressey(Author)
  • 2018(Publication Date)
  • Routledge

    (Publisher)

  The principles upon which nearly all methods for the production and application of electrical power depend were discovered by Michael Faraday between 1830 and 1832. He showed that if a magnet be moved so as to approach a coil of wire the coil has a current of electricity produced within it. If the magnet is withdrawn a current in the opposite direction to the first is produced. In fact any movement of the magnet such that the lines of force in its field cut the wire causes a current, the direction of which depends upon the direction in which the lines move. As a wire conveying a current has a magnetic field it is capable of acting on another wire in the same way, either by actually moving either wire, or by causing the strength of the current to alter so that the lines move inwards or outwards, and thus cut the second wire in their motion.

  If the reader can keep these actions and reactions in his mind he will have no difficulty in understanding how nearly all the apparatus employed in the practical applications of electricity work. He must always picture a conductor conveying a current as surrounded by a field of magnetic force, and he may, as a rule, assume that iron is used merely to concentrate this force and to give it direction. If either a wire carrying a current, or a piece of iron, is free to move, the movement will take place in such a way that the greatest possible number of lines of force pass through the iron. For example, a coil of wire through which a current of electricity is passing (Fig. 60 ) will “suck-up” an iron rod until the latter protrudes equally at either end, and will exert considerable force in so doing. Two conductors carrying currents act upon one another by reason of the magnetic fields with which each is accompanied. And every electrical machine may be regarded as a magnetic machine in which the magnetism is produced by electric currents.

  Fig. 63 . RECTANGULAR COIL ROTATING BETWEEN POLES OF FLELD MAGNETS .

  It is customary to express the strength of a magnetic field by the number of imaginary lines of force per square centimetre— measured at right angles to the direction of the force. Into the exact meaning of this it is not necessary to enter here, but it may be stated that the e.m.f. produced in a conductor is proportional to the number of lines cut per second; that is, to the strength of the field and the velocity with which the conductor moves. And as the movement of conductors in a magnetic field is the method by which electricity is invariably generated for practical purposes, we may proceed to consider the construction and mode of working of generators, or dynamos as they are more usually called.

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  Basic Physics

  • Kenneth W Ford(Author)
  • 2016(Publication Date)
  • WSPC

    (Publisher)

  * The reader need hardly be reminded that current in wires remains today a vitally important aspect of electromagnetism. It is probably fair to say that through the utilization of current more than in any other single way, physical science has had its impact on the everyday life of man. In the more technically advanced countries, electricity is available in nearly every home for heat, for light, for refrigeration, and for turning the motors of vacuum cleaners or mixers or power saws or a dozen other gadgets. Equally important, power supplied by electric current does most of the heavy work in the industries turning out manufactured goods. Those societies least advanced technically are the ones with the least utilization of electric power. In those areas of the world little touched by the implications of scientific discovery, the dominant impact of physical science in the future will likely come through the introduction of electric current and electric power.

  According to the fundamental laws of electromagnetism summarized in the preceding section, the magnetic field created by a moving charge is always weaker than its electric field. This should be especially true in wires, where the electrons move at speeds far less than the speed of light. However, in a wire the total positive charge is almost exactly equal to the total negative charge. Outside the wire the electric effects cancel because of the wire’s neutrality. But because the electrons in the wire are moving along the wire in one direction and the positively charged ions are not moving, the magnetic effects do not cancel. Outside the wire the countless tiny magnetic contributions add together to produce a possibly very strong magnetic field. In this important way the current in a wire differs from the flow of charged particles in a beam through space. The electrons in a cathode-ray tube produce electric as well as magnetic effects. The electrons in a wire produce purely magnetic effects.

  Surrounding a straight current-carrying wire is a magnetic field whose lines describe circles with the wire as an axis (Figure 16.16 ). As indicated in the figure, an alternative to moving the palm according to right-and-left rule 2 is curving the fingers to show the direction of the magnetic field. (This is not

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  eBook - ePub

  Electromagnetism

  • I. S. Grant, W. R. Phillips(Authors)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  B (sometimes called the magnetic induction field).

  The current I 1 in the wire shown on Figure 4.2 gives rise to a field B 1 which describes the forces on the moving charges constituting the current I 2 in the other wire. The current I 2 can be seen from Equation (4.3) to be proportional to the product of the mean speed of the electrons and the electronic charge. Experiments show that the force on the wire carrying the current I 2 is proportional to I 2 , for a fixed current in the first wire. Hence, in a constant magnetic field, the magnitude of the force on a moving charge is proportional to the product of the charge q and the speed υ of the charge.

  We may do more experiments to determine how the force on a moving charge depends on its velocity and the magnetic field in which it moves. Let us make the reasonable assumption that the field B in the middle of the gap between the parallel pole faces of a large permanent magnet is uniform and in a direction perpendicular to the pole faces, as shown in Figure 4.3 . Experiments with charged particles moving in the gap show that the particles always experience a force F which is perpendicular both to the field direction and to the velocity v of the particles. For example the electron beam of a cathode ray tube placed in the gap is deflected parallel to the pole faces. The magnitude of the force is found to be proportional to the speed of the particle, its charge q, and to the sine of the angle between the vectors v and B. These experiments are illustrated schematically in Figure 4.4

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