Basic Principles of Motors and Generators_2_No_Eskimo

Basic Principles of Motors and Generators_2_No_Eskimo -...

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1 Basic Principles of Motors and Generators Prof. George Shoane March 12, 2010 Introduction The relationship between electricity and magnetism are provided by two fundamental laws: Ampere’s and Faraday’s laws. Ampere’s law states that electric charge in motion (current) gives rise to a magnetic field. Conversely, Faraday’s law states that a changing magnetic field gives rise to a voltage (and thus a current). These laws provide the linkage between electricity and magnetism, and serve as the foundation for understanding electro- magnetic devices. When applied to certain classes of devices, these laws become embedded in other laws or specific equations, as in the case for moving coil transducers and motors. Moving-Coil Transducers An important class of electromagnetic devices is moving coil transducers, in which the conductor is free to move in a magnetic field. These include microphones, loudspeakers, electric motors and generators. In these transducers, the relationships between magnetic field, current, and the induced force is described by the Lorentz equation: f = q u x B (1) where force, f , is the force on charge q, u is the movement of q, and B is the magnetic field flux density, with vector direction emanating out of the north pole towards the south pole. For simplicity but without a loss of generality, we assume the three vectors, f , u and B , are mutually orthogonal. The directions of the vectors are often described by Fleming’s left hand rule for motors and right hand rule for generators. Fleming Left Hand Rule (LHR) for Motors The basic principle for the Fleming LHR is that current flow causes movement of a freely-moving conductor in space. Thus, if a current is applied through the conductor in a magnetic field B , where u is in the direction of current, then f would be in the direction of the induced deflection of the conductor. Fig. 1. Fleming’s Left Hand Rule.
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2 In applying the LHR rule, the THumb represents THrust (f), the First finger represents the magnetic Field (B), and the seCond finger represents C urrent. Hence, if one points the First finger in the direction of the magnetic Field (B, from N to S), and the seCond finger in the direction of Current flow, the Thumb will point in the direction of Thrust or displacement of the conductor. As an aid to remember when to use the LHR, remember that the Motorist sits on the Left side of the vehicle. Fleming Right Hand Rule (RHR) for Generators The basic principle for the Fleming RHR is that the forced movement of a freely-moving conductor in space generates an electrical potential. Thus, if a force f is applied to a freely-moving conductor in a magnetic field B , then a charge will be displaced in the direction of u. If one points the First finger in the direction of the magnetic Field B, and the Thumb in
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Basic Principles of Motors and Generators_2_No_Eskimo -...

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