Basic Working Of Generators

Basic Working Of Generators: An electric generator is a device designed to take advantage of electromagnetic induction in order to convert movement into electricity. The phenomenon of induction (introduced in Section 1.5.4) can be summarized as follows: an electric charge, in the presence of a magnetic field in relative motion to it—either by displacement or changing intensity—experiences a force in a direction perpendicular to both the direction of relative motion and of the magnetic field lines. Acting on the many charges contained in a conducting material—usually, electrons in a wire—this force becomes an electromotive force (emf ) that produces a voltage or potential drop along the wire and thus causes an electric current (the induced current) to flow. 

A generator is designed to obtain an induced current in a conductor (or set of conductors) as a result of the mechanical movement, which is utilized to continually change a magnetic field near the conductor. The generator thus achieves a conversion of one physical form of energy into another— the energy of motion into electrical energy— mediated by the magnetic field that exerts forces on the electric charges. In this sense, a generator is the opposite of an electric motor, which accomplishes just the reverse: the motor converts electrical energy into mechanical energy of motion, likewise mediated by the magnetic field. As far as the physical principles are concerned, electric generators and motors are very similar devices; in fact, an actual generator can be operated as a motor and vice versa. To achieve the best possible performance, however, there are many subtleties of design that specialize in a given machine for one or the other task.1 These subtleties have to do almost exclusively with geometry (though the choice of materials may also be important). Indeed, geometry is what distinguishes the many different types of specialized generators: the particular way in which the conducting wires are arranged within the generator determines the spatial configuration of the magnetic field, which in turn affects the precise nature of the current produced and the behavior of the machine under various circumstances.

A comprehensive discussion of the many specific types of generators in common use would far exceed the scope of this text. This chapter instead concentrates on three cases: first, we discuss a greatly simplified device that can work as a motor or generator (a small version of which the reader may actually build) in order to illustrate the basic principles of the generator function. Second, we describe at some length the standard type of generator used in utility power systems, the synchronous generator. Rather than going into the details of its construction and the many subtle variations among specific designs, we focus on the operation of generators in the system context, emphasizing the means by which synchronous generators control such variables as voltage, frequency, real and reactive power, and stressing, in particular, the interaction among generators, which is fundamental to the overall performance and stability of an alternating current (a.c.) system. Third, we briefly describe the induction generator, which is used in some specific applications such as wind turbines and which has some distinct and important properties. 

Unlike most engineering texts on this subject, we do not discuss in any detail the ramifications of various design choices for generators, nor do we use mathematical derivations beyond the initial description of the basic induction phenomenon. Rather, the goal is to develop a conceptual understanding of the workings of a generator, including its operating constraints and limitations, so as to appreciate its function from the perspective of the power system as a whole.

This chapter also omits discussion of the prime mover, or whatever energy source pushes the turbine because those aspects of power plants are well covered in standard texts on energy. The most common assumption for utility-scale generators is that in some sort of boiler water is turned to high-pressure steam and that while being guided to expand through a set of fanlike turbine blades, this steam forces the turbine to rotate. For our purposes of understanding the electromagnetic phenomena inside a generator, it is irrelevant what boils the water; it could be burning hydrocarbons, fissioning atoms, or concentrated sunshine, or the turbine might simply be pushed by cold water running downhill. All we assume in this chapter is that any generator’s rotor, or the part that rotates, is mechanically connected to something—usually by being mounted on a single rotating steel shaft—that continually exerts a force and expends energy to make it turn.