Читать книгу Handbook of Large Hydro Generators - Geoff Klempner - Страница 15

Certain materials found in nature exhibit a characteristic to attract or repel each other. These materials, called magnets, are also called ferromagnetic because they include the element iron as one of their constituent elements. Magnets always have two poles: one called north, the other called south. Two north poles will repel each other, as will two south poles. However, north and south poles will attract each other. A magnetic field is defined as a physical field established between two poles. Its intensity and direction determine the forces of attraction or repulsion existing between the two magnets.

Figures 1.1-1 and 1.1-2 are typical representations of two interacting magnetic poles and the magnetic field established between them.

Figure 1.1-1 Representation of two magnetic poles of opposite polarity, with the magnetic field between them shown as “lines of force.”

Magnets are found in nature in all sorts of shapes and chemical constitution. Magnets used in industry are artificially made. Magnets that sustain their magnetism for long periods of time are denominated “permanent magnets.” The magnetic field produced by the north and the south pole of a permanent magnet is directional from north to south as shown in Figure 1.1-3. These are widely used in several types of electric rotating machines, including synchronous machines. However, due to mechanical as well as operational reasons, permanent magnets in synchronous machines are restricted to those with ratings much lower than large hydraulic (“hydro”) turbine‐driven generators, which is the subject of this book. Hydro generators take advantage of the fact that magnetic fields can be created by the flow of electric currents in conductors, see Figure 1.1-4.

The direction of the lines of force is given by the “law of the screwdriver”: mentally follow the movement of a screw as it is screwed in the same direction as that of the current; the lines of force will then follow the circular direction of the head of the screw. The magnetic lines of force are perpendicular to the direction of current. A very useful phenomenon is that forming the conductor into the shape of a coil can augment the intensity of the magnetic field created by the flow of current through the conductor. In this manner, as more turns are added to the coil, the same current produces larger and larger magnetic fields. For practical reasons, all magnetic fields created by current in a machine are generated in coils as shown in Figure 1.1-5.

Figure 1.1-2 Representation of two north poles and the magnetic field between them. South poles will create similar field patterns, but the lines of force will point toward the poles.

Figure 1.1-3 Representation of a “permanent magnet” showing the north and south poles and the magnetic field between them flowing from north to south outside the magnet.

Figure 1.1-4 Representation of a magnetic field created by the flow of current in a conductor.

The use of coils to amplify the magnetic field intensity requires them to be constructed in a very specific manner so that the resulting flux is produced in an effective way. When the coil is operating in air, the magnetic field direction, shape, and intensity depends on the number of turns in the coil, the size of the coil, and the direction of electric current flow in the coil winding. The flux produced is basically divided into two types. One is the effective flux that links the entire coil and does the useful work, and the other is the leakage flux which is a more localized effect and does no useful work. In fact, the leakage flux creates additional losses that make the coil less efficient, electromagnetically speaking (see Figure 1.1-6). The principles illustrated here become very important later on as we discuss the magnetic field in the generator and stray losses.

Figure 1.1-5 Representation of a magnetic field produced by the flow of electric current in a coil‐shaped conductor.

Figure 1.1-6 Representation of a magnetic field produced by the flow of electric current in a coil‐shaped conductor operating in air, showing the effective and leakage flux components of the magnetic field produced.

To use the flux produced in a coil as effectively as possible, highly permeable ferromagnetic materials are used to capture and direct the flux so that the amount of leakage flux is minimized. This allows the coil to do more useful work and keeps losses to a minimum. Iron in various derivatives is by far the most widely used material because it has all the magnetic characteristics required. It is structurally suitable, and cost‐effective. When an “iron” core is used within the coil, and current is flowing, the magnetic field produced is shaped effectively, and the iron core essentially becomes a north–south magnet in the process (see Figure 1.1-7). This is why stator cores and rotor poles of generators are made of steel, containing iron and a few small quantities of additional elements. The iron allows the principles discussed above to become a reality and is one of the reasons generators can be built to at least 97.5% efficiency.

Figure 1.1-7 Representation of a magnetic field produced by the flow of electric current in a coil‐shaped conductor with an “iron” core. The majority of the field produced is effective flux and the leakage field is reduced to a minimum.