Читать книгу Handbook of Large Hydro Generators - Geoff Klempner - Страница 30

For a more in depth discussion of the operation and control of hydro generators, the reader is referred to Chapter 4. In this chapter, the most elementary principles of operation of synchronous machines will be presented. As mentioned above, hydro generators are almost always three‐phase machines. Thus, the best place to start describing the operation of a three‐phase synchronous machine is a description of its magnetic field.

Earlier, we described how a current flowing through a conductor produces a magnetic field.

Figure 1.6-5 Typical winding configurations.

Source: Courtesy of Voith.

It was also shown that by coiling the conductor, a larger field is obtained for the same current magnitude. Recall, that if the three phases of the winding are distributed at 120 electrical degrees apart, three balanced voltages are generated, creating a three‐phase system.

Now, a new element can be brought into the picture. By a simple mathematical analysis, it can be shown that three balanced currents (of equal magnitudes and 120 electrical degrees apart) flow in a balanced three‐phase winding when a magnetic field of constant magnitude is produced in the airgap of the machine.

This magnetic field revolves around the machine at a frequency equal to the frequency of the currents flowing through the winding (see Figure 1.7-1). As shown, a constant magnitude and constant rotational speed magnetic flux is created when three‐phase balanced currents flow through a three‐phase symmetrical winding. The sketch is for a four pole winding, however, similar result applies for any number of pairs of poles.

The importance of a three‐phase system creating a constant field cannot be stressed enough. The constant magnitude flux allows power, megawatts, to be transformed inside an electric machine from electrical to mechanical power, and vice versa. It is important to remember that a constant‐magnitude flux produces a constant‐magnitude torque.