Synchronous machines - General principles and structures

Electromechanical energy conversion plays an important role in energy processing in the broadest sense. Electric power generation, largely carried out by rotating groups comprising synchronous machines, consumes a third of the world's primary energy, and electric motors convert a significant proportion (around two-thirds in industrial applications) of this electricity [1] . The power range of electromechanical converters, all fundamentally reversible, extends over more than 10 decades, from thermomechanical production units of more than 1 GW to the smallest actuators of micromechanical systems of a few microwatts. Synchronous machines are widely represented and occupy an ever-growing share of the market. They encompass a wide variety of structures, but all share one immutable principle: the essential synchronism between the frequency of the supply currents and the relative movement of the fixed and moving parts. In fixed-frequency generator operation, they have been in use for over a century. Among the many qualities that have made them so successful are their naturally higher efficiency than other machines, their ability to regulate reactive power, and their considerable design freedom. Over the past few decades, however, these machines have been increasingly used in variable-speed motor operation, frequently employing permanent magnets in an ever wider power range (from the lowest powers of a few microwatts to several megawatts). Developments in power and control electronics have largely contributed to their widespread use. They can be found in electric traction, high-speed machining equipment, robotics, watchmaking, home automation, the automotive industry and computer peripherals [2] .

To meet this broad spectrum of applications, there is a huge variety of synchronous architectures and arrangements. However, this topological richness does not translate into fundamental differences in terms of physical operating principles. Electromechanical conversion from electromagnetic sources is always based on a single physical principle. The implementation of this principle, using different techniques, and the constraints linked to the envisaged application then naturally generate this architectural diversity.

First, the general operating principle of electromechanical converters, and more specifically synchronous converters, is reviewed, and their operation is characterized. Focusing on cylindrical machines with rotating fields, the main types of synchronous machines and their operating modes are then presented.

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