Double excited synchronous machines (DESM)

Electric drives based on permanent magnet synchronous machines (PMSMs) can be found in a wide range of industrial applications. Among the advantages of these devices, it's important to highlight the improvement in energy efficiency due to the use of permanent magnets. However, there are a number of disadvantages associated with the constant excitation flux created by permanent magnets. Indeed, variable-speed operation, and especially high-speed operation, of these machines is more problematic than that of machines with wound excitation, for example. High-speed operation requires the implementation of a so-called "defluxing" algorithm, which consists of injecting an armature current with a negative component in the direct axis. However, this type of operation is only possible if the converter connected to the machine is controllable, and it is not free from risk, namely the risk of demagnetizing the magnets. What's more, "defluxing" can only be effective if the machine's armature magnetic reaction is strong enough to counteract the excitation flux of the permanent magnets. It should be noted that a sufficiently strong armature magnetic reaction is synonymous with a poor power factor.

Double-excitation synchronous machines (DESMs) attempt to overcome these drawbacks by combining the advantages of permanent magnet machines (high energy efficiency) with those of controllable excitation machines (ease of operation at variable speed). The excitation flux in these machines is the sum of a flux created by permanent magnets and an excitation flux created by coils.

In this dossier, we outline the principle of operation and sizing of MSDEs with :

• the operating principle of double excitation machines, as well as the various structures based on this principle (§ 2 );

• the principles of variable speed MSDE control, using simple first harmonic models of the machine and assuming a sinusoidal power supply (§ 3