Draft:Machines synchrones à double excitation

Double Excitation Synchronous Machines :

Definition :

Alternateur sans balai auto-excite.

Double excitation synchronous machines (DESMs) are advanced electric machines that use two distinct sources of excitation to optimize their performance. They combine the benefits of permanent magnets, which provide a constant and lossless excitation flux, with wound excitation, which allows precise and variable magnetic flux control. This hybrid configuration enables more efficient management of load and speed variations, thus enhancing the overall stability and energy efficiency of the machine. DESMs outperform traditional synchronous machines in terms of operational flexibility, as they can adjust their excitation to meet the specific requirements of various industrial and energy applications. This makes them particularly suitable for electric propulsion systems, renewable energy generators, and any application requiring dynamic and efficient power control. By integrating these two excitation systems, DESMs allow for continuous performance optimization, ensuring more efficient energy use under varying operating conditions.

Operation

The operating principle of double excitation synchronous machines (DESMs) is based on the integration of two distinct excitation circuits: one using permanent magnets and the other using electromagnetic windings. The permanent magnets generate a constant magnetic flux, providing a stable and continuous base for the machine's magnetic field. In parallel, the electromagnetic windings allow dynamic modulation of the magnetic flux by adjusting the current flowing through them. This modulation enables precise control of the magnetic flux according to operational requirements.

The double excitation machine benefits from the combination of the two excitation sources. The permanent magnet excitation ensures a constant and lossless flux, while the wound excitation offers rapid and precise adjustment capability. This configuration allows efficient management of load and speed variations, enhancing the overall stability and energy efficiency of the machine. For example, when the load varies, the flux generated by the windings can be adjusted to maintain optimal magnetic flux, thus avoiding energy losses and increasing machine efficiency.

In a DESM, the wound excitation flux can either consolidate or weaken the flux from the permanent magnets by reversing the direction of the excitation current as needed. The active fluxes from each excitation source are illustrated by the main flux paths within the machine, showing how they interact to optimize machine performance. Additionally, the leakage flux generated by the permanent magnets, although not contributing to torque production, is considered in the overall design to minimize losses and maximize energy efficiency.

For synchronous machine alternator operation, three application cases can be identified:

Alternator directly connected to the electrical grid;

Alternateur connecté au réseau électrique (vitesse constante).

Alternator connected to a diode rectifier;

Alternateur connecté à un redresseur à diodes.

Alternator connected to a controllable rectifier.

Alternateur connecté à un redresseur contrôlable.

The first case corresponds to constant speed operation, with the electrical grid imposing its frequency. Two types of motivations can be identified in studies conducted on the use of double excitation machines for this type of application:

Improving the performance of conventional wound excitation alternators.

Exploring the performance of double excitation alternators for this type of application.

Dual Excitation Structures and Classification:

Structures :

The principle of double excitation offers the possibility of designing a wide variety of structures for synchronous machines. Many criteria can be used to classify these machines, similar to those used for other types of electric machines. Among these criteria are the distinction between radial or axial flux machines (for rotary machines), 2D structure machines (mainly flux in one plane) or 3D (flux in three dimensions), as well as the differentiation between rotary and linear machines. Several publications have focused on the classification of double excitation synchronous machine structures.

The different structures of DESMs allow for meeting specific performance and cost requirements. Classification criteria include winding configuration, cooling method, and the type of regulator used.

Classification criteria:

Two classification criteria seem particularly suitable due to the principle of double excitation, involving the presence of two magnetic excitation flux sources: the way the two excitation flux sources are combined and the location of the excitation flux sources in the machine. The first criterion distinguishes series and parallel double excitation machines, according to the analogy with electric circuits. The second criterion concerns the location of the excitation flux sources in the stator, the rotor, or a combination of both. Preferring stator excitation coils avoids slip rings and simplifies construction, offering functional advantages such as simplified cooling. Double excitation flux switching machines, with all excitation sources and armature windings in the stator, illustrate this approach. We opt for an updated synthesis by classifying the structures according to the first criterion, while considering the others for a comprehensive understanding.

Series Double Excitation Synchronous Machines (SDESM) :

Exemple d’une machine synchrone à double excitation série (Fodorean et al. 2007 ; Mizuno 1997)

Series Double Excitation Synchronous Machines (SDESMs) integrate two excitation sources: permanent magnets and wound excitation. The permanent magnets provide a constant magnetic flux, while the wound excitation allows dynamic and precise flux control by adjusting the current in the windings. This combination optimizes machine performance in real-time, improving stability and energy efficiency. The total flux in the armature is the vector sum of the fluxes generated by the permanent magnets and the wound excitation, allowing optimal flexibility and adaptation to varying operating conditions.

Parallel Double Excitation Synchronous Machines (PDESM)

Exemple d’une machine synchrone à double excitation parallèle (Akemakou et Phounsombat 2000)

Parallel Double Excitation Synchronous Machines (PDESMs) combine two excitation sources in parallel: permanent magnets and electromagnetic windings. This configuration benefits from a constant magnetic flux produced by the permanent magnets and an adjustable flux thanks to the windings. The permanent magnets provide a stable base flux, while the windings dynamically modulate the magnetic flux according to operational needs. This leads to better management of load and speed variations, optimizing machine stability and energy efficiency. The total flux in the armature is the vector sum of the fluxes generated by each excitation source, allowing increased flexibility and optimal adaptation to varying operating conditions. PDESMs are particularly suited for applications requiring rapid response and precise power control.

Benefits and Applications:

Benefits :

Effective Defluxing: DESMs allow for power loss reduction by adjusting the magnetic flux according to operating conditions. Energy Optimization: Double excitation enables improving the machine's energy efficiency, particularly important in applications requiring high efficiency.

Load Variation Management: DESMs can better handle load variations, maintaining optimal performance even under variable operating conditions.

Reduced Permanent Magnet Costs: By optimizing the use of magnets, DESMs can reduce dependence on costly materials, which can be particularly beneficial in large-scale applications.

Application :

DESMs are used in various industrial and technological applications, such as:

• Electric propulsion systems: In electric vehicles, DESMs can improve energy efficiency and provide better propulsion control.
• Renewable energy generators: Wind turbines and other renewable energy sources can benefit from the energy optimization offered by DESMs.
• industrial motors: DESMs are used in motors requiring high precision and efficiency, such as in automated manufacturing plants.

Equation :

Ratings:

These notations are used to describe the various variables and parameters involved in the operation of double excitation synchronous machines:

• • Ef: Excitation electromotive force
  • Er: Reaction electromotive force
  • If: Excitation current
  • Ir: Reaction current
  • id, iq: Components of the armature current in the d and q axes (A)
  • Lf: Excitation inductance
  • Lr: Reaction inductance
  • M: Mutual inductance
  • p: Number of pole pairs
  • ω: Angular speed
  • θ: Angular position
  • τ: Torque
  • V: Voltage
  • vd, vq: Components of the armature voltage in the d and q axes (V)
  • R: Resistance
  • Ψ: Magnetic flux
  • P: Power
  • Φexc max: Maximum excitation flux
  • Φa: Excitation flux of the permanent magnets
  • ФRMI: Magnetic reaction flux of the armature
  • Фb: Excitation flux of the excitation windings (Wb)

relations :

Equations for Calculating DESM Parameters:

Relations permettant de calculer les flux de la MSDE

The circuits :

Equivalent Electrical Circuits and Vector Diagrams for Simplifying the Study of a DESM :

Circuits électriques équivalents pour les machines synchrones à double excitation à pôles lisses (modèle sans pertes)

Vector Diagrams :

Diagramme vectoriel équivalent pour le modèle sans pertes des MSDE à pôles lisses (mode moteur)

This figure shows the vector diagram of the electromagnetic quantities (flux, FEM, voltage, current) in Park's coordinate system. By convention, the angles ψ and δ, as shown in this figure, are counted negatively and positively, respectively. The flux of the armature ФInduct is the vector sum of the excitation flux Фexc, and the flux of the magnetic reaction of the armature ФRMI

Power Feature:

Dual excitation synchronous machines (MSDEs) have distinctive power characteristics, influenced by the variation in torque and speed. The maximum normalized power varies according to the normalized speed:

Enveloppes « tension/vitesse », « puissance/vitesse » et « couple/vitesse » pour les entraînements électromécaniques

Caractéristiques de puissance pour une machine ayant Ln = 0,5, sans et avec double excitation

Usage:

Synchronous machines with dual excitation (MSDE) are used in various applications due to their flexibility and energy efficiency. They are particularly suited for electric vehicles, allowing optimal speed variation and good energy efficiency through the adjustment of the hybridization rate. MSDEs are also used in energy conversion systems and electric actuators, where they enhance performance and reduce costs. In industry, they enable precise speed control and direct start on the network. Additionally, they are an important research topic, offering interesting prospects for improving the performance of electric machines

Bibliography:

Document used as a source for this article

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