Milutin Jovanovic

Control of synchronous reluctance machines

A comprehensive approach to the control of inverter-fed synchronous reluctance machines is developed, based on the machines ideal model. From the theory, a control simulation is designed. Simulation results are presented.<<ETX>>

The use of doubly fed reluctance machines for large pumps and wind turbines

Brushless doubly-fed induction machines (BDFIMs) have been extensively researched over the last 15 years because of the possibility of using a partially rated inverter in many applications with limited speed variations. However, the special cage rotor construction and substantial rotor losses is one of the key deficiencies of these machines. A similar and extremely interesting machine, the brushless doubly-fed reluctance machine (BDFRM), has been largely ignored in comparison. This was mainly due to the fact that reluctance rotor designs were not capable of generating saliency ratios large enough to make the BDFRM competitive with other machines. However recent developments in reluctance rotors, spurred on by research into synchronous reluctance machines, have resulted in high saliency ratio cageless rotors that are economic to build. This, together with the promise of higher efficiency and simpler control compared to the BDFIM, means that further investigation of the BDFRM is warranted. This paper presents a comparative theoretical analysis of the important control properties and related machine performance/inverter size trade-offs for the BDFRM in the light of its most likely applications-large pump type adjustable speed drives and variable speed constant frequency wind power generation systems.

Introduction to the Space Vector Modeling of the Brushless Doubly Fed Reluctance Machine

The brushless doubly fed reluctance machine (BDFRM) is the least known of a group of electrical machines that include the classic cascaded induction machine, the brushless doubly-fed induction machine (BDFIM), and the double-fed slip ring induction machine (DFSRIM). Since its initial development some 30 years ago, the BDFRM has been largely ignored because of the performance limitations imposed by the reluctance rotor design. However, improvements in reluctance rotors, a by-product of the development of the synchronous reluctance machine, have resulted in renewed interest in the BDFRM. This together with the promise of higher efficiency and simpler control compared to the BDFIM means that further investigation of the BDFRM is warranted. This paper is designed to be a starting point for research into the BDFRM. It develops the fundamental modelling equations that are required to carry out research into its dynamics and control. The approach is partly tutorial in nature as it presents some “well-known” analysis techniques that are scattered throughout the literature on the machine. It develops from first principles the space vector model of the machine, which is then used to derive the steady-state BDFRM equations. Standard sinusoidal spatial variation and linearity assumptions are used throughout the analysis. Where relevant, the physical concepts behind the machine’s operation are emphasized. In addition to the full dynamic model of the machine, the paper also introduces a per-unit system that forms the basis for machine-independent performance expressions.

The brushless doubly fed reluctance machine and the synchronous reluctance machine-a comparison

The brushless doubly fed reluctance machine (BDFRM) is related to the better known brushless doubly fed induction machine (BDFIM). Research into doubly fed machines is motivated by the fact that they allow the use of a partially rated inverter in many variable-speed applications. Research into the BDFRM has been largely ignored in comparison to the BDFIM, despite the fact that it has the potential for greater efficiency as compared to the BDFIM, and the rotor is simpler to manufacture. This paper compares the BDFRM and its singly fed cousin, the synchronous reluctance machine. This is a natural comparison since both machines use the same reluctance rotor. The first part of the paper establishes relationships between the inductances of the two machines. This is then used to facilitate a comparison using the constraints that both machines have the same amount of active material, i.e., the same amount of copper and iron, and that the copper losses for both machines are the same. This analysis also allows an approximate comparison with the conventional squirrel-cage induction machine. The analysis is carried out using machine-independent normalizations.

Encoderless direct torque controller for limited speed range applications of brushless doubly fed reluctance motors

The paper proposes a direct torque control (DTC) algorithm suitable for low-variable-frequency operation of the brushless doubly fed reluctance machine (BDFRM) and considers aspects of its practical implementation. The simulation and experimental results show that a small BDFRM prototype can successfully operate down to zero supply frequency of the inverter-fed (secondary) winding, unlike cage induction or many other alternating current machines with DTC. This BDFRM advantage is a consequence of using a flux estimation technique not relying on the secondary voltage integration and therefore avoiding the well-known problems at low secondary frequencies, being typical for the BDFRM normal operation. The developed algorithm also offers the prospect for optimizing the machine performance in a manner similar to conventional vector controllers but with control actions executed in a stationary reference frame as usual for all traditional DTC methods. The maximum torque per inverter ampere control strategy has been chosen as a case study.

On the Possibilities of Using a Brushless Doubly-Fed Reluctance Generator in a 2 MW Wind Turbine

The paper will put a forward a description of a realistic design for a brushless doubly-fed machine that is of practical value. It takes the form a 4-pole / 8-pole machine aimed for use in a 2 MW wind turbine over a correct speed and torque range. Previous examples discussed laboratory machines and this study takes the outcomes from these studies to formulate a procedure for sizing and designing the machine. The latest design and analysis techniques are used, with practical rotor ducting considered. Control is also addressed in the paper, in particular the assessment of sensorless reactive power control. This paper is aimed at taking the machine from a small-scale laboratory example to consideration as a large-size industrial generation and therefore it represents a step-change in the literature on the machine.

Vector Control Methods for Brushless Doubly Fed Reluctance Machines

This paper is concerned with flux and voltage vector-oriented control of a promising brushless doubly fed reluctance machine (BDFRM) for generator and drive systems with limited adjustable speed ranges (e.g., wind turbines and/or pump drives). The BDFRM has been receiving increasing attention because of the low capital and operation and maintenance costs afforded by using partially rated power electronics and the high reliability of brushless design. Furthermore, it has the potential to offer competitive performance to its well-known slip-ring counterpart, i.e., doubly fed induction generator. The experimental studies have evaluated the control algorithms on a custom-built BDFRM in both motoring and generating modes under the maximum torque per inverter ampere conditions. The test results achieved should make a step forward toward the development of robust generic controllers for doubly excited machines.

Toward a Generic Torque and Reactive Power Controller for Doubly Fed Machines

A novel shaft-position sensorless algorithm for decoupled control of torque and reactive power (TRPC) of doubly fed machines, such as the classical wound-rotor induction machine (DFIM) and the emerging brushless reluctance machine (BDFRM), has been discussed and experimentally verified in this paper. The underlying control concept is derived from first principles of magnetization and torque production in the machines. For control purposes, only the grid-connected winding measurements and rough knowledge of its resistance value are required. Such a weak parameter dependence makes the TRPC inherently robust, structurally simple, and fast to execute even on low-cost DSPs. A variety of applications are possible including drive and generator systems with limited variable speed ranges (e.g., large pumps and wind turbines), where cost savings of using partially rated power electronics are significant. Two custom-designed and built BDFRM prototypes have served as case studies to evaluate the controller performance by computer simulations and through laboratory experiments.

Control of Brushless Doubly-Fed Reluctance Generators for Wind Energy Conversion Systems

This paper presents the conceptual analysis, comparative simulation, and experimental evaluation of voltage and flux vector-oriented control of a promising low-cost brushless doubly-fed reluctance generator (BDFRG) technology for variable-speed wind turbines with maximum power point tracking. The BDFRG has been receiving increasing attention because of the use of partially rated power electronics and the high reliability of brushless design, while offering performance competitive to its popular slip-ring counterpart, the doubly-fed induction generator. The development and viability of the two parameter-independent controllers have been validated on a custom-made BDFRG prototype using the maximum torque per inverter ampere strategy for the speed and loading conditions commonly encountered in wind power applications.

Practical Implementation of Sensorless Torque and Reactive Power Control of Doubly Fed Machines

A recently proposed shaft position sensorless method for torque and reactive power control of the brushless doubly fed reluctance machine has proven successful in simulation studies. In this paper, its real-time performance has been evaluated on a custom-designed machine prototype. The preliminary experimental results have confirmed the viability of the scheme and represent a serious step toward the development of a generic virtually parameter independent controller for doubly excited machines. A wide range of applications is possible, including drive and generator systems with limited speed ranges (e.g., large pumps and wind turbines), where the cost savings of using a partially rated power electronic converter are significant.