Brushless Doubly Fed Reluctance Machine Research Articles

Optimal Design and Comparative Analysis of Different Configurations of Brushless Doubly Fed Reluctance Machine

The simple and robust construction of reluctance machines and the fact that they need an unsophisticated control have reignited interest in these machines. The constructional modifications and new configurations such as brushless doubly fed reluctance machines (BDFRM) show superior performance compared to their traditional counterparts, especially in variable speed applications such as pumps and wind generators. The design of BDFRM is different from other machines, as there are unusual pole combinations of stator and rotor besides the absence of a winding on the rotor. The operation and performance of the machine greatly depends on the mutual interaction between the stator windings. This interaction is modulated by the rotor with increased saliency. This paper examines a few critical issues in the design of BDFRM. Design optimization is performed using the gradient method from non-linear programming for 6-4-2 pole and 8-6-4 pole configurations of BDFRM. The performance of these optimized machines is examined through finite element analysis employing MAXWELL 16 software and then compared with 2kW prototypes constructed for laboratory use. A comparative analysis of the two configurations are presented and experimental results show that the BDFRM with the 8-6-4 ducted rotor is superior to BDFRM with the 6-4-2 reluctance and ducted rotor configurations.

Design and Analysis of Brushless Doubly Fed Reluctance Machine for Renewable Energy Applications

This paper presents the analysis of a brushless doubly fed reluctance machine (BDFRM) for renewable energy applications with an emphasis on the rotor design. The BDFRM stator includes two sets of windings, i.e., the control winding (CW) and power winding (PW). The PW is connected to the grid, while the CW links the grid by a power converter of fractional rating to regulate the operating speed. This reduces the converter cost, and the brushless configuration enhances the reliability. In this paper, the reluctance rotor of the BDFRM, which is similar to that of a synchronous reluctance machine, is designed and analyzed in order to improve the performance. The influence of the total MMF and its allocation to the two stator windings are also discussed in relation to the machine performance. Finite-element analysis is used for performance simulation. The experimental study validates the proposed design for a small BDFRM.

An optimization-oriented sizing model for brushless doubly fed reluctance machines: Development and experimental validation

This paper deals with the electromagnetic modeling and design aspects of brushless doubly fed reluctance machines (BDFRM). It presents the development and the experimental validation of a semi-analytical model (SAM) used for sizing new machine designs. The SAM is an optimization-oriented model implemented using analytical approaches and a deterministic optimization algorithm to find an optimal machine by solving iteratively an objective function, whereas satisfying several output constraints simultaneously. It is fast and accurate for pre-design stages, premises that are confirmed by both simulation and experimental results.

Design of Novel Axial-Flux Dual Stator Doubly Fed Reluctance Machine

The most well-known application of a doubly fed machine is probably the use of a doubly fed induction generator (DFIG) with a wind turbine. A clear drawback to the use of DFIGs is the need for slip rings to supply power to the rotor circuit, which may result in increased operation and maintenance cost [1]. Nowadays, the brushless doubly fed reluctance machines (BDFRMs) have been developed as a class of machines that is controlled using a power converter that has a rating lower than the total power rating of the machine which makes it an attractive solution for applications where speed control over a limited operating range is required like in the case of wind turbines. It has two windings with different no. of poles. One of the windings called primary winding which is connected directly to the power grid, while the other winding called secondary winding is connected through a variable speed drive to grid and is used to control the output frequency [1]. The machine model proposed is an axial flux machine with dual stator topology. In this paper, axial flux brushless doubly fed reluctance machine (AF-BDFRM) is designed and analyzed and the initial FEA results are presented.

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.

High-efficiency control of brushless doubly-fed machines for wind turbines and pump drives

The paper is concerned with vector control of a promising brushless doubly-fed reluctance machine (BDFRM) technology for generator and drive systems with limited adjustable speed ranges such as wind turbines or pump-alike installations. The BDFRM has been receiving increasing attention because of the low capital and operation and maintenance costs afforded by the partially-rated power electronics and the high reliability of brushless construction, while offering performance competitive to its well-known slip-ring counterpart, a doubly-fed induction machine. The comprehensive comparative studies have evaluated the performance of two robust control algorithms by computer simulations and experimentally on a custom-made BDFRM under the maximum torque per inverter ampere conditions for improved efficiency of electro-mechanical energy conversion.

Saturation and Ducting Effects in a Brushless Doubly-Fed Reluctance Machine

The brushless doubly-fed reluctance machine has two sets of stator windings of different pole number linked by a reluctance type of rotor. The rotor modulates the stator MMF from one winding so that flux links the other winding creating mutual coupling. This is still a research machine and the design methodology is under development. Past works indicates that a ducted rotor with radial laminations is best suited to be the reluctance rotor. In this paper the geometry of the rotor is investigated. If the ducts are too thin then the reluctance ratio is low and the coupling is affected. If they are too wide then the reluctance path for the coupling flux is high and the machine will saturate quickly. It is found, using finite element analysis, that the duct ratio should be about 38% from the results in the paper. This is in line with previous work on synchronous reluctance machines that can also use ducted rotors (although they use axial lamination which are inappropriate for this application).

Design and Analysis of Brushless Doubly Fed Reluctance Machines

Brushless doubly fed reluctance machines (BDFRMs) are a class of machines that may be controlled using a power converter that has a rating lower than the total power rating of the machine. The attractive properties of these machines have, in the past, been offset by low power density and efficiency when compared to other types of machines. Recent advances have shown that, when well designed, these machines are, in fact, capable of operation at high torque density and efficiency. However, little guidance on how to design these machines is available in the literature. This paper presents analytical approaches to design a BDFRM with desirable qualities and the use of time-stepped finite-element analysis to validate the results of the design process.

Power control of brushless doubly-fed reluctance drive and generator systems

A new algorithm for shaft position sensor-less independent control of real and reactive power of the Brushless Doubly-Fed Reluctance Machine (BDFRM), an emerging competitive for applications with limited variable speed ranges (e.g. pumps or wind turbines), is proposed and experimentally verified in this paper. The algorithm is inspired by the fundamental operating principles of the machine and does not depend on any parameters. Therefore, it is more robust than model-based control approaches which often suffer from sensitivity issues and performance trade-offs associated with machine parameter variations or their inaccurate identification. In turn, the considered control method merely relies on the grid-connected winding voltage and current signals. Practical studies including speed tracking, power factor regulation and load response have demonstrated high performance of the scheme on a custom designed 1.5 kW prototype.

Sensored and sensorless speed control methods for brushless doubly fed reluctance motors

The study considers aspects of scalar V/f control, vector control and direct torque (and flux) control (DTC) of the brushless doubly fed reluctance machine (BDFRM) as a promising cost-effective alternative to the existing technological solutions for applications with restricted variable speed capability such as large pumps and wind turbine generators. Apart from providing a comprehensive literature review and analysis of these control methods, the development and results of experimental verification, of an angular velocity observer-based DTC scheme for sensorless speed control of the BDFRM which, unlike most of the other DTC-concept applications, can perform well down to zero supply frequency of the inverter-fed winding, have also been presented in the study.

SENSORLESS SPEED AND DIRECT TORQUE CONTROL OF DOUBLY FED RELUCTANCE MOTORS

The brushless doubly fed reluctance machine (BDFRM) is a potential low cost alternative to the existing solutions in applications with limited variable speed ranges. The cost effectiveness of the BDFRM drive can be attributed to the possibility of using a partially rated power electronic converter and reliable, maintenance-free operation afforded by the absence of brush gear. This paper proposes a load model based observer algorithm for speed and direct torque (and flux) control (DTC) of this machine without using a shaft position sensor. The preliminary results have demonstrated good controller performance over the desired speed range for different loading conditions.

Brushless Doubly Fed Reluctance Machines: An Emerging Technology

Brushless doubly-fed machines (BDFMs) allow the use of a partially rated inverter and therefore offer reliable, maintenance-free operation at low cost, especially in large power variable speed applications with limited speed ranges such as off-shore wind turbines, pumps etc. However, with a special cage rotor design, the machine has substantial rotor losses, poor efficiency and complicated control being difficult to implement.The BDFM reluctance type, the brushless doubly-fed reluctance machine (BDFRM), can overcome most of the above BDFM deficiencies while retaining all its merits. It has a "cold" reluctance rotor, which makes it more efficient and much easier to model/control than the BDFM.A theoretical study of the BDFRM is very important to gain proper understanding of its unusual operation, the main control features and compromises between optimal performance and the size of the inverter and machine required. This paper will attempt to develop, and provide a comprehensive comparative analysis of, different control strategies for the machine that are suitable for digital implementation.

Assessment of Losses in a Brushless Doubly-Fed Reluctance Machine

This paper investigates the iron losses that occur in a brushless doubly-fed reluctance machine. This is a novel machine that is still being researched. It is found that the iron losses are quite high when using an axially-laminated rotor (as often used in a synchronous reluctance machine) in this application. Simulations are carried out using finite element analysis and the level of rotor eddy-current loss verified with experimental tests

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.

Analysis of a polyphase synchronous reluctance motor with two identical stator windings

This paper presents the performance analysis of a cageless two-stator-winding reluctance motor which is capable of developing reluctance torque at good power factors in the absence of inverter control. The machine stator has two separate identical polyphase windings whose pole numbers are the same as that of the cageless rotor. The machine performance is investigated using the traditional circle diagrams, and the d– q rotor reference frame equations derived in space-vector form by applying the concept of winding functions. Core loss and saturation are accounted for in the developed dynamic model. It is shown that if one winding is connected directly to the supply and the other fed with a balanced capacitor, the developed torque is superior to a brushless doubly-fed reluctance machine (BDFRM) of similar size. Other motor characteristics, its line-start behaviour and torque ripples are also presented and discussed. Experimental results corroborate the simulations.

Performance Analysis and Control of a Brushless Doubly Fed Reluctance Machine

In this article, performance analysis and control technique for a brushless doubly-fed reluctance machine (BDFRM) with extensive simulation and experimental results is presented. The transient and steady-state governing equations are presented with appropriate assumptions. A speed control scheme based on field orientation control strategy is presented, and start-up behavior of the machine are studied extensively via digital simulation and experiments. Moreover, effects of both voltage and frequency of the control winding on the machine performance are studied. Digital simulation studies have been carried out using the proposed method of analysis and the results are compared to those obtained from an experimental machine.

Direct Torque Control of Brushless Doubly Fed Reluctance Machines

A brushless doubly fed reluctance machine (BDFRM) allows the use of a partially rated inverter and as such is an attractive cost-effective candidate for applications with limited speed ranges. While the Direct Torque Control (DTC) method has been successfully applied to almost all conventional AC machines, it has not been considered so far as a potential mean for controlling doubly-excited systems. This paper attempts to fill in this gap, by exploring the principles and implementation aspects of Direct Torque Control (DTC) for the BDFRM. The presented results demonstrate good controller performance over the entire supply frequency range for the maximum torque per inverter ampere control strategy.

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

Brushless doubly fed induction machines (BDFIMs) have been extensively researched over 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, developments in reluctance rotors, spurred on by research into synchronous reluctance machines, have resulted in high-saliency-ratio cageless rotors that are economical 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 and aspects of practical implementation of the important control strategies and associated machine performance/inverter size tradeoffs 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.

Theoretical analysis of control properties for the brushless doubly fed reluctance machine

The brushless doubly fed induction machine (BDFIM) has been extensively researched for approximately 30 years, but a related machine [the brushless doubly fed reluctance machine (BDFRM)], has not. 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, has resulted in high saliency ratio 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. A relatively limited amount of work to date has been published on the BDFRM. This paper attempts to fill this void by presenting a theoretical analysis of some of the important control properties of the ideal BDFRM.