Doubly-fed Machine Research Articles

Performance Description of Brushless Doubly-Fed Induction Machine in Its Asynchronous and Variable Speed Synchronous Modes

Brushless Doubly-Fed Machine has attracted considerable attention in recent years due to its advantages. It has the robustness of the squirrel cage induction machine, and the speed and power factor controllability of the synchronous machine as well as the absence of brushes and slip rings, and using a fractionally rated frequency converter. Hence, there are considerable benefits over the conventional machines, when the machine is applied to applications such as a wind turbine generator or high power adjustable speed drive. However, these benefits are obtained in slightly more complex structure, higher cost and larger dimensions in comparison to the conventional induction machine. This paper presents fundamental aspects of the three modes of operation of brushless doubly fed machine, i.e. simple induction mode, cascade induction mode, and synchronous mode. The investigation is performed by analyzing the spatial harmonic contents of the rotor magnetic flux density. The direct cross couplings between stator and rotor fields as well as, indirect cross coupling between stator fields by the special rotor of this machine is described. Furthermore, loss analysis of the machine in various modes is presented and the torque-speed curves for asynchronous modes are obtained. A 2-D magnetodynamic finite element model based on the D-180 4/8 pole prototype machine is extracted and simulated to verify the results.

A General Model for Describing the Performance of Brushless Doubly-Fed Induction Machines

This paper presents a generalized model, by which the dynamic and steady-state behaviour of the Brushless Doubly-Fed Induction Machine (BDFIM) can be precisely predicted. The investigated doubly-fed machine has two sets of three-phase stator windings with different pole numbers. The rotor is a squirrel-cage type with a simple modification in order to support the two air-gap rotating fields that are produced by the stator windings and have different pole numbers. The machine model is derived in the qdo-axis variables. The qdoaxes are attached to rotor and hence, it rotates at the rotor speed (

Radial Force Analysis of Radial Laminated Magnetic-Barrier Rotor Brushless Doubly-Fed Machine

In this paper, the respective harmonic magnetic field generated by the two sets of stator windings of radial laminated magnetic-barrier rotor brushless doubly-fed machine has been analyzed with the winding function method, the calculations of the radial forces on stator’s inner edge which are resulted from the fundamental and harmonic magnetic field that generated by the two sets of stator windings have been made, and the radial forces of three kinds of such machines with different rotor pole numbers have been compared, all the above can provide theoretical guidance for optimized design of radial laminated magnetic-barrier rotor brushless doubly-fed machine.

The DFM Control System Based on PLL

A specific structure of control system with double fed machine (DFM) is studied in the paper. The DFM is assumed to be a generator with stator winding connected to the power grid and it produces active and reactive powers. A command vector controlling DFM is transformed to the rotor-oriented coordinate system using angle of transformation. The angle of transformation is estimated in the system by applying a phase locked loop (PLL).Moreover, an example of application of pulse phase locked loop for estimation of voltage vector components is presented in the paper. In conclusion the properties of proposed PLL-based control systems are compared with properties of systems known from radio engineering and telecommunications described in the first chapter of the paper.

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.

Performance of BDFM as generator and motor

The performance of the brushless doubly-fed machine (BDFM) is analysed using a per-phase equivalent circuit. An expression for the rating of the machine as a function of magnetic and electric loadings is developed, and the rating is compared to those of the doubly-fed induction machine and cascaded induction machines. As the magnetic field in a BDFM is complex, the magnetic loading is considered in detail and a new generalised loading is derived. The BDFM suffers a reduction rating of about one-quarter in comparison to comparable conventional machines, arising from penalties in magnetic and electric loadings consequent on the presence of two stator to rotor couplings. The handling of reactive power has an important effect on the machine performance and this point is illustrated with experimental results from a frame size 180 BDFM. The tests were carried out at modest flux densities to avoid the effects of saturation, but the implications of saturation are considered.

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.

Stability analysis of the doubly fed machine with emphasis on saturation effects

The doubly fed machine (DFM) is presently given an increasing attention in high power variable speed drives and in wind power generation systems, where it exhibits high performances. This has been gained thanks to the stator flux oriented control. Nevertheless, beyond the effect of heating, the robustness of such control strategy is affected by saturation especially the main magnetic one. Accounting for the effect of the magnetizing branch saturation in steady‐state stability analysis, considering the case of a voltage‐controlled DFM and the case of a current‐controlled one, represents the aim of the study. To this end, a numerical procedure based on a combination of the eigenvalue and the fixed point methods has been developed. It has been found that, in both cases, accounting for saturation yields a stabilization effect which is more or less significant depending on the rotor supply parameters.

Brushless Doubly-fed Induction Machine: Operating Characteristics and Applications

This paper presents the main characteristics of the brushless doubly-fed machine. Its principle of operation is described and some of its possible applications are indicated. An extend bibliography is provided in order to allow for a better understanding of its potentialities.

Combined magnetising flux oriented control of the cascaded doubly-fed induction machine

A cascaded doubly-fed induction machine (CDFM) is a connection of two wound rotor induction machines. In comparison to a single doubly-fed (wound rotor) induction machine (SDFM) brushes are obsolete. Because of recent developments in brushless doubly-fed machine design, there is a renewed interest in associated control. The paper presents theoretical and experimental studies of a combined magnetising flux oriented control method for a CDFM. After the existing control method has been described it is then modified to be applicable to any CDFM machine combination. In a second step the control method is simplified and the performance is explored. Investigations of a power control loop for the simplified control show that this control method does not lead to natural decoupled active and reactive power control as known from the SDFM.

Performance evaluation of doubly-fed twin stator induction machine drive with voltage and current space vector control schemes

The doubly-fed twin stator induction machine (DFTSIM) is of interest as an adjustable speed drive (ASD) and a variable speed generator (VSG), as it combines the advantages of a brushless drive with the potential for high-voltage, high-power, and limited speed range applications. Previous work on the DFTSIM (or its equivalent brushless doubly-fed machine) has considered steady state performance with open loop control or dynamic performance with closed loop control, but has not considered an overall peformance evaluation with an appropriate closed loop speed control. The comparative dynamic and steady state performance of a laboratory DFTSIM drive using ‘voltage space vector control’ and ‘current space vector control’ schemes is discussed. While the magnitude of the control winding voltage (or current) space vector is dynamically controlled to obtain the desired torque in a speed control loop, its position is regulated to operate the machine in synchronous mode. Results show that the maximum efficiency for the voltage space vector control is slightly higher than that for the current space vector control scheme, but the current space vector control offers better dynamic response and minimum converter voltampere requirement. Although the reduction in the converter VA rating with the DFTSIM drive is similar to that achievable with a slip power recovery drive, the brushless DFTSIM drive has a greater potential for applications requiring high reliability and low maintenance. Further, the DFTSIM is a viable alternative in the context of non-conventional power generation such as wind and small hydro.

Doubly-fed induction machine differential cascade. II. Verification by test

In this paper, the doubly-fed differential cascade (DFDC) analysis, in the steady state, given in Part I, is verified by tests. The DFDC consists of two similar doubly-fed machines having the corresponding phases of the two rotors connected. Two sources are used to supply the DFDC stators. One is the mains and the other is a variable frequency converter. The DFDC is loaded with two DC machines acting as brakes. Operations similar to the ones required to provide propulsion to a vehicle are simulated. Variable speed and direction, when the vehicle is moving on a straight course, is simulated by varying the frequency of the frequency converter. Vehicle turn is simulated by distributing the load on the two machines. A nonmoving vehicle, held on a ramp by the DFDC developed torque, is simulated by applying the same frequency to both machines. The experimental results are compared with the results from the analysis.

Stator flux oriented control of a cascaded doubly-fed induction machine

A cascaded doubly-fed induction machine (CDFM) is a connection of two wound rotor induction machines. In comparison to a single doubly-fed induction machine (SDFM) brushes are obsolete. Due to recent developments in brushless doubly-fed machine design, there is a renewed interest in associated control. Theoretical and experimental studies of a stator flux oriented control method for a CDFM are presented. The use of vector control principles to control torque, speed, active and reactive power is investigated. It is found that the additional closed rotor circuit of the CDFM introduces a cross coupling between the d-axis and the q-axis. Nevertheless, the cross coupling effect remains small in relation to the overall control concept so that the control of the CDFM resembles that of the SDFM. List of symbols Abbreviations: SDFM - CDFM - SF-CDFM = - single doubly-fed induction machine cascaded doubly-fed induction machine single frame cascaded doubly-fed induc- tion machine brushless doubly-fed induction machine doubly-fed reluctance machine microcontroller pulse width modulation

Model Reduction and sTability Analysis of Doubly Fed Machine

This paper deals with the reduced order model of singularly perturbed systems applied to the Doubly Fed Machine (DFM) system. In order to draw the steady state stability limits, we have found that this study can be limited to the reduced order model of the machine. This results are an optimization of the computation quantity and of the computational time. In fact, it is convenient to study a second order systems than a sixth order one. Finally a discussion with respect the operating point and inertia is also included.

Experimental evaluation of a rotor flux oriented control algorithm for brushless doubly-fed machines

The brushless doubly-fed machine (BDFM) has shown potential for adjustable speed drive applications. Although the BDFM is readily applicable for industrial drives requiring only slow speed response, control methods for higher performance operation need to be explored. With the help of a suitable synchronous reference frame dq model, this paper defines a BDFM synchronous angle and employs an electric torque estimator to establish a rotor flux oriented control algorithm. Experimental results show the effectiveness of the proposed controller.

Generalised theory of the brushless doubly-fed machine. Part 2: Model verification and performance

The paper compares the performance predictions made using a new model of the brushless doubly-fed machine with measurements made on a laboratory test machine. The agreement between measurement and prediction is shown to be very good. The synchronous behaviour of the machine is shown to be very similar to that of a DC-excited synchronous machine, with the shaft torque determining the input power and the power factor being controlled by the voltage applied to one of the windings. The effect of magnetic saturation is discussed, and operating limits based on maximum flux and current limits are investigated.

Generalised theory of the brushless doubly-fed machine. Part 1: Analysis

The authors develop the concept of the brushless doubly-fed machine (BDFM), from consideration of the frequency and distribution of the EMFs induced in bars of a conventional doubly wound cage motor. These considerations lead to the development of a rigorous mathematical model which is suitable for the analysis of BDFM operating in the synchronous mode. Expressions for all impedances and electrical and mechanical quantities are derived from first principles and the concept of load angle is discussed.

Direct torque control for brushless doubly-fed machines

Work has illustrated the potential benefits of brushless doubly-fed machines in adjustable speed drive applications. While it has been shown that the drive is open-loop stable over a wide speed range, the resultant steady-state and dynamic performance characteristics are far from optimum. Thus, a closed-loop controller is desirable to achieve competitive drive performance. The controller proposed here is applicable for general purpose industrial drives in the medium to high power range. The nature of the doubly-fed machine, with two separate sources of excitation, only one of which is controllable, rules out field-oriented control strategies applied to conventional induction machines. However, the concept of direct torque control based on instantaneous error shows promise for this machine geometry. The present paper extends the concept of predictive torque control for induction machines to the doubly-fed machine. The controller calculates the value of converter voltage which leads to desired changes of flux and torque. Once the voltage is determined, conventional algorithms, such as space vector pulsewidth modulation (PWM), can be used to generate the inverter switching function.

Power balance considerations for brushless doubly-fed machines

This paper discusses the power balances in a brushless doubly-fed machine (BDFM). Equations of power in the two stator windings and the rotor circuit, showing the distribution of electrical and mechanical powers in the air-gap, are obtained in two different speed ranges of BDFM synchronous mode operation. Comparisons vis-a-vis power distribution are made between the wound-rotor induction machine (WRIM) and the BDFM. A brief analysis of the power balances, along with experimental data points obtained using a laboratory prototype machine, are given.

ROTOR MODELING AND DEVELOPMENT FOR BRUSHLESS DOUBLY-FED MACHINES

ABSTRACT The basic principles of brushless doubly-fed machine (BDFM) synchronous operation are used to establish the essential components of the rotor configuration and alternative types of construction are examined. Certain forms, which appear to be physically realizable only with wound or fabricated rotors, are dismissed in favor of others which may be fabricated or die-cast in a manner similar to the cage rotors of conventional induction machines. A detailed simulation model is used to predict the performance of a proof-of-concept laboratory machine in a motoring mode which has been correlated with test data. Various developments and options of the rotor configuration are then examined for relative performance at two different speeds using the modeling techniques. The design features considered include (i) alternative interpretations of the nested-loop configuration requirement, (ii) number of rotor loops per nest, (iii) location (span) of rotor loops, and (iv) conductor section of specific rotor bars ...