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 of BDFM as generator and motor

Brushless doubly fed induction generators (BDFIGs) show commercial promise for wind-power generation due to their lower capital and operational costs and higher reliability as compared with doubly fed induction generators. This paper presents a vector-control scheme for a BDFIG operating as a variable-speed generator. The proposed vector controller is developed on the power-winding stator-flux frame and can be used to control both the speed and the reactive power. The machine model and the control system are developed in MATLAB. The test results show that the proposed controller has good dynamic performance when changes in speed and reactive power are applied.

Stator-Flux-Oriented Vector Control for Brushless Doubly Fed Induction Generator

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.

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

The Brushless Doubly-Fed Machine (BDFM) shows commercial promise as a variable speed drive or
generator. However, for this promise to be realised the design of the machine must be improved beyond
that proposed to date. This dissertation contributes towards this goal through machine analysis,
design and control.
A generalised framework is developed for a coherent and rigorous derivation of models for a wide
class of BDFMs, of which machines with 'nested-loop' design rotors are a subset. This framework
is used to derive coupled circuit, d-q axis, sequence components and then equivalent circuit models
for the class of machines. Proofs are given for all derivations, exploiting the circulant properties
of the mutual inductance matrices. The coherence between the different models allows parameters
calculated for the coupled circuit model to provide parameter values for the other models.
A method of model order reduction is proposed for the class ofBDFMs with 'nested-loop' rotors,
and examples given of the efficacy of the procedure. The reduction method allows parameter values to
be computed for a simple equivalent circuit representation of the machine. These calculated parameter
values, and those for other BDFM rotor designs are verified by experimental tests on a prototype
BDFM.
The significance of particular equivalent circuit parameters is investigated from the model. Series
rotor inductance terms are found to have a significant and direct effect on machine petformance.
These terms are shown to relate directly to the design of the rotor, and are quantified using the previously
developed framework. Seven different rotor designs, including a new BDFM rotor design, are
considered to show how the values of these parameters change.
An experimental method of parameter estimation is developed for the equivalent circuit model,
and the relationship between these parameters and the parameters in other forms of the model derived.
The experimental method is shown to be applicable both to standard induction machines and to BDFM
machines, yielding accurate results in each case.
A synchronous reference frame model for the class ofBDFMs is derived and is used to analyse the
stability of the machine via a linearized model. Practical methods for the design of PID controllers
are proposed to stabilise the machine using voltage source inverters. Results are presented from
experimental implementations which show a significant improvement in petformance over previously
published results.
The non-linear control technique, feedback linearization, is applied to the BDFM and shown to
have some robustness to modelling enors, in a realistic simulation. An initial attempt at implementation
of the scheme is reported. Preliminary results are encouraging, and warrant further investigation.
Keywords: ac machines, BDFM, Brushless Doubly Fed Machines, control, coupled circuits, dq
axis, equivalent circuits, feedback linearization, model reduction, parameter estimation, synchronous
reference frame

A study of brushless doubly-fed (induction) machines : contributions in machine analysis, design and control

The paper describes a new type of variable speed doubly-fed reluctance machine and presents the model for the analysis of the machine. The stator of the machine is equipped with two sets of sinusoidally distributed windings having pole numbers P1 and P2. The rotor consists of simple saliencies of pole number Pr, which is shown to be constrained to particular values by P1 and P2. A d-q-n model and equivalent circuit is developed for this machine. Simulation results using the new model show that this machine can have the advantages of doubly-fed wound-rotor machine while eliminating the complicated wound rotor, brushes and slip rings normally associated with doubly fed machines.

d-q ANALYSIS OF A VARIABLE SPEED DOUBLY AC EXCITED RELUCTANCE MOTOR

A brushless a.c. generator is described which has a mode of operation similar to two wound-rotor induction machines connected in cascade. This cascade machine, which has previously been considered for application as a motor, has single stator and rotor windings in a conventional induction-machine frame. The machine is one of inherently high pole number. As an alternator, therefore, it is suitable for the generation of medium- to high-frequency supplies. The direct excitation current and the alternating output current flow in its one stator winding. Its cylindrical rotor iswound with an extremely simple short-circuited polyphase winding. Since no brushes, slip rings or rotating diodes are needed, the machine may be driven reliably at high speeds. A theoretical treatment is developed which takes account of magnetic saturation both in the steady state and in the transient state. Design and performance details of a prototype 400 Hz, 3000 rev/min., 3-phase machine are also included.

P.W. Neal

Papers

Brushless cascade alternator