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Natural field orientation (NFO) is a technique for generating a rotating reference frame position for an induction machine aligned with the stator flux. The term 'natural' is applied because there is an implicit tendency for the rotating frame to realign with the correct stator flux frame position if there is a perturbation away from this position. However, under regeneration conditions this realignment property does not occur, and the frame position, if perturbed, will move the control reference frame away from stator flux alignment. This paper examines this frame alignment stability problem and proposes a solution that retains the features and simplicity of the NFO concept. Index Terms--Natural field orientation (NFO), reference frame stability, vector control.

Current control in inverter-driven machine systems is the inner most component of the hierarchy of control loops. If the control of current in the machine is not fast and accurate then it is difficult, if not impossible, to build a high-performance drive system. Unfortunately, the implementation of current control in power electronic systems is not ideal. Practical effects can have a significant influence on its performance. This paper examines one of these effects, dead time, and considers the influence it has on the performance of predictive current controllers (PCCs). The paper presents analysis that shows that a PCC implicitly compensates for voltage loss due to dead time. Also, a modified PCC is introduced that reduces the zero-current-clamp problem caused by dead time. Simulation and experimental results are presented to verify the analysis and confirm the performance of the new algorithm. Index Terms--Current clamp, inverter dead time, predictive current controller (PCC), pulsewidth modulation (PWM).

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 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. Index Terms--Brushless doubly fed machine, electric machine control, reluctance machine, self-cascade 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. Index Terms--Brushless doubly fed machine, electric machine control, reluctance machine, self-cascade machine.

This paper presents the design of an induction machine current controller that is entirely implemented in digital hardware. A hardware current controller allows high switching frequencies with only modest processor power, as well as simplified hardware and software. The paper briefly presents the concepts of the algorithm implemented, and then outlines the changes that are made to make the digital implementation even more efficient. It then discusses the architecture used for the hardware design. Experimental results are presented to demonstrate the algorithm's performance. Index Terms: Current control, induction machine control, pulsewidth modulation.

This paper investigates a new control approach for longwall armored face conveyors (AFC's) using variable-speed drives (VSD's). Traditionally, AFC's have used fixed-speed or two-speed motors, with various mechanical solutions employed to try to solve the problems that this causes. The VSD approach to the control problem promises to solve all the significant problems associated with the control of AFC's. This paper will present the control algorithms developed for a VSD-based AFC drive system and demonstrate potential performance via computer simulation. A full discussion of the problems involved with the control of AFC's can be found in the companion paper [1]. Index Terms - Armored face conveyor, inverters, vector control.

This paper is a tutorial discussion of the current difficulties being experienced with the performance of armored face conveyor (AFC) drive systems, as used in longwall mining. It presents the traditional approaches to the design of the drive system and highlights the inadequacies. The final part of the paper presents a possible solution approach using variable-speed drive systems, emphasizing the advantages of this approach. The paper is significant, in that it discusses, in one document, a number of problems related to the operation of longwall AFC's. Furthermore, it presents a solution path for these problems. The details of the control strategies to solve the problems highlighted are left to a companion paper [4]. Index Terms - Armored face conveyor, control problems, inverters.

A new high-performance sensorless speed vector controller that implements the maximum torque per ampere control strategy for the inverter-driven synchronous reluctance machine is presented in this paper. It is based on a parameter-dependent technique for on-line estimation of rotor position and angular velocity at the control rate. The current ripple principle is used to estimate position. The estimates are fed to a conventional closed-loop observer to predict the new position and angular velocity. The very high accuracy of the sensorless control algorithm at both low and high speeds is confirmed by experimental results. Index Terms - Angular velocity observer, sensorless control, synchronous reluctance machines.

Longwall armored face conveyors (AFC's) have traditionally been designed using a combination of heuristics and simple models. However, as longwalls increase in length, these design procedures are proving to be inadequate. The result has either been a costly loss of production due to AFC stalling or component failure, or larger than necessary capital investment due to overdesign. In order to allow accurate estimation of the power requirements for an AFC, this paper develops a comprehensive model of all the friction forces associated with the AFC. Power requirement predictions obtained from these models are then compared with measurements from two mine faces. Index Terms - Armored face conveyor, longwall mining, power requirements.

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. Index Terms--Cascade machines, doubly fed machines, slip recovery, synchronous reluctance machines.

A low cost software implementation of an angular velocity observer based control scheme for a space vector PWM inverter fed synchronous reluctance motor is presented. A standard saturated machine d-q model neglecting iron losses is used for torque control. Excellent performance using the maximum torque per ampere strategy is confirmed by simulation and experimental results on an axially-laminated machine. Keywords -Synchronous reluctance machine, control, torque control.

The performance of a synchronous reluctance motor digital controller based on a saturated dq model without compensation for slotting and iron loss influences is investigated. The model predictions for the machine torque characteristics at various speeds including standstill, and the effects of using simplified control strategies are verified by experimental results. Issues related to the low cost software implementation of the control algorithm are discussed. Quality speed control of an axially-laminated machine has been achieved. Keywords -- Reluctance motor drives, Programmable control, Observers, Estimation, Error analysis.

Experimental techniques for determining the machine parameters and results demonstrating their performance on an axially-laminated synchronous reluctance motor are presented. A new software implementation of the control algorithm for conducting an instantaneous flux-linkage test is developed. A low frequency AC test allowing better estimation accuracy compared to a standard 50-Hz test is proposed. Effects of stator slotting on the quality of dc torque test estimates are examined. Keywords -- Rotating machine testing, Inductance measurement, Parameter estimation.