Slobodan N. Vukosavic

SRM inverter topologies: a comparative evaluation

Five inverter power circuits suitable for switched reluctance motor (SRM) drives are analyzed. The peak voltage and current ratings of the power switches and the size and peak ratings of the DC link components of each circuit are compared. As an example, a converter analysis and selection are performed for a high-speed, 6/2 SRM, suitable for a spindle drive, and a high-torque, 8/6 motor. Experimental results obtained for a high-torque drive are presented. Of the five circuits, only two are found suitable for industrial SRM drives, supplied from 380 or 460 V AC lines. The remaining circuits all use dual rail topology, requiring that the inverter active devices have a rating at least twice the motor supply voltage. A discussion of basic motor requirements is included.<<ETX>>

Suppression of torsional oscillations in a high-performance speed servo drive

This paper deals with the problem of mechanical resonance in modern servo drive systems having the speed control loop bandwidth and resonance frequency above 100 Hz. To enable the extension of the range of stable gains in the presence of flexible coupling, a simple and straightforward modification of the speed loop controller is proposed in order to augment the performance of present speed controllers and to make them accordant with mechanical structure suffering from the compliance problem. This paper is comprised of the analytical considerations, straightforward design guidelines, and results of experimental verification obtained by an experimental setup with the elastically coupled 7-N m synchronous servo motor and mechanical resonance frequency of 160 Hz.

A novel concept of a multiphase, multimotor vector controlled drive system supplied from a single voltage source inverter

Since variable speed electric drive systems are supplied from power electronic converters, it is possible to utilize ac machines with a phase number higher than three. It is shown in the paper, using general theory of electrical machines, that an increase of the stator phase number to at least five (or more) enables completely independent vector control of two (or more) multiphase machines that are supplied from a single current-controlled voltage source inverter. In order to achieve such an independent control it is necessary to connect multiphase stator windings of the machines in series and perform an appropriate phase sequence transposition. The concept is equally applicable to any multiphase ac machine type and its major advantage, compared to an equivalent multimotor three-phase drive system, is the saving of a certain number of inverter legs. The actual saving depends on the number of phases. The concept is introduced using an n-phase induction machine as the starting point and further analysis is restricted to an odd number of phases, for the reason explained in the paper. Classification of all the possible cases that may arise is given, followed by presentation of connection diagrams for selected phase numbers. Detailed verification of the proposed concept is provided by simulating the operation of a seven-phase three-motor drive system, controlled using indirect rotor flux oriented control principles. Some preliminary experimental results, which confirm the feasibility of a two-motor series-connected drive system, are included as well. The main advantages and drawbacks of the concept, when compared with an equivalent three-phase multimotor drive system, are finally addressed.

A stator resistance estimation scheme for speed sensorless rotor flux oriented induction motor drives

Accurate knowledge of stator resistance is of utmost importance for correct operation of a number of speed sensorless induction motor control schemes in the low speed region. Since stator resistance inevitably varies with operating conditions, stable and accurate operation at near-zero speed requires an appropriate online identification algorithm for the stator resistance. The paper proposes such an identification algorithm, which is developed for the rotor flux-based model reference adaptive system (MRAS) type of the speed estimator in conjunction with a rotor flux oriented control scheme. In this speed estimation method, only one degree of freedom (out of the two available) is utilized for speed estimation. It is proposed to utilize the second available degree of freedom as a means for adapting the stator resistance online. The parallel stator resistance and rotor speed identification algorithm is developed in a systematic manner, using Popovs hyperstability theory. It increases the complexity of the overall control system insignificantly and enables correct speed estimation and stable drive operation at near-zero speeds. The proposed speed estimator with parallel stator resistance identification is at first verified by simulation. Extensive experimentation is conducted next at low speeds of rotation and successful stator resistance identification is achieved down to 0.5-Hz frequency of rotation.

Generalised Sinusoidal PWM with Harmonic Injection for Multi-Phase VSIs

Application of power electronics in electric drives enables utilisation of ac machines with a phase number higher than three. Such multi-phase motor drives are nowadays considered for various applications. Multi-phase drives are invariably supplied from multi-phase voltage source inverters (VSIs) and adequate methods for VSI pulse width modulation (PWM) are therefore required. This paper deals with carrier-based PWM, where harmonic injection enables an improvement in the dc bus utilisation without moving into the over-modulation region, while still providing sinusoidal output phase voltages. A generalised concept of n th harmonic injection for n-phase inverters is presented. The viability of the proposed concept is proved by simulation taking five-phase VSI as an example. The method of fifth-harmonic injection in five-phase VSIs enables operation with pure sinusoidal voltages up to a certain reference voltage value which is smaller than the maximum achievable with the given dc link voltage. Sinusoidal PWM is further modified, by injecting other harmonics, in order to enable full utilisation of the dc bus voltage. The correlation of this PWM scheme with space vector PWM, based on utilisation of large vectors only, is established in this way. Some experimental results, illustrating application of the sinusoidal PWM in series-connected five-phase two-motor drives, are included.

Space-vector PWM voltage control with optimized switching strategy

Since space vector modulation offers superior performance with respect to other modulation techniques, it is important to establish the sequencing strategy which is best suited for variable-frequency AC drives. The characteristics of regular direct-direct (DD) and inverted direct-inverse (DI) sequencing strategies are investigated both analytically and experimentally. The two strategies are compared with respect to the current ripple, the current spectrum, and the commutation losses. It was found that DD sequence offers superior performance for the majority of loads, such as induction motor drives. For other types of inverter loads, with small or leading power factors, the optimal modulation strategy consists in using the DI sequence when the output voltage is low and the DD sequence if the modulation index is between m(M) and 1. For best results, the switch-over point M has to be evaluated as a function of load and inverter design.<<ETX>>

Modeling, Control, and Experimental Investigation of a Five-Phase Series-Connected Two-Motor Drive With Single Inverter Supply

This paper analyzes a recently introduced two-motor five-phase drive system with series connection of stator windings. It has been shown, using physical reasoning, that the introduction of an appropriate phase transposition in the series connection of two machines leads to a complete decoupling of the flux/torque-producing currents of one machine from the flux/torque-producing currents of the second machine. Consequently, independent vector control of the two machines becomes possible while using a single current-controlled five-phase voltage source inverter as the supply. The drive system modeling and control are first elaborated in this paper by taking both machines as induction motors. It is shown, using rigorous mathematical derivations, that the independent control of the two machines results due to the placement of machines in two different subspaces of the five-dimensional space. This is enabled by phase transposition in series connection. The models of the complete drive in the stationary common reference frame and in the rotor-flux-oriented reference frames of the two machines are developed. An associated vector control scheme for the two-motor drive is presented next. The second part of this paper describes an experimental setup used further on to evaluate the dynamic behavior of the two-motor drive. Performance is investigated by extensive experimentation for various transients (acceleration, deceleration, reversing, and disturbance rejection). Excellent decoupling of control of the two machines is achieved. Both the concept of the drive and the approach to modeling and control are thus fully verified experimentally. Finally, the advantages and shortcomings of the series-connected five-phase two-motor drive are discussed, and potential application areas are highlighted

A novel six-phase series-connected two-motor drive with decoupled dynamic control

This paper analyzes a six-phase two-motor drive, consisting of a six-phase voltage-source inverter (VSI), a six-phase induction machine, and a three-phase induction machine. Stator windings of the two machines are connected in series in an appropriate manner. This enables full decoupling of the dynamics of the two machines by means of vector control. Detailed mathematical modeling of the system is performed and the set of d-q equations, describing the two machines connected in series to a six-phase VSI, is developed. The resulting model clearly shows the possibility of independent vector control of the two machines, although a single VSI is used as the supply. An experimental rig is constructed and existence of the decoupled dynamic control of the two machines supplied from a single VSI is fully verified by extensive experimental testing.The paper proposes a novel six-phase two-motor drive, consisting of a six-phase voltage source inverter (VSI), a six-phase induction machine and a three-phase induction machine. Stator windings of the two machines are connected in series in an appropriate manner. This enables full decoupling of the dynamics of the two machines by means of vector control. Detailed mathematical modeling of the system is performed and the set d-q equations, describing the two machines connected in series to a six-phase VSI, is developed. The resulting model clearly shows the possibility of independent vector control of the two machines, although a single VSI is used as the supply. An experimental rig is constructed and the existence of the decoupled dynamic control of the two machines supplied from a single VSI is fully verified by extensive experimental testing.

A method for magnetizing curve identification in rotor flux oriented induction machines

Operation of an indirect rotor flux oriented induction machine in the field weakening region is usually realized by varying the rotor flux reference in inverse proportion to the speed of rotation. In order to provide the correct stator d-axis current reference at all speeds, it is necessary to incorporate the inverse magnetizing curve of the machine in the controller. The paper proposes an experimental method for identifying the inverse magnetizing curve, specifically developed for the type of vector controlled drives described. The method utilizes the same indirect vector controller and PWM inverter that are used in subsequent normal operation of the drive. It requires that the machine can run under no-load conditions and that the fundamental component of the stator voltage can be measured. The simplicity and accuracy of the method make it well suited for use during commissioning of the drive. The method is verified by extensive experimentation.

Operating principles of a novel multiphase multimotor vector-controlled drive

Independent flux and torque control of an ac machine can be achieved by means of vector control, utilizing only two stator d-q current components. Consequently, in ac machines with a phase number greater than three, there exist additional degrees of freedom. Although they can be used to enhance the torque production of a multiphase machine through injection of higher stator current harmonics, an entirely different purpose is possible as well. The additional degrees of freedom can be utilized to control independently other machines within a multimotor drive system. In order to do so, it is necessary to connect stator windings of all the multiphase machines in series, with an appropriate phase transposition, apply a vector control algorithm to each machine separately, and supply the stator windings of the multi-machine system from a single current controlled voltage source inverter (VSI). Inverter current control is performed in the stationary reference frame, using inverter phase currents. The foundations of the concept are set forth in the paper, for an arbitrary odd n-phase case, using the general theory of electrical machines. Further analysis is performed for all the theoretically possible odd phase numbers and it is shown that the number of machines connectable in series depends on the properties of the phase number. Connection diagrams are illustrated next for some selected phase numbers and vector control, including the inverter reference current generation, is detailed for the multimotor drive system. The main advantages and drawbacks of the concept are discussed and verification is provided by simulation of a nine-phase four-motor drive system.