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The optimization of both DC link voltages and switching angles of fundamental switching strategy for the output waveform of a multilevel cascade inverter is investigated in this study. Previous work in the area of fundamental switching for multilevel inverters is highlighted by the pulse width optimization technique, where the switching angle at each voltage level is optimized so that the resulting staircase output voltage has the minimum harmonic content. The DC link voltage of each level, however, is assumed to be the same and constant. As an extension to this work, the optimization of both DC link voltages and switching angles of the staircase output voltage is carried out. Harmonic performance of the proposed switching strategy is simulated and results are compared with that of the previous work. It is confirmed theoretically that substantial improvement in the harmonic minimization can be obtained. In practice, the proposed method is especially suitable for the control of multilevel cascade inverters, the latest development of its kind, where each DC voltage can be self-maintained and independently controlled. The description of the multilevel cascade inverters for applications of AC drives, as well as Static Var Generation, is also given in this article.

Numerous techniques have been reported in the literature to accomplish the task of locating the rotor flux axis for the purpose of direct field orientation. This paper first summarizes the existing sensorless techniques using high-frequency test signal injection to determine the flux spatial location in an induction machine. A novel approach employing balanced and unbalanced excitation is then investigated to track the flux. It is first demonstrated that the rotating flux vector in the machine causes saturation-induced saliency in the phases under normal operating conditions. By employing high-frequency test signals, this saliency is monitored and then used to track the flux in the machine under test. The feasibility of the proposed approach is verified with experimental results.

In this paper a new, improved induction motor control topology is proposed which allows for continuous, disturbance-free operation of the drive with a failed short-circuited switch in one leg of the inverter. The analysis of this control strategy and the computer simulation are explored. The experimental results from hardware are attached to verify the simulation results. A complete control strategy combined with short-circuit fault and open-circuit fault is also included.

New doubly salient ac machines in which the field excitation is developed by stationary permanent magnets has recently been undergoing development at the University of Wisconsin. Because of the unique geometry of the stator, these permanent magnets are readily accessible. Therefore, for experimental purposes, simple iron blocks have been used to "short" a large portion of the permanent magnet flux and thus reduce the back emf developed within the machine. However, for practical applications, field weakening schemes involving miniature linear actuators or high torque stepper motors are being explored, as well as machine designs with combined PM and electrical excitation. Preliminary experimental results show that a substantial reduction in the back emf can be readily obtained thus lending credibility to the concept of weakening air gap fields produced by stationary permanent magnets.

This paper contributes to the understanding of the behavior of the saturation third harmonic and how energy is converted from fundamental to third harmonic form in a squirrel-cage induction machine in me presence of magnetic saturation. It is shown that the power transferred via the third harmonic airgap flux is accounted for by a change in the magnitude and phase-shift in the fundamental airgap flux. A procedure is proposed to link the rotor third harmonic circuit quantities to a set of equivalent fundamental component circuit quantities which leads to a modified equivalent circuit for the induction machine.

This paper describes a practical method for achieving optimal efficiency over the complete operating range of a variable speed drive. The proposed system adaptively adjusts the flux level in the motor based upon a direct measurement of the power input to the drive. An internal field orientation torque control loop and a speed regulator are employed to maintain the load speed requirements. Experimental results describing the efficiency optimization and the dynamic behavior of the drive at reduced flux are presented. The influence of the tuning of the field oriented controller on the efficiency of the drive is experimentally investigated.

A fundamental component model of current-regulated naturally sampled PWM inverters operating in the pulse-dropping mode is proposed. Analytical and test results are compared for purposes of model verification and regulator evaluation. The results demonstrate the problems with the classical (stationary reference frame) current regulator that occur in this region of operation. In contrast, the synchronous reference frame regulator is shown to have ideal steady-state response in this region.

A new method for the transient analysis of induction machines with saturating leakage reactances is presented. The equations which define operation under this condition are arranged so that the saturation of stator and rotor leakage as well as magnetizing reactance can be readily modeled with three function generators. The method is equally applicable to the analysis of synchronous machines.

An investigation of the maximum speed capability of a current source inverter (CSI) drive in both a three-and a six-phase connection is presented. It is shown that a six-phase configuration has distinct advantages over a three-phase connection in high speed capability for the same peak motor spike voltages. In addition, with proper control of the firing pulses fed to the inverters supplying the six-phase machine, the speed range can be substantially extended beyond the maximum limit predicted using conventional control. Predicted analytical results are correlated with results obtained by a detailed hybrid computer simulation.

Minimum input power and maximum efficiency operation occur at characteristic slip values which can be realized for any induction motor operating at part load by properly adjusting the amplitude of the applied stator terminal voltages. These two criteria are shown to yield perceptibly different results when the motor is driven from a silicon-controlled rectifier (SCR) voltage controller. In addition, it is demonstrated that a constant power factor controller results in an operating regime which is substantially poorer than operation at either minimum input power or maximum efficiency. It is further shown that minimum stator current and minimum power factor angle criteria yield results which are closer to the ideal than the constant power factor controller.

A new method of air-gap flux sensing in induction machines is introduced which is based on sensing the voltage across individual motor coils. By subtracting the voltage across two properly located motor coils a signal is obtained which is independent of stator ir drop and nearly independent of motor leakage reactance drop. By combining the proposed flux sensing method with conventional current transducers, the electromagnetic torque is readily computed. The method has been implemented in the laboratory and good correlation has been obtained for both sinusoidal and inverter power supplies.

The factors which influence the current response to a current command for a normal autosequentially commutated current link inverter (ASCI) are discussed. A simple new method of notching the output current waveform for the ASCI is presented, which is used to control the ac output current from the ASCI. To demonstrate the improvement in the current response of the ASCI with notching (or NASCI), two typical classes of control systems of ac induction machine drives are considered. A performance comparison is made between ASCI and NASCI induction machine drives. Analysis and analog computer simulation results show that substantial improvement in dynamic response is achieved when the notching is utilized.

The design and performance of a unique concentrated winding machine specifically designed for operation with a static power converter is described. When operating in tonjunction with a converter supply, the machine is theoretically capable of 15 percent more power output for the same active copper and iron than a conventionally designed synchronous machine of the same rating. The performance of the machine operating as a generator feeding a rectifier load is described, the theoretical predictions are compared with test. The potential of such a machine operating as a motor is also described.

Switched reluctance motors are typically constructed such that the coupling between phases is relatively small compared to conventional induction and synchronous machines. While small, this coupling is not negligible and could have important influence on transfer of current between motor phases during both normal and abnormal conditions. This paper presents, for the first time, a coupled circuit model which allows for mutual coupling between motor phases. Comparison is made between simulation and test results and good correlation is obtained.

A new force commutated three level boost type rectifier is proposed in this paper. The rectifier has the characteristics of drawing nearly sinusoidal current from utility with unity fundamental power factor. High reliability is possible due to its shoot-through free structure. A doubled dc bus voltage compared to a normal two-level rectifier is also possible because of the neutral point clamping. The operating principle, steady state analysis, input current and neutral point voltage control schemes, as well as detailed experimental results are presented in this paper.

A new variable reluctance motor is introduced to help solve the energy circulation problems which exist during commutation in conventional variable reluctance motors (VRM's). The new motor design enables the energy stored in the magnetic field to be retained and utilized within the motor, instead of being returned to the source. The operating principles of the motor and associated converters are presented. The analysis shows that by employing both short pitch and full pitch windings, this new motor can eliminate the two problems without extra conductors in the slots. As a result, the new motor has the following important performance advantages over conventional VRM's: (1) It allows a significant improvement in the turn-off process and therefore allows higher speed capacity; (2) it has higher output with the same or higher efficiency with the same output because of the improvement on the turn-off performance and due to the utilization of the trapped energy; (3) the machine can have a higher output with the same power converter because of the improvement of the energy conversion ratio.

A self-controlled, current-fed inverter excited linear inductor machine is described, in which the field current and inverter switching angles are programmed such that the machine operates at constant air-gap flux and unity displacement factor under all operating conditions. The machine can operate in wide speed range and constant predetermined torque is developed during both motoring and regeneration. The control incorporates overlap angle compensation for the inverter and permits modulation of inverter firing angle with resolution of a fraction of a degree. The dynamic d-q axis model of the machine with the power converters and the proposed control strategy has been simulated on a hybrid computer, and static and dynamic performance have been studied in detail. The control circuit has been designed and tested with a 112-kW prototype inductor machine. The experimental results give good agreement with the theory and with simulation.

Conventional methods of controlling an induction motor utilize regulation of stator current and motor slip frequency in order to maintain system stability. This control strategy requires a shaft speed feedback and fast-response current regulation. An alternative method of controlling an induction motor is presented which achieves the necessary system stabilization by controlling only the motor frequency. The control inherently regulates the motor torque angle by properly adjusting the phase of the converter firing signals. By synchronizing the inverter firing pulses to the motor back electromotive force (EMF) possible adverse inverter operating modes are avoided. The concept of synchronous control eliminates the preprogrammed functional relations previously required and allows the control to adapt to any desired motor flux level. Any desired outer regulating loop can be incorporated to form a fast-response wide-range ac drive system.

A dynamic model for current-controlled induction motor drives is developed, and a transfer function approach to the transient response investigation is formulated by means of d-q variables in the synchronously rotating reference frame. A sample control strategy is discussed, and transfer functions for various combinations of input and output variables are presented. It is shown that both dynamic and static instabilities exist for open-loop operation, but a well-damped closed-loop response is possible if slip frequency and current magnitude control are imposed. A comparison between the analytical transfer function transient performance predictions, the transient respontse predicted by the hybrid computer simulation, and actual laboratory tests is made using frequency response techniques. Bode plots are used to correlate' the results between laboratory and analytical techniques. Hybrid computer and laboratory results are presented to show typical steady-state characteristics and waveforms.

A method of analyzing the constant speed steady-state performance of a reluctance motor fed from voltages of an arbitrary waveform is presented. The solution is obtained directly from Park's equations by a time-domain harmonic balancing approach related to the method of multiple reference frames. Both synchronous and asynchronous operation can be obtained from the same theory. The method is readily extended to incorporate any type of synchronous machine. Harmonic balancing is applied to the performance calculations of a reluctance motor supplied by a square wave voltage source inverter. Comparisons are made with more conventional approaches.

A load-commutated inverter synchronous motor drive system employing a simple auxiliary commutation circuit for machine startup analyzed, and results hybrid computer simulation are presented. The commutation circuit employs a single commutation capacitor connected to the neutral of the machine and two auxiliary thyristors, which are used only during machine starting. A practical operating scheme is developed for the forced commutated inverter, which insures commutation over all load currents by actively allowing the commutation capacitor to charge to a voltage proportional to load, current. Results of key computer runs are given including inverter waveforms, transient waveforms during transition from forced to load commutation, as well as the effect of forced commutation and load commutation on pulsating torque. The forced-commutation circuit is used only for synchronous machine startup. However, due to its simplicity it also is an attractive alternative to be considered for other types of current-fed inverter ac drives.