The technology of solid-state adjustable speed ac drives was launched in the 1960's. Since then many innovations in devices, circuits, control theory, and signal electronics have made a considerable contribution to this technology. With the heritage of past experience, and the projection of present trends, an almost certain prediction can be made that adjustable speed ac drives will find widespread applications in industry before the end of this decade. The paper reviews the present status of ac drives technology in which the salient technical features of ac machines, converters, controls, and performances of the integrated drive systems have been discussed, and wherever possible, the appropriate trend of the technology has been indicated.

Adjustable speed AC drives&#8212;A technology status review

Multilevel converters (MLCs) have emerged as standard power electronic converters in high power as well as quality demanding applications. They are classified into current-fed MLCs and voltage-fed MLCs. Voltage-fed MLCs have widely researched whereas the current-fed MLCs are the recent topic of research. Based on the principle of duality between voltage and current sources, several current-fed MLCs analogous to voltage-fed MLCs have been identified. Current-fed MLCs offer several advantages in terms of high power capability, transformerless operation, short-circuit protection, and excellent quality of output current waveform. The goal of this paper is: 1) to present review of circuit topologies, modulation schemes, and applications of current-fed MLCs; and 2) to review an emerging low-device switching frequency modulation technique known as synchronous optimal pulsewidth modulation for current-fed MLCs. The circuit configuration and advantages of each topology along with various modulation techniques are discussed in detail. Compared to voltage-fed MLCs, the operation of current-fed MLCs need to satisfy additional switching constraints. A survey of classical methods for realization of these operational constraints has been done and a new generalized method has been proposed. Finally, future scope of research has been presented to encourage further development of topologies and modulation techniques for current-fed MLCs.

Current-Fed Multilevel Converters: An Overview of Circuit Topologies, Modulation Techniques, and Applications

Speed control of ac motors requires variable frequency, variable current, or variable voltage supply. Variable frequency supply can be obtained directly from a fixed frequency supply by using a frequency converter or from a dc source using inverters.

A Control Strategy for Reference Wave Adaptive Current Generation

Unbalanced DC currents in parallel current source inverters cause unequal power division in the inverters, higher harmonic content at the AC load side and generally, system performance decline. In this paper, a novel Space Vector Modulation is introduced for a 5-level parallel current source inverter with DC current balance control. The method is working by synthesizing the rotating current reference vector in the inverter's space vector plane with three adjacent switching vectors. One Medium vector is employed as one of adjacent vectors to balance the input DC currents. The switching state for each switching vector is chosen to provide the balanced DC link currents. In addition, lower switching frequency is achievable due to switching design which minimizes the switching loss. Finally, effectiveness of the proposed method is verified by simulation.

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5-level parallel current source inverter for high power application with DC current balance control

Novel six-step and twelve-step current-source inverters (CSI) with dc side commutation and energy rebound capability are presented with detailed explanation of the circuit operation. The proposed inverters can operate in a very wide range of frequency and load variation by employing dc side commutation. Also, the energy rebound makes the use of low voltage silicon controlled rectifiers (SCR's) possible and increases the inverter efficiency. Unlike the dual current-source inverter, one auxiliary inverter with a power requirement of about one-half that of the main inverter is simply added to the six-step CSI in order to obtain a twelve-step CSI. Motor operation is possible in four quadrants in both six-and twelve-step inverters. The advantages of the proposed CSI's over conventional ones are described, and experimental results are given in oscillograms.

Novel Six-Step and Twelve-Step Current-Source Inverters with DC Side Commutation and Energy Rebound

The principle of operation of a dual current source converter is briefly explained. The combination of two single current source converters (SCSC) to form a ``dual (duplex) current source converter'' (DCSC) is proposed. The DCSC is shown to have the following merits: 1) it retains all the advantages of the SCSC; 2) it reduces the harmonic content of the current waveform considerably; and 3) since the load current is shared equally between two current source converters, ratings of the individual components employed in the circuit are considerably lowered. A DCSC can be an attractive choice for sophisticated large horsepower drives where a good performance of the drive rather than cost is a prime factor. An open-loop control scheme employing the DCSC for an ac motor drive has been successfully implemented in the laboratory. Oscillograms of the improved load current waveforms are shown.

Principle of a Dual Current Source Converter for AC Motor Drives

The modified McMurray Inverter with Pulse Forming Network (PFN) has been explained. The current and voltage waveshapes of the PFN commutation ci rcuit have been compared with conventional L-commutation circuit. The design method of PFN has been explained. Advantages of this type of commutation have been discussed. Experimental results are given.

Modified McMurray Inverter with Pulse Forming Network Commutation Circuits

This paper describes a variable-frequency source employing the cycloconverter principle. SCR's have been used as the principal switching and voltage-regulating devices. The logic of the firing circuit has been discussed. The factor affecting output frequency stability has also been presented. A three-phase variable low-frequency oscillator has been used as the reference to achieve the variation of frequency. Voltage variation has been affected by phase delaying of the firing pulses in a manner similar to that in a phase-controlled rectifier. Provision has been made for changing the phase sequence at the output in order to facilitate reversal of the direction of rotation of the motor. Experimental results include output waveforms of various frequencies (0-30 Hz), and speed variations of the experimental motor (1 hp, 100 V, 1700 r/min) in either direction of rotation thereby providing the practical feasibility of the SCR cycloconverter for speed control of induction motors.

Samir K. Datta

Papers

Principle of a Dual Current Source Converter for AC Motor Drives

Modified McMurray Inverter with Pulse Forming Network Commutation Circuits

A Static Variable-Frequency Three-Phase Source Using the Cycloconverter Principle for the Speed Control of an Induction Motor