The author summarizes important developments in AC drive design that have occurred in the past several years. He discusses: converter technology, covering the matrix converter, PWM voltage and current link converters; and resonant link converters; AC motor technology, covering stepping motor drives, DC brushless motor drives, and synchronous reluctance motors; and control technology for AC drives, covering online and offline parameter identification and efficiency-maximizing control. >

Recent progress in the development in solid-state AC motor drives

An AC/DC conversion scheme is presented which eliminates the need for self-commutated devices and requires only 12 thyristors for full double bridge AC to AC power conversion. The system utilizes a series resonant DC link between the AC/DC and DC/AC converters. This series resonant scheme is, in effect, the dual of the parallel DC resonant converter. The DC resonant circuit can be essentially considered as a commutating circuit which ensures turn-off of all twelve thyristors by providing the necessary zero-current instants. A significantly improved sinusoidal current waveform can be obtained at both the input and output compared to conventional high-power converters by the use of high-frequency pulse density modulation. >

High-frequency series-resonant DC link power conversion

The paper evaluates the potential of a brushless doubly-fed generation system for wind power applications. A 1.5 kW proof-of-concept laboratory prototype is used to investigate the feasibility of the proposed variable-speed generation principle. Experimental results show that the prototype system can achieve high efficiency over a range of speeds. The system efficiencies achieved at the power levels considered compare favorably with conventional squirrel cage induction machines. It is experimentally demonstrated that the brushless doubly-fed system achieves variable-speed operation with a power converter of reduced rating. For the prototype under consideration, a 2:1 speed range can be covered with a power converter rated at approximately 25% of system capacity. While reactive power control can be realized with the proposed system, the magnitude is limited by the desired low power converter rating. The prototype system also is also shown to exhibit excellent output current waveforms. >

Experimental evaluation of a variable-speed, doubly-fed wind-power generation system

A comprehensive review and a comparative evaluation of AC/AC converter topologies are presented. The aim is to provide guidelines for selecting an optimum technology for a given application. The topologies are discussed and evaluated based on the type of link they use. The essential features associated with the links are included. Mixed topologies, which are the focus of significant research activity, are also discussed. >

A guide to the application-oriented selection of AC/AC converter topologies

The more widespread adoption of adjustable-speed drives to promote process efficiency through energy conservation is currently hampered by the high cost of power electronic conversion units for use with AC motors. The examination of alternative methods of obtaining speed control in both synchronous and induction machines leads to the conclusion that a self-cascaded machine has considerable promise. The motor has six input terminals to the same stator windings: one three-phase set presenting a different pole number to the other three-phase set. The rotor is a specialized form of a robust cage construction without slip rings. When operated in a doubly-fed mode, this machine requires only a fraction of its input power to be processed by a power electronic converter. The resulting system has the potential to provide a low cost, robust, fault-tolerant, precise speed, and adjustable speed drive. >

The potential of brushless doubly-fed machines for adjustable speed drives

Results of study and measurements are presented for a variable-speed generation (VSG) system that uses a series-resonant converter to excite the rotor windings of a doubly-fed generator The capabilities of this system include maximum-efficiency operation and limited converter rating for a speed range from stand-still (zero RPM) to well beyond synchronous speed, as well as flexibility for line-voltage control, reactive power generation, and synchronization at zero RPM Measurements of the harmonic distortion are provided showing the superiority of this converter as compared with a VSG system using a conventional cycloconverter or a DC-link converter A convenient tool for analyzing the operating modes of the system is discussed that can be developed under varying resource conditions >

D.B. Seely

Papers

Variable-speed generation with the series-resonant converter