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Publication Title | An Ultra-High-Speed, 500000 rpm, 1 kW Electrical Drive System

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An Ultra-High-Speed, 500000 rpm, 1 kW Electrical Drive System

Abstract—New emerging micro gas turbine generator sets and turbocompressor systems push the speed limits of rotating machinery. To directly connect to these applications, ultra- high-speed electrical drive systems are needed. Therefore a 1 kW, 500000 rpm machine and the according power and control electronics are designed and built. This paper includes design considerations for the mechanical and electromagnetic machine design. Furthermore, a voltage source inverter with an additional dc-dc converter is described, and sensorless rotor position detection and digital control is used to drive the machine. Finally, the hardware and experimental results are presented.

I. INTRODUCTION

The trend for air-compressors and miniature gas turbine generator sets is towards smaller power ratings and with the scaling of turbomachinery therefore towards higher speeds [1]–[4]. The definition of super-high-speed electrical machines is dependent on the required power level and the rated speed of the machine. Figure 1 shows the dividing line between high- speed and super-high-speed electrical machines as defined by [1] and the application areas for turbomachinery such as industrial and micro gas turbines and compressors. Extrapo- lating the turbomachinery line (dotted) predicts the operating range for ultra-high-speed drive systems as having speeds from 300000 to 1 million rpm and power levels between 10 W and 3 kW.

For the industrial gas turbines in the MW power range the grid-connected generator is coupled to the turbine through a gearbox and operates at a lower speed. Micro gas turbines with tens of kW of power that have direct connected high- speed permanent magnet (PM) generators/starters delivering 10 to 100 kW are becoming more prevalent [2], for example, the Capstone micro gas turbine operates at 90000 rpm with a power output of 30 kW. Several international research groups are investigating ultra-micro gas turbines with power outputs up to a 100 W for use in portable power applications [3]. Only a few of these projects consider the electrical system, although the electrical drive system is an integral part of the total system in order to start and generate electrical power from the turbines. For compressor systems the power and speed trends are similar to the turbines. One application is a fuel cell air compressor that requires 120000 rpm at 12 kW [5] and another is a 70000 rpm, 131 kW turbo compressor connected to a PM machine and an inverter [6]. Future automotive fuel cells will require low-power compressors which are small and lightweight and directly driven by high-speed electrical drives. All these emerging applications require an ultra-high-speed electrical drive system. Therefore, this paper presents a new machine and power electronics interface that is capable of

108 106 104 102 100

industrial gas turbines

ultra-high speed region

turbomachinery trend

micro turbines and compressors

emerging turbines and compressors

1-4244-0844-X/07/$20.00 ©2007 IEEE. 1577

C. Zwyssig, M. Duerr, D. Hassler, J.W. Kolar Power Electronic Systems Laboratory

ETH Zurich

8092 Zurich, Switzerland zwyssig@lem.ee.ethz.ch

power (W)

high-speed region

104

ETH project

[1]

105 106 rotational speed (rpm)

Fig. 1: Power and speed ratings of turbines and compressors and the trend line towards smaller power and higher speed. Systems referenced are plotted with circles, while future trends and planned systems are given as dashed circles. The ultra-high-

speed electrical drive system is highlighted.

operating at 500000 rpm with a power output of 1 kW (see Figure 1).

The complete drive system is basically composed of three main parts as indicated in Figure2. First, there is the electromechanical part, the permanent-magnet synchronous machine. The main challenges are the mechanical rotor design and the machine loss minimization. Further, the machine has to be supplied with currents of 8.3 kHz fundamental frequency by the power electronics. The drive system should run from a dc-voltage bus, and bi-directional power capability is needed. And last but not least the power electronics converter is driven by the control system, which includes sensorless rotor position detection and a cascaded torque and speed controller.

These main parts of the system are described in this paper and an experimental setup and measurements are presented. Section II describes the mechanical and electromagnetic ma- chine design and presents the loss analysis. Section III covers the power electronic topology including the selection of the power semiconductors and the dimensioning of the passive components. Section IV deals with the control system including the common torque and speed control cascade, the sensorless rotor position detection and the realization on a

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