312IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 55, NO. 1, JANUARY 2008Induction Motor Drives Equipped With DiodeRectifier and Small DC-Link CapacitanceMarko Hinkkanen, Member, IEEE, and Jorma Luomi, Member, IEEEAbstractThis paper deals with sensorless vector-controlled induction motor drives that are fed by a frequency converterthat is equipped with a diode front-end rectifier. A small dc-link capacitance is used, which makes it possible to replace the elec-trolytic capacitors with film capacitors. The natural frequency of the dc link is chosen to be considerably higher than six times the mains frequency but lower than the switching frequency. A re- cently proposed sensorless controller can be exploited; only minormodifications for small capacitances are needed. The simulation and experimental results of a 2.2-kW drive that is equipped with a capacitance of only 24 F demonstrate operation in a wide speed range.Index TermsControl, dc link, film capacitors, sensorless.I. INTRODUCTION FREQUENCY converters that are used in induction motor drives are usually equipped with a cost-effective six-pulsediode rectifier, a voltage-stiff dc link, and a pulsewidth- modulated (PWM) inverter. Conventionally, the dc link consists of electrolytic capacitors and a dc choke (or an ac choke atthe input of the rectifier). The natural frequency of the dc link is much lower than six times the mains frequency, resulting in a stiff dc-link voltage. Electrolytic capacitors have a large capacitance per volume. There is an increasing interest inreplacing these capacitors with film capacitors having a longer lifetime and no explosion risks, but the dc-link capacitance has to be considerably reduced in order to conserve the size of the capacitor bank. Small dc-link capacitances have been applied in drives thatare equipped with active rectifiers 1, 2, where both therectifier and the inverter can be used to control the power balance of the dc link. Commercial drives that are equippedwith a diode rectifier and a small dc-link capacitance are available 3; a small capacitance is advertised to reduce the mains harmonics, while the dynamic performance of the drive has not been used as an argument. In research publications, theconcept of a diode rectifier and a small dc-link capacitance has received little attention, with the exception of recent studies from the viewpoint of pulsewidth modulation 4, 5.Manuscript received June 8, 2005; revised June 14, 2007. This work was supported by ABB Oy. An earlier version of this paper was presented at the 31st Annual Conference of the IEEE Industrial Electronics Society, Raleigh, NC, November 610, 2005. The authors are with the Power Electronics Laboratory, Department of Elec- trical and Communications Engineering, Helsinki University of Technology,02015 Espoo, Finland (e-mail: marko.hinkkanentkk.fi).Color versions of one or more of the figures in this paper are available online at http:/ieeexplore.ieee.org.Digital Object Identifier 10.1109/TIE.2007.903959In this paper, the dimensioning of the dc link is based on a natural frequency that is higher than six times the mains frequency but lower than the switching frequency. In practice,the mains inductance is sufficient, and no additional choke isneeded. The vector control with rotor flux orientation is used. The effects of the small dc-link capacitance on the control system design and control performance are studied. The ripple in the dc-link voltage, particularly, at six times the mains frequency, is considerably higher than usual. At low speeds, this ripple does not deteriorate the performance since the duty cycles of the pulsewidth modulator are calculated based onthe measured dc-link voltage. At high speeds, the flux level is controlled based on the pulsating dc-link voltage in order to maximize the torque 6, 7. To avoid a braking resistor, the braking scheme that was proposed in 7 is exploited. Furthermore, restrictions on the dynamics may be needed to prevent an overvoltage in the dc link, except in the case of low- power drives.II. SYSTEMMODELA. Diode Rectifier and DC LinkThe models of the drive system components are presented in the following. Fig. 1(a) shows a three-phase model of themains, the diode rectifier, and the dc link. The mains resistance and inductance are denoted by Rgand Lg, respectively, and the dc-link inductance, resistance, and capacitance are denoted by L?d, R?d, and Cd, respectively. The phase-to-neutral mains voltages are uga, ugb, and ugc, having peak value ugand angularfrequency g.Thecurrentattheoutputoftherectifieris idi, while the current and voltage at the input of the inverter are idand ud, respectively. The inverter is not shown in the figure. It is modeled by three ideal changeover switches.The model in Fig. 1(a) is approximated by a simplified modelin Fig. 1(b). The ideal rectified voltage isudi= maxuga,ugb,ugc minuga,ugb,ugc(1)or in terms of its harmonic componentsudi= udN?1 ?n=12 (6