Abstract
This paper presents a Z-source inverter system and control for general-purpose motor drives. The Z-source inverter system employs a unique LC network in the dc page link and a small capacitor on the ac side of the diode front end. By controlling the shoot-through duty cycle, the Z-source can produce any desired output ac voltage, even greater than the line voltage. As a result, the new Z-source inverter system provides ride-through capability duringvoltage sags, reduces line harmonics, improves power factor and reliability, and extends output voltage range. Analysis, simulation, and experimental results will be presented to demonstrate these new features.
Index Terms—Line harmonics, motor drives, voltage sags, Z-source inverter.
I. INTRODUCTION
THE TRADITIONAL general-purpose motor drive (or adjustable speed drive—ASD) system is based on the voltage-source inverter (V-source inverter), which consists of a diode rectifier front end, dc page link capacitor, and inverter bridge, as shown in Fig. 1. In order to improve power factor, either an ac inductor or dc inductor is normally used. The dc page link voltage is roughly equal to 1.35 times the line voltage, and the V-source inverter is a buck (or step-down) converter that can only produce an ac voltage limited by the dc page link voltage. Because of this nature, the V-source inverter based ASD system suffers the following common limitations and problems. 1) Obtainable output voltage is limited quite below the input line voltage. Fig. 1 illustrates voltages of a three-phase 230-V drive system. The diode rectifier fed by the 230-V ac line produces about 310-V dc on the dc-link, which is roughly 1.35 times the line-to-line input voltage under the assumption of heavy load and continuous “double-hump” input current for large ( 50 kW) drives that typically have an approximately 3% of inductance on the ac or dc side. For light load operation or small drives with no significant inductance, the line current becomes discontinuous “double-pulse,” and the dc voltage is closer to 1.41 times the line-to-line input voltage (i.e., 325-V dc for a 230-V ac input). The inverter can only produce a maximum 190-V ac in the linear modulation range given the 310-V dc under the heavy load operation when the voltage is needed the most. For a 230-V motor, the low obtainable output voltage significantly limits output power that is proportional to the square of the voltage. This is a very undesirable situation for many applications because the motor and drive system has to be oversized for a required power. 2) Voltage sags can interrupt an ASD system and shut down critical loads and processes. Over 90% of power quality related problems are from momentary (typically 0.1–2 s) voltage sags of 10–50% below nominal (Fig. 2 illustrates voltage sags). The dc capacitor in an ASD is a relatively small energy storage element, which cannot hold dc voltage above the operable level under such voltage sags. Lack of ride-through capacity is a serious problem for sensitive loads driven by ASDs [1]–[6]. [6] details the vulnerability of a ASD and the dc voltage under three-phase and two phase voltage sags. Solutions have been sought to boost ride-through [2]–[6]. The ASD industry provides options using flyback converter or boost converter with energy storage or diode rectifier (Fig. 3) to achieve ride-through; however, these options come with penalties of cost, size/weight, and complexity. 3) Inrush and harmonic current from the diode rectifier can pollute the line. Low power factor is another issue of the traditional ASD system. 4) Performance and reliability are compromised by the V-source inverter structure, because 1) miss-gating from EMI can cause shoot-through that leads to destruction of the inverter, 2) the dead time that is needed to avoid shoot-through creates distortion and unstable operation at low speeds, and 3) common-mode voltage causes shaft current and premature failures of the motor.

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