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Two-level three-phase voltage source inverter fed low-power AC induction motor based on unipolar pulse-width modulation method Abstract: Space vector modulation is a well-established theory. Generally, a three-phase voltage source inverter generates eight switching states, including six active and two zero states, by the bipolar method. This study proposes a novel way to synthesise the output voltage vector by turning off both switches in a leg. Accordingly, new leg switching states are defined. A unipolar method is used to achieve discontinuous pulse-width modulation. The complex reference vector is located by one active vector and two newly defined leg switching states. The most notable features of the proposed method are that it completely overcomes the problem of dead time without current polarity detection. These features make it very attractive for low-cost, low-power constant volts per hertz (V/f), open-loop, three-phase AC induction motor applications. Existing system: Two-level voltage source inverter (VSI) fed three-phase AC induction motors are widely used in low-power applications. The standard topology of a three-phase VSI. It consists of three inverter legs with two switches in parallel with a freewheel diode per leg. Each leg’s output can easily be connected to either the upper or the lower DC bus by the bipolar method. One of the earliest used modulation techniques is sub-oscillation pulse-width modulation (PWM). As a modulator’s performance characteristics primarily depend on the voltage utilisation level, i.e. modulation index, various highperformance carrier-based PWM methods have been developed. Proposed system: A novel modulation of a three-phase two-level VSI based on the unipolar PWM method is presented. It is similar to the discontinuous method DPWM0. Both techniques have similar characteristics. The inverter leg switching state ‘×’ with both switches OFF is used not only for blanking time but also for producing the space voltage vector. The advantage of the new method is that the requirement of dead time in every sector is eliminated. Thus, the traditional adverse effects of dead time are avoided. It is unnecessary to detect the current polarity of each inverter leg. Considering each inverter leg put out correct voltage depending on the current polarity, an AC induction motor must be connected to provide the current path. Since the power factor of most low-power induction motors is <0.866, it is suitable for low-power induction motor load with more than 30° lag for the power factor angle. Circuit diagram: Reference: 1 Holmes, D.G., Lipo, T.A.: ‘Pulse width modulation for power converters: principles and practice’ (Wiley-IEEE Press, Hoboken, 2003) 2 Chung, D.-W., Kim, J.-S., Sul, S.-K.: ‘Unified voltage modulation technique for real-time three-phase power conversion’, IEEE Trans. Ind. Appl., 1998, 34, (2), pp. 374–380 3 Ojo, O.: ‘The generalized discontinuous PWM scheme for three-phase voltage source inverters’, IEEE Trans. Ind. Electron., 2004, 51, (6), pp. 1280–1289 4 Narayanan, G., Zhao, D., Krishnamurthy, H.K., et al.: ‘Space vector based hybrid PWM techniques for reduced current ripple’, IEEE Trans. Ind. Electron., 2008, 55, (4), pp. 1614–1627