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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