10-07-2012, 02:55 PM
A Three-Phase Multilevel Converter for High-Power Induction Motors
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INTRODUCTION
HIGH-POWER induction motor drives using classical
three-phase converters have the disadvantage of poor
voltage and current qualities. To improve these values, the
switching frequency has to be raised which causes additional
switching losses. Another possibility is to put a motor input
filter between the converter and motor, which causes additional
weight. A further inconvenience is the limited voltage that
can be applied to the induction motor determined by the
blocking voltage of the semiconductor switches. For highpower
semiconductors, the switching frequency is limited by
the maximal power loss. In this paper, a three-phase multilevel
converter is described. The aim is to generate motor currents
of high quality by using more semiconducting devices.
THREE-PHASE MULTILEVEL CONVERTER
The basic element of the converter is the full-bridge converter.
The full bridge is supplied with a floating dc voltage,
either a dc voltage from a regulated [1] or unregulated dc–dc
converter, or by a series connection of a transformer output
[3]. The output on the other side of the full bridge can now be
a positive or negative output step. With a series connection of
several full-bridge cells, the positive and negative dc voltages
can be superposed to create a high-voltage output for each
phase on the induction motor. In the example in Fig. 3, two
cells in series per phase were used.
MODULATION METHODS
For the multilevel converter, different modulation methods
can be used to drive an induction motor. The following
methods have been developed and tested.
The first method described is a PWM modulation. For one
full-bridge converter, two auxiliary triangular functions have
to be generated and compared with a sine-wave signal (one
for the half bridge generating the positive voltage and the
other for the half bridge generating the negative step). So, for
the first phase of the multilevel converter, two steps with two
triangles per step are taken using the phase angles 0 , 90 ,
180 , and 270 .
CONCLUSIONS
A new converter topology has been presented for drive
applications in high-power fields. With a high number of semiconducting
devices, current quality is improved and weight
reduced by avoiding heavy current filters. With the series con-
nection of steps, a redundant motor drive can be designed. In
case of a power failure of one step, the other multilevel steps of
the same phase can compensate the missing voltage step. The
proposed solution has many advantages, especially for highvoltage
motor applications. With the new modulation method,
the losses are distributed equally on each full-bridge cell. The
simulations and experimental results show the feasibility of
such a system.