Abstract
The application of multilevel converters for traction drive systems is being investigated. The main
advantage of this kind of topology is that it can generate almost perfect current or voltage waveforms,
because it is modulated by amplitude instead of pulse-width. That means that the pulsating torque
generated by harmonics can be eliminated, and power losses into the machine due to harmonic
currents can also be eliminated. Another advantage of this kind of drive is that the switching frequency
and power rating of the semiconductors is reduced considerably. The amplitude modulation is based
on a cascade of N converters scaled in a “trinary” form (three-state “H” converters). In the chain of N
converters of each phase (N-Stage Converter), there is a “Master converter” that manages more than
80% of the total power, and N-1 “Slave converters” that take the rest of the power (less than 20%).
One important drawback of this kind of arrangement is that it needs isolated power sources for each
one of the N converters, and also for each phase. This paper shows that this problem can be overcome
by using isolated motor windings for each phase of the traction motor (which is easy to get in normal
machines), and by using low-power high-frequency, bidirectional switching power supplies for the
“slave converters”. Simulations using PSIM (Power Electronics Simulator) have demonstrated the
feasibility to build drive converters for electric vehicles using multilevel inverters. They are being
compared with inverters using the conventional PWM technique. The multilevel converter used in the
simulations, works with only four inverters (N=4): one Master and three Slaves. In both the cases
(PWM and multilevel), the traction motors have a rating of 80 kW, and the battery pack supply is 240
Vdc. The battery pack is connected to the master converters of each phase in parallel, and to the slaves
through isolated bi-directional switching power supplies. Copyright 2002 EVS19
Keywords: Inverter, Drive, Converter, Control System, Electric Drive.
1. Introduction
Power Electronics technologies contribute with important part in the development of electric vehicles.
On the other hand, the PWM techniques used today to control modern static converters for electric
traction, do not give perfect waveforms, which strongly depend on switching frequency of the power
semiconductors. Normally, voltage (or current in dual devices) moves to discrete values, forcing the
design of machines with good isolation, and sometimes loads with inductances in excess of the
required value. In other words, neither voltage nor current are as expected. This also means harmonic
contamination, additional power losses, torque ripple, and high frequency noise that can affect the
controllers. All these reasons have generated many research works on the topic of PWM modulation
Multi-stage converters [5-7] work more like amplitude modulation rather than pulse modulation, and
this fact makes the outputs of the converter very much cleaner. This way of operation allows having
almost perfect currents, and very good voltage waveforms, eliminating most of the undesirable
harmonics. And even better, the bridges of each converter work at a very low switching frequency,
which gives the possibility to work with low speed semiconductors, and to generate low switching
frequency losses. The objective of this paper is to show the advantages of multi-stage converters for
all kind of applications. The drawbacks of requiring isolated power supplies is solved using different
techniques, which depend on the type of application, and based on the fact that the first converter,
called Master, takes more than 80% of the total power delivered to the load. A four-stage converter
using three-state power modules, which gives 81 different levels of voltage amplitude, is studied. The
results are compared with conventional PWM modulators working at a switching frequency of 10
kHz. All the load parameters of both types of converters are set at the same values.
2. Basics of Multi-Stage Converters
2.1. Basic Principle
The circuit of fig.1 shows the basic topology of one converter used for the implementation of multistage
converters. It is based on the simple, four switches converter, used for single-phase inverters or
for dual converters. These converters are able to produce three levels of voltage in the load: +Vdc, -
Vdc, and Zero.

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