5
PREFACE
The aim of this master’s degree thesis, developed in collaboration with STMicroelectronics in
Catania, has been the investigation of the thermal properties of the newest device of the
family of IGBT Intelligent Power Modules (IPMs) manufactured by the company. As is known,
international treatises between countries like the most-cited “Kyoto protocol” established the
commitment by the countries signers of the agreement in reducing the emission of several
greenhouse-gases (GHG) like carbon dioxide, methane, nitrous oxide, sulphur hexafluoride. A
key role in the reduction of the emission of carbon dioxide is covered by electric motors. In
fact they are responsible of the largest consumption of electric energy and, because most of
electric energy produced in the world is derived by coal, gas and other non-renewable sources,
they take charge of the greatest “carbon footprint”. For this reason, even small improvements
in the field of electric motors would greatly benefit the environment and lead to more self-
sustainable energy supply for the countries. An indirect but strategic role is instead played by
power electronics. The conversion and control of the power electrical/electronic systems cover
a fundamental role for the efficient operation of whatever larger system: electrical,
electromechanical, electronic and so on. Improvements in the field of power electronics can
regard several parts of it, like circuit topologies, control techniques, power devices, CAD
software and so on. As mentioned in the beginning, this thesis has regarded the analysis of the
thermal properties of a particular power electronic device belonging to the great family of
Intelligent Power Modules, of which the ultimate goal is a more efficient conversion of the
6
electric energy and a better control for electric drives. The device taken into examination was
the smallest of the family, used for a large range of home appliances like small fans, sewing
machines, dishwashers, washing machines and so on. the device is rated for the management
of 100 W of electrical power at its input. By using it together with a microcontroller, the
operation of an electric motor can be efficiently controlled in an Adjustable Speed Drive (ASD)
fashion. As we’ll see in chapter 1, these type of drives, working in a smarter way respect to
traditional drives, afford great energy-savings potential. Let’s summarize briefly the content of
the next chapters. The thesis starts with an introductive chapter to electric drives, a view of
the scenario of electric motors is given along with their basic functioning, and a glance to the
power converters, motor-mechanical load matching and feedback control is provided. The
chapter 2 take more closely in detail the PMSM motors: they are a particular type of
synchronous motors where the wound-rotor is substituted by permanent magnets for a more
efficient operation. PWM control and sensorless techniques for reliable control of electric
motors are also illustrated in chapter 2. In chapter 3 we delved into the interesting field of
Intelligent Power Modules (IPMs), looking at them from a general perspective and examining,
in particular, the ST SLLIMM family of IPMs. Chapter 4 deals with the design of circuits to
implement a power converter along with its control for a PMSM motor, this chapter describes
how to size the auxiliary components for the IPM, which constitutes the main block of the
power conditioning for PMSM control. Chapter 5 reports the experimental results of the work
carried out in the “Market and Application Development laboratory” at the Catania site of
STMicroelectronics. Chapter 6 draws the conclusions of the experimental work. Appendixes
are also given with some useful MATLAB
TM
scripts for the abc to dq0 transformation and
general information about the resources regarding the SLLIMM
TM
-nano intelligent Power
Module.
Catania, Italy Giuseppe Attilio Laudani
9
CHAPTER 1
INTRODUCTION TO ELECTRIC DRIVES
1. Generalities about electric drives
Electrical machines are electromechanical devices capable to convert energy between
electrical and mechanical form. The name electrical machines incorporates both electric
generators, that take in energy by work and expel a fraction of that energy by electrical
transmission, and electric motors, that in the opposite way to generators, take in energy by
electrical transmission and expel a fraction of it by work. Electrical machines are a very
important sector in the field of electrical engineering: they are involved in the entire chain of
generation, transmission, distribution, utilization or storage of energy at large, medium and
small scale. Of particular importance is to point out that the limitation of Carnot’s theorem for
a heat engine doesn’t apply to electrical machines. Very high efficiencies are thus obtainable
with electrical machines, making them the first choice in applications with energy savings
priorities.
Electric motors, converting electrical energy into mechanical energy, are able to power a large
range of industrial and consumer products. They can be found in many applications like fans,
pumps, blowers, washing machines, dishwashers, electric vehicles (EV), power machine tools,
disk drives, etc.
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In general, electric drives can be distinguished in two large categories: servo drives, mainly
used in robotics, and adjustable speed drives (ASDs), known also with the names variable
speed drives (VSDs), or variable frequency drives (VFDs). The main requirement that is
demanded for a servo drive system is high-precision in positioning, hence a very accurate
control of the drive is needed. In the opposite way, an accurate control of an ASD is not
required, because the time constant associated with the feedback path of the controlled
system is so large, so that a precise control is not feasible or needed.
In order to implement an ASD, an electric motor requires an appropriate control of its
operation by means of a power electronic converter (PEC) circuit. Power converter circuits lie
at the heart of the ASDs (Figure 1).
Figure 1 – Block representation of an electrical drive system
Recently, the power management of motors has become one of the most discussed and
attracting topic in the public sector of several countries, because of their impact on energy-
saving and environmental issues. In the aim to investigate the impact of the electric motors on
11
energy consuming and environment, it urges to say that electric motors themselves take
account of a large amount of the consumed energy and emitted greenhouse-gases.
As a matter of fact, nowadays, electric motors are still consuming an amount of energy of
about 46% of all the global electricity consumption, as shown in Figure 2, and 69% of all the
electricity used by industry.
Figure 2 – Split of main sources of energy consumption
Scientists and engineers are continuously looking for new higher quality materials, innovative
design and construction techniques to increase the efficiency of electric motors, despite of an
increase of the cost. Improving the efficiency of electric motors would be interesting for
several reasons: first inefficient electric motors increase electrical demand and associated
electricity costs, second because energy is often produced from fossil fuels (Figure 3 and Figure
4), inefficient motors increase the “carbon footprints” and emission of greenhouse-gases.
46%
19%
10%
3%
19%
3%
Main sources of energy comsuption
Motors
Light
Electronics
Electrolysis
Heat
Standby
12
Third, the losses in the motor worsen the motor reliability while increasing maintenance costs.
Therefore, some methods to increase efficiency in electric motors are exploited, since a few
percentage points (1% or 2%) of efficiency improvement can lead to great energy savings. The
interested reader can consult the points [1] and [2] of bibliography for further investigation.
For the scope of this thesis, it suffices to say that in an ASD, the power converter circuit and its
control cover a major role in the total efficiency of the electrical drive. Improvements in the
field of power devices, driving techniques and circuit topologies can all benefit the system
performance and contribute to lower power losses both in the driving circuit and in the electric
motor.
Figure 3 – World electricity generation by sources
41,30%
5,54%
21,16%
13,10%
16,12%
0,30%
0,06%
1,11%
1,31%
2008 World electricity generation
Coal
Oil
Gas
Nuclear
Hydro
Geothermal
Solar
Wind
Bio & other
13
Figure 4 – Italy electricity generation by sources
The energy saving properties of ASDs can be quickly understood considering that a traditional
electrical drive spends all of its operating time working with a fixed acceleration and reducing
its speed by braking.
Figure 5 – Torque-speed characteristics of traditional and adjustable speed drives showing the minor energy
consumption of ASDs respect to the traditional ones
12,91% 2,45%
41,73% 13,23%
1,63%
3,12%
2,85%
8,89%
13,20%
2011 Italy electricity generation
Coal
Oil
Gas
Hydro
Geothermal
Solar
Wind
Bio & other
Import
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Instead, an ASD operates in a smarter way adjusting its speed with an electronic controller
that, on the base of the feedback coming from the sensors, regulates the motor speed as
required by the load. The main torque-speed characteristic of traditional drive vs. an ASD is
shown in Figure 5. For example, a heat pump that takes advantage of an ASD can set its speed
after evaluating the temperature detected by a temperature sensor, in this way the heat flow
rate is dynamically regulated accordingly to the environment temperature. The same
argument can involve many different applications, all exploiting the great saving energy
properties of ASDs.
The control of electrical drives is one of the most challenging topics, given the multidisciplinary
nature of it. In fact it requires at least a good knowledge of mechanical concepts together with
electrical concepts, and a specific knowledge of power electronics for motor drives. Further
knowledge in the fields of electrical machines, control theory, embedded systems,
microcontrollers, digital electronic systems is always more needed, given the expanding use of
digital control circuitry in the control stage of electric motors. In these days, many
semiconductor companies are coming out with always new products to make the electrical
drives more efficient and reliable than ever. Many new circuits both for the control part and
for the power part of the electric drive are frequently introduced by many manufacturers.
Before diving in the exploration of Intelligent Power Modules (IPMs) (Chapter 3) for the control
of PMSM motors (Chapter 2), a brief description of the total electrical drive is presented, just
to provide the main ideas regarding motor control, focusing on what is needed for the
discussion of the following chapters, since the electrical drives in all their generalities are not
the main objective for the thesis.
2. Overviews on electrical drives
As illustrated in the Figure 1, an electrical drive is constituted by five components:
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power converter/control stage,
electric motor,
coupler,
mechanical load,
feedback.
a. Power converter/control stage
The first block is a Power Electronic Converter (PEC) with its control circuitry. The PEC topology
required to drive an electric motor depends on what type of motor we are going to drive. For
example, to drive a DC motor many classical DC-DC topologies (along with an input bridge
rectifier for AC mains) are used. Thus for a DC motor is common to use topologies like the
step-down or buck converter (1-quadrant operation in the torque-speed plane: current and
voltage can only assume positive values, that means that the motor is not able to brake and
not able to reverse its speed) or half-bridge (2-quadrant operation: current can assume
positive or negative values but voltage can only assume positive values, the motor is able to
brake but not to reverse its speed) or full-bridge (4-quadrant operation: current and voltage
can assume both positive and negative values, that means that the motor can brake and
reverse its speed). When DC motors are supplied from AC mains, especially with large power
ratings, it can be economically convenient to use line-frequency controlled rectifiers
(employing SCRs) in the first conversion stage, for regenerative breaking [3].
Without going in the details of the entire AC motors panorama we mention that there are
mainly two types of AC motors: synchronous and asynchronous (or induction) motors. They
employ mostly DC-AC power converters, more commonly the Voltage Source Inverter (VSI) for
their driving in variable-speed operations, with eventually other prior stages for the AC mains-
to-DC conversion and power factor correction (PFC) (Figure 6). Furthermore, both in the field
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of DC and AC synchronous and induction motors and apart from them, there are many other
types of motors that add up to constitute the entire scenario of the electric motors (Figure 7).
Load
3-phase
AC
+
Figure 6 – Three-phase bridge inverter with upstream smoothing capacitor and three-phase bridge rectifier
Figure 7 – Types of electric motors
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