B2377 - Electric Energy Conversion

Learning outcomes

The course delivers the principles of electromechanical energy conversion and suitable skills for the analysis of the main electrical machines for residential and industrial applications, along with the principles of power electronics devices and basic architectures.

Course contents

The course provides the fundamental knowledge required to understand power electronic converters and electromechanical energy conversion systems used in modern electrical drives. It is divided into two main parts: the first part introduces the principles, devices and converter topologies of power electronics; the second part focuses on the conversion of electrical energy into mechanical energy through different types of electric machines, including their operating characteristics and basic control strategies.

Basic knowledge of electrical circuits, electromagnetism and three-phase electrical systems is recommended.

The course is organized as follows.

Part I – Power Electronics

This part introduces the semiconductor devices and converter topologies used to control and convert electrical power in electrical drive systems.

Fundamentals of semiconductor physics

Basic principles of semiconductor materials. Energy bands, charge carriers and elementary conduction mechanisms. Relationship between semiconductor properties and the operation of power electronic devices.

Power semiconductor devices

Operating principles and characteristics of the main power semiconductor devices used in converters:

• power diodes, for rectification and protection functions;
• Silicon-Controlled Rectifiers, SCRs, for controlled rectification and AC/DC conversion;
• Insulated Gate Bipolar Transistors, IGBTs, for high-efficiency switching applications in power converters and drives.

AC/DC converters

Single-phase and three-phase rectifiers. Operating principles, output voltage characteristics and typical applications in low-power and industrial power conversion systems.

DC/DC converters

Basic DC/DC conversion principles, with particular reference to the buck converter as a step-down topology for efficient DC voltage regulation.

DC/AC converters and inverters

Half-bridge and full-bridge inverter topologies. Single-phase and three-phase inverters. Generation of AC voltages from a DC source and role of inverters in AC motor drives.

Modulation techniques

Pulse Width Modulation, PWM, for the control of power converters. Use of PWM techniques to regulate power delivery and synthesize AC voltages from a DC source in electrical drive applications.

Part II – Electromechanical Energy Conversion

This part focuses on the interaction between electrical systems and mechanical motion, with reference to the principles of energy conversion and the operating behaviour of the main types of electrical machines.

Principles of electromechanical energy conversion

Fundamental principles of electromechanical energy conversion. Force and torque production, energy flow, losses and efficiency in electrical machines and drive systems.

DC machines

Dynamic equations of DC machines, including armature and field circuits. Steady-state characteristics and speed control methods.

The following topics are covered:

• armature control, based on voltage variation for speed regulation at constant field;
• field control and field-weakening operation for extended speed range;
• constant-torque and constant-power operating regions;
• regenerative and dynamic braking;
• single-, two- and four-quadrant operation;
• torque and power operating modes in low-speed and high-speed drive applications.

Synchronous machines

Magnetic circuit analysis of synchronous machines, including stator and rotor field interactions. Machine equations, torque production and steady-state operating characteristics.

The following topics are covered:

• d-axis and q-axis reactance models;
• electromagnetic torque and machine equations;
• voltage regulation, load angle and stability in steady-state operation;
• open-loop voltage source inverter drives for synchronous machines;
• typical applications in propulsion systems, power generation and variable-speed drives.

Brushless DC machines, BLDC

Structure and operating principles of Brushless DC machines. Analysis of machines with trapezoidal back-EMF and electronic commutation.

The following topics are covered:

• construction features and rotor position dependence;
• trapezoidal back-EMF and phase current waveforms;
• electromagnetic torque production;
• six-step operation with two phases supplied at a time;
• commutation sequences as a function of rotor position;
• switching transients and torque ripple;
• block diagram of the BLDC drive;
• typical applications in variable-speed drives and motion control systems.

Induction machines

Steady-state modelling of induction machines using the equivalent circuit. Torque production, rotor dynamics and performance analysis.

The following topics are covered:

• torque-speed characteristics, slip and efficiency;
• starting methods, including direct-on-line, soft-start and star-delta starting;
• scalar V/f control strategy;
• constant terminal volts/hertz operation for flux regulation;
• low-frequency operation and compensation issues;
• current-controlled PWM inverter with slip-frequency control;
• constant-power operating region for variable-speed drive applications.

Single-phase induction motors

Structure and operating principles of single-phase induction motors. Analysis of the main construction types and of the methods used to produce the starting torque.

The following topics are covered:

• construction features and operating principles;
• equivalent magnetic field interpretation;
• starting torque production;
• capacitor-start motors;
• capacitor-start and run motors;
• permanent split capacitor motors;
• split-phase motors;
• shaded-pole motors;
• typical applications in low-power drive systems.

The course links the theoretical analysis of converters and electrical machines with their use in practical drive systems, highlighting the role of power electronics in the control, regulation and operation of electromechanical energy conversion systems.

Readings/Bibliography

PDF files of the slides used during the lectures can be downloaded from the University of Bologna Virtuale platform.

The following books are recommended as reference texts for studying and further exploring the topics covered in the course:

• A.E. Fitzgerald, C. Kingsley, S.D. Umans, Electric Machinery, McGraw-Hill.
• M. Rashid, Power Electronics Handbook, Butterworth-Heinemann.
• N. Mohan, T. Undeland, W. Robbins, Power Electronics: Converters, Applications and Design, John Wiley & Sons Inc.

For exam preparation, students should refer primarily to the lecture slides and teaching materials made available through Virtuale. The books listed above are intended as supporting references and sources for further study on specific course topics.

Teaching methods

Lectures are supported by numerical simulations of the main electrical machines and the main power electronic converters covered in the course.

The simulations are used to help students understand the operating behaviour of machines and converters, relate theoretical models to practical applications, and analyse the effects of different control strategies and operating conditions.

Teaching materials, including PDF files of the lecture slides and any simulation files prepared by the instructor, will be made available through the University of Bologna Virtuale platform.

Assessment methods

Student learning is assessed through a written examination

The assessment of learning is based on a written examination, designed to evaluate the acquisition of the knowledge and skills outlined in the course syllabus.

The written examination consists of:

• 7 true/false questions;
• 1 numerical exercise;
• 2 open-ended questions.

During the examination, the student’s ability will be assessed with reference to the following aspects:

• correct use of the tools and concepts related to electrical engineering and electromechanical energy conversion;
• clear and accurate description of the operating principles of static power converters and electrical machines;
• effective representation of the fundamental principles governing the operation of electrical machines;
• ability to apply theoretical concepts to simple numerical and application-oriented problems.

The final grade will reflect the correctness and completeness of the answers, the student’s ability to present concepts clearly, the degree of independent reasoning, and the appropriate use of technical language.

Teaching tools

Lessons and exercises are carried out with the support of a personal computer, PowerPoint slides and MATLAB.

These tools are used to present theoretical concepts, develop numerical examples, and support the analysis and simulation of electrical machines, power electronic converters and electromechanical energy conversion systems.

Teaching materials, including lecture slides and any MATLAB files prepared by the instructor, will be made available through the University of Bologna Virtuale platform.

Office hours

See the website of Michele Mengoni