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

This course is divided into two main sections. The first part focuses on the fundamentals of Power Electronics, introducing key devices and converters essential for modern electrical drive systems. The second part covers the Electromechanical Energy Conversion, exploring how electrical energy is transformed into mechanical energy through various types of electric machines and their control techniques.

Part I – Power Electronics

This section introduces the electronic components and converter topologies used to control and convert electrical power in electric drive systems.

1. Basics of Semiconductor Physics

Understanding the behavior of semiconductors is essential for analyzing and designing power electronic devices. Topics include energy bands, charge carriers, and basic conduction mechanisms.

2. Semiconductor Devices

• Diodes: Unidirectional devices used for rectification and protection.

• Silicon-Controlled Rectifiers (SCRs): Used for controlled rectification and switching in AC-DC conversion.

• Insulated Gate Bipolar Transistors (IGBTs): High-efficiency switches combining the advantages of MOSFETs and BJTs.

3. AC to DC Converters

• Single-phase Rectifier: Converts single-phase AC to DC, used in low-power applications.

• Three-phase Rectifier: Converts three-phase AC to DC, suitable for industrial and high-power systems.

4. DC-DC Converters

• Buck Converter: Step-down converter used to reduce voltage efficiently.

5. DC-AC Converters

• Half-Bridge Inverter: Basic configuration for generating AC from DC.

• Full-Bridge Inverter: More versatile, capable of full voltage swing and bidirectional power flow.

6. Inverters

• Single-phase and Three-phase Inverters: Key components in AC motor drives, capable of generating AC waveforms from a DC source.

7. Modulation Techniques

• Pulse Width Modulation (PWM): A technique for controlling power delivery and synthesizing AC voltages from DC, essential for motor control.

Part II – Electromechanical Energy Conversion

This section focuses on the interaction between electrical systems and mechanical motion, exploring different types of electric machines and their behavior under various operating conditions.

1. Principles of Electromechanical Energy Conversion

Introduces the fundamental principles behind converting electrical energy into mechanical energy, focusing on force production, energy flow, and system efficiency.

2. DC Machines  

Develops the dynamic equations governing DC machine operation, considering armature and field circuits.

Steady-State Characteristics

Analysis of performance under constant load, with:

• Armature Control: Varying voltage to control speed at constant field.

• Field Control: Varying field strength for higher-speed operation.

Operating Regions

• Constant Torque Region: Speed control through voltage variation.

• Field-Weakening Region: Extended speed range at reduced torque.

Braking and Quadrant Operations

• Transition to Braking: Understanding regenerative and dynamic braking modes.

• Single-, Double-, and Four-Quadrant Operation: Covers motoring and braking in both rotational directions.

Torque and Power Modes

• Constant Torque Operation: Common in low-speed drives.

• Constant Horsepower Operation: Applicable in high-speed regimes.

3. Synchronous Machines Magnetic Circuit Analysis

Covers the magnetic design of synchronous machines, including stator and rotor magnetic field interactions.

Reactance Models

• d-axis and q-axis Reactances: Key to understanding torque production and transient behavior.

Torque and Machine Equations

Describes how electromagnetic torque is generated and controlled in synchronous machines.

Steady-State Characteristics

Focuses on voltage regulation, load angle, and stability under constant speed operation.

Open-loop Voltage Source Inverter Drives

Explains how synchronous machines can be driven using open-loop control from a voltage source inverter.

Applications

Typical applications in propulsion systems, power generation, and variable-speed drives.

4. Induction Machines Steady-State Model

Uses the equivalent circuit to analyze machine behavior under steady conditions.

Torque and Machine Equations

Derives expressions for electromagnetic torque and rotor dynamics.

Performance Analysis

• Steady-State Characteristics: Efficiency, slip, and torque-speed curves.

• Starting Methods: Direct-on-line, soft start, and star-delta starting.

V/f Control Strategy

• Constant Terminal Volts/Hertz Operation: Maintains magnetic flux for efficient performance.

• Low-frequency Operation: Adjustments needed to maintain torque at low speeds.

Advanced Control

• Current-controlled PWM Inverter with Slip Frequency Control: Allows precise torque and speed regulation.

• Constant Horsepower Operation: For applications where power remains constant over a speed range.

Readings/Bibliography

The pdf files of the slides utilized during the lessons can be downloaded from "Insegnamenti On line".

1. A. E. Fitzgerald, C. Kingsley, S,D. Umans: Electric Machinery. McGraw-Hill

2. M. Rashid, "Power Electronics Handbook", Butterworth-Heinemann

3. N. Mohan, T. Undeland, W. Robbins, "Power Electronics: Converters, Applications and Design", John Wiley & Sons Inc

Teaching methods

The lessons are supported by numerical simulations of main electric machines and of main power electronics conveters.

Assessment methods

Student learning is assessed through a written examination designed to evaluate the acquisition of the knowledge outlined in the course syllabus.

The written exam consists of:

• 7 true/false questions,

• 1 numerical exercise,

• 2 open-ended questions.

During the exam, the student’s ability will be assessed in the following areas:

• Correct use of 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.

The final grade will reflect the student’s ability to clearly articulate their answers, demonstrate independent reasoning, and use appropriate technical language.

Teaching tools

Lessons and exercises are carried out with the help of a personal computer and slides (Power Point, MATLAB).

Office hours

See the website of Michele Mengoni