Lecturer: Lucia Frosini  

Course name: Diagnostica di macchine e azionamenti elettrici
Course code: 064187
Degree course: Ingegneria Elettrica
Disciplinary field of science: ING-IND/32
University credits: CFU 3
Course website: http://www.unipv.it/dmae/

Specific course objectives

The aim of this course is to introduce the students to the study of the diagnostic systems for the identification of the actual or incipient faults in electrical machines and drives. The diagnostic tools used to detect a fault condition, to identify the damaged element and to determine the cause of fault (mechanical, thermal, electrical or environmental) will be analysed.

Course programme

1. Introduction. Aim of a diagnostic system: detection, isolation and identification of the fault. Relationship among the diagnostics and the concepts of protection, maintenance, condition monitoring. Reactive, preventive, predictive and proactive maintenance. Reliability, availability, probability density of fault, failure rate, criteria for the classification of the faults.

2. Types of fault in electrical machines and drives and methods for their detection
Types of fault: in the stator (windings and core), in the bearings, in the rotor, static and dynamic irregularities of the air-gap, others. Electrical methods: measurement of voltage, current, power, magnetic flux (at the air-gap and leakage), shaft voltage, bearing current, partial discharge. Mechanical methods: measurement of vibration, acoustic noise, force, torque, speed. Other methods: chemical measurements, temperature, etc. Signal analysis in the time and frequency domain. Advanced techniques for the signal analysis (e.g. neural networks).

3. Vibrations in electrical machines and drives
Vibration measurement as diagnostic tool. Longitudinal, flexional (Jeffcott rotor) and torsional vibrations. Natural and forced vibrations. Electromagnetic forces: Maxwell tensor, Lorentz forces. Static and dynamic eccentricity. Vibrations in the end-windings. Practical examples.

4. Bearing faults
Rolling and friction bearings. Possible damages in the bearing faults and their causes. Methods to detect bearing faults: vibration and stator current analysis. Practical examples.

5. Faults in the stator windings
Construction characteristics, possible faults and diagnostic methods for low voltage stator windings (supplied by the grid or by inverter): stator current and leakage flux analysis. Construction characteristics of medium voltage stator windings. Ground insulation failure. Diagnostic methods for medium voltage stator windings: insulation resistance, polarization index, tan-delta or insulation power factor, AC and DC hi-pot test. Partial discharge (PD) measurement: positive or negative polarity of the PD, comparison of the effectiveness of the PD method with respect to the other diagnostic methods, PD detection by measurement of the electric pulses, interpretation of the experimental measurements, PD as direct cause or as symptom, effect of the load and the temperature on the PD.

6. Rotor faults
Rotor short circuits in the generators and methods for their detection: measurement of the air-gap flux, recurrent surge oscillation, dynamic impedance. Shaft voltages and currents: methods for their monitoring in turbo-generators and in motors supplied by inverter. Broken rotor bars in induction motors and their detection by stator current, vibration and leakage flux analysis. Inter-bar current phenomenon. Partial demagnetization in brushless motors.

Course entry requirements

Knowledge of the functional and construction aspects of the electrical machines and drives.

Course structure and teaching

Lectures (hours/year in lecture theatre): 20
Practical class (hours/year in lecture theatre): 6
Practicals / Workshops (hours/year in lecture theatre): 0
Project work (hours/year in lecture theatre): 0

Suggested reading materials

Notes prepared by the lecture. To study in depth, the following books can be consulted:

Peter Tavner, Li Ran, Jim Penman and Howard Sedding. Condition Monitoring of Rotating Electrical Machines, 2nd Edition. IET, 2008.

Stone G., Boulter E.A., Culbert I., Dhirani H.. Electrical Insulation for Rotating Machines: Design, Evaluation, Aging, Testing, and Repair. Wiley-IEEE Press, 2004.

Geoff Klempner, Isidor Kerszenbaum. Handbook of of Large Turbo-Generators. Operation and Maintenance. Wiley-IEEE Press, 2008.

Testing and exams

The exam consists of an oral test on the contents of the course.