Henrik Lavrič

Modeling, analysis and detection of rotor field winding faults in synchronous generators

The paper presents an approach to modeling of shorted turns in rotor field winding of synchronous generator using finite element method. It enables detailed analysis of magnetic field at several operating conditions under healthy and faulty states which are difficult or even impossible to carry out by available measurement methods in industrial environment. Modeling of field winding faults are performed for both typical generator designs - turbo and hydro, and analysis reveals some differences, which are significant for practical use in fault detection procedures. It is confirmed that an extensive analysis should be performed to assure accurate healthy/faulty state predictions, since the level of diagnostic signal is considerably influenced by a combination of many machine and operating parameters. The scheme of developed on-line diagnostic system with its hardware and software concept is also presented and discussed.

Computations of Magnetic Field Anomalies in Synchronous Generator Due to Rotor Excitation Coil Faults

The paper presents an approach to model shorted turns in rotor excitation winding of synchronous generator using finite element analysis. It enables detailed examination of magnetic field at several operating conditions under healthy and faulty states which are difficult or even impossible to carry out by available measurement methods in industrial environment. It is confirmed that an extensive analysis should be performed to assure accurate healthy/faulty state predictions, since the level of diagnostic signal is considerably influenced by a combination of many machine design and operating parameters.

Diagnostic system for on-line detection of rotor faults in induction motor drives

The paper presents an on-line condition monitoring and diagnostic system for induction motor drives. It enables detection of many different faults, which may arise during the lifetime of the motor, although special attention was devoted to identify broken rotor bars at an early stage of the fault propagation. The method is based on the analysis of stator current frequency spectrum, which can be measured without disturbing normal motor operation, therefore it is completely non-invasive and easy to implement in industrial environments. The presented diagnostic system is being applied on 17 high-voltage motors (range of power 1000 ÷ 6400 kW) in two thermal power plants to increase operational reliability of induction motors and to reduce costs of their maintenance. Concepts of hardware and software configurations are explained in details as well as some of the numerous monitoring results during the last five-years period. The paper also discusses two examples of a timely and efficient detection of rotor electric asymmetry due to cracked end-ring segments of induction motors, which exhibited no obvious problems during operation.

Induction motor parameters in case of rotor electrical asymmetry

The paper presents estimation of parameters alternation in a three-phase induction motor due to broken rotor bars, which cause electric and magnetic asymmetry in the machine. Using d-q model of induction motor this asymmetry is reflected as pseudo-saliency through different resistances and inductances in both axes. An extensive study has been performed to simulate various cases of faulty cage, which cannot be achieved by measurements on actual industrial drives. Simulation results can be efficiently used for analysis of closed-loop induction motor drives, in which determination of rotor faults, due to the intrinsic speed correction performed by the controller, is much harder to achieve.

Novel approach to closed-loop control of wire bending machine

This paper describes the problems in nowadays production of ribs, the most important assembling part of leverarch mechanism. The process of wire bending machine upgrade is presented and the control principle for ribs geometry stabilization is proposed. The overall control scheme consists of inner - force control loops and outer - geometry control loop. The inner loops control the force on respective roller of the horizontal and vertical plane of wire straightener according to the predefined reference values. These values are modified with the superior loop in respect of geometry variations of the rib. Required displacements of rollers are obtained using wedge systems driven by stepper motors. The final geometry of rib is analyzed by image processing algorithm inside the image acquisition system.

A Three-Phase Power Source with Low THD for Energy Meters Calibration

The paper presents an experimental three-phase electronic phantom power source with special requirements, designed for calibrating mostly electronic energy meters. The three-phase power calibrator arrangement generates a variable phantom power of up to 19.2 kVA per phase. Two different power amplifiers, i.e. a voltage and a transconductance amplifier, and other individual components of the power calibrator are introduced and explained. A control method suitable for reducing effects of nonlinear periodic disturbances on electrical quantities, i.e. the output waveforms of voltage and current sources, is explained. The focus is on the theory of a modified repetitive control method based on a procedure with a recurrent integral action distributing the controlled signal in a certain number of intervals and subsequently correcting each interval separately and independently of the remaining intervals. Results acquired with the help of the experimental source are presented.

Flux Linkages in Squirrel-Cage Motor

Modeling of squirrel-cage induction motor has traditionally relied on generalization of wound-rotor induction motor claiming their nearly identical operational characteristics. However, the distribution of rotor windings differs profoundly among the two types, as the squirrel-cage rotor features non-sinusoidal distribution of bars along the air-gap. In this paper, a model of squirrel-cage induction motor is derived based on the actual geometry of stator windings and rotor loops. Various types of inductances which are instrumental to the description of the model are computed by means of winding function. Consequently, a winding asymmetry can be easily incorporated in the model as shown for the case of broken rotor bar.

Efficient testing of electric drive components on a test bed with energy recuperation

Complexity of modern electric drives demands high reliability of its components. In this paper an energy efficient testing of converted-fed electric drives is presented. A DC link connection between two converters, which supply braking and tested machine, enables energy recuperation. The software solution provides five different testing modes such as: manual control of only one motor, manual control of synchronized machines, fully automated measurement of various characteristics (torque, efficiency...), loading according to user-defined function and loading using periodical reference signal. Experimental results of presented tests confirm versatility of the test bed. Such test bed is not only useful for industrial testing but can also be exploited in educational process and in research activities.

Assessment of electrical energy production in small hydropower plant with ultra-low head

With intentions of preserving cultural heritage and biotic diversity near less technically exploitable Slovenian rivers, plans for sustainable energy production in small hydropower plants are examined. New technologies of water turbines allow the exploitation of ultra-low heads down to 1 m. We have developed a computer program, which makes assessment of energy performance and economic efficiency of small hydropower plant possible. The program has many options from selection of turbine type to variation of number of turbines. Through calculation of efficiency, exploitable flow, net head and other factors, the program calculates nominal power of the power plant and its energy production over a chosen time period. Considering many different conditions we have found out that for power plant in question two units in parallel arrangement of overall nominal power not exceeding 50 kW is preferred solution. Results enable good insight into project viability and thus offer support to investors or sales technicians, while planning investments in hydropower plants on ultra-low head and moderate flow watercourses.

Evaluation of simplified dq model of induction machine for development of diagnostic procedures

This paper presents evaluation of simplified dq model of induction machine. Model is particularly useful for simulation of induction machines with broken rotor bars. Investigation is made in how useful and accurate this model is compared to experimental measurements. Comparison of simulation results with experimental data, don for two different machines, show that the model is very accurate and useful. Along with its fast execution speed it makes a good foundation for further development of advanced detection and diagnostic techniques.