B. Szabados

Design and implementation of a calorimetric measurement facility for determining losses in electrical machines

The measurement of the total losses of electrical machines is of most interest to designers for verifying their calculations. These measurements can be done in various ways, however, some lead to more accurate results than others. We will show why the conventional input-output procedure does not result in accurate loss figures, An alternate method is proposed. For this purpose a calorimetric measurement facility was designed and implemented. This paper is intended to present the calibration procedure for this facility, as well as results of tests performed on a 10 hp induction motor.

Measurement of the torque-speed characteristics of induction motors using an improved new digital approach

A measurement technique for determining the torque-speed characteristics of induction motors is presented. This technique is in compliance with the IEEE standard test procedure for polyphase induction motors and generators. It is based on the acceleration method which is performed under no-load conditions. A fast data-acquisition system is used to record the speed and other signals of interest. The data is processed digitally with the objective of removing undesired extraneous signals while preserving the accuracy of the machine characteristics over the complete range of speed. To achieve this objective, algorithms for performing adaptive window-size average filtering and numerical differentiation are developed. Experimental tests performed on 1 HP and 600 HP induction motors confirm that this approach has several advantages over other methods. It produces very accurate results over the full dynamic speed range of the machine and permits precise correlation between the switching transients on the line with the variation of the speed signal. >

Determining Total Losses and Temperature Rise in Induction Motors Using Equivalent Loading Methods

Conventional loading of induction motors is an extremely difficult and expensive process for large machines. In those cases, full load losses and temperature rise can be estimated by means of equivalent loading methods, which provide an accurate alternative, without the need of a mechanical load applied to the shaft. This paper describes three such methods, all using a commercial PWM inverter. The methods examined are described in detail and the results of tests performed on a 10 hp induction motor are presented. A calorimetric measurement facility was used in order to ensure consistency of result comparison between the different methods, as well as high accuracy for total loss measurement. The measured values of losses and temperature rise, although marginally larger, are in good agreement with those obtained through conventional loading.

Error Recovery Service for the IEEE 802.11B Protocol

We develop a service that allows the current IEEE 802.11b MAC protocol to perform dynamic packet sizing and forward error correction. Our service, called ERSMAC, is designed to allow the deployment of the IEEE 802.11b protocol in industrial environments characterized by high BER and fast time variation. ERSMAC uses a maximum likelihood estimate of the BER to solve for the optimal packet size that maximizes the success probability of transmissions while minimizing the overhead cost. ERSMAC also implements an adaptive forward error correction scheme using Reed-Solomon code such that every retransmission attempt has a higher probability of success than the previous attempt due to its association with a stronger RS code. Finally, we show, through simulations, that ERSMAC outperforms the original unmodified IEEE 802.11b protocol in terms of average throughput, average delay and efficiency

High-power cycloconverter drive for double-fed induction motors

Two cycloconverters independently supply stator and rotor windings of a wound rotor induction machine. The control strategy to stabilize this inherently unstable system is derived from the double-fed machine matrix (DFM) and the resulting algorithm is implemented in hardware with a simple field calculator. A practical implementation of a 2 kW drive illustrates the feasibility of the drive, and practical performances of full speed in both directions, as well as speed ramps of up to 700 r/min, are realized. The system benefits from the simplicity of line-commutated converters associated with a relatively high frequency of operation of each individual cycloconverter, thereby avoiding saturation effects appearing at low frequencies, combined with control of harmonic generation using the extra degree of freedom provided by the DFM. >

Interactive Fortran for Power Engineering Education

The advantages and disadvantages of FORTRAN are described in relation to engineering education. The development of personal computers with an attendant emphasis on interactive capabilities and screen graphics is discussed together with the lack of these features in current versions of FORTRAN. A set of FORTRAN callable routines is described which has been developed to provide these interactive and graphing features. These routines have been written in machine language for Intel 8086 compatible microprocessors for efficiency and a high degree of portability. The ease with which these routines may be used is demonstrated with an example commonly used in electric power engineering.

Three Dimensional Flux Calculation on a Three-Phase, Transformer

Increases in the ratings of large power transformers have demanded more accurate methods of predicting leakage fields which give rise to excessive losses in the devices. This paper addresses the issue, giving the development of a technique which can be used with minimal computing power to determine the distribution of the magnetic fluxes. Estimates for the additional stray load losses in the iron parts can be calculated with reasonable accuracy after the flux distribution has been found. For developing large transformers and reactors, it is necessary to know the distribution of the magnetic leakage field to calculate the electro-dynamic forces and the stray losses due to eddy currents in the winding conductors and in steel parts such as core, tank, pressing beams, etc. In general, the magnetic field in the transformer is calculated using the assumption that the structure is two-dimensional. But actual transformers are three-dimensional. Thus, the customary two-dimensional approximations are inadequate. In this paper, the three-dimensional magnetic flux distribution on the tank wall of a three phase transformer has been calculated. In order to perform the calculation, a finite element scheme has been adopted to obtain a two-dimensional solution at 15 meridian planes of the transformer coil of a single phase transformer for any arbitrary value of current density. Using the actual values of current densities, the magnetic field has been computed for each phase at two selected instants of time in the cycle. Then, the results are manipulated to obtain an overall solution.

High-Resolution Precision Digital Tachometer

A new approach to the digital measurement of angular velocity is presented. This new instrument performs the division by time that has been carefully avoided in the existing instruments. The accuracy and resolution of the transducer does not depend upon the number of quantizations/revolution. The measurements can be made in a very short time, leading to up to 14700 samples per second. An important feature of this instrument is that even at speeds close to zero (or at zero), the error is less than 0.02 percent and 1600 samples of the angular velocity can be obtained/second. This makes the instrument particularly suitable for the study and control of the transient behavior of motors and other rotating devices.

A novel method of measurement of synchronous machine losses using synthetic loading

Synthetic loading of synchronous machines can be used to test machines for temperature rise and total losses without the need for attaching mechanical load to the shaft. A new phase modulation technique of the voltage is proposed in lieu of the traditional dual frequency method which suffers from cumbersome equipment and also predicts higher losses. A simple bang-bang DC switch is used to modulate the excitation current of an induction generator. System modelling was performed using the traditional 2 axis method, and has proven that the method is feasible. Practical test results are shown to validate the simulated performance. It is shown that the only constraint of this synthetic loading is to maintain the RMS values of the current and voltage at rated values.

Magnetic field analysis in squirrel cage induction motors

A method for analyzing the field in a squirrel-cage induction motor is discussed, starting from a classical design scheme but using the finite element technique. The approach considered allows the calculation of the flux density distribution in an induction motor at different operating points. A magnetostatic approach is used to model the no-load situation; for the locked rotor situation, a time harmonic solution is used. This leads to the calculation of induced currents, accounting for the real saturation. The parameters deduced from these calculations are influenced by the local saturation, and overall correction factors may be avoided. This approach may be used for all three-phase systems, having a flux pattern that remains virtually the same as a function of time. It is shown that a combination of a correct flux distribution calculation method with appropriate input and output routines may lead to a design procedure which can be used by industrial manufacturers. >