Willy Geysen

Calculation of the flux density and the unbalanced pull in two pole induction machines

ContentsIn this paper, the relationship between the homopolar flux generated by a statically eccentric rotor and the unbalanced magnetic pull in two-pole induction motors is considered. We will show that due to this kind of eccentricity, a homopolar flux is generated having the supply frequency, yielding a vibrational component of the unbalanced magnetic pull with the double supply frequency.ÜbersichtIn der vorliegenden Arbeit wird auf die Verknüpfung der Unipolarflüsse mit den Radialkräften bei 2poligen Drehstromasynchronmaschinen näher eingegangen. Es wird nachgewiesen, daß sich bei statisch exzentrischer Verlagerung des Läufers in zweipoligen Drehstromasynchronmaschinen netzfrequente Unipolarflüsse ausbilden können, die Rüttelkräfte von doppelter Netzfrequenz zur Folge haben.

Electro-mechanical analysis of the audible noise of an inverter-fed squirrel-cage induction motor

The audible noise emitted by a three-phase squirrel-cage induction motor fed by a frequency inverter is determined by the electromagnetic and mechanical behavior of the components of the drive. A computerized analysis method is described which uses theoretical as well as experimental data for predicting the audible noise spectrum components. Starting from the inverter voltage and the motor data, the electromagnetic forces are predicted and analyzed. The mechanical natural frequencies are considered, either in a theoretical (finite-element calculation) or in an experimental (modal analysis) methodology, in order to predict the vibrational behavior of the machine.<<ETX>>

Analysis of the Audible Noise of Three-Phase Squirrel-Cage Induction Motors Supplied by Inverters

One of the major problems in inverter-fed motors is the high level of audible noise produced by harmonic current and voltage components. To analyze these phenomena the field in the machine airgap is calculated using the rotating field theory together with the Maxwell stress theorem. This analysis yields a way for predicting the spectrum components produced by the motor and for relating it to the airgap flux density distribution time harmonics caused by the nonsinusoidal supply. The theoretical approach is used for calculating the airgap flux density distribution and the frequency spectrum components of a small three-phase squirrel-cage induction motor fed by a six-step voltage source inverter (VSI) and by a pulsewidth-modulated (PWM) inverter. The theoretical results are compared with experiments.

Overvoltages in inverter fed induction machines using high frequency power electronic components

Inverter supplied induction motors have become common. One of the tendencies that can be observed over the years, is that the switching frequencies of the power electronic components increase. This allows the manufacturers to build more compact inverters with an improved sinusoidal wave shape of the output voltage and/or current. The increase in the switching frequency is made possible by the progress made in the domain of the switching devices. For the user of such inverters, the advantage would be an improved efficiency of the motor and a lower audible noise level. One of the unexpected drawbacks of this new generation of inverters is that considerable overvoltages appear when supplying a motor with a long cable. These voltage peaks may damage the insulation of the motor. Measurements and calculations were carried out to understand the origin of this problem.<<ETX>>

An acoustic model for a permanent magnet machine: modal shapes and magnetic forces

A modal analysis concept for calculating the vibrational level of an electrical machine stator is presented. Using this concept, the designer is freed from calculating real magnet forces, which is normally difficult and inaccurate. Only flux linkages have to be calculated which can be easily done using finite element methods and the vector potential as the unknown function. Furthermore, the energy method also deals with any random or tangential movement of the stator surface. The main disadvantage is the need to know the mode shapes accurately. These can only be measured with great effort in the low-frequency region (up to 1 kHz). In the higher frequency regions, hundreds of modes exist, although, at this point, other approximations can be made.<<ETX>>

Technique for finding the optimum geometry of electrostatic micromotors

A computer-based method for finding the optimum geometry of very small electrostatic motors based on the variable capacitance principle is presented. The geometry is taken into account and different excitation schemes are studied in order to find the maximum average output torque and/or the minimum vibration component in the output torque. Different combinations of stator and rotor poles are treated. The most important part of this optimization is automation of the procedure. For the actual calculation of the electrostatic field distribution, the finite element method is used. The mesh generation for the machine as a function of the rotor position and the design parameter is automated. The extraction of the torque and capacitance values is done in a batch procedure, constructed around a commercial finite element CAD (computer-aided design) package.<<ETX>>

An approach to modelling of magnetically excited forces in electrical machines

A distinction is made between the vibrations produced by the fundamental current supplying the machine and the much smaller higher harmonic currents, with frequencies ranging into the kilocycles range. The small harmonic currents are emphasized. An appropriate mathematical modeling technique is presented. A mechanical model is selected based on modal analysis techniques. A method is developed for finding the driving force for each mode shape. This is illustrated with measurements on a permanent magnet machine. Experimental verifications of the model show the importance of a correct knowledge of eigenfrequencies and damping coefficients. >

Application of field oriented control in crane drives

It is shown how a combination of several PWM inverter fed induction motors, both with scalar and field oriented control, may be used for the different motions in crane drives. The concept of a common DC bus for the supply of the individual invertors yields particular advantages, very useful in this type of application. A design scheme is discussed, resulting in the choice of the appropriate motor, inverter, and rectifier ratings. The scheme starts from the requirements of the ratings. The scheme starts from the requirements of the different crane motions and environmental problems. The advantages of using modern control techniques are discussed for all motions. Indoor and outdoor operations are compared.<<ETX>>

Influence of Torsional Vibrations on Lateral Oscillations of Induction Motor Rotors

The combination of torsional vibrations in induction motor drive systems and the inevitable eccentricity of the machine rotor may lead to inadmissible lateral oscillations of this rotor. In this paper we will discuss the influaence of the torsional vibrations in an analytical way and using digital simulation techniques. Special attention will be given to the frequency-content of the vibrations. The theoretical results are compared with experimentally obtained values.An adaptive power factor controllerfor three-phase inducti on generators (and also for motors) is proposed in this paper. The controller senses the reactive current drawn by the machine and accordingly provides the needed reactive power to improve the power factor to as close to unity as possible. The controller is a modular, low-cost, harmonic free device. It does not create any transients in line current. It is designed to eliminate the self-excitation problems associated with induction generators. The controller is tested on an induction generator, which is being used in wind energy and similar applications.

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. >