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Transient Voltage Distribution in Stator Winding of Electrical Machine Fed from a Frequency Converter

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ABSTRACT Standard induction motors are exposed to steep-fronted, non-sinusoidal voltages when fed from frequency converters. These wave patterns can be destructive to the insulation. The aim of the present work is to develop methods of predicting the magnitude and distribution of fast voltage within the stator winding of an electric machine fed from a frequency converter. Three methods of predicting the magnitude and distribution of fast voltages within form windings commonly used in medium and high voltage machines are described. These methods utilise some aspects of previously published works on the surge propagation studies to achieve simplification of the solution without loss of accuracy. Two of these methods are applied to the voltage calculation in random winding commonly used in low voltage machines. Multi-conductor transmission line theory forms the basis of the methods described in this work. Computation of the voltage distribution using either of these methods requires the calculation of the parameters for the slot and the end (over-hang) part of the winding. The parallel plate capacitor method, the indirect boundary integral equation method and the finite element method are the three possible methods of calculating the capacitance also described in this work. Duality existing between the magnetic and the electric field has been used for the inductance calculation. Application of these methods to the voltage calculation in the first coil from the line-end of a 6 kV induction motor is shown to be successful. From the computed and measured voltage results it is evident that the improved accuracy for the capacitance values is sufficient to give good agreement between the measured and calculated inter-turn voltages without the need to infer the presence of a surface impedance effect due to the laminated core. Application of two of these methods for the transient voltage calculation on the first coil from the terminal-end of low voltage induction motors with random windings is also shown to be successful. Comparison between the computed and the measured results shows that the turn-to-ground capacitance matrix obtained in over-hang part of the coil can be assumed the for the slot part of the coil. With this assumption modelling the first five turns in the line-end coil produce turn and coil voltage that match well with the corresponding measuring result. The methods of voltage computation described in this work should be of great help to engineers and researchers concerned with the turn strength and over-voltage protection in high and low voltage motors. Acta polytechnica Scandinavica. El, Electrical engineering series, ISSN 0001-6845; 100

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