Chapter 20 Operational Deflection Shapes of a PWM-Fed Traction Motor Hanna Amlinger, Fredrik Botling, Ines Lopez Arteaga, and Siv Leth Abstract Operational deflection shapes of an asynchronous traction motor for railway applications are investigated. The radiated noise from the traction motor on a train is, especially at low speeds, dominated by noise generated by electromagnetic forces. The tested motor is fed by a pulse-width-modulated (PWM) frequency converter for which the voltage is modulated as a series of pulses that are switched with a certain frequency. In this case, PWM force lines can be expected to influence the radiated noise. Therefore, detailed knowledge about the frequencies and deflection shapes of vibrations generated by PWM forces is of great importance for understanding and controlling the radiated noise and its spectral content. Vibration levels are measured on the stator shield and the operational deflection shapes are studied for several PWM switching frequencies and motor speeds. The deflection shapes with the largest vibration levels are determined. These are then compared to the expected excitation resulting from the pure PWM force lines. Changing the switching frequency, will shift the frequencies of the exciting forces. An appropriate selection of the PWM switching frequency is therefore important for the resulting acoustic radiation from the motor. Keywords Traction motor • Pulse-width-modulation (PWM) • Operational deflection shapes (ODS) • Vibrations • Electromagnetic noise 20.1 Introduction As the attention for acoustic comfort increases for rolling stock, a deep knowledge of the emitted acoustic noise is important. Already on the design stage, the acoustic noise needs to be evaluated and accounted for. There are many types of acoustic noise on a train, e.g. rolling noise, aerodynamic noise from fans, mechanical noise from bearings, and electromagnetic noise from the drive system caused by magnetic forces acting on structures. Especially at low speeds, when rolling noise is very low, the noise of electromagnetic origin is dominating and it is often annoying because of high tonalities in the spectrum. An important contributor to the electromagnetic noise on trains is the traction motor that also passengers are exposed to. Since this noise is dominating at low speeds, passengers on platforms as well as in the compartments are exposed to this noise. The generation of acoustic noise of electromagnetic origin in a traction motor is a process involving several different physical domains. The motor current induces a magnetic field that generates magnetic forces in the air-gap of the motor. These forces act radially on the stator and cause vibrations, i.e. structural deflections of the stator, that will result in noise radiated from the motor. The frequency spectra of the radiated noise, the vibrations, and the exciting forces are identical. The electromagnetic noise from induction motors increases when they are operated from pulse-width modulated (PWM) converters (in comparison to sinusoidal operation). PWM has a large impact on the force spectrum since the modulation strategy adds many harmonics to the air-gap Maxwell forces. Therefore, the influence of PWM force lines is important to understand in order to describe the electromagnetic generated acoustic noise. H. Amlinger ( )• F. Botling • S. Leth KTH Royal Institute of Technology, The Marcus Wallenberg Laboratory for Sound and Vibration Research (MWL), KTH Railway Group, SE 100 44 Stockholm, Sweden Center of Competence Acoustics and Vibration, Bombardier Transportation Sweden AB, SE 722 14 Västerås, Sweden e-mail: amlinger@kth.se I.L. Arteaga KTH Royal Institute of Technology, The Marcus Wallenberg Laboratory for Sound and Vibration Research (MWL), KTH Railway Group, SE 100 44 Stockholm, Sweden © The Society for Experimental Mechanics, Inc. 2016 J. De Clerck, D.S. Epp (eds.), Rotating Machinery, Hybrid Test Methods, Vibro-Acoustics & Laser Vibrometry, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-30084-9_20 209
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