J. Le Besnerais

Characterization and Reduction of Audible Magnetic Noise Due to PWM Supply in Induction Machines

This paper derives the analytical characterization of the Maxwell radial vibrations due to pulsewidth modulation (PWM) supply in induction machines and, particularly, in traction motors supplied with an asynchronous switching frequency. The number of nodes and the velocity of these particular force waves are experimentally validated by visualizing some operational deflection shapes of the stator. It is shown that according to the switching frequency, these forces can be responsible for high magnetic-noise levels during starting and braking. A simple rule to avoid PWM noise is then proposed and applied to an industrial traction motor. Experimental results show that the choice of the switching frequency can have a 15-dB impact on the sound power level emitted by the motor during starting and that a lower switching frequency can sometimes lead to lower magnetic noise. In agreement with analytical predictions, the new proposed switching frequency that avoids resonances between PWM exciting forces and corresponding stator modes reduces the magnetic noise of 5 dB during starting.

Multiphysics Modeling: Electro-Vibro-Acoustics and Heat Transfer of PWM-Fed Induction Machines

The design of variable-speed electrical machines involves several fields of physics, such as electromagnetism, thermics, mechanics, and also acoustics. This paper describes the analytical multiphysics models of a computer-aided-design software which is applied to inverter-fed traction induction machines. The electromagnetic model computes rotor and stator currents, the induction-machine traction characteristics, and the radial air-gap flux density. The mechanical and acoustic models compute the motors audible magnetic noise level due to Maxwell forces. The thermal model based on 3-D nodal network computes the transient temperature of different parts of the motor. These fast models make it possible to couple the software with some optimization tools. Some simulation results are presented on a self-ventilated closed motor and compared to experiments.

Multiobjective Optimization of Induction Machines Including Mixed Variables and Noise Minimization

Induction motor design requires making numerous tradeoffs, especially when including electromagnetic noise criterion besides usual criteria like efficiency and cost. Moreover, adding the noise objective significantly increases computational time as it must be evaluated at variable speed in order to take into account resonance effects. In that case, the application of multiobjective optimization algorithms can be hard for their computational cost as for the difficulty to interpret multidimensional results in both design variables and objectives spaces. This paper first describes a fast analytical model of a variable-speed induction machine which calculates both motor performances and sound power level of electromagnetic origin. This model is then coupled to Nondominating Sorting Genetic Algorithm (NSGA-II) in order to perform global constrained optimizations with respect to several objectives (e.g., noise level, efficiency and material cost). As induction machine design involves both continuous and discrete variables, a modified NSGA-II algorithm handling mixed variables is detailed. Finally, some optimization results are presented and analyzed by the aid of several visualization tools.

Optimal Slot Numbers for Magnetic Noise Reduction in Variable-Speed Induction Motors

Although many empirical rules have been established for correctly choosing the number of stator and rotor slots so as to limit the audible magnetic noise level radiated by induction machines, these rules never take into account the stator natural frequencies or the fact that the motor is run at variable speed. In this paper, we present a fast simulation tool for the variable-speed magnetic noise emitted by induction machines, based on fully analytical models. On the basis of these models, we derive and experimentally validate an analytical expression for magnetic vibrations due to slotting reluctance harmonics, confirming the prime importance of slot combination in magnetic noise radiation. We ran simulations on a 700-W squirrel-cage motor in order to quantify the noise emitted by all possible combinations of slot numbers in two- and three-pole pairs, including odd slot numbers. We thus obtained a database that efficiently replaces the old empirical rules for slot combination numbers and helps in designing quiet induction motors. Similar databases can be built for other power ranges.

Characterization and Reduction of Magnetic Noise Due to Saturation in Induction Machines

This paper derives the analytical characterization of Maxwell radial vibrations due to saturation effects in induction machines, and especially in traction motors. The number of nodes and the velocity of these particular force waves are experimentally validated by visualizing some operational deflection shapes of the stator. It is shown that according to the stator and rotor slot numbers, and stator natural frequencies, these forces can be responsible for high magnetic noise levels during starting and braking. A simple rule to avoid saturation magnetic noise is then proposed, and applied to an industrial motor. Simulation results show that the new proposed motor improves magnetic noise level up to 20 dB, whereas experiments give a 15 dB improvement.

Optimal Slot Opening Width for Magnetic Noise Reduction in Induction Motors

This paper presents a method to characterize the main magnetic force waves occurring in a sinusoidally fed induction machine. Three main force types are identified: slotting force waves, winding force waves, and saturation force waves. Slotting force waves are characterized in terms of number of nodes, velocity, propagation direction, and magnitude. On the ground of the expression of these forces magnitude, a method to cancel a given magnetic force wave by properly choosing the rotor slot or stator slot opening width is presented. This new method is validated with both simulations and experiments. Contrary to the common design rule that advices to decrease rotor and stator slot openings width in order to reduce magnetic noise, it is shown that a wider slot opening can lower the global noise level when properly chosen.

Multiphysics modeling: electro-vibro-acoustics and heat transfer of induction machines

The design of electrical machines involves several fields of physics, such as electromagnetism, thermics, mechanics but also acoustics. This paper describes the analytical multiphysics models of a computer-aided design (CAD) software which is applied to inverter-fed traction induction machines. The electromagnetic model computes rotor and stator currents, the induction machine traction characteristics, and the radial airgap flux density. The mechanical and acoustic model computes the motor audible magnetic noise level due to Maxwell forces. The thermal model based on 3D nodal network, computes the transient temperature of different parts of the motor. These fast models make it possible to couple the software with some optimization tools. Finally, some simulations are presented on a self-ventilated closed motor, and an example of multi-objective optimization is exposed.

Characterization of the audible magnetic noise emitted by traction motors in railway rolling stock

This paper first presents the analytical models of an ALSTOM simulation software, DIVA, which computes the vibratory and acoustic behavior of a variable-speed induction machine due to Maxwell forces. This radial magnetic pressure in the air-gap makes the stator vibrate in the audible range, creating the so-called magnetic noise characterized by high tonality. On the basis of these analytical models, the main magnetic vibrations due to slotting, pulse-width modulation (PWM) harmonics and their interaction are then analytically characterized. Their number of nodes, velocity and propagation direction are experimentally validated by visualizing the stator deflection shapes. Finally, some experimental validations of the simulation tool are presented. Both analytical results and simulations show that some quieter motors can be designed with motor geometry (especially slot numbers) and PWM strategy.

Fast prediction of variable-speed acoustic noise due to magnetic forces in electrical machines

This paper presents a new multiphysic model and simulation environment for the fast calculation and analysis of acoustic noise and vibration levels due to Maxwell forces in variable-speed rotating electrical machines. In the first part, some numerical methods for the prediction of electromagnetic noise are analyzed and compared to analytical or semi analytical techniques. In the second part, a new coupling of electrical, electromagnetic and vibro-acoustic models based on analytical and semi-analytical modeling techniques is presented. This model is validated by comparing simulation results to experimental results on several electrical machines at variable speed, including surface permanent magnet (SPMSM), interior permanent magnet (IPMSM) and squirrel cage induction machines (SCIM). The main resonances and noise levels are correctly estimated by the models implemented in MANATEE® simulation software, and the calculation time at variable speed varies from one second to a few minutes including harmonics up to 20 kHz.

Effect of magnetic wedges on electromagnetically-induced acoustic noise and vibrations of electrical machines

The articles studies the effect of stator magnetic wedges on the electromagnetically-induced acoustic noise and vibration of a squirrel cage induction machine. Firstly, a review of previous studies analysing the effects of magnetic wedges both on electromagnetics and vibro-acoustic domain is done. Then, simulations are performed with MANATEE® software to quantify the reduction of noise and vibrations due to Maxwell forces with the use of magnetic wedges in a squirrel cage induction machine presenting a strong resonance due to slotting effect. A sensitivity study is carried on the magnetic permeability of the wedge and it is shown that the maximum sound pressure level and vibration reduction only reaches 2 dB with a relative magnetic wedge permeability of 20.