A detailed analysis of electromagnetic noise in external rotor permanent-magnet synchronous motors is presented in this paper. First, the spatial distribution and frequency characteristics of the electromagnetic force acting on the surface of the permanent magnet are discussed. Then, calculation models for electromagnetic force, structural vibration, and acoustic radiation are developed to predict noise by taking an external rotor in-wheel motor as example. The uneven distribution of electromagnetic force on the surface of the permanent magnet is taken into account by means of loading nodal force into the structural model which is verified by modal test. Through the mode superposition method, the vibration on the surface of the outer rotor is calculated, and the acoustic boundary element method is used to predict the acoustic radiation. Noise test is conducted to validate the simulated noise. It is shown in both simulation results and noise test that the electromagnetic force due to slotting effects contributes the most remarkable component to the overall noise. Finally, slot opening width is optimized to reduce the amplitude of magnetic force close to resonance frequencies, and the overall sound pressure level decreases by 6 dB(A) after optimization.

Noise Analysis, Calculation, and Reduction of External Rotor Permanent-Magnet Synchronous Motor

This paper presents a multiphysics modeling of a switched reluctance motor (SRM) to simulate the acoustic radiation of the electrical machine. The proposed method uses a 2-D finite-element model of the motor to simulate its magnetic properties and a multiphysics mechatronic model of the motor and controls to simulate operating conditions. Magnetic forces on the stator are calculated using finite-element analysis and are used as the excitation on a forced response analysis that contains a finite-element model of the motor stator structure. Finally, sound power levels are calculated using the boundary element method. Simulation results of the model are shown and compared with experimental measurements for a four-phase 8/6 SRM.

Multiphysics NVH Modeling: Simulation of a Switched Reluctance Motor for an Electric Vehicle

Using magnetic flux barriers in the stator yoke of electric machines with fractional slots, tooth-concentrated winding, it is possible to reduce or even to cancel some space harmonics of low order in the air-gap flux density resulting in lower rotor losses induced by the armature reaction field. In this paper, this new technique is applied during the design and analysis of two permanent magnet machines with different 12-teeth/10-poles concentrated windings. Considering the main machine performances, such as the electromagnetic torque, machine losses, and also the field-weakening capability, the new stator design shows significant advantages over the conventional design. According to the new technique, a prototype machine is built and some measurement results are given.

Reduction of Low Space Harmonics for the Fractional Slot Concentrated Windings Using a Novel Stator Design

Permanent-magnet synchronous motors (PMSMs) have been widely used as the driving core in electric vehicles due to the high power/torque density and high efficiency. Recent years have witnessed a great effort on the vibration and noise of PMSMs in order to ensure the driving comfort. In this paper, the state of the art and recent progress of the electromagnetic vibration and noise of PMSMs for electric vehicles are presented, with emphasis on the induced mechanism, prediction method, suppression method, and sound quality. The future trends faced are also described.

Electromagnetic Vibration and Noise of the Permanent-Magnet Synchronous Motors for Electric Vehicles: An Overview

The influence of slot-and-pole-number combinations on the leakage inductance of double-layer fractional-slot concentrated nonoverlapping winding (i.e., tooth-coil winding) permanent-magnet synchronous machines is studied. A Fourier-analysis-based method for calculating the harmonic air-gap leakage inductance applied to the current-linkage waveform (winding function) in the air gap is suggested. Different slot-and-pole combinations are considered, and their influence, in particular, on the air-gap leakage inductance is indicated. In tooth-coil machines, the air-gap harmonic leakage can have a surprisingly large impact on the machine performance.

Effect of Slot-and-Pole Combination on the Leakage Inductance and the Performance of Tooth-Coil Permanent-Magnet Synchronous Machines

In external rotor permanent magnet synchronous motors with non-overlapping windings, the higher frequency harmonics of the radial forces generate considerable resonant vibrations and acoustic noise. Therefore, diversity of spatial and specifically frequency harmonic ordinal numbers of representative slot and pole number combinations are derived analytically with open circuit and under load. Amplitudes of radial force waves are calculated by means of the finite element method and 2-D Fourier analysis. The obtained results confirm the analytical investigation. Determining factors on amplitudes are studied on machines having different slot and pole numbers and double-layer windings. Higher frequency harmonics significantly depend on the pole and tooth shape as well as the current profile. With open circuit, the calculations are validated by experiments. The findings are applied to reduce the noise of a centrifugal fan.

J. Krotsch

Papers

Radial Forces in External Rotor Permanent Magnet Synchronous Motors With Non-Overlapping Windings