Electric vehicle powertrain noise analysis and optimization

summary:

In order to find out the source of order noise of electric vehicle powertrain, this paper uses order analysis method to analyze reducer noise, and finds that the possible order of reducer noise is 9.5, 21 and integer multiples of the two. Finite element software is used to establish a two-dimensional electromagnetic simulation model of the drive motor, and the radial electromagnetic force of the drive motor is analyzed and calculated. The radial electromagnetic force is decomposed in time and space by two-dimensional Fourier transform, and the spatial order of the radial electromagnetic force and the frequency order it contains are obtained. It is identified that the radial electromagnetic force of spatial order 0 and frequency order 48 contributes the most to the electromagnetic noise of the drive motor. The acoustic wrapping method is proposed to optimize the powertrain noise. The experimental results show that the near-field noise of the drive motor and the reducer output stage is reduced by 6.1 and 4.9 dB respectively after wrapping. The method in this paper has reference significance for the optimization of powertrain noise.

0 Introduction

With the continuous development of automobile manufacturing technology, automobile comfort has become the main demand of consumers. The powertrain is the power source of pure electric vehicles, and its vibration and noise performance are key factors affecting automobile comfort. The powertrain of pure electric vehicles consists of motors and reducers. Permanent magnet synchronous motors are widely used in electric vehicles due to their small size and high power density. The electromagnetic noise of permanent magnet synchronous motors and the howling noise of reducers are common problems in the development of pure electric vehicle NVH (noise vibration and harshness). Optimizing the above two noises is an important means to improve the NVH performance of pure electric vehicle powertrains.

At present, there are many studies on reducer gear whine noise and permanent magnet synchronous motor electromagnetic noise at home and abroad. Reducer whine is a kind of medium and high frequency noise caused by the unsteady exciting force on the internal gears in the meshing transmission and the transmission error in the meshing process. Its optimization is mostly through micro-modification of the gears to improve the gear meshing condition.

The source of electromagnetic noise in permanent magnet synchronous motors is the alternating electromagnetic force generated by the harmonic magnetic fields in the air gap inside the motor. The electromagnetic force has tangential and radial components. The radial electromagnetic force plays a major role in causing electromagnetic vibration and noise. It causes the stator core to vibrate radially. The noise generated by radial vibration is the main component of the electromagnetic noise of the motor.

There are two main ways to optimize the electromagnetic noise of permanent magnet synchronous motors: 1. Changing the mechanical structure of the motor; 2. Reducing the harmonic content of the armature current.

This paper takes the powertrain of a certain model of pure electric vehicle as the research object. First, the order noise of the powertrain reducer is analyzed; then the radial electromagnetic force characteristics of the powertrain drive motor are analyzed, and the Maxwell software is used for simulation to identify the noise order that the motor may generate; finally, optimization measures for reducing powertrain noise by using acoustic packaging are proposed, and experimental verification is carried out.

1 Analysis of powertrain noise sources

The powertrain studied in this paper is shown in Figure 1.

Figure 1 Powertrain

1.1 Reducer noise

The most common noise of electric vehicle reducer is gear whine noise. Gear whine noise is a signal related to the speed, and is often analyzed using the equal-angle sampling order analysis method. The whine noise order is related to the number of gear teeth and the gear ratio of each level. The order calculation formula is:

(1)

Among them, O is the order; f is the gear meshing frequency; n is the motor output shaft speed.

The powertrain reducer studied in this paper is a two-stage gear reduction, with the first-stage gear ratio of 21/53 and the second-stage gear ratio of 24/79. Therefore, the order of gear whine noise is 21, 9.5, and integer multiples of the two.

1.2 Permanent Magnet Synchronous Motor Noise

1.2.1 Analysis of the electromagnetic force of the motor

Electromagnetic noise is the main source of noise in permanent magnet synchronous motors, which is mainly caused by the alternating electromagnetic force generated by the air gap magnetic field between the stator and the rotor acting on the stator surface. Therefore, to analyze electromagnetic noise, it is necessary to calculate the electromagnetic force first.

In a permanent magnet synchronous motor, the tangential component of the electromagnetic force is much smaller than the radial component. To simplify the calculation, the influence of the tangential component on the motor noise is usually ignored, and only the effect of the radial electromagnetic force is considered. According to the Maxwell stress tensor method, the instantaneous value of the radial electromagnetic force pn(θ,t) per unit area on the stator surface is:

(2)

Among them, μ0=4π×10-7H/m; bn(θ,t) is the air gap magnetic flux density; t is time; θ is the spatial angle.

When magnetic saturation is ignored, the expression of air gap magnetic flux density bn(θ,t) is:

bn(θ,t)=f(θ,t)λ(θ,t)

(3)

Among them, f(θ,t) is the air gap magnetic potential; λ(θ,t) is the air gap magnetic permeability.

The powertrain drive motor studied in this paper is a built-in permanent magnet synchronous motor. When the motor is operating normally, the air gap magnetic potential f(θ, t) is composed of the stator winding harmonic magnetic potential, the rotor permanent magnet harmonic magnetic potential and its fundamental wave synthetic magnetic potential. The motor rotor is smooth and the stator is slotted, and the air gap permeance λ(θ, t) can be expressed as:

(4)

Among them, Λ0 is the constant part of the air gap permeability per unit area; Λk is the kth harmonic amplitude of the air gap permeability; Z is the number of stator slots; δ is the air gap length; Kc is the Carter coefficient. The size of the mth time harmonic of the spatial r-order radial electromagnetic force wave is:

pr,m=pmcos(mω1t-rθ-αm)

(5)

Among them, pm is the amplitude of the radial electromagnetic force wave, m=1,…,n.

The size of the synthesized spatial r-order radial electromagnetic force wave is:

pr=pr,1+pr,2+…+pr,m+…+pr,n=

(6)

Among them, am(t) and bm(t) are coefficients related to the motor speed.

It can be seen from formula (6) that, except for the 0th order spatial radial electromagnetic force wave, the other rth order spatial radial electromagnetic force waves are composed of a spatial sine waveform sin(rθ) and a spatial cosine waveform cos(rθ) superimposed.

The expression of the radial electromagnetic force wave pn(θ,t) obtained by synthesizing the radial electromagnetic force waves of all spatial orders is:

(7)

Where R is the spatial order of the force wave.

For integer slot permanent magnet synchronous motors, the main source of electromagnetic noise is the interaction between the stator and rotor high-order harmonic magnetic fields. The harmonic order of the stator winding magnetic field is:

v=(6k1+1)p,k1=±1,±2,±3,…

(8)

Where p is the number of pole pairs of the motor.

The harmonic order of the rotor harmonic magnetic field is:

μ=(2k2+1)p,k2=1,2,3,…

(9)

Therefore, the radial electromagnetic force wave order generated by the interaction between the stator and rotor harmonic magnetic fields is:

(10)

From equation (10), we can see that the spatial order of the radial electromagnetic force wave of an integer-slot permanent magnet synchronous motor may be 0 or an integer multiple of the number of motor poles. The powertrain drive motor studied in this paper is an 8-pole 48-slot permanent magnet synchronous motor, so the spatial order of its radial electromagnetic force wave may be 0, 8, 16, etc.

1.2.2 Simulation analysis of radial electromagnetic force

The radial electromagnetic force is distributed periodically in space, and the radial electromagnetic force at each point in space changes periodically in time. In the past, many scholars only performed one-dimensional harmonic analysis on the radial electromagnetic force in time or space, that is, only performed harmonic analysis on the radial electromagnetic force that changes with time at a certain point in space or only performed harmonic analysis on the radial electromagnetic force that changes with the spatial angle at a certain moment, which cannot well analyze the temporal and spatial distribution law of the radial electromagnetic force of the motor. This paper establishes a two-dimensional electromagnetic finite element model of the motor and uses the time-step finite element method to simulate the distribution of the radial electromagnetic force in time and space under the conditions of the maximum speed of 11,000 r/min and the peak power of 110 kW. The simulation parameters of the motor are listed in Table 1, and its winding form is a double-layer winding. The two-dimensional electromagnetic finite element model of the motor is shown in Figure 2.

Table 1 Motor simulation parameters

Figure 2 Electromagnetic finite element model of drive motor

The spatial and temporal distribution of the radial electromagnetic force of the motor is shown in Figure 3.

It can be seen from the literature [15] that the motor will resonate only when the spatial order of the radial electromagnetic force is equal to the radial modal order of the motor and the frequency contained in this order of radial electromagnetic force is close to the modal frequency of the motor of the corresponding order. Therefore, the radial electromagnetic force that changes periodically in time and space is decomposed in time and space by two-dimensional Fourier transform to obtain the spatial order of the radial electromagnetic force and the frequency contained in each order, as shown in Figure 4a; for rotating machinery, the order analysis method is often used to analyze noise, and the rotation frequency of the motor output shaft is selected as the reference frequency. The frequencies of each order obtained by the spatial and temporal decomposition of the radial electromagnetic force are transformed into the corresponding frequency order, as shown in Figure 4b.