Electric vehicle powertrain noise analysis and optimization

Fig.3 Spatiotemporal distribution of radial electromagnetic force

Figure 4 Two-dimensional space-time decomposition of radial electromagnetic force

The simulation analysis results of the radial electromagnetic force of the motor show that the spatial orders of the radial electromagnetic force are 0 and 8, which are consistent with the analytical analysis results; the frequency orders of the radial electromagnetic force are 0, 8, 16, etc., which are integer multiples of the number of motor poles.

The lower the spatial order of the radial electromagnetic force, the farther the distance between the two adjacent nodes of the motor deformation caused, and the greater the radial deformation of the motor. The vibration displacement caused by the radial electromagnetic force acting on the stator surface is inversely proportional to the fourth power of the spatial order, so usually only the contribution of the radial electromagnetic force with a spatial order of 0 to 4 to the motor vibration noise is considered. As shown in Figure 4, the spatial order of the radial electromagnetic force of the drive motor of the powertrain studied in this paper only exists in the 0th order between 0 and 4. Therefore, the radial electromagnetic force with a spatial order of 0 is the main source of the drive motor noise, and the frequency orders it contains are 0, 24, and 48. Among them, the frequency order of the radial force wave is 0, which means that the force wave does not change with time, and the contribution to the noise is 0, and the amplitude of the 48th order electromagnetic force is about twice the amplitude of the 24th order electromagnetic force. The radial electromagnetic force with a spatial order of 0 and a frequency of 48th order contributes the most to the motor noise.

2 Powertrain Noise Test Analysis

The powertrain was mounted on the vehicle, and the near-field noise test was conducted on the powertrain using the data acquisition equipment of Miller-Baum under the condition of full throttle and uniform acceleration to 80 km/h. Two microphones were used to collect the noise data of the near-field of the drive motor and the near-field of the reducer output stage. The microphone and the motor shaft were on the same horizontal plane, and the microphone head was facing the reducer housing and the motor housing, respectively, with a distance of 20 cm, as shown in Figure 5.

Figure 5 Powertrain test layout

When the vehicle is accelerated from a stationary state to 80 km/h at full throttle, the time-frequency diagram of the A-weighted sound pressure level in the near field of the drive motor and the near field of the reducer output stage is shown in Figure 6.

The test results in Figure 6 show that the main order noises in the motor near field and the reducer output stage near field are 9.5, 19, 21, 42, and 48. Among them, 9.5, 21, 19, and 42 are the whistling noises and their frequency multiples generated by the meshing of the reducer gears; the 48th order is the electromagnetic noise caused by the radial electromagnetic force of the motor.

When the motor speed is 2660 r/min, the motor near-field noise changes suddenly at a frequency of 2145 Hz. This is because the frequency corresponding to the radial electromagnetic force of spatial order 0 and frequency order 48 is close to the driving motor breathing modal frequency of 2173 Hz obtained from the powertrain modal test, which causes motor resonance. The reducer has obvious howling noise in the motor speed range of 4000-5550 r/min.

Figure 6 A-weighted sound pressure level time-frequency diagram

The analysis results of the noise contribution of each order of the motor near-field noise and the reducer output stage near-field noise are shown in Figure 7.

As can be seen from Figure 7, the motor near-field noise reaches its peak value when the motor speed is 2 660 r/min, and the total sound pressure level is 102.7 dB, among which the 48th-order electromagnetic noise contributes the most, and the 24th-order electromagnetic noise contributes relatively less; the reducer output stage near-field noise reaches its peak value when the motor speed is 5 335 r/min, and the total sound pressure level is 98.0 dB, and the contribution of each order noise generated by gear meshing is roughly the same.

Therefore, optimizing the noise of this powertrain mainly involves improving the 48th-order electromagnetic noise of the drive motor and the reducer gear meshing noise.

Figure 7 Order noise contribution analysis results

3 Noise optimization measures and experimental verification

Noise optimization generally starts from two aspects: noise source control and noise propagation path control.

Starting from controlling the noise propagation path, this paper uses sound-absorbing materials to acoustically wrap the powertrain, and uses the sound absorption characteristics of the sound-absorbing materials to reduce the radiated noise of the powertrain. The powertrain and test layout after acoustic wrapping are shown in Figure 8, and the test layout is the same as before wrapping.

Figure 8: Powertrain test layout after wrapping

After the powertrain is acoustically wrapped, when the vehicle is accelerated uniformly from a stationary state to 80 km/h at full throttle, the time-frequency diagram of the A-weighted sound pressure level in the near field of the drive motor and the near field of the reducer output stage is shown in Figure 9.

The total sound pressure level of the motor near-field noise before and after the package, the comparison of the 48th-order electromagnetic noise, and the total sound pressure level comparison of the reducer output stage near-field noise before and after the package are shown in Figure 10.

As shown in Figure 9, after the powertrain is acoustically wrapped, the mutation point of the motor near-field noise at a speed of 2 660 r/min and a frequency of 2 145 Hz disappears; the noise of each order in the near-field of the reducer output stage is significantly reduced.

As shown in Figure 10, the peak value of the mutation point of the motor near-field noise has improved after wrapping. The total sound pressure level reaches a peak value of 96.6 dB at a speed of 3620 r/min, which is 6.1 dB lower than the noise before wrapping. The mutation of the 48th-order electromagnetic noise contained at a speed of 2660 r/min has been significantly improved; the near-field noise of the reducer output stage reaches a peak value at a speed of 5145 r/min, and the total sound pressure level is 93.1 dB, which is 4.9 dB lower than before wrapping.

The above results show that acoustic packaging has a significant optimization effect on the noise of the powertrain.

Figure 9 Time-frequency diagram of A-weighted sound pressure level after wrapping

Figure 10 Noise comparison before and after wrapping

4 Conclusion

This paper takes an electric vehicle powertrain with a rated power of 40 kW and a peak power of 110 kW as the research object, and conducts an order analysis on the source of reducer noise; uses finite element software to analyze the radial electromagnetic force of the drive motor, and uses two-dimensional Fourier transform to perform two-dimensional space-time decomposition to obtain the main spatial order and frequency order of the electromagnetic noise of the drive motor; tests the powertrain noise, proposes the method of acoustic wrapping to optimize its noise, and conducts experimental verification.

The research in this paper draws the following conclusions:

(1) The reducer gear whine noise and the drive motor electromagnetic noise are the main sources of powertrain noise. When the frequency contained in the spatial 0th-order radial electromagnetic force of the drive motor is close to the breathing mode frequency of the drive motor, it will cause the motor to resonate and deteriorate the noise level of the powertrain.

(2) The near-field noise of the motor and reducer output stage was reduced by 6.1 and 4.9 dB, respectively, by acoustically wrapping the powertrain, which has a significant optimization effect. This method has certain reference significance for the optimization of powertrain noise.