## Abstrakt

The work in this thesis is focused on the acoustic noise generated by electrical motors driven by a pulse width modulated (PWM) power electronic inverter. In a usual inverter based electrical drive, the modulation uses fixed switching frequency; that introduces a set of harmonics in the acoustic spectra transforming the acoustic noise generated by the motor in a strong whistling noise. To maintain high efficiency for the entire drive, the switching frequency is typically kept around 4 kHz. However, this is the range where the human ear is the most sensitive. The main goal of this thesis is to ameliorate this whistling acoustic noise, while maintaining the efficiency of the drive.

The first chapter of the report is an introductory chapter where the motivation, objectives, limitations and an overview on electrical motor acoustics are presented. A list of main contributions of this PhD project is also presented here. The second chapter starts with an overview of the most widely used two level inverters and presentation of the basic modulation principles. A theoretical elaboration of the line-to-line voltage and vibration spectrum is presented in the next chapter, where a new unified analytical solution is proposed. The proposed unified analytical solution can be used for most of the carrier based PWM techniques. Starting from the unified analytical solution of the line-to-line voltage, a mathematical equation is proposed which describes the spectral components of the acoustical noise generated by the motor.

A cost effective solution to reduce the annoyance of the acoustic noise generated by the modulation in an inverter-fed electric motor, is the random PWM. After the presentation of the existing random modulation methods in Chapter 4, a new random PWM technique is proposed. The advantage of the new modulation method is that it can be easily retrofit in a prior implemented open- or closed-loop control algorithm, without adding any extra hardware components.

Typically, for application in heating ventilation and air condition (HVAC), the main focus is on cost, efficiency and acoustic noise, while high performance is not required for shafttorque dynamics. In order to investigate the acoustic performance of the new proposed random modulation method, a ventilation system has been used. In Chapter 5, various methods to measure the frequency response of the ventilation system were evaluated. The first method was based on exciting the system with a force impulse. The second excitation method was based on injection of a sinusoidal current with variable frequency into the motor. The third, most promising method to measure the frequency response of the ventilation system, was based on random PWM.

Chapter 6 presents the acoustic measurements of the ventilation system using various modulation techniques. A new trend in HVAC is to use permanent magnet synchronous motor (PMSM) instead of the asynchronous motor, in order to increase the efficiency of the electrical drive. In this chapter, the acoustic performance of the two different motor structures is also analyzed.

A relatively new cost effective solution for HVAC applications is to decrease the size of the capacitance form the DC-link of the inverter. This will cause a large DC-link voltage ripple, and resonances between the line inductance and DC link capacitor can appear. These effects decrease the acoustic performance of the drive. A new compensation method for the DC-link voltage fluctuation is proposed in Chapter 7. The compensation removes the main frequency component introduced by the DC-link voltage ripple from the acoustic spectra.

The last chapter of this report presents the conclusions based on the theoretical and experimental results performed during the PhD work. Finally, a list of future work is proposed.

The first chapter of the report is an introductory chapter where the motivation, objectives, limitations and an overview on electrical motor acoustics are presented. A list of main contributions of this PhD project is also presented here. The second chapter starts with an overview of the most widely used two level inverters and presentation of the basic modulation principles. A theoretical elaboration of the line-to-line voltage and vibration spectrum is presented in the next chapter, where a new unified analytical solution is proposed. The proposed unified analytical solution can be used for most of the carrier based PWM techniques. Starting from the unified analytical solution of the line-to-line voltage, a mathematical equation is proposed which describes the spectral components of the acoustical noise generated by the motor.

A cost effective solution to reduce the annoyance of the acoustic noise generated by the modulation in an inverter-fed electric motor, is the random PWM. After the presentation of the existing random modulation methods in Chapter 4, a new random PWM technique is proposed. The advantage of the new modulation method is that it can be easily retrofit in a prior implemented open- or closed-loop control algorithm, without adding any extra hardware components.

Typically, for application in heating ventilation and air condition (HVAC), the main focus is on cost, efficiency and acoustic noise, while high performance is not required for shafttorque dynamics. In order to investigate the acoustic performance of the new proposed random modulation method, a ventilation system has been used. In Chapter 5, various methods to measure the frequency response of the ventilation system were evaluated. The first method was based on exciting the system with a force impulse. The second excitation method was based on injection of a sinusoidal current with variable frequency into the motor. The third, most promising method to measure the frequency response of the ventilation system, was based on random PWM.

Chapter 6 presents the acoustic measurements of the ventilation system using various modulation techniques. A new trend in HVAC is to use permanent magnet synchronous motor (PMSM) instead of the asynchronous motor, in order to increase the efficiency of the electrical drive. In this chapter, the acoustic performance of the two different motor structures is also analyzed.

A relatively new cost effective solution for HVAC applications is to decrease the size of the capacitance form the DC-link of the inverter. This will cause a large DC-link voltage ripple, and resonances between the line inductance and DC link capacitor can appear. These effects decrease the acoustic performance of the drive. A new compensation method for the DC-link voltage fluctuation is proposed in Chapter 7. The compensation removes the main frequency component introduced by the DC-link voltage ripple from the acoustic spectra.

The last chapter of this report presents the conclusions based on the theoretical and experimental results performed during the PhD work. Finally, a list of future work is proposed.

Originalsprog | Engelsk |
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Forlag | Department of Energy Technology, Aalborg University |
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Antal sider | 157 |

ISBN (Trykt) | 978-87-89179-94-0 |

Status | Udgivet - sep. 2010 |