Two-level inverters are widely used in variable frequency motor drive systems of electric vehicles. High-frequency PWM modulation technology can generate high-frequency and high amplitude common mode voltage in the inverter, which will endanger the normal operation of the motor. Especially at low speeds, using traditional fixed switching frequency asynchronous modulation PWM will result in high switching losses with low output power and inverter efficiency. Therefore, a multi-mode modulation strategy is proposed based on TSPWM (Tri-State PWM). By improving the TSPWM modulation algorithm and adopting a segmented variable carrier wave ratio modulation strategy, the inverter efficiency of the motor drive system is significantly improved, and the common mode voltage is reduced. Firstly, the TSPWM algorithm, which uses three switch states to synthesize the reference voltage in each PWM cycle, reduces the number of switching actions by one-third. Secondly, select the clamping phase according to the amplitude of the current, increase the overlap time between the maximum current and the inactivity of the switching device, and reduce the switching loss of the inverter. Thirdly, considering the system efficiency and current waveform quality, the carrier frequency is optimized at different speed ranges. Moreover, based on the calculation of carrier period angle, a phase angle compensation algorithm of the voltage vector is proposed to avoid the current shock when the modulation mode is switched. By accurately analyzing the impact of changing the modulation mode on the voltage vector angle, the compensation angle after switching is calculated and compensated to the reference space voltage angle, thus achieving smooth switching of different modes. The experimental results show that the common-mode voltage of TSPWM in a PWM period is one-third of that of SVPWM, whether the voltage vector is in a high or low modulation ratio region. Using the maximum current phase clamping technique, the clamping phase is precisely the phase with the maximum phase current, and the vertex position of the current amplitude is in the middle of the clamping region. The simulation results show that the angle compensation algorithm of the voltage vector can achieve a smooth transition of the angle of the voltage vector during the carrier frequency switching process. Through the experimental results of the multi-mode modulation strategy based on TSPWM at different switching frequencies, the current waveform oscillation without angle compensation is significant, and about 9% of torque ripple is detected during the modulation mode switching. The current waveform using angle compensation has no impact on the switching process, and the torque fluctuation collected by the system is less than 2%, which verifies the feasibility of the switching strategy based on angle compensation. Compared with the traditional SVPWM algorithm, the proposed multi-mode modulation algorithm based on TSPWM can increase the maximum efficiency of the inverter by about 10%, and the overall efficiency of the inverter can be improved by an average of 5%. The high-efficiency area, where the inverter efficiency is 95% or more, has increased by 40.1%. The following conclusions can be drawn through simulation and experiments. (1) Compared with the traditional SVPWM strategy, this paper optimizes the switching frequency at different speeds and adopts TSPWM to reduce the number of switching actions, significantly reducing the switching loss and improving the efficiency of the inverter. (2) The switching loss can be further reduced by dynamically changing the clamping mode according to the magnitude of the phase current. (3) The angle compensation algorithm of the voltage vector effectively overcomes the voltage phase hopping problem in the PWM modulation mode switching to realize the smooth transition. (4) The proposed algorithm can also reduce the peak-to-peak value of high-frequency common mode voltage in one PWM cycle. Therefore, it is suitable for small and medium-sized inverters with a 100 kW or less power output, represented by electric vehicle drive systems. © 2024 China Machine Press. All rights reserved.
