Double-Reference PWM for Low-Speed AC Machine Torque Ripple
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Solution Overview
Problem
Conventional AC machines experience vibrations and jerking at low speed regions due to high torque and current ripples, which are not effectively addressed by existing methods such as SVPWM and 3L-VSI with limited switching frequency and efficiency.
Innovation Solution
A method for generating double-reference pulse-width modulation (DRPWM) control signals for multi-level power converters, using two different reference voltages for each phase to generate multi-level PWM signals, reducing torque and current ripples, and ensuring smooth operation at low speeds without changing the converter's topology or switching frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional PWM methods (SVPWM, 3L-VSI) are used to control AC machines at low speed, then the basic speed control function is maintained, but torque and current ripples increase causing vibrations and jerking
Solution Approach 1:
The patent segments the reference voltage generation into two separate reference voltages (first reference voltage and second reference voltage) for each phase instead of using a single reference voltage. This segmentation allows independent optimization of each reference voltage to minimize different components of torque ripple, thereby reducing overall torque ripple and improving low-speed operation smoothness
Solution Approach 2:
The patent changes the modulation parameters by introducing two different modulation indices (first modulation index and second modulation index) for each phase. By dynamically adjusting these modulation indices based on operating conditions, the system optimizes the PWM control to minimize torque ripple at low speeds while maintaining effective speed control
2Object-generated harmful factors
If switching frequency is increased to improve current quality, then torque ripple is reduced, but switching losses increase reducing efficiency
Solution Approach 1:
The patent optimizes the modulation indices and reference voltage parameters to achieve minimum torque ripple at conventional switching frequencies. By carefully tuning these parameters, the system reduces current ripple without requiring increased switching frequency, thereby avoiding additional switching losses and maintaining efficiency
3Device complexity
If conventional single-reference PWM is used, then the control system remains simple, but torque quality and current quality deteriorate at low speeds
Solution Approach 1:
The patent segments the control approach by using two separate reference voltages and two separate modulation indices for each phase. While this increases control complexity compared to conventional single-reference PWM, the segmented approach enables independent optimization of each reference voltage to target different torque ripple components, resulting in significantly improved torque quality at low speeds
Data Source
AI summary
A method for generating a set of pulse-width modulation control signals for a multi-level power converter. The method includes generating a base reference signal for each of three or more reference phases and determining a maximum reference and minimum reference. The method includes calculating a reference sum of the maximum reference and the minimum reference and generating a first offset and a second offset based on the reference sum. The method includes for each of the three reference phases generating an upper PWM output and a lower PWM output. The method includes combining the upper PWM output and lower PWM output to generate a multi-level PWM control signal for the reference phase and outputting a set of multi-level PWM control signals generated for the three or more reference phases.