Inverter AC Current Sampling to Suppress Six-Step Harmonic Aliasing
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Solution Overview
Problem
In six-step inverter operations for electric or hybrid vehicles, unwanted (6n-1) and (6n+1) harmonics appear, leading to alternating current estimation inaccuracies and torque estimation inaccuracies due to aliasing effects caused by insufficient sampling frequency.
Innovation Solution
A current acquisition method that increases the number of samples per subperiod, transforming and averaging them in a rotating coordinate system to suppress unwanted harmonics, thereby improving the reconstruction of frequency components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If only one current value is sampled per PWM period (6 values per fundamental period), then the sampling frequency is 6*f0 which meets the minimum requirement, but the Nyquist criterion cannot properly reconstruct frequencies above 3*f0 and aliasing effects occur causing improper reconstruction of fundamental frequency components
Solution Approach 1:
The fundamental period is divided into six subperiods (PWM periods), and each subperiod is further divided into multiple sampling intervals. Instead of taking only one sample per PWM period, the method takes multiple samples (at least two) within each PWM period, effectively segmenting the sampling process to capture more frequency information and avoid aliasing while maintaining system simplicity
Solution Approach 2:
The method employs periodic sampling at multiple points within each PWM period, with sampling frequencies that are integer multiples of the fundamental frequency. This periodic multi-point sampling approach ensures proper reconstruction of frequency components above 3*f0 by satisfying the Nyquist criterion across the entire fundamental period
2Loss of energy
If six-step operation is used to reduce switching losses and achieve maximum fundamental frequency AC output voltage amplitude, then switching losses are minimized, but unwanted (6n-1) and (6n+1) harmonics appear causing current estimation inaccuracies and torque estimation inaccuracies
Solution Approach 1:
The method extracts and isolates the fundamental frequency component from the current signal by transforming sampled values into a rotating coordinate system and averaging them. This extraction process separates the desired fundamental component from unwanted harmonics, eliminating their influence on current and torque estimation while maintaining the energy-efficient six-step operation
Solution Approach 2:
A rotating coordinate system transformation is introduced as an intermediary step between sampling and measurement. This transformation mediates the relationship between the sampled current values and the fundamental frequency component, allowing accurate reconstruction of the fundamental component while filtering out harmonics through the coordinate transformation and averaging process
3Measurement precision
If multiple values are sampled per PWM period and transformed to rotating coordinate system for averaging, then unwanted harmonics are suppressed and frequency component reconstruction is improved, but the processing complexity increases
Solution Approach 1:
The signal processing is segmented into distinct steps: sampling multiple values per PWM period, transforming each sample to the rotating coordinate system using Park transformation, and then averaging the transformed values. This segmentation of the processing workflow makes the complex operation more manageable and implementable while achieving superior frequency component reconstruction accuracy
Data Source
Figure 1~2
Figure 3
AI summary
The present invention relates to a method of AC acquisition (1) for an electric machine, comprising: - a step (2) in which an inverter is operated according to a six-step operation, during which a fundamental period of 2π is divided into six equal time periods, maximum one switching action occurring during each; - a step (8) in which a plurality of values of an AC of the inverter from a first reference frame are sampled for each equal time period; - a step (10) in which the sampled values are rotated from the first reference frame to a second reference frame; - a step (12) in which for each equal time period, the values from the second reference frame are averaged; - a step (14) in which a current value is estimated according to the averaged sampled values.