Cascaded Three-Phase VFD Modulation for Motor Overvoltage
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Solution Overview
Problem
Cascaded three-phase VFDs experience overvoltage issues at the motor end due to voltage reflection caused by cable distribution parameters, leading to accelerated motor insulation aging and reduced service life, which existing modulation methods fail to adequately address without increasing system volume and cost through the use of dv/dt filters.
Innovation Solution
A method of modulating cascaded three-phase VFDs by obtaining and arranging space voltage vectors into large, medium, and small vectors, determining action times, and generating switching signals to synthesize a reference voltage vector, thereby reducing the number of switching levels and minimizing overvoltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If space vector modulation is directly used with eight basic space voltage vectors, then the modulation scheme is simple, but multiple bridge arms operate simultaneously causing two-level voltage jumps and overvoltage at the motor end
Solution Approach 1:
The patent segments the traditional eight basic space voltage vectors into four categories: large vectors, medium vectors, small vectors, and zero vectors. This segmentation allows for controlled switching where only one bridge arm operates at a time, eliminating simultaneous bridge arm switching and the resulting two-level voltage jumps that cause overvoltage at the motor end.
Solution Approach 2:
The patent applies different switching strategies to different vector types. Large vectors are used when high voltage output is needed, medium vectors for intermediate levels, small vectors for fine control, and zero vectors for maintaining current. This local quality approach ensures that voltage transitions are smooth and controlled, preventing overvoltage spikes at the motor end while maintaining effective motor control.
2Object-affected harmful factors
If dv/dt filter is used to suppress overvoltage at the motor end, then overvoltage is reduced, but system volume and cost increase
Solution Approach 1:
The patent extracts and eliminates the root cause of overvoltage (simultaneous bridge arm switching causing two-level voltage jumps) through its modulation strategy, rather than adding a dv/dt filter to suppress the symptom. By redesigning the switching sequence to operate only one bridge arm at a time, the system achieves overvoltage suppression without the additional volume and cost of external filtering components.
3Object-affected harmful factors
If dv/dt filter is used to suppress overvoltage at the motor end, then overvoltage is reduced, but system cost increases
Solution Approach 1:
The patent removes the need for expensive dv/dt filters by extracting and eliminating the fundamental cause of overvoltage through intelligent modulation. The segmented space vector modulation strategy controls switching to prevent harmful voltage transitions, achieving the same protective effect as a dv/dt filter but through control software rather than additional hardware, thereby reducing system cost.
Data Source
AI summary
A method of modulating a cascaded three-phase VFD, including: obtaining space voltage vectors according to states and output levels of switches of power units in each stage, and dividing the vectors into large vectors, medium vectors, small vectors and zero vectors according to their lengths; arranging the vectors into a vector space, and dividing the vector space into sectors, wherein each sector corresponds to a group of the large vector, the medium vector, the small vector and the zero vector; determining a sector in which a reference voltage vector is located, wherein the reference voltage vector is composed by the group of vectors; calculating action time of the vectors in the group; allocating an action order to the vectors; and generating a three-phase modulated wave signal based on the action orders and the action time.