Normalized Modulation Control for Phase-Imbalanced Adjustable Drives
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Solution Overview
Problem
Adjustable frequency drives (AFDs) face issues with unbalanced DC bus voltages due to imbalances in multilevel cascaded inverter topologies and DC bus overvoltage conditions during deceleration or overhauling, leading to undesirable temperature rises and limited braking capabilities.
Innovation Solution
A control circuit generates a representative maximum DC voltage signal to normalize modulation control signals for inverter circuits, scaling them based on individual DC bus voltages to balance output and limit DC bus voltages, and adjusts output frequency to manage voltage levels, using an inverse Clarke transformation and gain factors to ensure balanced operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If separate DC links are used for respective phases in MLCI topology, then inverter control flexibility is improved, but DC bus voltage imbalance occurs leading to unbalanced output
Solution Approach 1:
The control circuit continuously monitors individual DC bus voltages and uses this feedback information to dynamically adjust modulation control signals for each inverter circuit, ensuring voltage balance is maintained despite separate DC links providing control flexibility
Solution Approach 2:
The system changes modulation parameters (amplitude and phase) based on detected DC bus voltage conditions, adjusting these parameters in real-time to compensate for voltage imbalances while maintaining the flexibility benefits of separate DC links
2Device complexity
If conventional vector control is used with unbalanced DC voltages, then control simplicity is maintained, but motor current unbalance and temperature rise occur
Solution Approach 1:
The control circuit applies individualized modulation control to each phase based on its specific DC bus voltage condition, allowing localized compensation for voltage imbalances without requiring complete redesign of the control system
Solution Approach 2:
The control circuit preemptively adjusts modulation signals to compensate for detected DC bus voltage imbalances before they can cause significant motor current unbalance and temperature rise
3Speed
If output frequency is not controlled during deceleration, then braking response speed is improved, but DC bus overvoltage occurs
Solution Approach 1:
The control circuit dynamically adjusts the output frequency based on real-time DC bus voltage conditions during deceleration, creating a adaptive response that prevents overvoltage while maintaining effective braking performance
Data Source
AI summary
An apparatus includes a plurality of DC buses and a plurality of inverter circuits, respective ones of which are coupled to respective ones of the plurality of DC buses. The apparatus further includes a control circuit configured to generate a representative DC voltage signal based on DC voltage signals for respective ones of the DC buses, to generate a first modulation control signal from the representative DC voltage signal, to generate second modulation control signals for the respective inverter circuits from the representative DC voltage signal and the first modulation control signal that are normalized based on the DC voltage signals for the buses and the representative DC voltage signal, and to modulate the inverter circuits responsive to respective ones of the second modulation control signals. The representative DC voltage signal may represent a maximum of the DC voltages for the respective DC buses.