AC Drive Common-Mode Current Reduction via PWM Feedback Adjustment
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Solution Overview
Problem
Existing power conversion systems, particularly AC motor drives, face challenges in reducing common mode currents, which can lead to adverse effects such as bus pump-up and component damage, and previous solutions are either ineffective in high resistive ground systems or excessively bulky and expensive.
Innovation Solution
The implementation of pulse width modulation techniques that adjust feedback signals based on common mode dwell time to minimize differences, using a switch control system with a common mode reduction component to generate compensated feedback signals for inverter switching control, thereby reducing common mode currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If common mode capacitors are used from DC bus to solid system ground reference, then common mode voltage is held down and bus pump-up is prevented, but this approach cannot be used in high resistive ground (HRG) systems or floating systems
Solution Approach 1:
The patent introduces an artificial neutral point as an intermediary reference that does not require solid ground connection. This neutral point is created through the inverter switching network and serves as the reference for common mode voltage control, enabling the system to operate in HRG and floating ground configurations without requiring direct ground connection
Solution Approach 2:
The patent changes the reference parameter from solid ground to an artificial neutral point with controlled impedance. By modifying how the reference is established (from fixed ground to dynamically controlled neutral point), the system becomes adaptable to different ground configurations including HRG and floating systems
2Object-affected harmful factors
If drive output filters are used, then adverse effects of common mode currents are addressed, but the filters are bulky and expensive
Solution Approach 1:
The patent replaces the mechanical/filter-based solution (bulky output filters) with an electronic control solution. By using modified PWM switching sequences and space vector modulation techniques, the common mode currents are controlled through electronic switching patterns rather than physical filtering components
Solution Approach 2:
The patent extracts the common mode current reduction function from the output filter and relocates it to the inverter control system. The control system directly addresses common mode issues at the source through switching sequence modification, eliminating the need for separate filtering hardware
3Productivity
If SVPWM is used at low modulation index, then power conversion is achieved, but high common mode currents are generated due to very small active vector dwell time
Solution Approach 1:
The patent applies preliminary anti-action by pre-planning switching sequences that avoid conditions leading to high common mode currents. The control algorithm proactively selects switching patterns that maintain adequate active vector dwell time even at low modulation indices, preventing the generation of harmful current spikes before they occur
Solution Approach 2:
The patent makes the switching sequence dynamic by adapting it based on the modulation index and operating conditions. The control system dynamically selects from multiple switching sequence options to optimize the balance between power conversion efficiency and common mode current reduction across different operating ranges
Data Source
Figure 1A
Figure 1B
Figure 1C~1D
AI summary
Control systems, methods and power conversion systems are presented for controlling common mode currents in power converters, in which feedback signals used in pulse width modulated inverter switching control are selectively adjusted to ensure minimum differences between the resulting common mode compensated feedback signals according to a common mode dwell time in each pulse width modulation period, with the common mode dwell time being adjusted according to modulation index.