BLDC Phase Advance Control for Flat Back-EMF Zero-Crossing
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Solution Overview
Problem
BLDC motors with non-ideal, trapezoidal induced voltages experience instability due to a flat region around the zero-crossing point, causing issues with phase advance monitoring and leading to oscillations in motor speed.
Innovation Solution
The method involves detecting the flat region through direct measurement of induced voltage or measurement of freewheel current extinction time, allowing for increased phase advance to bypass nonlinearities and stabilize motor control, using a gradual increase in the Hcoef parameter and applying a fixed increment to the H_coef parameter when the flat region is detected.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If phase advance monitoring is performed using zero-crossing detection of induced voltage, then motor position can be identified, but instability and oscillations occur when induced voltages present a flat region near zero-crossing
Solution Approach 1:
The patent extracts the problematic flat region around zero-crossing from the induced voltage waveform and performs phase advance monitoring at a different electrical angle where the voltage waveform is linear and stable. This avoids the nonlinear and unpredictable effects in the zero-crossing region while maintaining accurate position identification.
Solution Approach 2:
The patent introduces an intermediary parameter (induced voltage at a specific electrical angle offset from zero-crossing) to mediate the phase advance monitoring process. Instead of directly measuring at the problematic zero-crossing point, the system uses this intermediary measurement point that provides stable and predictable voltage values for accurate position detection.
2Speed
If conduction angle is increased from 120 degrees to 150 degrees to extend rotation speed, then average voltage applied to motor increases, but zero-crossing detection becomes unreliable
Solution Approach 1:
The patent performs phase advance monitoring at a predetermined electrical angle before the zero-crossing point occurs. This preliminary action allows the system to determine motor position and adjust conduction angle while avoiding the unreliable zero-crossing region, enabling extended rotation speeds with maintained detection accuracy.
3Speed
If open loop motor drive is used to extend driving angle up to 180 degrees, then average voltage and rotation increase, but control becomes unstable when load angle exceeds 90 degrees
Solution Approach 1:
The patent implements a feedback mechanism using phase advance monitoring at a stable electrical angle to continuously track motor position and adjust control parameters. This feedback allows the system to operate at extended conduction angles (up to 180 degrees) while maintaining stability by detecting position changes and correcting control actions, preventing the instability that occurs when load angle exceeds 90 degrees in open-loop systems.
Data Source
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AI summary
The present invention refers to the use of phase advance in motors with non-ideal induced voltages, especially in motors with induced voltages that present a flat region, to avoid instability in the control, wherein the first step is to detect the passage of the voltage induced by the flat region ofinstability, and then applya fixed increment in the phase advance in order to avoid the flatregion. Detection can be done in two ways: by direct measurement of induced voltage; and measuring the extinguishing time of the freewheel current.