Brushless Motor Control via High Ripple Rectification
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Solution Overview
Problem
Brushless motors require active power factor correction (PFC) circuits and high-capacitance DC link capacitors to achieve sinusoidal current drawn from the AC supply, which increases costs and physical size, and complicates control due to back EMF from permanent-magnet rotors.
Innovation Solution
A control method that rectifies alternating voltage with a ripple of at least 50%, exciting the motor winding until current exceeds a threshold proportional to the rectified voltage, and either freewheels or continues excitation with an overrun period, allowing for sinusoidal current waveform without active PFC or high-capacitance capacitors, and adjusts conduction periods based on motor speed and voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If an active power factor correction circuit is used to achieve sinusoidal current from AC supply, then the power factor is improved, but the cost and device complexity increase significantly
Solution Approach 1:
The patent extracts and eliminates the active power factor correction circuit from the control system, replacing it with a simplified approach that uses the inherent ripple from a passive rectifier to achieve sinusoidal current waveform without requiring complex active correction electronics
Solution Approach 2:
The patent changes the operating parameters by allowing high ripple (at least 50%) in the rectified DC voltage, which would traditionally be considered a defect to be eliminated. Instead, this ripple is exploited as a feature to naturally shape the AC supply current into a sinusoidal waveform, thereby improving power factor without active correction
2Power
If a high-capacitance DC link capacitor is used to provide regular feedback voltage, then the power factor is improved, but the physical size and cost increase
Solution Approach 1:
The patent removes the requirement for high-capacitance DC link capacitors by eliminating the active PFC circuit that depends on them. The system achieves good power factor performance using only a small capacitor sufficient for basic rectification, leveraging the controlled freewheeling process to maintain current continuity
Solution Approach 2:
The patent replaces expensive, large high-capacitance capacitors with a smaller, more economical capacitor that works in conjunction with the freewheeling diode and controlled excitation process. The system accepts higher voltage ripple in exchange for dramatically reduced capacitor size and cost
3Reliability
If the winding is immediately freewheeled when current exceeds threshold, then magnetic saturation is avoided, but motor acceleration may be insufficient at low speeds
Solution Approach 1:
The patent implements a dynamic control strategy with two distinct processes: immediate freewheeling for low speeds to prevent magnetic saturation, and overrun period freewheeling for higher speeds to maintain adequate current and improve acceleration. The system automatically transitions between these modes based on operating conditions
Solution Approach 2:
The patent applies preliminary action by continuing to excite the winding for an overrun period beyond when current first exceeds the threshold. This ensures that sufficient current is driven into the winding during the critical acceleration phase before freewheeling begins, preventing inadequate motor response
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method achieves a high power factor and low harmonic content in the current waveform, reducing the need for costly PFC circuits and large capacitors, while maintaining constant average power over a range of speeds and voltages, and ensuring compliance with harmonic standards.
Implementation Method 1
rectifying an alternating voltage to provide a rectified voltage having a ripple of at least 50%
Implementation Method 2
exciting a winding of the motor with the rectified voltage
Implementation Method 3
performs one of a first process and a second process in response to current in the winding exceeding a threshold, wherein the first process comprises freewheeling the winding
Data Source
AI summary
A method of controlling a brushless motor that includes rectifying an alternating voltage to provide a rectified voltage having a ripple of at least 50%, exciting a winding of the motor with the rectified voltage, and performing a first process or a second process in response to current in the winding exceeding a threshold that is proportional to the rectified voltage. The first process includes freewheeling the winding, while the second process includes continuing to excite the winding for an overrun period and freewheeling the winding at the end of the overrun period. Additionally, a control system that implements the method, and a motor system that incorporates the control system.