Three-Phase Brushless Motor Drive Phase Current Detection
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Solution Overview
Problem
Existing motor drive devices equipped with low-spec microcomputers cannot perform phase current detection for three-phase brushless motors, as they can only conduct one A/D conversion per PWM carrier cycle, limiting the application of existing phase current detecting methods.
Innovation Solution
A motor drive device and phase current detecting method that calculates a phase current for one phase based on a current sensor output and estimates currents for the remaining phases using a peak value, motor angle, and phase shift, allowing for phase current determination with a single A/D conversion per PWM control cycle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If two current detection periods are set within one PWM carrier cycle to detect two current values, then phase current detection accuracy is improved, but the number of A/D conversions increases to two per PWM carrier cycle
Solution Approach 1:
The patent extracts only the essential information needed for phase current detection by setting one detection timing per PWM carrier cycle instead of two, reducing A/D conversion frequency while maintaining detection capability through strategic timing selection
Solution Approach 2:
The patent changes the detection timing parameter from two periods per PWM carrier cycle to one period per PWM carrier cycle, adjusting the detection strategy to match the capabilities of low-spec microcomputers while preserving phase current detection functionality
2Loss of information
If two A/D conversions are performed per PWM carrier cycle to determine three-phase currents, then complete phase current information is obtained, but hardware complexity and processing load increase
Solution Approach 1:
The patent applies self-service by using the single detected current value along with electrical angle information to calculate and derive the remaining phase current values through computational methods, allowing the system to obtain complete three-phase current information from a single detection point
Solution Approach 2:
The patent introduces electrical angle information as an intermediary element that bridges the gap between single-point current detection and complete three-phase current determination, enabling accurate phase current calculation without requiring multiple direct measurements
3Productivity
If one A/D conversion is performed per PWM carrier cycle to reduce processing load, then hardware requirements are simplified, but phase current detection accuracy may be compromised
Solution Approach 1:
The patent performs preliminary action by selecting the optimal detection timing based on the PWM carrier cycle characteristics, ensuring that the single A/D conversion occurs at the most informative moment to maximize the utility of the limited measurement
Solution Approach 2:
The patent employs periodic action by synchronizing the single A/D conversion with the PWM carrier cycle period, performing detection at regular intervals that align with the AC power supply frequency to maintain accurate phase current measurement
Data Source
AI summary
The present invention relates to a motor drive device and a phase current detecting method for a three-phase brushless motor, and more particularly to a technique of detecting each phase current through A/D conversion that is carried out once per PWM carrier cycle, by use of a current sensor for detecting a DC bus current of an inverter. The control unit of the present invention calculates a phase current for one phase, an absolute phase current value of which becomes maximum at a detection timing that is set once per PWM control cycle, from an output of the current sensor, and estimates phase currents for the remaining two phases based on a peak value of a phase current detected at each detection timing, a motor angle, and a phase shift of an actual phase current waveform relative to the motor angle.