Motor Current Vector Control Under Flux-Weakening Limits
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Solution Overview
Problem
Existing motor control devices face challenges in increasing motor speed and torque output in regions where d-axis current is limited, leading to voltage saturation and inefficient magnetic-flux weakening control, which restricts torque generation and speed increase.
Innovation Solution
A motor control device that includes a current vector controller, q-axis current command generator, magnetic-flux weakening controller, current limiter, and reference voltage correction unit, which separates motor current into d-axis and q-axis components, limits d-axis current, and corrects reference voltage to enhance output voltage and reduce harmonic components during overmodulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If magnetic-flux weakening control is performed by increasing negative d-axis current to suppress voltage saturation, then induced voltage is reduced and speed can be increased, but reaction magnetic field causes irreversible demagnetization of permanent magnet and motor efficiency deteriorates
Solution Approach 1:
The patent changes the control parameter from d-axis current magnitude to voltage command magnitude. By controlling the voltage command magnitude to not exceed the reference voltage, the system achieves magnetic-flux weakening control without requiring excessive negative d-axis current, thereby preventing demagnetization and efficiency deterioration while still enabling speed increase.
Solution Approach 2:
The patent implements feedback control by comparing the voltage command magnitude with the reference voltage and adjusting the d-axis current command accordingly. The magnetic flux weakening controller generates a d-axis current command that ensures the voltage command does not exceed the reference voltage, creating a closed-loop control system that optimizes the balance between speed and efficiency.
2Reliability
If d-axis current command is increased to achieve magnetic-flux weakening control, then voltage saturation is suppressed, but the magnitude of combined current exceeds current limit and current limiter reduces q-axis current command
Solution Approach 1:
The patent performs preliminary control by adjusting the d-axis current command through the magnetic flux weakening controller before the current limiter acts on the combined current. By pre-controlling the voltage command magnitude to not exceed the reference voltage, the system prevents the need for aggressive current limiting, thereby maintaining torque generation capability while suppressing voltage saturation.
Solution Approach 2:
The patent changes the control approach from direct current magnitude control to voltage command magnitude control. This parameter change allows the system to achieve voltage saturation suppression while maintaining better control over the combined current magnitude, reducing the impact of current limiting on torque generation.
3Reliability
If q-axis current command is reduced by current limiter to maintain combined current within limit, then current magnitude is controlled, but the room for increasing d-axis current command is reduced and magnetic-flux weakening control effect is diminished
Solution Approach 1:
The patent performs preliminary adjustment of the d-axis current command through the magnetic flux weakening controller based on voltage command magnitude comparison. This preliminary action ensures that the d-axis current command is optimized before the current limiter processes the combined current, thereby maintaining magnetic-flux weakening control effectiveness while still controlling current magnitude within limits.
Solution Approach 2:
The patent implements feedback control where the magnetic flux weakening controller continuously monitors the voltage command magnitude and adjusts the d-axis current command accordingly. This feedback mechanism ensures that the d-axis current command is optimized in real-time, maintaining magnetic-flux weakening control effectiveness while coordinating with the current limiter to control combined current magnitude.
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
The solution allows for increased motor speed and output even under current limitations, preferentially performing magnetic-flux weakening control to minimize overmodulation regions and reduce harmonic components, ensuring stable current control and efficient torque generation.
Implementation Method 1
magnetic-flux weakening control is used in which a negative d-axis current is passed to weaken a magnetic flux generated by a permanent magnet and suppress the increase in the induced voltage
Implementation Method 2
an induced voltage generated during the drive of the motor increases in proportion to the drive speed
Data Source
Figure 1
Figure 2~3(c)
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AI summary
A motor control device controls a current of a motor based on a torque command, the current being separated into a d-axis current and a q-axis current orthogonal to the d-axis current, the torque command being a target value of a torque of the motor. The motor control device includes a current vector controller that receives input of a d-axis current command and a q-axis current command, and generates a d-axis voltage command and a q-axis voltage command, a difference between a value of the d-axis current and a value of the d-axis current command being zero, a difference between a value of the q-axis current and a value of the q-axis current command being zero, a q-axis current command generator that generates the q-axis current command based on the torque command, a magnetic-flux weakening controller that generates the d-axis current command based on a difference between a voltage command and a reference voltage, the voltage command being a vector with the d-axis voltage command output from the current vector controller as a d-axis component and the q-axis voltage command as a q-axis component, an amplitude of the voltage command not exceeding the reference voltage, a current limiter that limits a magnitude of the d-axis current command according to a magnitude of the q-axis current command, the d-axis current command being a d-axis component of a current command vector of the motor, the q-axis current command being a q-axis component of the current command vector of the motor, a magnitude of the current command vector of the motor not exceeding a current limit value, and a reference voltage correction unit that corrects the reference voltage based on a difference between a value of the d-axis current command before limitation and a value of the d-axis current command after the limitation.