Synchronous Motor Current Control for Field Weakening Stability
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Solution Overview
Problem
Conventional current control methods for synchronous motors fail to adequately perform field weakening during high-speed operations, leading to instability and heat generation due to inadequate d-phase current control, especially when the short-circuit current ratio is lower than the amplifier's maximum current, and do not consider voltage saturation effectively.
Innovation Solution
A current control device that calculates a provisional d-phase current command based on torque and load conditions, uses a voltage command vector from the previous sampling period to determine a target d-phase current command, and applies a correction value through a low-pass filter to adjust the d-phase current command, ensuring optimal field weakening and stability by accounting for voltage saturation and fluctuations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If d-phase current is increased to perform field weakening during high-speed operation, then motor terminal voltage is reduced below amplifier maximum voltage, but excessive d-phase current causes heat generation in the motor
Solution Approach 1:
The patent implements feedback control by using the voltage command output from the current control in the previous sampling period to calculate a correction value for the d-phase current command. The correction value is determined by passing the difference between the provisional d-phase current command and the target d-phase current through a low-pass filter, creating a feedback mechanism that dynamically adjusts d-phase current to prevent both voltage saturation and excessive heat generation
Solution Approach 2:
The patent applies dynamic adjustment by continuously updating the d-phase current command based on the voltage ratio calculated from the previous sampling period. The correction value calculation unit dynamically modifies the d-phase current command in response to changing operating conditions, allowing the system to adapt to varying load and speed conditions while maintaining optimal field weakening without excessive heat generation
2Reliability
If d-phase current is increased to prevent voltage saturation, then motor terminal voltage is maintained within amplifier limits, but inadequate d-phase current control leads to current control instability
Solution Approach 1:
The patent applies preliminary action by calculating the target d-phase current command in advance using the voltage command from the previous sampling period. This pre-calculation allows the system to proactively adjust the d-phase current before voltage saturation occurs, maintaining stable current control while preventing power loss due to voltage limiting
Solution Approach 2:
The feedback mechanism uses the voltage command output from the previous sampling period to continuously adjust the d-phase current command. By passing the difference between provisional and target d-phase current through a low-pass filter, the system provides continuous feedback that maintains voltage within amplifier limits while ensuring current control stability
3Device complexity
If conventional linear approximation is used to calculate d-phase current based on load, then calculation is simple, but excessive d-phase current flows causing heat generation
Solution Approach 1:
The patent improves upon simple linear approximation by adding a feedback mechanism that uses the voltage command from the previous sampling period to calculate a correction value. This feedback approach maintains relatively simple calculation while dynamically adjusting d-phase current to prevent excessive heat generation, avoiding the need for complex lookup tables or iterative calculations
Data Source
AI summary
A current control device of a synchronous motor comprises a provisional d-phase current command calculation unit; a voltage amplitude calculation unit calculating a magnitude of a voltage command vector in a previous sampling period; a voltage ratio calculation unit determining a voltage ratio between the magnitude of the voltage command vector and a maximum output voltage of an amplifier; a target d-phase current calculation unit obtaining a d-phase current command in the previous sampling period, and calculating a target d-phase current command from the voltage ratio and the d-phase current command; a correction value calculation unit determining a correction value by passing a difference between the provisional d-phase current command and the target d-phase current command through a low-pass filter; and an adder adding the correction value to the d-phase current command in the present sampling period to calculate a new d-phase current command.