Delta Wound Brushless Motor Current Control for Demagnetization Prevention
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Solution Overview
Problem
Conventional delta wound three-phase permanent magnet brushless motors using a single current sense resistor experience inaccuracies in current measurement due to recirculating currents, leading to potential demagnetization of permanent magnets and excessive thermal stress, especially in the second and fourth quadrants of the motor speed versus torque curve.
Innovation Solution
A method that determines a transition motor speed above which the motor controller reduces the requested phase current using a current control modification manner, allowing for accurate phase current control through closed-loop control with the single current sense resistor, thereby mitigating demagnetization and thermal stress risks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional motor controllers use a single current sense resistor to measure phase current, then the complexity and expense of motor current sensing is reduced, but the current measurement becomes inaccurate due to recirculating currents, leading to potential demagnetization and excessive thermal stress
Solution Approach 1:
The controller determines transition motor speed and establishes current control modification manner before actual motor operation in problematic quadrants. This preliminary configuration allows the system to proactively adjust requested phase current when entering second or fourth quadrants above transition speed, preventing measurement inaccuracies from causing demagnetization or thermal stress while maintaining single resistor simplicity
Solution Approach 2:
The controller dynamically changes the requested phase current parameter based on motor operating quadrant and speed. When motor speed exceeds transition speed in second or fourth quadrants, the controller modifies the requested phase current to account for recirculating current effects, thereby maintaining accurate effective current control despite using only a single current sense resistor
2Speed
If the motor operates in the second and fourth quadrants at high speed, then the motor can continue to increase motor speed while motor current/torque decreases, but the inaccurate current measurement causes excessive thermal stress and potential demagnetization
Solution Approach 1:
The controller implements closed-loop feedback by continuously monitoring measured phase current from the single sense resistor and comparing it against the modified requested phase current. When operating in second or fourth quadrants above transition speed, the feedback mechanism allows the controller to detect actual current conditions and adjust control signals to prevent excessive thermal stress and demagnetization while maintaining high speed operation
Solution Approach 2:
The controller applies preliminary anti-action by reducing the requested phase current before recirculating currents can cause harmful effects. By detecting when the motor enters second or fourth quadrants above transition speed, the controller preemptively modifies the current command to counteract the inevitable measurement inaccuracies, preventing demagnetization and thermal stress before they occur
Data Source
AI summary
Method for operating a delta wound three-phase permanent magnet brushless motor. A transition speed is determined in a quadrant of a speed versus torque curve above which speed continues to increase while current/torque decreases wherein actual phase current can be controlled in a current control modification manner which reduces requested phase current. The modification manner is determined for a quadrant, wherein a controller can use the modification manner to reduce the requested phase current to control the actual phase current when the speed is above the transition speed. The motor is controlled in a quadrant wherein, when the motor has a speed above the transition speed, the controller reduces the requested phase current in the modification manner and the controller supplies the actual phase current to the motor using the reduced requested phase current and using measured phase current of the motor derived from using a single current sense resistor.