Permanent Magnet Motor Speed Control via Phase Offset Adjustment
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Permanent magnet motors experience undesirable periodic variations in the amplitude of the motor phase current waveform due to steady errors in speed and current PI controllers, leading to torque ripple and vibration, especially when the reference speed is not reached due to lower inverter DC bus voltage or higher load torque.
Innovation Solution
A method that compares the measured q-axis stator current to a reference q-axis stator current, adjusts the reference speed by setting it to a temporary value if the difference exceeds a threshold, and changes the phase offset angle between stator and rotor magnetic fields to prevent saturation of the current PI controller and reduce amplitude variations, thereby avoiding periodical variations in the phase current waveform.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If flux weakening is applied by increasing d-axis stator current to increase motor speed, then the motor speed increases, but periodical variations in the amplitude of the motor phase current waveform occur causing torque ripple and vibration
Solution Approach 1:
The patent employs feedback control by continuously monitoring the q-axis stator current and comparing it with the reference q-axis stator current. When the difference exceeds a threshold, the system adjusts the reference speed and modifies the phase offset angle to eliminate steady errors in the PI controllers, thereby preventing current waveform distortions and torque ripple while maintaining the desired motor speed.
Solution Approach 2:
The patent dynamically changes the phase offset angle between stator and rotor magnetic fields as a control parameter. By adjusting this phase offset angle in response to detected current errors, the system modifies the flux weakening effect to prevent current waveform distortions while maintaining the desired motor speed, thus resolving the contradiction between speed increase and torque ripple reduction.
2Speed
If the reference speed is set to application reference speed, then the motor operates at desired speed, but steady errors in speed and current PI controllers cause maximum reference q-axis current leading to controller saturation
Solution Approach 1:
The patent applies preliminary action by proactively detecting steady errors in the PI controllers through continuous monitoring of q-axis current differences. When errors are detected before they cause complete controller saturation, the system preemptively adjusts the reference speed and phase offset angle to prevent the harmful effects of saturation, thereby maintaining reliable operation.
Solution Approach 2:
The patent uses feedback control to continuously monitor the difference between measured and reference q-axis stator currents. When this difference exceeds a threshold indicating steady error, the system feeds back this information to adjust the reference speed and phase offset angle, preventing PI controller saturation and maintaining reliable motor control operation.
3Speed
If steady error exists in speed and current PI controllers, then the motor cannot reach reference speed, but periodical variations in phase current waveform amplitude occur causing undesirable vibration and noise
Solution Approach 1:
The patent employs feedback control to detect steady errors by continuously comparing measured q-axis stator current with reference q-axis stator current. When steady errors are detected that would cause current waveform variations, the system adjusts the phase offset angle and reference speed to eliminate these errors, thereby preventing vibration and noise while achieving the desired motor speed.
Solution Approach 2:
The patent modifies the phase offset angle between stator and rotor magnetic fields as a dynamic parameter to counteract steady errors in the PI controllers. By changing this parameter in response to detected current errors, the system eliminates the conditions that cause periodical variations in phase current waveform amplitude, thereby preventing vibration and noise while maintaining speed accuracy.
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 approach prevents saturation of the current PI controller and reduces torque ripple by smoothly adjusting the motor speed to match the reference speed while maintaining the current error close to the threshold value, thereby minimizing amplitude variations in the phase current waveform.
Implementation Method 1
This flux weakening is usually achieved by increasing the value of the d-axis stator current. However, if only this flux weakening is applied without adopting any other measures, a problem with possible variations in the amplitude of the motor phase current waveform can still arise
Data Source
Figure 1
Figure 2A
Figure 2B
AI summary
The present invention provides a method of controlling a permanent magnet motor (101). The permanent magnet motor may be a brushless direct-current (BLDC) motor or a permanent magnet synchronous motor (PMSM). The method comprises applying closed-loop control (301) to the speed of the motor to match it to a reference speed (ωref) by comparing a measured q-axis stator current of the motor with a reference q-axis stator current of the motor and deriving a difference therebetween. This difference represents the current error. The current error is compared (302) to a threshold value at which a phase current amplitude of the motor starts to exhibit distortions. If the current error is not greater than this threshold value, the reference speed (ωref) of the motor is set (303) equal to an application reference speed of the motor for its particular application and the closed-loop control is continued. If, on the other hand, the current error is greater than the threshold value, the reference speed (ωref) is set (304) equal to a temporary reference speed, which is then compared (305) to the application reference speed. If the temporary reference speed is not less than the application reference speed, the reference speed (ωref) is set (306) equal to the application reference speed and the closed-loop control is continued. If, on the other hand, the temporary reference speed is less than the application reference speed, a phase offset angle (theta) between stator and rotor magnetic fields of the motor is changed (307), in order to adjust the actual speed of the motor by flux weakening, before the closed-loop control continues. In this way, the actual speed of the motor may be made to approach the reference speed (ωref) without inducing periodical variations in the amplitude of the motor phase current waveform. The invention also provides a system for performing such a method and computer program product or a program code for executing such a method.