Brushless DC Motor Speed Control for Electric Power Tools
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Solution Overview
Problem
In electric power tools with brushless DC motors using permanent magnets, the number of rotations abnormally increases as the motor temperature rises, leading to user discomfort and degraded usability due to decreased magnetic force.
Innovation Solution
A control unit detects the motor's number of rotations and temperature, shifting drive control to rotation control when the upper limit is reached, using a bonded magnet rotor and setting control amounts based on operation and temperature thresholds to prevent excessive rotation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If PWM drive control is used to increase motor speed, then productivity is improved, but the motor rotates abnormally fast when temperature rises, causing user discomfort
Solution Approach 1:
The control unit continuously monitors the motor's actual rotation speed and compares it with the target speed. When the motor temperature rises and causes speed deviation, the feedback mechanism detects this and automatically adjusts the PWM duty ratio to maintain the target speed, preventing abnormal high-speed rotation and ensuring user comfort.
Solution Approach 2:
The system dynamically changes the PWM duty ratio parameter based on motor temperature and actual speed feedback. When temperature rises causing magnetic force degradation, the control unit adjusts the duty ratio to compensate for the speed increase, thereby maintaining stable motor operation within acceptable speed limits.
2Power
If permanent magnet rotor is used to maintain magnetic force, then motor performance is improved, but magnetic force decreases when temperature rises, causing abnormal rotation
Solution Approach 1:
The control unit monitors motor temperature and rotation speed continuously. When temperature rises and magnetic force decreases, the feedback system detects the resulting speed deviation and adjusts the drive control accordingly, compensating for the temperature-induced magnetic force degradation.
Solution Approach 2:
The system changes the drive control parameters (PWM duty ratio) in response to temperature changes. When rotor temperature increases and permanent magnet magnetic force decreases, the control unit adjusts parameters to maintain stable motor performance and prevent abnormal rotation.
3Productivity
If high operation amount is set to maximize motor output, then productivity is improved, but motor speed exceeds upper limit, degrading usability
Solution Approach 1:
The control unit continuously compares actual motor speed with the upper limit speed threshold. When high operation amount causes speed to approach or exceed the upper limit, the feedback mechanism activates rotation control to reduce speed back to the target value, ensuring reliable speed control and maintaining usability.
Solution Approach 2:
The system dynamically switches between drive control and rotation control modes based on real-time speed conditions. When motor speed approaches the upper limit during high operation, the control unit transitions to rotation control mode, making the control system adaptive and ensuring speed remains within reliable boundaries.
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 solution effectively suppresses abnormal motor rotation increases due to temperature rises, improving usability and preventing rotor detachment by maintaining controlled motor speed and reducing magnetic force degradation.
Implementation Method 1
since a permanent magnet is used as a rotor in the brushless DC motor, when the electric power tool is used for a long time or at short intervals and the temperature of the rotor rises, magnetic force of the permanent magnet constituting the rotor is decreased
Data Source
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AI summary
An electric power tool (1) includes a motor (4) with a rotor (5) having a permanent magnet; a control unit (46) that sets a control amount of the motor (4) in accordance with operation of an operation portion (21a) and that drive-controls the motor (4) based on the control amount; and a number-of-rotations detection unit (50) that detects the number of rotations of the motor (4). During driving of the motor (4), when the number of rotations detected by the number-of-rotations detection unit (50) has reached an upper limit number of rotations set in advance, the control unit (46) shifts drive control of the motor (4) to rotation control, in which the control amount is made to be increased or decreased so that the number of rotations of the motor (4) becomes a predetermined target number of rotations.