Brushless Motor Control Unit Duty Cycle Management
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Solution Overview
Problem
In electric tools with brushless motors, turning off the manipulation switch can lead to high counter-electromotive voltage being applied to the switching element due to rotor inertia, potentially shortening the switching element's lifespan and causing breakage.
Innovation Solution
A control unit that gradually lowers the duty of the driving signal to a predetermined value and then to 0, allowing the brushless motor to stop through natural deceleration, thereby reducing the risk of high counter-electromotive voltage and extending the switching element's life.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of stationary object
If the inverter circuit is turned off to allow natural deceleration, then the switching element lifetime is extended, but the rotor inertia causes high counter-electromotive voltage that can break the switching element
Solution Approach 1:
The control unit performs preliminary action by detecting when the manipulation switch is turned off and proactively controlling the switching element duty cycle before the motor stops. It maintains duty at a predetermined value (greater than 0) for a set period after switch-off, preventing the generation of high counter-electromotive voltage that would occur if the circuit were immediately turned off, thus protecting the switching element while allowing controlled deceleration
Solution Approach 2:
The system dynamically adjusts the duty cycle of the switching element based on the operational state. When the manipulation switch is turned off, the duty is maintained at a predetermined value rather than immediately set to 0 or 100%, creating a dynamic transition state that balances motor deceleration with protection against counter-electromotive voltage, optimizing both lifetime and safety
2Speed
If braking is actively performed when the manipulation switch is turned off, then the rotating tool stops rapidly, but the user cannot perform tasks using inertia
Solution Approach 1:
The control unit dynamically adjusts the duty cycle based on operational needs. By maintaining duty at a predetermined value (not 0) after the manipulation switch is turned off, the system allows the motor to continue running at a reduced level, enabling users to complete tasks using inertia while still progressing toward natural deceleration, thus balancing stopping speed with operational flexibility
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 extends the lifespan of the switching element, suppresses breakage, and maintains good operability by allowing tasks to be completed using inertia after the switch is turned off while preventing excessive stress on the switching element.
Implementation Method 1
a high counter-electromotive voltage occurring in a stator coil due to rotation of a rotor caused by inertia after the manipulation switch has been turned off
Implementation Method 2
the brushless motor stops through natural deceleration if the duty is 0
Data Source
Figure 1~3(B)
Figure 4
Figure 5
AI summary
In order to provide an electric tool with good operability that can extend the life of and suppress breakage of a switching element for controlling energization of a brushless motor, a control unit (50), after an operation switch (5) is turned off, does not immediately turn off an inverter circuit (43) (does not set the duty of a PWM signal to 0), and instead normally operates the inverter circuit (43) with a low-duty PWM signal and waits until a rotation speed of a brushless motor (6) reaches a predetermined rotation speed or less to turn off the inverter circuit (43) (to set the duty of the PWM signal to 0).