BLDC Power Tool Braking with Two-Stage Commutation and Coasting
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Solution Overview
Problem
Existing methods for braking brushless DC motors in power tools are either time-consuming, potentially dangerous, cause premature wear, or risk damaging electrical components due to high currents.
Innovation Solution
A two-stage braking method that alternately commutates and coasts the motor to reduce speed, followed by actuating all high or low side switches to completely stop the motor, preventing high bus voltage and current damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the BLDC motor is allowed to coast to a stop, then electrical switches and circuit components are not damaged, but the stopping time is excessive and safety risks increase
Solution Approach 1:
The braking process is divided into two distinct stages: Stage 1 uses alternating commutation and coasting to rapidly reduce speed from high to low, while Stage 2 uses all switches actuated to complete the stop. This segmentation allows the system to achieve fast stopping without subjecting electrical components to damaging high currents throughout the entire braking process.
Solution Approach 2:
The controller dynamically adjusts the commutation pattern based on motor speed. At higher speeds, the motor is commutated to slow down rapidly; at lower speeds, the motor is allowed to coast. This dynamic adjustment optimizes the braking process at different stages, achieving fast stopping while protecting electrical components.
2Reliability
If brushless motor commutation is used to slow the motor to a stop, then electrical components are protected, but high bus voltage capacitance is required and motor components experience premature wear
Solution Approach 1:
Instead of continuously commutating the motor throughout the braking process, the system applies commutation partially - only during Stage 1 when the motor is at higher speeds. During Stage 2 and during coasting periods in Stage 1, commutation is reduced or stopped. This partial application of commutation achieves sufficient braking while avoiding the need for excessive bus voltage capacitance and reducing motor component wear.
3Speed
If all high or low side switches are actuated to stop the motor, then the motor stops quickly, but high currents flow through electrical switches causing damage
Solution Approach 1:
The braking process is divided into two stages with different switch actuation strategies. In Stage 1, switches are alternately actuated during commutation periods and left open during coasting periods, preventing continuous high current flow. In Stage 2, all switches are actuated only when the motor has already been slowed to low speed, minimizing the energy dissipation and current stress on switches.
Solution Approach 2:
The controller performs preliminary speed reduction through alternating commutation and coasting in Stage 1 before actuating all switches in Stage 2. This preliminary action reduces the motor speed and kinetic energy beforehand, so that when all switches are actuated, the resulting currents are significantly lower and less damaging to electrical components.
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
Rapidly brings the motor to a stop without damaging electrical components, improving usability, lifespan, and efficiency.
Implementation Method 1
a motorized ratchet wrench, drills, sanders, and drivers, driven by brushless DC (BLDC) motors
Implementation Method 2
allowing the BLDC motor to coast to a stop
Data Source
AI summary
A two-stage braking method for a brushless DC (BLDC) motor. The method includes control a switching array to drive the BLDC motor according to a commutation scheme to decrease a speed of the BLDC motor, allow the BLDC motor to coast, determine the speed of the BLDC motor, and actuate high side switches and/or low side switches to substantially stop rotational movement of the BLDC motor.