Cycle-By-Cycle Current Limiting in Corded Brushless Power Tools
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Solution Overview
Problem
Handheld power tools, particularly those with brushless electric motors, often exceed the rating of the AC power outlet's circuit breaker during high-demand tasks, leading to tripping and a need for increased power output without complex and expensive power conversion circuits.
Innovation Solution
A power tool with a controller that enforces current limits by comparing instantaneous current measures to a predetermined limit, interrupting motor switches during excessive current conditions and resuming current flow at the end of a fixed time interval, and using capacitors to manage DC bus voltage and current, allowing for efficient operation within AC power source limits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the power output of handheld power tools is increased to meet high-demand tasks, then the tool can perform more demanding work, but the current demand exceeds the circuit breaker rating causing the breaker to trip
Solution Approach 1:
The patent applies periodic action by using pulse-width modulation (PWM) to switch the motor power supply in controlled pulses rather than continuous operation. The controller switches between full power and reduced power states in periodic cycles, allowing the tool to deliver high peak power when needed while maintaining average power within circuit breaker ratings. This enables the tool to perform demanding tasks intermittently without tripping the breaker.
Solution Approach 2:
The patent implements dynamics by making the motor power supply variable and adjustable rather than fixed. The controller dynamically adjusts the duty cycle of PWM signals to vary the average power delivered to the motor, enabling the system to adapt between high-power demand modes and low-power conservation modes based on task requirements and circuit breaker constraints.
2Power
If complex power conversion circuits are used to increase power output, then the power output can be increased, but the circuits become complicated and expensive
Solution Approach 1:
The patent applies self-service by using the existing brushless DC motor controller to also perform power conversion and current regulation functions. Rather than adding separate complex power conversion circuits, the controller utilizes its existing PWM capability and current sensing to directly regulate power delivery from the rectified AC input, making the system self-sufficient and avoiding additional complexity.
Solution Approach 2:
The patent implements universality by making the motor controller perform multiple functions: it controls motor speed and torque as usual, while also functioning as a power converter and current regulator to manage AC input power. This multi-functionality eliminates the need for dedicated power conversion circuits, reducing overall system complexity while achieving the desired power output increase.
3Reliability
If the current limit is enforced continuously without time intervals, then the current is strictly controlled within limits, but the power output is reduced and the tool cannot meet high-demand tasks
Solution Approach 1:
The patent applies periodic action by enforcing current limits in periodic cycles rather than continuously. The controller allows current to exceed the average rating during brief intervals within each PWM cycle, then enforces limits during other intervals. This periodic enforcement pattern maintains reliability over time while permitting high-power bursts during task-demanding moments.
Solution Approach 2:
The patent implements preliminary action by pre-setting the PWM duty cycle and time interval parameters to anticipate and manage current demand patterns. The controller is configured with predetermined switching frequencies and duty cycles that prepare the system to deliver appropriate power levels in advance, ensuring current limits are met while maximizing power output within those constraints.
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
The solution effectively increases power output of handheld power tools while preventing circuit breaker tripping, ensuring safe and efficient operation within the AC power source limits, improving power factor and reducing harmonic content in AC current.
Implementation Method 1
the capacitor has a nominal capacitance in the range of 200-400 micro Farads for tool having a current rating of approximately 10 Amps
Data Source
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AI summary
A handheld AC power tool is provided. The power tool is comprised generally of: a brushless DC motor; a power cord connectable to an AC power socket; a converter circuit configured to receive input power from the power cord and operable to output a DC bus voltage, a switching arrangement interposed between the electric motor and the converter circuit; a motor drive circuit interfaced with the motor switches; and a power switch operable by a user to selectively energize the motor drive circuit and thereby power on the tool. The converter circuit includes a rectifier and a capacitor electrically coupled across the rectifier, such that the capacitor has capacitance sized to produce a DC bus voltage whose magnitude from an AC power source is substantially same as magnitude of voltage from a DC power source.