Brushless Motor Capacitor Sizing for Voltage Smoothing Stability
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Solution Overview
Problem
Conventional power tools with small capacitors in rectifying and smoothing circuits experience non-energization periods due to insufficient voltage smoothing, leading to unstable motor rotation speeds, resulting in uneven cut surfaces during operations like sawing.
Innovation Solution
A power tool configuration that includes a brushless motor, a supplying circuit with a rectifying circuit, a capacitor, and a switching circuit controlled by a controller to maintain a smoothed voltage always greater than or equal to the induced voltage, ensuring stable rotation speeds through constant-number-of-rotation control, and includes capacitors with a capacitance of at least 500 microfarads for efficient voltage regulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If a small capacitor is used in the rectifying and smoothing circuit, then the device size is reduced, but the voltage smoothing becomes insufficient causing non-energization periods and unstable motor rotation
Solution Approach 1:
The patent changes the capacitance parameter from a small value to a large value (500μF or more) to ensure sufficient voltage smoothing. This parameter change eliminates non-energization periods by maintaining capacitor voltage above the motor's induced voltage throughout the AC cycle, thereby preventing unstable motor rotation while accepting the increased capacitor size.
2Reliability
If a large capacitor is used in the rectifying and smoothing circuit, then voltage smoothing is sufficient, but the device size increases
Solution Approach 1:
The patent specifies a minimum capacitance value of 500μF as the optimal parameter that provides sufficient voltage smoothing while minimizing capacitor size. This parameter setting ensures the capacitor can maintain voltage above the motor's induced voltage without excessive size increase, representing a balanced optimization point.
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 ensures stable motor rotation speeds, preventing uneven cut surfaces by maintaining a consistent driving voltage, even under varying loads, and effectively limits current to prevent damage, ensuring reliable performance.
Implementation Method 1
The rectifying circuit is configured to rectify an alternating voltage and output a rectified voltage
Implementation Method 2
The capacitor is configured to smooth the voltage inputted via the rectifying circuit
Implementation Method 3
The brushless motor is configured to drive and rotate when a voltage applied to the brushless motor is larger than or equal to an induced voltage
Data Source
AI summary
A power tool includes a brushless motor, a supplying circuit, and a controller. The brushless motor is configured to drive and rotate when a voltage applied to the brushless motor is larger than or equal to an induced voltage. The supplying circuit is configured to apply a driving voltage to the brushless motor. The controller is configured to control the supplying circuit. The supplying circuit includes a rectifying circuit, a capacitor, and a switching circuit. The rectifying circuit is configured to rectify an alternating voltage and output a rectified voltage. The capacitor is configured to smooth the voltage inputted via the rectifying circuit. The switching circuit is configured to perform a switching operation based on a PWM signal to adjust a period during which the driving voltage is applied. The controller is configured to set a duty ratio within a prescribed range, and output the PWM signal of the set duty ratio to the switching circuit to control the switching operation. The controller is configured to perform a constant-number-of-rotation control for controlling the brushless motor to rotate at a target rotation number by changing the duty ratio. The capacitor has a capacitance allowing a smoothed voltage to be always larger than or equal to the induced voltage during the constant-number-of-rotation control.