Brushless Power Tool Inverter Control for Pulsating DC Drive
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Solution Overview
Problem
Conventional electric power tools with large capacity smoothing capacitors deteriorate the power factor of AC power, and while power-factor improvement circuits can address this, they increase tool size and cost, whereas tools with small capacity smoothing capacitors produce pulsation voltages that hinder motor operation.
Innovation Solution
The electric power tool employs a small capacity smoothing capacitor that generates pulsation voltages with maximum voltage higher than the induced voltage and minimum voltage lower than the induced voltage, allowing the motor to operate by cyclically driving it in sections where the pulsation voltage is higher, and controls the inverter circuit to prevent energy reversal into the capacitor, thus improving power factor without a power-factor improvement circuit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a large capacity smoothing capacitor is used, then the motor can operate stably, but the power factor of AC power deteriorates
Solution Approach 1:
The invention extracts the harmful function of the large smoothing capacitor (power factor deterioration) while retaining its useful function (motor operation stability). By removing the capacitor and replacing it with a small capacity alternative, the harmful effect is eliminated while the useful effect is maintained through control circuitry that compensates for the reduced capacitance.
Solution Approach 2:
The invention changes the parameter of capacitor capacity from large to small, and introduces control parameters (duty cycle adjustment) to compensate. The control circuit adjusts the duty cycle of the inverter circuit based on the pulsation voltage characteristics, enabling stable motor operation with a small capacity capacitor that does not deteriorate power factor.
2Object-generated harmful factors
If a power-factor improvement circuit is added, then the power factor is improved, but the size and cost of the electric power tool increase
Solution Approach 1:
The invention extracts and removes the power-factor improvement circuit entirely, achieving power factor improvement through an alternative approach. By eliminating this separate circuit and integrating the solution into the existing inverter control, the tool size and cost are reduced while still achieving the desired power factor improvement.
Solution Approach 2:
The inverter circuit is given multiple functions: it not only controls motor operation but also inherently improves power factor through its control strategy. The control circuit adjusts the duty cycle to utilize pulsation voltage characteristics, making the inverter circuit serve both motor control and power factor improvement functions simultaneously.
3Object-generated harmful factors
If a small capacity smoothing capacitor is used, then the power factor is improved, but pulsation voltages are generated that hinder motor operation
Solution Approach 1:
The invention converts the harmful pulsation voltages generated by the small capacity capacitor into a beneficial effect. Instead of filtering out the pulsation voltages, the control circuit adjusts the duty cycle to utilize these pulsations, enabling the motor to operate stably while maintaining good power factor characteristics.
Solution Approach 2:
The invention employs periodic duty cycle adjustment that synchronizes with the pulsation voltage frequency. By periodically adjusting the duty cycle in response to the pulsation voltages, the control system ensures continuous stable motor operation while leveraging the pulsation characteristics rather than being hindered by them.
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 configuration enables improved power factor without increasing tool size or cost, while preventing damage to the smoothing capacitor and ensuring continuous motor operation through inertia, albeit with controlled inverter circuit operation.
Implementation Method 1
a rectifier circuit 10, and a smoothing capacitor 11. When a trigger switch 3 is operated, AC voltage outputted from a commercial power source 2 is rectified and smoothed by the rectifier circuit 10 and the smoothing capacitor 11
Implementation Method 2
a smoothing capacitor 11 having small capacity, by design, that outputs pulsation voltage having maximum voltage larger than the induced voltage and minimum voltage smaller than the induced voltage
Implementation Method 3
pulsation voltage having maximum voltage larger than the induced voltage and minimum voltage smaller than the induced voltage
Data Source
Figure 1
Figure 2(a)~2(c)
Figure 3
AI summary
An electric power tool includes: a brushless motor having a plurality of stator windings and configured to rotate in accordance with voltages applied to the plurality of stator windings, an induced voltage being generated in accordance with a rotation of the brushless motor; a rectifier circuit configured to rectify an AC voltage; a smoothing capacitor configured to smooth the AC voltage rectified by the rectifier circuit to a pulsation voltage having a maximum value larger than the induced voltage and a minimum value smaller than the induced voltage; and an inverter circuit configured to perform switching operations to output the pulsation voltage to the plurality of stator windings by rotation.