Brushless Motor PWM Control for Torque Pulsation
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Solution Overview
Problem
Electrically powered tools with brushless motors experience torque pulsation and noise due to pulsating voltage from small smoothing capacitors, leading to harmonic components in the current, which increases the load on power transmission equipment and requires high-processing microcomputers for PWM control.
Innovation Solution
An electrically powered tool with a rectifier circuit, a smoothing circuit using a small capacitor, and a controller that maintains continuous current during no-load rotation and intermittent current during load application, reducing PWM duty ratio to minimize torque pulsation and harmonic components, and using an inexpensive microcomputer for control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a large capacity smoothing capacitor is used to smooth the pulsating DC voltage, then the voltage is sufficiently smoothed, but the current flows from the rectifier circuit to the smoothing capacitor only during a short period, causing harmonic components in the current and increasing the load on power transmission equipment
Solution Approach 1:
The patent changes the capacity parameter of the smoothing capacitor from large to small, which fundamentally alters the voltage smoothing characteristic and current flow pattern. This parameter change allows the current to flow for a longer period, reducing harmonic components while accepting some voltage pulsation that is then managed through motor control strategies.
2Object-generated harmful factors
If a small capacity smoothing capacitor is used to reduce harmonic components, then the current flows for a longer period reducing harmonics, but the voltage supplied from the smoothing capacitor to the motor largely pulsates, causing torque pulsation and noise
Solution Approach 1:
The patent ensures continuous current flow to the motor by coordinating the rectifier circuit operation with the motor control. The controller adjusts the inverter circuit to maintain continuous current even when the smoothing capacitor voltage pulsates, thereby eliminating torque pulsation and noise while keeping the capacitor small to reduce harmonics.
Solution Approach 2:
The patent uses feedback control where the controller monitors the motor operation and adjusts the inverter circuit accordingly. This feedback mechanism allows the system to compensate for voltage pulsations from the small smoothing capacitor, maintaining stable motor torque and preventing noise while preserving the harmonic reduction benefit.
3Volume of stationary object
If the capacity of the smoothing capacitor is set to be relatively small to save accommodating space, then the rectifier circuit and smoothing capacitor can be built in the electrically powered tool, but the voltage supplied from the smoothing capacitor to the motor largely pulsates, causing torque pulsation
Solution Approach 1:
The patent replaces the mechanical approach of using a large capacitor for torque stabilization with an electrical control approach. The controller and inverter circuit electronically manage the motor current to maintain stable torque despite the small capacitor's voltage pulsations, achieving space savings without sacrificing torque stability.
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 minimizes noise from the power transmission mechanism and reduces harmonic components in the current, allowing for stable torque and efficient operation with less expensive hardware.
Implementation Method 1
a rectifier circuit configured to rectify an AC voltage supplied from a commercial power supply to a pulsating DC voltage
Implementation Method 2
a smoothing circuit having a smoothing capacitor configured to reduce a pulsation state of the pulsating DC voltage
Implementation Method 3
a brushless motor, a rotational position of a rotor is detected using a magnetic sensor and then controlled by a controller to supply a drive current from an inverter circuit to a predetermined winding of the motor
Data Source
Figure 1
Figure 2~3
Figure 4(1)~4(4)
AI summary
A rectifier circuit for rectifying an AC voltage supplied from a commercial power supply into a DC voltage, a smoothing capacitor for smoothing the output of the rectifier circuit, and an electrically powered tool for supplying the smoothed pulsating DC voltage to a motor by an inverter circuit, wherein when the motor is driven while no load is being applied to a tip tool, the smoothed DC voltage repeatedly generates a maximal value and a minimum value so as to synchronize with the fluctuation in the AC voltage. The duty ratios 161-166 of the PWM control when no load is being applied are restricted so as to be low, so that an interval in which the current is zero does not occur in the vicinity of the minimum value in the fluctuation. During no-load rotation, control is performed so that driving is performed at a value that is less than a threshold value D1 for the duty ratio at which the generation of a gear sound starts in any of the speed dial setting values 1-6. It is thereby possible to suppress a gear sound generated by a power transmission mechanism during no-load rotation.