Power Tool Motor Control With Small DC Link and Back-EMF Limiting
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Solution Overview
Problem
Existing power tools face issues with poor power factor and high harmonic content when using a large DC link, and torque ripple and increased ohmic power loss when using a small DC link, due to conventional motor control methods.
Innovation Solution
A power tool with a small DC link that follows the rectified mains voltage, using a control unit to calculate a torque curve with corresponding half-waves and adjust current values based on induced back EMF to reduce torque ripple and maintain a good power factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If the DC link is dimensioned large to maintain constant voltage, then the torque curve becomes nearly constant and phase currents are stable, but the power factor deteriorates and harmonic content in input current increases
Solution Approach 1:
The patent applies dynamics by transitioning from a static, constant DC link voltage to a dynamic voltage that follows the rectified mains voltage profile. The control unit dynamically adjusts the motor control parameters based on the instantaneous DC link voltage, which varies in sync with the mains voltage. This dynamic approach allows the system to maintain good power factor while avoiding excessive harmonic content, as the motor current is continuously adapted to the available voltage rather than demanding a constant voltage supply.
Solution Approach 2:
The patent changes the key parameter of DC link voltage from constant to variable, specifically designing it to follow the rectified mains voltage. This parameter change fundamentally alters the motor control strategy, requiring the control unit to continuously adjust torque and current references based on the varying voltage level. This approach resolves the contradiction by accepting voltage variation as a means to improve power factor and reduce harmonics, while compensation through adaptive control maintains acceptable torque delivery.
2Object-generated harmful factors
If the DC link is dimensioned small to follow the rectified mains voltage, then the power factor improves, but torque ripple increases and ohmic power loss in motor windings increases
Solution Approach 1:
The patent implements feedback by continuously monitoring the DC link voltage and using this information to adjust the motor control parameters. The control unit measures the instantaneous voltage and feeds this information back into the control algorithm, which then adapts the current references and torque commands accordingly. This feedback mechanism allows the system to operate efficiently with a small DC link by continuously compensating for voltage variations, thereby reducing ohmic losses compared to uncontrolled operation.
Solution Approach 2:
The patent applies preliminary action by proactively adjusting the motor control parameters before torque ripple and excessive current can occur. The control unit anticipates voltage variations by synchronizing the motor control cycle with the mains voltage frequency, and pre-adjusts current references to prevent problematic current peaks. This preliminary adjustment of control parameters based on expected voltage behavior reduces ohmic power loss and minimizes torque ripple.
3Object-generated harmful factors
If the DC link is dimensioned small to follow the rectified mains voltage, then the power factor improves, but braking torques and torque ripple occur when back EMF exceeds DC link voltage
Solution Approach 1:
The patent applies dynamics by implementing a control system that continuously adapts to the varying DC link voltage. When the back EMF exceeds the DC link voltage, the control unit dynamically adjusts the motor phase currents to prevent reverse current flow that would cause braking torque. This dynamic control approach, rather than a fixed control strategy, allows the system to maintain smooth torque delivery while operating with a small, voltage-following DC link that provides good power factor.
Solution Approach 2:
The patent applies preliminary anti-action by proactively preventing the condition that would lead to braking torques. The control unit monitors the relationship between back EMF and DC link voltage and, before reverse current can flow, adjusts the motor control parameters to maintain current flow in the correct direction. This preemptive control action prevents torque ripple and braking effects before they occur, while still allowing the DC link to follow the mains voltage for good power factor.
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
Achieves a good power factor of at least 0.70 or 0.75, reduces torque ripple and ohmic power loss, and allows for a smaller electric motor design.
Implementation Method 1
a rectifier arrangement (12) with an intermediate circuit (15) for providing an intermediate circuit voltage (V2) based on a mains voltage (V1)
Implementation Method 2
the induced back EMF of the motor becomes greater than or equal to the DC link voltage, preventing current from flowing into the motor
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
The invention relates to an electric tool (1), in particular a hand-held electric tool, e.g. a polisher (1A), a grinder and/or a saw (1B), comprising a tool (2), an electric motor (3) for driving the tool (2), and a control unit (4) for controlling the electric motor (3), the electric tool (1) being designed to be connected to a mains voltage (V1) and comprising a rectifier assembly (12) with a DC link (15) for supplying a DC link voltage (V2) on the basis of the mains voltage (V1), wherein the DC link voltage (V2) includes a plurality of successive voltage half-waves (16), the control unit (4) is configured to provide a torque curve (28) for controlling the electric motor (3), said torque curve (28) comprising a torque half-wave (36) for each voltage half-wave (16), and the control unit (4) is configured to ascertain a back electromotive force (34) induced in the electric motor (3), and if the back electromotive force (34) is greater than or equal to the DC link voltage (V2), to reduce a current torque value of the torque curve (28).