Motor Control Device Common Mode Noise Reduction via PWM Phase Shifting
Solution Overview
Problem
Motor control devices for three-phase electric motors with two systems experience high-frequency common mode noise due to currents flowing through stray capacitance between the motor and frame ground, leading to noise radiation in the power supply lines.
Innovation Solution
A motor control device that includes a PWM count computation unit, a PWM count setting unit, and a common mode noise reduction unit, which cancels out currents flowing through stray capacitance by adjusting PWM counts in each phase, ensuring that currents from one phase cancel out those from other phases in each PWM cycle, thereby reducing common mode noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If PWM control is applied to a two-system three-phase electric motor, then motor control performance is improved, but common mode noise increases due to currents flowing through stray capacitance
Solution Approach 1:
The patent converts the harmful common mode noise currents into a beneficial effect by intentionally introducing opposite-polarity noise currents through coordinated PWM control of the two systems. The control unit adjusts PWM duty cycles to generate canceling noise currents that neutralize the original common mode noise, transforming the harmful electromagnetic interference into a useful noise-cancellation mechanism
Solution Approach 2:
The patent changes the PWM control parameters (duty cycles and switching timing) of the two systems to achieve noise cancellation. By dynamically adjusting these parameters, the control unit ensures that noise currents from one system oppose and cancel the noise currents from the other system, thereby reducing overall common mode noise while maintaining motor control performance
2Power
If two-system drive circuits are used to control a two-system motor, then motor drive capability is enhanced, but the frequency of common mode noise generation increases
Solution Approach 1:
The patent introduces asymmetry in the PWM control parameters between the two systems to achieve noise cancellation. By deliberately creating asymmetric duty cycles and switching patterns, the noise currents generated by each system have opposite characteristics that enable them to cancel each other, thereby reducing common mode noise frequency while preserving the enhanced motor drive capability provided by the two-system configuration
3Manufacturing precision
If PWM switching is performed in each current control cycle, then current control precision is improved, but noise radiation in power supply lines increases
Solution Approach 1:
The patent converts the harmful noise radiation caused by frequent PWM switching into a beneficial noise-cancellation effect. By coordinating the switching actions of the two systems, the control unit generates opposite-polarity noise currents that cancel each other, transforming the harmful electromagnetic radiation into a useful mechanism for reducing overall noise while maintaining precise current control
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 reduces common mode noise by ensuring that currents through stray capacitance in both systems cancel each other out, minimizing noise radiation in the power supply lines.
Implementation Method 1
a pulse width modulation unit that generates first U-phase, V-phase, and W-phase PWM signals and second U-phase, V-phase, and W-phase PWM signals based on first and second three-phase voltage command values
Implementation Method 2
a current flows through a stray capacitance that is present between the three-phase electric motor and a frame ground at the time of rise and the time of fall of output voltages
Implementation Method 3
a common mode noise reduction unit that reduces common mode noise generated in a motor control device that controls a two-system three-phase electric motor
Data Source
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AI summary
A common mode noise reduction section performs a first noise reduction process in a first system, a second noise reduction process in a second system and a third noise reduction process in the case where the two systems have a phase, a current for which that flows through a stray capacitance is not canceled out in at least one PWM cycle in a current control cycle after the first noise reduction process is performed and the second noise reduction process is performed. The third noise reduction process includes shifting a PWM count for each phase in the first system by a first predetermined amount and shifting a PWM count for each phase in the second system by a second predetermined amount such that the currents for such a phase which flow through the stray capacitances in the two systems cancel out each other in the relevant PWM cycle.