Motor Control Device Common Mode Noise Reduction
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Solution Overview
Problem
Motor control devices for two-system multi-phase electric motors experience high-frequency common mode noise due to currents flowing through stray capacitance between the motor and frame ground, which radiates noise through power supply lines, especially in electric power steering systems.
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 adjusts PWM counts in each phase to cancel out currents through stray capacitance by inverting the output voltage waveform of one phase in another system within the same PWM cycle, without changing the total PWM count, thereby reducing common mode noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If PWM control is applied to two-system multi-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 applies preliminary anti-action by computing PWM counts in advance for multiple PWM cycles within a current control cycle, and by inverting output voltage waveforms of phases from different systems to preemptively cancel out noise currents before they flow through the stray capacitance to ground. This prevents common mode noise generation rather than treating it after occurrence.
Solution Approach 2:
The patent merges the control of two-system multi-phase motor coils by coordinating PWM signals across different systems. Specifically, it combines the output voltage waveforms from phases of different systems and inverts them to work together in canceling noise currents, thereby achieving noise reduction while maintaining motor control performance.
2Object-generated harmful factors
If output voltage waveforms are inverted to cancel noise currents, then common mode noise is reduced, but PWM control complexity increases
Solution Approach 1:
The patent applies preliminary action by pre-computing PWM counts for each phase in each system for multiple PWM cycles within a current control cycle. This advance computation simplifies the real-time control process by preparing the PWM count data beforehand, reducing the computational burden during actual PWM signal generation despite the complexity of waveform inversion.
3Object-generated harmful factors
If PWM counts are adjusted to invert output voltage waveforms, then noise current cancellation is achieved, but total PWM count must remain constant
Solution Approach 1:
The patent applies parameter changes by adjusting PWM counts for specific phases in one system while compensating with corresponding changes in other phases or systems to maintain the total PWM count constant. This allows the output voltage waveform inversion needed for noise cancellation while preserving the overall motor control characteristics through parameter coordination.
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
Effectively reduces common mode noise by canceling out noise currents through stray capacitance, minimizing noise radiation in power supply lines and improving the operational efficiency of electric power steering systems.
Implementation Method 1
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 (phase voltages)
Data Source
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AI summary
An electric motor that is controlled by a motor control device has two-system multi-phase motor coils. The motor control device includes a common mode noise reduction unit that changes a PWM count in a PWM cycle for at least one phase in one of two systems such that a current that flows through a stray capacitance because of an output voltage for one phase in the other system is canceled out with a current that flows through the stray capacitance because of an output voltage for the at least one phase in the one system in at least one PWM cycle in a current control cycle.