Motor Control Device Common Mode Noise Cancellation
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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, particularly in electric power steering systems, leading to noise radiation through power supply lines.
Innovation Solution
A motor control device with two drive circuits for three-phase motor coils, each with a phase difference of 60 degrees, 180 degrees, or 300 degrees, where the switching elements are controlled using specific patterns to synchronize the PWM signals across systems, effectively canceling out common mode noise by inverting the waveform of one system relative to the other.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If two-system drive circuits are used to control three-phase electric motor, then motor control capability is improved, but common mode noise increases due to currents flowing through stray capacitance
Solution Approach 1:
The patent applies preliminary anti-action by inverting the PWM waveform of one system relative to the other system. This creates opposite switching actions that generate equal and opposite common mode currents, which cancel each other out before radiating as noise. The waveform inversion is performed in advance through the PWM count computation units that calculate complementary PWM signals for the two systems.
Solution Approach 2:
The patent converts the harmful common mode noise into a beneficial cancellation effect. By deliberately creating two systems with opposite PWM waveforms, the harmful stray capacitance currents are transformed into useful counteracting forces. The noise that would normally be generated by each system is converted into a canceling mechanism where the same stray capacitance becomes part of the solution rather than the problem.
2Measurement precision
If PWM switching frequency is increased to improve motor response, then control precision is improved, but common mode noise frequency increases leading to more noise radiation
Solution Approach 1:
The patent applies preliminary anti-action by pre-calculating inverted PWM counts for the two systems. The PWM count computation units generate complementary switching signals before the actual switching occurs, ensuring that high-frequency switching improves control precision while the inverted waveforms simultaneously generate opposing common mode currents that cancel noise radiation.
3Adaptability or versatility
If long power supply lines are used in electric power steering system, then system adaptability is improved, but noise current intrusion into power supply lines increases
Solution Approach 1:
The patent converts the long power supply lines from being a noise propagation path into a configuration where the two-system inverted PWM approach creates equal and opposite noise currents. These opposing currents cancel each other out along the power supply lines, transforming the previously harmful long-line configuration into a noise-canceling structure.
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 approach significantly reduces common mode noise by ensuring that currents flowing through stray capacitances in both systems are opposite in direction, thereby canceling each other out, thereby minimizing noise radiation and improving motor control efficiency.
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
Implementation Method 2
Consequently, common mode noise is generated
Data Source
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AI summary
A first PWM count computation unit computes a first PWM count for each of three phases in a first system. A second PWM count computation unit computes a second PWM count for each of three phases in a second system. Upper and lower switching elements for each phase in the first system are controlled in accordance with a first pattern in which the upper and lower switching elements are varied in the order of an upper on state, a lower on state, and the upper on state from the time of start of PWM cycles. Upper and lower switching elements for each phase in the second system are controlled in accordance with a second pattern in which the upper and lower switching elements are varied in the order of a lower on state, an upper on state, and the lower on state from the time of start of PWM cycles.