AC Rotary Machine Current Detection Under High PWM Modulation
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Solution Overview
Problem
Conventional motor control devices and frequency converters for AC motors face issues with current detection precision due to switching noise, particularly when the modulation factor exceeds 90%, leading to uneven heat generation and performance deterioration in electric power steering systems.
Innovation Solution
A control device for AC rotary machines and electric power steering systems that includes a DC power supply, AC rotary machine with n-phase windings, a current detector, and a power converter with high and low potential side switching elements. The control unit calculates voltage commands and outputs ON/OFF signals based on a carrier wave, allowing current detection at multiple timings to avoid errors caused by switching noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the ON time of the low potential side switching element is set to be shorter than the phase current detection time to secure current detection timing, then switching noise is prevented from becoming intermixed in the phase current values, but the modulation factor is limited to 90% or below resulting in uneven heat generation
Solution Approach 1:
The patent dynamically adjusts the current detection timing based on the modulation factor. When the modulation factor exceeds 90%, the system shifts the current detection timing to a different phase period, allowing high modulation factors while maintaining accurate current detection without switching noise interference.
Solution Approach 2:
The patent changes the detection timing parameter adaptively. Instead of fixing the detection timing, the system modifies the timing based on the modulation factor value, enabling operation across a wider modulation factor range while preserving current detection precision.
2Adaptability or versatility
If the duty of the intermediate phase is adjusted to exceed Dth to enable operation at modulation factors above 100%, then the modulation factor range is expanded, but current detection precision deteriorates due to switching noise from the intermediate phase
Solution Approach 1:
The patent implements dynamic timing adjustment where the current detection timing is adaptively changed based on the modulation factor. When operating above 100% modulation factor, the system detects currents during phases where switching noise from the intermediate phase does not interfere, thus maintaining detection precision while enabling extended modulation range.
Solution Approach 2:
The system preliminarily determines the appropriate detection timing based on the modulation factor before performing current detection. This preliminary timing selection ensures that detection always occurs during noise-free periods, preventing precision deterioration even when operating at high modulation factors.
3Measurement precision
If the PWM output shifts before and after to advance the carrier wave, then current detection precision is improved, but voltage harmonics are generated causing noise and vibration in sound-sensitive applications
Solution Approach 1:
The patent converts the potential harm of fixed timing limitations into a benefit by adaptively selecting detection timings based on modulation factor. This approach achieves high current detection precision without requiring PWM output shifts, thereby avoiding voltage harmonic generation while maintaining detection accuracy.
Data Source
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AI summary
A control device for an AC rotary machine includes an AC rotary machine having m sets of n-phase windings, a current detector, a power converter, and a control unit that calculates voltage commands on the basis of respective differences between a current command for the AC rotary machine and current detection values obtained by the current detector, and outputs ON/OFF signals to high potential side switching elements and low potential side switching elements of the power converter by comparing applied voltages calculated on the basis of the voltage commands with a carrier wave signal, wherein the current detector, when detecting currents flowing through the n-phase windings on the basis of currents flowing through current detection resistance elements that are inserted in series into the low potential side switching elements, obtains current detection values at two or more fixed timings over a single period of the carrier wave signal.