Single-Shunt Motor Current Measurement With PWM Window Compensation
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Solution Overview
Problem
Conventional single-shunt current measurement techniques for three-phase motors face challenges in forming current measurement windows without distorting phase voltage, leading to signal distortion and inaccurate current measurements.
Innovation Solution
Adjusting the duration of current measurement windows within pulse width modulation periods using a time delta value to ensure a minimum time for accurate current measurement, allowing for measurement of two out of three motor phases and calculating the third phase using Kirchhoff's law, with compensation periods to correct for voltage distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If current measurement windows are extended to obtain accurate current measurements, then measurement precision is improved, but phase voltage distortion increases
Solution Approach 1:
The patent dynamically adjusts the duration of current measurement windows based on the PWM period and motor operating conditions. The measurement window duration is calculated as a proportion of the PWM period, allowing the system to adapt measurement timing to different operating speeds and load conditions, thereby achieving accurate measurements without fixed window extensions that cause voltage distortion
Solution Approach 2:
The patent changes the timing parameters of current measurement windows relative to PWM switching edges. By positioning measurement windows at specific intervals after PWM edges and adjusting their duration as a fraction of the PWM period, the system obtains accurate current measurements while avoiding the voltage distortion that occurs during PWM transitions
2Stability of the object's composition
If current measurement windows are shortened to avoid voltage distortion, then phase voltage stability is improved, but measurement precision deteriorates
Solution Approach 1:
The system dynamically determines measurement window duration as a proportion of the PWM period, allowing shorter windows at high speeds where voltage distortion is less problematic, and longer windows at low speeds where more measurement time is available, thus maintaining both voltage stability and measurement precision across different operating conditions
Solution Approach 2:
The patent performs preliminary calculation of optimal measurement window parameters before actual current measurement. The controller pre-determines the appropriate measurement window duration and positioning based on the current PWM period and operating conditions, ensuring that measurements are taken at optimal times without causing voltage distortion
3Measurement precision
If measurement windows are adjusted to meet minimum measurement time, then measurement precision is improved, but PWM period duration must be extended
Solution Approach 1:
The patent makes the measurement window duration dynamic relative to the PWM period, setting it as a specific proportion (e.g., 0.2 to 0.5 of the PWM period). This allows the measurement window to automatically scale with the PWM period, ensuring sufficient measurement time without requiring fixed extensions of the PWM period across different operating conditions
Solution Approach 2:
The system uses periodic PWM switching with measurement windows positioned at specific phases of each PWM cycle. By utilizing the periodic nature of PWM operation and placing measurement windows at optimal points within each cycle, the system achieves adequate measurement time through efficient use of the existing PWM period rather than extending it
Data Source
AI summary
Methods and apparatus for controlling a three-phase electric motor by determining a duration of current measurement windows within a first pulse width modulation period for measuring motor current using a single shunt coupled to the motor. A duration of a first one of the current measurement windows can be adjusted by a time delta value to meet a minimum time to measure the motor current. A next PWM period can include compensation for the time delta value. In embodiments window adjustments are made by modifying the high state of a middle phase signal for phase signals to switches that energize the motor.