Phase Current Determination in PWM Inverter Drives
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Solution Overview
Problem
Existing methods for determining phase currents in rotating multiphase electrical machines fed by PWM-controlled inverters face challenges in accurately measuring rotor position at low speeds and are prone to noise, especially when using anisotropy-based methods without sufficient signal-to-noise ratio.
Innovation Solution
The method involves determining an evaluation direction for phase current measurements based on the direction with the greatest information content, optimizing the number of measurements in each passive switching state to improve signal-to-noise ratio, and using multiple A/D converters with time offsets to minimize noise and maximize information capture.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If phase currents are measured multiple times in sequence within one passive switching state (oversampling), then the signal-to-noise ratio is improved, but the measurement time and complexity increase
Solution Approach 1:
The patent applies preliminary action by pre-determining the evaluation direction based on injection voltages and operating conditions before performing current measurements. This allows the system to know in advance which phase currents need to be measured and how many times, avoiding unnecessary measurements and optimizing the use of limited passive switching state time.
Solution Approach 2:
The patent implements partial action by measuring only the necessary phase currents with the required number of samples based on the evaluation direction, rather than uniformly oversampling all phase currents. This selective approach improves signal-to-noise ratio for critical measurements while minimizing total measurement time and computational overhead.
2Quantity of substance
If phase currents are measured in both passive switching states, then more current measurements can be obtained, but the complexity of the measurement system increases
Solution Approach 1:
The patent applies local quality by assigning different measurement strategies to different phase currents based on their individual information content and the evaluation direction. Some phase currents are measured multiple times while others are measured fewer times or calculated from other measurements, optimizing the overall measurement efficiency without uniformly increasing system complexity.
Solution Approach 2:
The patent uses Kirchhoff's current law as an intermediary to calculate certain phase currents from other measured phase currents and the neutral point current. This reduces the number of direct measurements needed while still obtaining all necessary current information, thereby reducing measurement system complexity.
3Measurement precision
If injection voltages are used for rotor position determination, then rotor position can be determined at low speeds, but noise in current measurements increases
Solution Approach 1:
The patent converts the harmful effect of injection voltages causing noise into a beneficial approach by using intelligent measurement distribution. Instead of avoiding injection voltages, the system strategically measures phase currents during periods when injection voltages are applied, using the known injection patterns to extract rotor position information while compensating for the induced noise through selective sampling and averaging.
Solution Approach 2:
The patent implements dynamic measurement strategies where the number and timing of current measurements are adaptively adjusted based on the evaluation direction and operating conditions. This dynamic approach allows the system to optimize measurements during injection voltage periods, capturing necessary information while minimizing noise impact through real-time adaptation of measurement parameters.
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 enhances the accuracy of rotor position estimation by improving the signal-to-noise ratio and reducing noise in current measurements, allowing for more precise determination of phase currents and rotor position, even at low speeds.
Implementation Method 1
An inverter with pulse width modulation (PWM) is usually employed in highly dynamic drives to generate the adjustable phase voltage of the electrical machines
Implementation Method 2
a position-dependent current change is produced through the deliberate modification of the voltage vector, and this is detected by current measurements at suitable points in time. The voltages required here to adjust the manipulated variables are, for example, overlaid with a high-frequency voltage. The high-frequency current changes generated by the high-frequency voltage contain corresponding rotor position information.
Data Source
AI summary
The disclosure relates to a method for determining phase currents of a rotating multiphase electrical machine fed by means of a PWM-controlled inverter. In this case, injection voltages applied in at least one stipulated PWM period are determined. An evaluation direction for a phase current vector is also determined and a division of current measurements for the individual phase currents is determined on the basis of the evaluation direction. The phase currents are then determined on the basis of the previously determined division of the current measurements.