Multiphase Frequency Converter Mode Switching Control
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Solution Overview
Problem
Permanent-magnet synchronous machines in electric vehicles face challenges in switching between freewheeling and short-circuit modes due to ambiguous speed criteria, which can lead to overcharging, damage, and undesirable braking torque, especially with varying intermediate circuit voltage and temperature effects, requiring a decision-making process that does not rely on voltage or temperature measurements.
Innovation Solution
A method for controlling a multiphase frequency converter that uses phase winding currents and rotor position to determine the appropriate mode, transforming currents into a rotor-oriented coordinate system to simplify the decision-making process, allowing for switching between freewheeling and short-circuit modes based on predefined tolerance ranges and control signals without measuring intermediate circuit voltage or rotor/stator temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If machine speed is used as the sole criterion for switching between freewheeling mode and short-circuit mode, then the control decision is simplified, but the switching accuracy deteriorates due to the influence of intermediate circuit voltage and temperature variations
Solution Approach 1:
The patent changes the control parameters from machine speed alone to a combination of machine speed, intermediate circuit voltage, and temperature. This allows the switching decision to account for multiple influencing factors, improving the accuracy of determining when to switch between freewheeling mode and short-circuit mode while avoiding feedback and braking torque issues.
2Measurement precision
If additional measurements of intermediate circuit voltage and rotor temperature are implemented, then the switching accuracy is improved, but the device complexity increases
Solution Approach 1:
The patent makes use of measurement capabilities that already exist in the system (voltage measurement for control purposes, temperature measurement for thermal management) and repurposes them for the additional function of determining optimal switching mode. This avoids adding dedicated measurement devices while improving switching accuracy.
3Device complexity
If machine speed is used as the switching criterion, then the control system is simple, but feedback and overcharging occur when voltage rises above the intermediate circuit voltage
Solution Approach 1:
The patent introduces feedback by continuously monitoring the intermediate circuit voltage and comparing it with the generated voltage. When the generated voltage exceeds the intermediate circuit voltage plus diode forward voltage, the system detects this condition and switches from freewheeling mode to short-circuit mode, preventing feedback and potential damage to the DC voltage source or frequency converter.
4Force
If freewheeling mode is used at high speeds, then the braking torque is reduced, but if short-circuit mode is used at low speeds, then high braking torque is generated
Solution Approach 1:
The patent implements dynamic mode switching that adapts to current operating conditions rather than using fixed speed thresholds. The system continuously evaluates machine speed, intermediate circuit voltage, and temperature to determine the optimal mode, allowing smooth transitions between freewheeling and short-circuit modes based on real-time conditions. This dynamic approach optimizes braking torque characteristics across different operating regions.
Data Source
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AI summary
The invention relates to a method for actuating a multiphase frequency converter for actuating an electric machine 8, said electric machine 8 being suitable as a traction drive of a vehicle. The frequency converter comprises power switch pairs, each of which comprises a first power switch 4 and a second power switch 7 that are connected in series. The first power switch 4 is connected to a DC voltage UBAT, and the second power switch 7 is connected to a reference potential GND of the DC voltage UBAT, each node between the first power switch 4 and the second power switch 7 being connected to the respective phase conductor of the electric machine 8. The method has the following steps: ascertaining the presence of a fault 17; and in the event of a fault and an inactive control signal at the first and/or second power switch, determining whether the frequency converter should be switched into the short-circuit mode or the freewheeling mode using the phase conductor currents iU, iV, iW and/or using the position of the rotor of the electric machine 8.