Inverter Control Unit for Stable Motor Restart
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Solution Overview
Problem
Medium voltage inverters face challenges in restarting electric motors stably due to measurement errors in output voltage, leading to inrush currents and malfunctions, especially when using passive elements like resistors for voltage detection.
Innovation Solution
The apparatus includes a control unit that determines a command voltage by adding an offset or using a proportion controller to compensate for measurement errors, ensuring a voltage larger than the residual voltage is applied to the electric motor during restart, preventing inrush currents and malfunctions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a voltage measuring device consisting of passive elements such as a resistor is employed to save cost, then device cost is reduced, but measurement precision deteriorates due to error in resistance
Solution Approach 1:
The control unit continuously monitors the measured output voltage and compares it with the commanded voltage. When a discrepancy is detected (indicating measurement error), the control unit automatically adjusts the commanded voltage to compensate for the error. This closed-loop feedback mechanism maintains measurement precision without requiring expensive passive elements.
Solution Approach 2:
The control unit dynamically changes the commanded voltage parameter based on the detected measurement error. By adjusting the voltage command in real-time, the system compensates for resistance errors in the passive measurement elements, thereby maintaining accurate voltage control despite using cost-effective components.
2Productivity
If voltage is applied according to voltage/frequency ratio during rotation of electric motor to reduce restarting time, then productivity is improved, but harmful factors increase due to large inrush current causing malfunction
Solution Approach 1:
The control unit uses feedback from the measured output voltage to determine the appropriate commanded voltage during restart. By continuously monitoring the actual voltage and comparing it with the desired voltage/frequency ratio, the control unit applies voltage smoothly without causing large inrush currents, thus preventing malfunction while maintaining fast restart capability.
Solution Approach 2:
The control unit dynamically adjusts the commanded voltage based on real-time measurements and motor state. Instead of applying fixed voltage/frequency ratio that may cause inrush current, the system adaptively changes voltage application according to actual motor conditions, enabling fast restart without harmful current spikes.
3Device complexity
If passive element voltage measurement is used to reduce cost, then device complexity is reduced, but reliability deteriorates due to voltage drop factors such as PWM and dead time in low switching frequency inverters
Solution Approach 1:
The control unit implements a feedback mechanism that continuously monitors the measured voltage and compares it with the commanded voltage. When discrepancies arise due to PWM effects or dead time in low switching frequency inverters, the control unit automatically compensates by adjusting the commanded voltage, thereby maintaining measurement reliability without increasing system complexity.
Solution Approach 2:
The control system performs self-correction by using its own measurement and control capabilities to compensate for errors introduced by PWM and dead time. The control unit automatically adjusts the voltage command based on detected errors, enabling the simple passive measurement system to maintain high reliability through intelligent self-compensation.
Data Source
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AI summary
Disclosed herein is an apparatus for controlling an inverter in an inverter system. The apparatus includes: a first determining unit (42) to determine a magnitude, a phase and a frequency of an input voltage to an electric motor in the inverter system; and a second determining unit (43) to determine a restart command voltage for generating an inverter driving voltage larger than a residual voltage in the electric motor using the magnitude of the input voltage determined by the first determining unit (42), when the inverter system is restarted.