Isolating Current Limiter for Inrush Current Reduction
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Solution Overview
Problem
Inrush currents caused by voltage sags in electronic equipment are not effectively limited by thermistors, as they disengage after startup, leaving equipment vulnerable to large current surges when voltage returns to normal.
Innovation Solution
A current limiting circuit that monitors power voltage and adds impedance during a voltage sag, timing the removal of this impedance at a zero crossing or when the power voltage returns to nominal, minimizing inrush currents by ensuring the impedance is removed before significant voltage differentials occur.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thermistor is used to limit inrush current during startup, then inrush current is limited when the thermistor is cool, but after startup the thermistor is heated and the inserted resistance is reduced, so it no longer functions as a current inrush limiter and cannot protect against current surges after voltage sags
Solution Approach 1:
The patent introduces a relay as an intermediary component that works in conjunction with the thermistor. The relay is controlled by a control circuit that detects voltage sags and triggers the relay to insert or remove impedance elements. This intermediary mechanism extends the protection duration beyond what the thermistor alone can provide, as the relay can actively respond to voltage sags occurring at any time after startup, not just during the thermistor's cool phase.
Solution Approach 2:
The patent employs a control circuit that continuously monitors the power voltage and provides feedback to determine when voltage sags occur. Based on this feedback, the control circuit activates the relay to insert impedance during detected voltage sags, even after the thermistor has heated up. This feedback mechanism ensures ongoing protection against inrush currents caused by voltage sags throughout the equipment's operation, not just during startup.
2Reliability
If impedance is continuously inserted to protect against inrush currents, then inrush currents are limited, but the equipment cannot operate normally during voltage sags due to the inserted impedance
Solution Approach 1:
The patent uses a dynamic impedance insertion/removal mechanism controlled by a relay and control circuit. Instead of continuously inserting impedance, the system dynamically inserts impedance only when voltage sags are detected and removes it when normal operation is detected. This dynamic approach allows the equipment to operate normally during typical voltage variations while providing protection only when necessary, resolving the contradiction between continuous protection and normal operation.
Solution Approach 2:
The control circuit periodically monitors the power voltage to detect voltage sags and triggers periodic impedance insertion only during detected sag events. This periodic, event-driven action ensures that impedance is present to limit inrush currents during voltage sags while being absent during normal operation, allowing equipment to function normally between sag events.
3Reliability
If a relay is used to control impedance insertion, then inrush currents can be limited by inserting impedance during voltage sags, but the relay may not respond fast enough to the sudden voltage return to prevent inrush currents
Solution Approach 1:
The patent implements preliminary action by having the control circuit continuously monitor voltage conditions and prepare for rapid impedance insertion upon detecting a voltage sag. The system is in a ready state with the relay poised to insert impedance immediately when a sag is detected, rather than waiting for the sag to fully develop. This preliminary monitoring and preparedness enables faster overall response to voltage sags and their subsequent returns to normal, reducing the window for inrush currents.
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
Effectively limits inrush currents by strategically removing impedance at the optimal point in the power cycle, preventing damage from sudden voltage returns and reducing the risk of inrush surges, thus protecting electrical components.
Implementation Method 1
the impedance is removed from the load when the power voltage has reached a predefined point in the power voltage cycle after the power voltage has returned to the nominal voltage, such as at approximately a zero crossing
Implementation Method 2
a thermistor might be operated to inject an impedance such as a resistance into a power circuit to limit the inrush current when the thermistor is cool at startup of the electronic equipment. However, after startup, a thermistor is heated, thereby reducing the inserted resistance
Data Source
AI summary
Various systems and methods are provided for minimizing an inrush current to a load after a voltage sag in a power voltage. In one embodiment, a method is provided comprising the steps of applying a power voltage to a load, and detecting a sag in the power voltage during steady-state operation of the load. The method includes the steps of adding an impedance to the load upon detection of the sag in the power voltage, and removing the impedance from the load when the power voltage has reached a predefined point in the power voltage cycle after the power voltage has returned to a nominal voltage.