Precharging Circuit Safety Control via Undervoltage Threshold
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Solution Overview
Problem
In systems with precharging circuits, when a precharging relay fails to close, the precharging resistor overheats, leading to undervoltage and potential ignition, causing damage to the frequency converter and posing safety risks during UL testing, with existing methods lacking reliable protection and resulting in permanent damage and high costs.
Innovation Solution
A safety control method that issues an error alarm and stops the system when the number of undervoltage occurrences reaches a predetermined threshold (m times) within a specific duration (τ seconds), using a first determining device to count undervoltage events and a second determining device to measure the duration, thereby preventing resistor damage and ignition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the precharging relay is manually opened to simulate fault situation, then the precharging resistor will become overheated, but the system safety cannot be ensured as the resistor may ignite surrounding materials
Solution Approach 1:
The control device performs preliminary detection of undervoltage conditions and counts the number of occurrences before the precharging resistor can overheat and cause ignition. By detecting undervoltage events (which indicate precharging relay failure) and comparing the count against a preset threshold, the system takes preventive action before the harmful thermal effect can materialize into ignition risk.
Solution Approach 2:
The system implements feedback control by continuously monitoring the precharging circuit voltage, detecting undervoltage conditions, counting their occurrences, and using this information to control the precharging relay. When the undervoltage count reaches the threshold, the control device opens the precharging relay to prevent further overheating and potential ignition, creating a closed-loop safety mechanism.
2Object-affected harmful factors
If the precharging relay is opened to prevent overheating, then ignition risk is reduced, but the frequency converter will be damaged permanently due to undervoltage
Solution Approach 1:
The control device opens the precharging relay in advance when the undervoltage occurrence count reaches the threshold, preventing the catastrophic failure of permanent system damage. This preliminary protective action sacrifices temporary operational continuity to avoid complete system failure and permanent damage from prolonged overheating or cotton ignition.
Solution Approach 2:
The system provides beforehand cushioning by implementing a threshold-based protection mechanism that accumulates undervoltage event counts and triggers protective relay opening before the thermal damage becomes irreversible. This cushioning mechanism absorbs the stress of repeated undervoltage events and prevents the system from reaching a catastrophic failure state.
3Object-affected harmful factors
If additional protection devices are added to prevent overheating, then system safety is improved, but the device complexity and cost increase
Solution Approach 1:
The control device performs multiple functions: it controls the precharging relay operation, detects undervoltage conditions, counts undervoltage occurrences, compares the count against thresholds, and triggers protective relay opening. By consolidating these diverse functions into a single control device rather than adding separate protection components, the system achieves comprehensive safety without proportionally increasing complexity.
Solution Approach 2:
The control device utilizes its existing voltage detection capability to monitor precharging circuit conditions and automatically count undervoltage events. The system serves itself by using its own monitoring resources to detect protection needs and trigger protective actions, eliminating the need for separate dedicated protection circuits or additional sensing hardware.
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 solution effectively prevents combustion and ensures system reliability by identifying precharging relay faults in real-time, avoiding permanent damage and reducing costs without requiring additional hardware, while ensuring compliance with UL testing standards.
Implementation Method 1
when precharging relay S1 can not be closed, during the period of charging and discharging of capacitor C1, due to voltage dividing of resistor R1, resistor R1 will become overheated
Data Source
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AI summary
The object of the present application is to ensure the safety of a device when a precharging relay is not closed and a load current passes through a precharging resistor, so as to ensure the customer's safety. Provided is a safety control method for a system with a precharging circuit, comprising: giving out an error alarm and stopping operating the system when the number of times of undervoltage of a precharging circuit achieves m times and the duration between the first undervoltage and the mth undervoltage is less than or equal to τ seconds, where m and τ are positive integers. Compared with the previous method, the present invention has the following advantages of no danger of combustion and high reliability; and cost saving and no need of adding additional hardware.