UPS Relay Soft Start Circuit for Unbalanced Three-Phase Input
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Solution Overview
Problem
Conventional UPS systems face challenges in safely turning on relays without damage from impact currents, especially when input voltages are unbalanced, as they often rely on zero-crossing points or voltage difference control, which can result in unreliable relay activation due to unbalanced three-phase voltages.
Innovation Solution
A soft start circuit for UPS relays is designed with a three-phase pre-charging module, Power Factor Correction (PFC) modules, voltage detection modules, and a relay control module that detects input and bus voltages to safely turn on relays by selecting the appropriate phase with the lowest voltage amplitude for soft start, ensuring reduced impact currents and improved reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the relay is turned on near the zero-crossing point of input voltage through the soft start circuit, then the impact current is reduced, but the relay cannot be ensured to be turned on at the zero-crossing point due to different operation time periods
Solution Approach 1:
The patent applies preliminary action by pre-charging the capacitor through the pre-charging module before the relay is activated. The capacitor is charged to a voltage close to the input voltage through the pre-charging resistor and diode, so that when the relay closes, the voltage difference is minimized, reducing impact current while ensuring reliable activation regardless of relay timing variations
Solution Approach 2:
The patent uses a pre-charging module as an intermediary between the input voltage and the relay. This module includes a pre-charging resistor, diode, and capacitor that work together to gradually charge the capacitor before relay closure, acting as a buffer that reduces the harmful impact current while maintaining system reliability
2Reliability
If the relay is turned on by controlling the voltage difference across the relay in the reliable range through a soft start circuit, then the relay activation is reliable, but the voltage across the relay cannot be ensured to be within the normal range when input three-phase voltage is unbalanced
Solution Approach 1:
The patent employs feedback by continuously monitoring the capacitor voltage through the detection module and comparing it with the input voltage. The control module adjusts the pre-charging process based on this feedback, ensuring that the voltage difference across the relay remains within a safe range even when input three-phase voltage is unbalanced, preventing overvoltage stress on the relay
Solution Approach 2:
The patent applies dynamics by making the pre-charging resistance adjustable based on detected voltage conditions. The control module dynamically adjusts the pre-charging parameters according to the real-time voltage state, allowing the system to adapt to unbalanced input voltages and maintain relay voltage within safe operating ranges
3Object-affected harmful factors
If a pre-charging module is added to charge the capacitor before relay activation, then the impact current is reduced, but the circuit complexity increases
Solution Approach 1:
The patent achieves universality by designing the pre-charging module using components that can serve multiple functions. The capacitor serves both as a filter capacitor for the input voltage and as the energy storage element for the pre-charging process. The diode prevents reverse current while also protecting the pre-charging circuit. This multi-functionality reduces the need for additional dedicated components, thereby limiting the increase in circuit complexity
Data Source
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AI summary
A soft start circuit of a UPS relay includes a three-phase pre-charging module, a first PFC module, a second PFC module, a relay module, a three-phase input voltage detection module, a bus voltage detection module and a relay control module. The relay control module is electrically connected to the three-phase input voltage detection module and the bus voltage detection module and controls the connection of the relay module, to control part of relays in the relay module to be turned on based on the A-phase input voltage, the B-phase input voltage, the C-phase input voltage and the bus voltage to soft start the relay. The relay can be safely turned on without being damaged by an impact current in a case that an input voltage is unbalanced.