UPS Relay Soft-Start Circuit for Unbalanced Three-Phase Input
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Solution Overview
Problem
Conventional UPS relays face damage from impact currents due to unbalanced input voltages, as they either miss the zero-crossing point or exceed voltage limits, leading to unreliable operation.
Innovation Solution
A soft start circuit for UPS relays incorporating a three-phase pre-charging module, PFC modules, voltage detection, and relay control to ensure safe relay activation by balancing input voltages and controlling relay engagement based on detected voltage levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the relay is turned on near the zero-crossing point of input voltage through the soft start circuit, then the impact current damage is avoided, but the relay action time variability causes unreliable turning on at zero-crossing point
Solution Approach 1:
The patent applies preliminary action by pre-charging the capacitor through the pre-charging module before relay activation. The control module monitors the voltage across the capacitor and only activates the relay when the pre-charging voltage reaches a predetermined threshold, ensuring the relay turns on at the optimal moment regardless of action time variability. This preliminary voltage buildup prepares the circuit in advance to guarantee reliable zero-crossing point activation.
2Reliability
If the relay is turned on by controlling the voltage difference across the relay in the reliable range through the soft start circuit, then the relay activation is controlled, but the voltage across the relay cannot be ensured to be within normal range when input three-phase voltage is unbalanced
Solution Approach 1:
The patent implements feedback by continuously monitoring the voltage across the capacitor through the detection module and using this information to control relay activation. The control module compares the detected voltage with the predetermined threshold and adjusts the activation timing accordingly. This feedback mechanism ensures that the relay is activated only when the voltage conditions are suitable, adapting to unbalanced three-phase voltage conditions and maintaining reliable operation.
3Object-affected harmful factors
If conventional soft start circuits are used, then relay protection is attempted, but impact currents still damage the relay under unbalanced input voltage conditions
Solution Approach 1:
The patent applies beforehand cushioning by introducing a pre-charging module that gradually charges a capacitor before relay activation. This pre-charging process cushions the voltage transition and limits inrush current by building up voltage gradually through the capacitor rather than applying full voltage abruptly. The predetermined voltage threshold ensures the relay activates only after this cushioning process has prepared the circuit, protecting against impact currents even under unbalanced voltage conditions.
Data Source
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.