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There are four main components in any online double conversion uninterruptible power supply (UPS) system: Rectifier; UPS Batteries; Inverter; and Static Bypass Switch.  

Rectifier  

The rectifier carries out several key functions. The first is to convert the input power from AC (Alternating Current) to DC (Direct Current). Its second main role is to recharge the batteries, while the DC power routes to the inverter too. 

Depending on the size of the UPS, the rectifier module may incorporate the battery charger. With smaller uninterruptible power supply systems (i.e. below 3 kVA) it is not uncommon for the rectifier and charge to be separate.  

UPS rectifiers can accept wide input voltage fluctuations, meaning the system can handle overloads or surges without having to engage the batteries.  

UPS Batteries 

The batteries in a UPS system provide emergency power when the mains supply fails. Either the rectifier or a separate charger ensures that the batteries are always charged.  

UPS battery systems have at least one string of batteries, with the number of batteries required depending on the DC voltage of the UPS. Batteries within a string are connected in series, so if a single battery fails, so too does the entire string.  

Inverter 

This component fulfils the second half of the double conversion by switching the DC voltage from the rectifier or battery back to an AC output that powers the critical load. 

This conversion process (AC to DC to AC) and filtering smooths out events such as spikes, sags, surges, and electrical noise, ensuring the final output is a pure sine waveform.  

Static Bypass Switch 

This component is a safeguard in case there’s a failure within the UPS system. In the event of a UPS failure or fault, the static switch automatically connects the load to the mains supply, bypassing the rectifier, batteries, and inverter.  

Having to transfer to mains supply isn’t ideal as the power won’t be filtered or conditioned as usual in an online double conversion UPS, but it does enable equipment to continue functioning while the UPS is repaired or replaced.  

 

Depending on the size and type of UPS, there are several other components that may be included in a UPS system, including External Maintenance Bypass, which enables the UPS to be removed and/or replaced without interrupting the load, Transient Volt Surge Suppressors (TVSS), and Simple Network Management Protocol (SNMP)-compliant monitoring and communications applications. 

 

Learn more about how uninterruptible power supply systems work with this handy ‘UPS Basics’ video: 

 

Further reading: