UPS vs VFD — Double Conversion

You're comparing two different animals...the UPS is only used when the power dips below a certain voltage level...the VFD is used continuously...

VFD...

UPS systems explained

Two basic types of UPS:

1) Off-line UPS - This unit passes power from the utility/source directly through some sort of relay or switching device to the UPS output with very low losses under normal power conditions. The inverter controller circuits are sometimes powered but the inverter switches are off (not switching) so no DC bus power (battery usually) is consumed. When the utility power is interrupted, the inverter switches are enabled to begin producing AC power at the output of the UPS from the battery or other DC source. There is a delay in producing power which results in an AC power drop-out until the inverters are fully functioning.

2) On-line UPS - This unit operates the inverters continuously. The utility input power is rectified into DC to charge/maintain the batteries (or other DC storage device) and also supplies the inverters with DC current to produce the required AC voltage/current at output of the UPS. When AC power is lost or droops too low, the battery seamless supplies current to the inverters so there is never any drop-out of AC power from the UPS when the utility power is lost.

" I soiled my armor I was so scared."

"seamlessly" grammar police

For some units the UPS inverter always supplies the AC output independent of the incoming supply which charges the DC battery/batteries. This way the output is isolated from any transients and brownouts without ever interrupting the output which is inverter fed.

This type is almost always used in Supplies to operator consoles, OIU and DCS/PLC control in industrial operations without compromising the control system..

The original operational concept of large capacity UPS is for the load to be always online regardless the commercial power is available or not... Which means that its operating frequency is fixed..

Small capacity UPS was adopted for computers to provide enough time that enables the line operated computers to properly shutdown whenever there is a power outage... This type has adopted the cheap emergency light application approach..

I believe the latest UPS was designed like a VFD, intended for the solar power systems that are tied to the power grid.. Which in my view enables a stable and fast synchronization with the ever changing load dependent commercial powerline frequency..

Which is also why the double conversion is done...

Both devices need a dc power source.

The UPS has a battery typically at low voltage for the dc power source, be chopped up to make ac. No ac input required at all times.

The VFD has capacitors at high voltage to be chopped up to make ac. Ac input is required at all times.