What are the three common designs of UPS systems?

• A. Delta conversion on-line, rotary, stand-by
• B. Double conversion online, standby, line-interactive
• C. Standby, line-interactive, flywheel
• D. Online, offline, capacitor-based

Answer: (A)

Think of UPS designs as different ways power is conditioned and delivered to the load, especially how they handle normal operation, minor disturbances, and outages. The three most commonly taught designs are standby (offline), line-interactive, and online (double-conversion).

Standby or offline keeps the load directly on the mains most of the time. When a power problem is detected, it quickly switches to battery-powered inverter. This is simple and efficient, but the load isn’t protected from small sags or transients while waiting for the switch, and there’s no voltage conditioning during normal operation.

Line-interactive adds a level of voltage regulation to the standby idea. It can adjust the output during modest input sags or surges using an automatic voltage regulator, reducing battery wear for common disturbances. The load still runs off the inverter only when the problem is beyond the regulator’s reach.

Online or double-conversion takes the most protection a UPS can offer. The incoming AC is always rectified to DC and then inverted back to AC for the load, so the load sees a clean, isolated power source regardless of input conditions. This design provides the strongest protection against disturbances but at the cost of some efficiency due to continuous conversion.

Other terms you might see, like rotary or flywheel, refer to specific hardware implementations, and terms like delta conversion or capacitor-based aren’t standard ways to categorize the three broad topologies above. So the well-recognized trio you’d study for UPS designs is standby/offline, line-interactive, and online/double-conversion.