The Best Inverter Solution
Best practices for selecting a UPS
The primary function of an uninterruptible power supply (UPS) is to supply power to connected loads, such as computers, when the utility power fails or has poor quality. In either instance, the quality of UPS output waveform must be suitable for your computer's power supply. Most server power supplies contain input fault detection circuitry that verifies the quality of the UPS output waveform. Server power supplies also use active power factor correction (PFC) to improve their operation. An active PFC power supply is especially affected by the quality of a UPS output waveform. This article describes the characteristics of UPS output waveforms and recommends best practices when selecting a UPS.
There are three basic UPS topologies - standby, line-interactive, and double conversion --- but they all have a few things in common. Each consists of input circuitry, a battery bank and charger, as well as a DC/AC inverter. The input circuitry filters the source current and protects against power surges. The battery bank supplies power if utility power fails. And the DC/AC inverter circuit converts the DC power from the battery bank to AC power for the connected devices. Line interactive and double conversion UPSs include additional circuitry as described below.
Square wave inverter
The square wave inverter is commonly used in low-cost standby UPS devices. Some UPS manufacturers also refer to output from a square wave inverter as a modified sine wave, step approximated sine wave, or pulse width-modulated (PWM) sine wave. The waveform includes zero voltage dead zones between each positive and negative half-cycle. Square wave inverters work well with pure resistive loads (lamps and heaters) or pure inductive loads (mixers and blenders), but not with computer power supplies. Using a UPS with a square wave inverter will cause computer power supplies to run hotter, operate less efficiently, and have a shorter life than using a UPS with a pure sine wave inverter.