HP Pro UPS 500 240V ISS Technology Update, Volume 9, Number 4 - Page 3

Sine wave inverter A true sine wave inverter outputs a smooth, highquality waveform that is practically the same as a utility AC power sine wave (Figure 3). A true sine wave DC/AC inverter is preferred when used with computer power supplies with active PFC technology. This waveform is characteristic of high-end lineinteractive and on-line UPS topologies. Figure 3. Representation of output from a true sine wave inverter Active power factor correction technology Power factor is the ratio of power used by a load (real power) to the power supplied by the utility (apparent power). The utility's apparent power is the voltage times the current, measured in volt-amperes (VA). However, the load's real power is different because the capacitive and inductive nature of AC circuits changes the phase relationship of current and voltage. Server power supplies contain active PFC circuitry that forces the input current to match the input voltage (brings input voltage and current into phase), resulting in a power factor near unity (~1). Normally, the utility feed is an almost perfect sine wave, so the detection circuits within the power supply have been optimized to achieve a power factor near unity with a sinusoidal input. If the voltage input to the server is not a sine waveform (that is, it includes voltage steps or waveform discontinuities), the power supply detection circuits try to approximate what the waveform should be. The PFC circuitry limits the additional stress induced by voltage steps or waveform discontinuities but may not be able to maintain unity power factor under such conditions. Conclusion For the best protection during unexpected utility outages, use a UPS that has a true sine wave inverter. A line-interactive UPS with a stepped sine inverter may be satisfactory if the dead zone is less than 2 ms. Do not use a UPS that has a square wave inverter. If you use a square wave UPS, some issues may occur in computers that have an active PFC power supply. If you have a question about the type of UPS you have or its output waveform, contact the manufacturer. Please answer the three questions in this survey (http://www.zoomerang.com/Survey/WEB22AXPUC6JFB) to let us know if the information in this article was useful and to help us provide future articles that may be of benefit to you. Additional resources For additional information on the topics discussed in this article, visit: Resource Power basic for IT professionals HP power and cooling technologies for the data center URL http://h20000.www2.hp.com/bc/docs/support/SupportManual/c01234421/ c01234421.pdf http://h20000.www2.hp.com/bc/docs/support/SupportManual/c02018535/ c02018535.pdf 80 PLUS HP Power Protection and Distribution http://www.80plus.org/ http://www.hp.com/products/ups 3