HP Pro UPS 500 240V ISS Technology Update, Volume 9, Number 4 - Page 2

Line-interactive A line-interactive UPS is similar to the standby UPS, except that it includes a multi-tap, variable-voltage transformer in the line circuitry. The UPS can change the tap on the transformer to decrease or increase the line voltage supplied to the load if the utility voltage is too high or too low, respectively. If the utility voltage cannot be manipulated to meet the output specifications, the battery will power the load through the inverter until the utility voltage is within spec or the battery is depleted. This class of UPS is further divided into low-end and high-end designs. Low-end designs generally use a ‗push-pull' inverter resulting in a stepped or modified sine wave output (see ―UPS output waveforms‖). Alternatively, high-end designs output a true sine wave. Double conversion (online) A double conversion UPS fully isolates the load from the utility and continuously powers the load through the DC/AC inverter. The term double conversion means that the UPS converts the line power to DC through a rectifier and converts it back to AC through the inverter. This UPS design provides the cleanest AC waveform. Double conversion UPSs are the most expensive design due to the additional rectifier circuitry, which also makes the device bulkier and heavier. It is the least efficient UPS design due to the continuous power conversion. UPS output waveforms Each type of UPS outputs a voltage waveform based on the design of its DC/AC inverter: a square wave, a stepped sine wave, or a true sine wave. Square wave inverter The square wave inverter is commonly used in lowcost 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. Figure 1. Output from a square wave inverter HP UPSs do not output a square waveform. Stepped or modified sine wave inverter A stepped or modified sine wave inverter inserts a zero-voltage step between the positive and negative sections of the output waveform (Figure 2). This waveform has a ‗dead zone' between +/-20 V. The circuitry in active PFC power supplies is designed to compensate for this type of waveform as long as the dead zone is less than 2 ms. This waveform is controlled to keep a nominal RMS value of 115 V or 120 V; however, the peak voltage may be different from that of a true sine wave. This type of design is used in low-end line-interactive UPSs and some standby UPSs. Figure 2. Representation of output from a stepped or modified sine wave inverter 2