Uninterruptible Power Supply Tutorial

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One would not normally expect to find power protection or backup power on the mind of the average person, unless they have recently suffered through a power event that caused the loss of valuable computer files or worse, equipment damage. Preventing a reoccurrence can be even more frustrating as one has to research enough information in a power protection market that is so large and competitive; it has become unintentionally misleading. One may go down to his local home improvement center, describe the problem and get a surge protected plug strip as the solution. He goes home thinking the problem is solved. It’s not. Next one may go to a computer store where he’s told he needs an uninterruptible power supply (UPS), a UPS battery backup or online UPS. After clarifying a UPS has nothing to do with package delivery, he is shown thirty different models and sizes. This results in more research. In the end this “one solution fits all” approach often does not prevent the problem from happening again.

The situation is not hopeless and one does not have to become an Electrical Engineer to resolve the majority of the power problems encountered by the average end-user.

The following will guide you through successfully determining the type and level of backup power protection, UPS battery backup, or a high quality online UPS, required for your specific environment.

What is a Standby Power Supply (SBS), an Uninterruptible Power Supply (UPS) and why do they not provide the same level of power protection?

This is often not clearly understood, because the term Uninterruptible Power Supply is often used to refer to a wide range of power protection products. It is often deceptively used to describe the Standby Backup Supply (SBS), which only solves a minimal number of power quality problems. It is appropriately used to define the true On-line Uninterruptible Power Supply (UPS), capable of removing or eliminating the greatest number of power quality problems.

To better understand this in today’s UPS market, one can purchase an SBS for as little as $50.00, while a true On-line UPS will cost more. There is a big difference between the performances of the SBS and On-line UPS.

There are three basic design types, each offering more power protection than the proceeding. If manufacturer is honest, they state the design type clearly on the product box or specification sheet. The three design types are:

OFF-LINE (SBS), the lowest grade

LINE-INTERACTIVE (SBS), the middle grade

ON-LINE (UPS), the highest grade

The Off-line SBS

The Off-line SBS offers the bare bones power protection of basic surge protection and battery backup. Through this type of SBS your equipment is connected directly to incoming utility power with the same voltage transient clamping devices used in a common surge protected plug strip connected across the power line. When the incoming utility voltage falls below a predetermined level the SBS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The SBS then mechanically switches the connected equipment on to its DC-AC inverter output. The switch over time is stated by most manufacturers as being less than 4 milliseconds, but typically can be as long as 25 milliseconds depending on the amount of time it takes the SBS to detect the lost utility voltage.

When selecting this type of an SBS, be aware that your computer equipment, as well as most electronic equipment is designed for use in the United States. As such it was designed to operate from a 120 volt, 60 Hertz (Hz), sinewave utility source. Most Off-line SBS products on the market today only provide a sinewave output to your equipment when operating normally from the utility line. When they switch to their internal DC-AC inverter they may only provide a square wave, modified square wave or quasi-sinewave, not a pure sinewave. In many cases your equipment may appear to operate normally on these waveforms, but over time may be damaged by them. If you decide only minimal protection is needed, an off-line SBS offers, it is always best to select an SBS or UPS that states it has an inverter with a true sinewave output. You should also be aware that most off-line SBS units will not be capable of accepting additional battery packs for extended battery operation. To keep the cost down and prevent overheating, their inverters are designed to only operate as long as the internal battery capacity allows. For your reference units of all three design types typically provide from 5 to 15 minutes of battery back-up time when loaded to their full output capacity. Slightly longer backup times can be achieved by overrating the SBS or UPS size.

The Line-Interactive SBS

The Line-interactive SBS offers the same bare bones surge protection and battery back-up as the offline, except it has the added feature of minimal voltage regulation while the SBS is operating from the utility source. This SBS design came about due to the off-line SBSs inability to provide an acceptable output voltage to the connected equipment during “brown-out” conditions. A “brown-out” happens when the utility voltage remains excessively low for a sustained period. Under these conditions the off-line SBS would go to battery operation and if the brown-out was sustained long enough, the SBS battery would become fully discharged, turn the power off to the connected equipment and not be able to be turned back on until the utility voltage returned to normal. To prevent this from happening a voltage regulating transformer was added, hence the term line-interactive was born. This feature really does help as low voltage utility conditions are common. The down side for this design, most of the units available have to switch to battery momentarily when making transformer voltage adjustments and this can be a bit annoying in a quiet home office on a bad power day.

Again when selecting a Line-interactive SBS it is always best to select a model with a true sinewave output. Several manufacturers have models available that will accept extended battery packs to provide additional battery runtime. This type of SBS typically costs more than the off-line type, but is worth the additional cost.

The On-line UPS

The On-line UPS provides the highest level of power protection for the serious home office user. It does typically cost more, but like all electronic equipment today the cost is coming down as the technology advances. The true advantage to the on-line UPS is its ability to provide an electrical firewall between the incoming utility power and your sensitive electronic equipment. While the off-line and line-interactive designs leaves your equipment connected directly to the utility power with minimal surge protection, the On-line UPS provides an electronic layer of insulation from power quality problems. This is accomplished inside the UPS in several tiers of circuits.

First the incoming AC utility voltage is passed through surge protected rectifier stage where it is converter to a Direct Current (DC) and is heavily filtered by large capacitors. This tier removes line noise, high voltage transients, harmonic distortion and all 50/60 Hertz frequency related problems. The capacitors also act as an energy storage reservoir giving the UPS the ability to “ride-through” momentary power interruptions. The battery is also connected to this tier and takes over as the energy source in the event of a utility loss. This makes the transition between utility and battery power seamless, without an interruption.

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