Uninterruptible power supply
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A small free-standing UPS
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An uninterruptible power supply (UPS), also known as a continuous power supply (CPS) or a battery backup is a device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when utility power is not available. It differs from an auxiliary power supply or standby generator, which does not provide instant protection from a momentary power interruption. Integrated systems that have UPS and standby generator components are often referred to as emergency power systems.
There are three distinct types of UPS: off-line, line-interactive and double conversion (also called on-line). An off-line UPS remains idle until a power failure occurs, and then switches from utility power to its own power source, almost instantaneously. An on-line UPS continuously powers the protected load from its reserves (usually lead-acid batteries or stored kinetic energy), while simultaneously replenishing the reserves from the AC power. It also provides protection against all common power problems, and for this reason it is also known as a power conditioner and a line conditioner.
While not limited to safeguarding any particular type of equipment, a UPS is typically used to protect computers, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption or data loss. UPS units come in sizes ranging from units which will back up a single computer without monitor (around 200 VA) to units which will power entire data centers or buildings (several megawatts).
Historically, UPSs were expensive and were most likely to be used on expensive computer systems and in areas where the power supply is interrupted frequently. As prices have fallen, UPS units have become an essential piece of equipment for data centers and business computers, and are also used for personal computers, entertainment systems and more.
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[edit] Common power problems
There are various common power problems that UPS units are used to correct. They are as follows (with a typical example of damage that might be caused):
- Power failure — Total loss of utility power: Causes electrical equipment to stop working.
- Voltage sag — Transient (short term) under-voltage: Causes flickering of lights.
- Voltage spike — Transient (short term) over-voltage i.e. spike or peak: Causes wear or acute damage to electronic equipment.
- Under-voltage (brownout) — Low line voltage for an extended period of time: Causes overheating in motors.
- Over-voltage — Increased voltage for an extended period of time: Causes light bulbs to fail.
- Line noise — Distortions superimposed on the power waveform: Causes electro magnetic interference.
- Frequency variation — Deviation from the nominal frequency (50 or 60 Hz): Causes motors to increase or decrease speed and line-driven clocks and timing devices to gain or lose time.
- Switching transient — Instantaneous undervoltage (notch) in the range of milliseconds to seconds: May cause erratic behavior in some equipment, memory loss, data error, data loss and component stress.
- Harmonic distortion — Multiples of power frequency superimposed on the power waveform: Causes excess heating in wiring and fuses.
UPS units are divided into categories based on which of the above problems they address. Some manufacturers categorize their supplies as a level 3, 5, or 9, if they address the first 3, 5, or 9 power problem respectively.
[edit] UPS Technologies
The general categories of modern UPS systems are on-line, line-interactive, and standby. An on-line UPS uses a "double conversion" method of accepting AC input, rectifying to DC for passing through the battery (or battery strings), then inverting back to AC for powering the protected equipment. A line-interactive UPS maintains the inverter in line and redirecting the battery's DC current path from the normal charging mode to supplying current when power is lost. In a standby ("off-line") system the load is powered directly by the input power and the backup power circuitry is only invoked when the utility power fails. Most UPS below 1 kVA are of the line-interactive or standby variety which are usually less expensive.
For large power units, Dynamic Uninterruptible Power Supply are sometimes used. A synchronous motor/alternator is connected on the mains via a choke. Energy is stored in a flywheel. When the mains power fails, an Eddy-current regulation maintains the power on the load. DUPS are sometimes combined or integrated with a diesel-genset.
Fuel cell UPS have been developed in recent years using hydrogen and a fuel cell as a power source, potentially providing long run times in a small space. A fuel cell replaces the batteries used in other UPS designs.
[edit] Offline / Standby
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| Typical protection time: 0 - 20 minutes Capacity expansion: Usually not available |
The Offline / Standby UPS offers only the most basic features, providing surge protection and battery backup. Usually the Standby UPS offers no battery capacity monitoring or self-test capability, making it the least reliable type of UPS since it could fail at any moment without warning. These are also the least expensive, selling for as little as US$75. The Standby UPS may be worse than using nothing at all, because it gives the user a false sense of security of being assurred protection that may not work when needed the most.
With this type of UPS, a user's equipment is normally 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 UPS 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 Standby UPS to detect the lost utility voltage.
[edit] Line-interactive
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| Typical protection time: 5 - 30 minutes Capacity expansion: Yes, several hours |
The Line-Interactive UPS is similar in operation to a Standby UPS, but with the addition of a multi-tap variable-voltage autotransformer. This is a special type of electrical transformer that can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field and the output voltage of the transformer.
This type of UPS is able to tolerate continuous undervoltage brownouts and overvoltage surges without consuming the limited reserve battery power. It instead compensates by auto-selecting different power taps on the autotransformer. Changing the autotransformer tap can cause a very brief output power disruption, so the UPS may chirp for a moment, as it briefly switches to battery before changing the selected power tap.
Autotransformers can be engineered to cover a wide range of varying input voltages, but this also increases the number of taps and the size, weight, complexity, and expense of the UPS. It is common for the autotransformer to only cover a range from about 90v to 140v for 120v power, and then switch to battery if the voltage goes much higher or lower than that range.
In low-voltage conditions the UPS will use more amperage than normal so it may need a higher amperage circuit than a normal device. For example to power a 1000 watt device at 120 volts, the UPS will draw 8.32 amps. If a brownout occurs and the voltage drops to 100 volts, the UPS will draw 10 amps to compensate. This also works in reverse, so that in an overvoltage condition, the UPS will need fewer amps of current.
[edit] Double-Conversion / Online
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The Online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations. Although once previously reserved for very large installations of 10kW or more, advances in technology have permitted it to now be available as a common consumer device, supplying 500 watts or less. The Online UPS is generally more expensive but may be necessary when the power environment is "noisy" such as in industrial settings, for larger equipment loads like data centers, or when operation from an extended-run backup generator is necessary.
The basic technology of the Online UPS is the same as in a Standby or Line-Interactive UPS. However it typically costs much more, due to it having a much greater amperage AC-to-DC battery-charger/rectifier, and with the rectifier and inverter designed to run continuously with improved cooling systems. It is called a Double-Conversion UPS due to the rectifier directly driving the inverter, even when powered from normal AC current.
In an Online UPS, the batteries are always connected to the inverter, so that no power transfer switches are necessary. When power loss occurs, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When power is restored, the rectifier resumes carrying most of the load and begins charging the batteries, though the charging current may be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte.
The main advantage to the on-line UPS is its ability to provide an electrical firewall between the incoming utility power and sensitive electronic equipment. While the Standby and Line-Interactive UPS merely filters the input utility power, the Double-Conversion UPS provides a layer of insulation from power quality problems. It allows control of output voltage and frequency regardless of input voltage and frequency.
[edit] Ferro-resonant
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Ferro-resonant units operate in the same way as a standby UPS unit with the exception that a ferro-resonant transformer is used to filter the output. This transformer is designed to hold energy long enough to cover the time between switching from line power to battery power and effectively eliminates the transfer time. Many ferro-resonant UPSs are 90-93% efficient and offer excellent isolation.
While this used to be the dominant type of UPS, they are no longer used for common applications. Power factor correcting equipment found in newer computer systems interacts with static ferro-resonant transformers, causing potentially damaging oscillations, and the transformer itself can create distortions which yield power less acceptable than poor quality line AC. These units are still used in some industrial settings, but have mostly disappeared from use with general computer equipment. Many ferro-resonant UPSs utilizing controlled ferro technology may not interact with power-factor-correcting equipment.
[edit] DC-Power Supply
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A UPS designed for powering DC equipment is very similar to an online UPS, except that it does not need an output inverter, and often the powered device does not need a power supply. Rather than converting AC to DC to charge batteries, then DC to AC to power the external device, and then back to DC inside the powered device, some equipment accepts DC power directly and allows one or more conversion steps to be eliminated.
Many systems used in telecommunications use 48 volt DC power, because it is not considered a high-voltage by most electrical codes and is exempt from many safety regulations, such as being installed in conduit and junction boxes. DC has typically been the dominant power source for telecommunications, and AC has typically been the dominant source for computers and servers.
There has been much experimentation with 48v DC power for computer servers, in the hope of reducing the likelihood of failure and the cost of equipment. However, to supply the same amount of power, the amperage must be greater than an equivalent 120v or 240v circuit, and greater amperage requires larger conductors and/or more energy to be lost as heat.
High voltage DC (380 volts) is finding use in some data center applications, and allows for small power conductors, but is subject to the more complex electrical code rules for safe containment of high voltages.[1]
[edit] Rotary UPS
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A Rotary UPS uses the inertia of a high-mass spinning flywheel to provide short-term ride-through in the event of power loss. The flywheel also acts as a buffer against power spikes and sags, since such short-term power events are not able to appreciably affect the rotational speed of the high-mass flywheel. It is also one of the oldest designs, predating vacuum tubes and integrated circuits.
It can be considered to be online since it spins continuously under normal conditions. However, unlike an electronic double-conversion UPS, it is only capable of providing reserve power for a few seconds before the flywheel has slowed and the protection fails. It is traditionally used in conjunction with standby diesel generators, providing backup power only for the brief period of time the engine needs to start running and stabilize its output.
The Rotary UPS is generally reserved for applications needing more than 10,000 watts of protection, to justify the expense of an extremely large and heavy power system that can only be transported by forklift or crane. A larger flywheel or multiple flywheels operating in parallel will increase the reserve running time, but at greatly increasing cost due to the size and weight of the precision-balanced flywheels.
Because the flywheels are a mechanical power source, it is not necessary to use an electric motor or generator as an intermediary between it and a diesel engine designed to provide emergency power. By using a transmission gearbox, the rotational inertia of the flywheel can be used to directly start up a diesel engine, and once running, the diesel engine can be used to directly spin the flywheel. Multiple flywheels can likewise be connected in parallel through mechanical countershafts, without the need for separate motors and generators for each flywheel.
They are normally designed to provide very high amperage output compared to a purely electronic UPS, and are better able to provide inrush current for inductive loads such as motor startup or compressor loads, as well as medical MRI and cath lab equipment. It is also able to tolerate short-circuit conditions up 17 times larger than an electronic UPS, permitting one device to blow a fuse and fail while other devices still continue to be powered from the Rotary UPS.
Its life cycle is usually far greater than a purely electronic UPS, up to 30 years or more. But they do require periodic downtime for mechanical maintenance (ball bearing replacement), while solid-state designs, using batteries, do not require downtime if the batteries can be hot-swapped, which is usually the case for larger units.
Typically, the high-mass flywheel is used in conjunction with a motor-generator system. These units can be configured as:[2]
- 1. A motor driving a mechanically connected generator,
- 2. A combined synchronous motor and generator wound in alternating slots of a single rotor and stator,
- 3. A Hybrid Rotary UPS, designed similar to an Online UPS, except that it uses the flywheel in place of batteries. The rectifier drives a motor to spin the flywheel, while a generator uses the flywheel to power the inverter.
In case #3 the motor generator can be synchronous/synchronous or induction/synchronous. The motor side of the unit in case #2 and #3 can be driven directly by an AC power source (typically when in inverter bypass), a 6-step double-conversion motor drive, or a 6 pulse inverter. Case #1 uses an integrated flywheel as a short-term energy source instead of batteries to allow time for external, electrically coupled gensets to start and be brought online. Case #2 and #3 can use batteries or a free-standing electrically coupled flywheel as the short-term energy source.
[edit] UPS Applications
The basic technology of UPS hardware can have many forms when applied for different purposes. Any of the technologies may be recombined as redundant systems or designed for special needs.
[edit] N+1 UPS
In large business environments where reliability is of great importance, a single huge UPS can also be a single point of failure that can disrupt many other systems. To provide greater reliability, multiple smaller UPS modules and batteries can be integrated together to provide redundant power protection equivalent to one very large UPS.
It is not normally possible to take the AC output of two separate UPS units and combine their output voltage, because the output waveform of one UPS inverter can be leading or lagging the other inverter, causing severe power fluctuations that can damage both the UPS units and the powered devices.
In an N+1 UPS, a special synchronization signal is shared amongst the inverter modules to assure that all are producing a sinewave output that is in synchrony, without leading or lagging waveforms. Additional monitoring circuits assure all inverters and batteries are operating correctly within tolerances.
Generally an N+1 UPS is designed to supply more power than is actually required by the load, so that in the event of a fault, at least one of the inverters or battery modules can be disabled and removed from powering the load. An internal crossbar bus can permit any battery module to be connected to any different inverter module, to bypass potential failures.
An N+1 UPS can permit easy, centralized expansion of enterprise load capacity. In contrast, by purchasing small separate UPS units, eventually the server room fills with a collection of many different UPS models with many different batteries all aging at different rates and needing lots of care and monitoring. Buying a single huge UPS means wasted capacity until it is full, and then another huge UPS must be added which again has wasted capacity. With the N+1 UPS, as capacity grows, expansion just requires purchasing additional inverter modules and battery modules, and adding them to the N+1 chassis.
[edit] Multiple, Redundant UPS
Many computer servers offer the option of redundant power supplies, so that in the event of one power supply failing, one or more other power supplies are able to power the load.
While it is common to plug each of these individual power supplies into one single UPS, redundant protection can be extended further yet by connecting each power supply to its own UPS. This provides double protection from both a power supply failure and a UPS failure, so that continued operation is assured.
These additional layers of protection also add complexity and cost to the design of an enterprise server room environment. It also requires handling only by experienced professionals, since the multiple redundant cabling can appear confusing and unnecessary to an untrained person.
[edit] Outdoor UPS
When a UPS system is placed outdoors, it should have some specific features that guarantee that it can tolerate weather with a 'minimal to none' effect on performance. Factors such as temperature, humidity, rain, and snow among others should have been considered by the manufacturer when designing an outdoor UPS system. Operating temperature ranges for outdoor UPS systems could be around -40°C to +55º C.
An outdoor UPS system is normally made of several components designed for this particular task:
- Outdoor enclosure: provides protection against the elements to all the components placed within. Quality outdoor enclosures are powder coat finished for corrosion resistance and long life. Outdoor enclosures are normally NEMA 3R compliant
- Power Module: is the UPS itself. The boards of this power module should be conformal coated to avoid humidity damage. This UPS unit is normally based on Line Interactive or Double Conversion topology. Some manufacturers prefer Line Interactive because it provides a better Mean Time Between Failures (MTBF), and that is a critical part of an outdoor UPS system.
- Batteries: The batteries used in outdoor UPS systems must operate in a wide temperature range, usually from -40°C to +60°C. Batteries normally used in outdoor UPS systems are Gel Cell Batteries. The outdoor UPS's Power Module should provide a temperature compensated battery charging mechanism to optimize the life of the batteries.
A proper outdoor UPS system requires that all its components are designed for this environment. As seen from the features of the components above, an outdoor UPS system is not an indoor UPS inside an outdoor enclosure.
Outdoor UPS systems can be pole, ground (pedestal), or host mounted. Outdoor environment could mean extreme cold, in which case the outdoor UPS system should include a battery heater mat, or extreme heat, in which case the outdoor UPS system should include a fan system or an air conditioning system.
Outdoor UPS systems are ideal for protection of WiFi/GSM/CDMA/satellite base stations, wireless communications/perimeter surveillance and security/gate control systems, LED traffic light/roadway display systems and remote terminal units (RTUs).
[edit] Internal-PC UPS
Internal UPS are a group of uninterruptible power supplies (UPS) designed to be placed inside computer chassis. There are two types of Internal UPS. First type is miniaturized regular UPS that are made small enough to fit into a 5.25” CD-ROM slot bay of a regular computer chassis. The other type is re-engineered switching power supplies that utilize dual power sources of AC and/or DC as power inputs and have an AC/DC built-in switching management control units.
The first type often requires extra connection wires between the internal UPS and computer's power supply. Some internal UPS of this group output high voltage (110 V - 220 V) direct current (DC) and some output nine-step table wave AC. Neither design is safe or energy efficient. As of 2006, there are only a couple of companies still selling this type of internal UPS in Australia, Asia and some part of Europe
The second group of internal UPS replaces the regular switching power supplies. There are three main design mechanisms:
- Optic-coupling that imitates AC during AC outages. This mechanism was first introduced by American Advanced Power of USA and Magnum Power of UK in 1997, as well as Apollo Power of Taiwan in 1998. This design provides a low-cost solution but its efficiency is low and it has a very low overall wattage limit (<300 W).
- An analog-circuitry-controlled AC/DC switching mechanism. This design also provides a low-cost solution. However, because of the bulky component circuit board, little space is available for increasing wattage output. Plus, the final products are very sensitive to factors such as local heat and causing frequent operational errors. Nevertheless, because of its low cost, it is still popular in China. Most Asian internal UPS manufacturers belong to this category.
- A CPU controlled AC/DC switching mechanism. This design was first introduced by American Advanced Power Inc. of USA and Amsdell of Canada. It provides error-free switching control and a complicated communication protocol between the power supply and computer.
[edit] Disposing of UPS batteries
Many UPS units contain sealed lead-acid batteries and electronics which can be detrimental to the environment. In the United States, it is illegal to dispose of lead-acid batteries in a landfill, and they must be properly recycled. Sealed lead-acid batteries are recycled in the same manner as car batteries, so any auto shop that accepts used car batteries for recycling will also accept sealed lead acid batteries.
[edit] UPS Limitations
[edit] Using a generator with a UPS
Some types of UPS cannot function reliably with emergency power generators and the UPS will fail to work correctly with the generator power. Only a UPS that is specifically rated to work with a generator can be trusted to function properly.
Due to the limited output capacity of an emergency generator, it is common for the generator to produce temporary surges and dropouts as devices are turned on and off. These surges and dropouts become larger as the capacity of the emergency generator decreases compared to the total load it must supply.
For example, if a small business were powered by a 25 kilowatt standby generator, the engine would only run fast enough to provide a stable 50/60 Hz output sinewave for the currently operating loads. If a 5,000 watt water heater suddenly turns on, there is a temporary sag in voltage and a drop in frequency because the generator power draw is suddenly much higher than the engine output. The engine control detects the drop and opens the engine throttle to compensate, increasing engine RPM and bringing the voltage and frequency back up to normal. This stabilization may take a few seconds to occur if the sudden power draw is large, or the device is a motor and normally draws much more power for a few moments when starting as compared to stable running.
Similarly if a large load such as a water heater turns off, the load on the generator is suddenly much lower, and the voltage and frequency rises as the generator RPM quickly increases. The engine control again detects this increase and backs off the engine throttle to bring the generator back down to normal voltage and frequency.
A simple Standby UPS cannot deal with these surges and dropouts, and will constantly transfer to battery, which will quickly discharge and cannot recharge quickly enough to compensate. A Line-Interactive UPS will have fewer problems but may still run down the battery due to the droupots. An Online or Double-Conversion UPS is potentially capable of handling these variations, but only if the power supply is designed to tolerate the wide ranging frequency and voltage variations.
Note that these frequency and voltage variations occur normally as a part of standard powerline generation. However the overall size of the distributed electrical grid and the huge capacity of generation stations help to buffer these surges and dropouts so the effects are not as severe as for a system running on emergency generators.
[edit] Power strip surge-protection hazard
For the basic Standby and Line-Interactive UPS, there is an often-unmentioned difficulty with adding additional power connections. Many UPS models only include a few closely-spaced outlets that cannot accommodate large power bricks or a large number of low-wattage devices all with separate power cables.
While a generic power strip without surge protection can be used to add additional room to the surge-protected outlets, a power strip with surge protection can interact badly with the UPS cutover switch and cause severe damage to the UPS. When the UPS switches from line power to battery and then back to line power, the cutover occurs so quickly that it can appear to be a power surge to a power strip plugged into the UPS. Most surge protection is sacrificial, in that the protection devices will create a temporary direct-short to create an alternate path for the surge to follow, and over time the protection eventually fails from stress.
[edit] How false surge triggering occurs
False triggering can occur because the inverter is usually not synchronized with the line current, and it is possible for a switchover to occur where the inverter has just reached the bottom of a negative voltage sine curve and the line current is reaching the exact top of a positive voltage sine curve (or vice-versa).
120v AC is an averaged Root Mean Square number, and has an actual peak near 170 volts. During the bypass switch cutover in this worst-case scenario, the voltage suddenly swings from -170v to +170v in 4 millisconds, which appears to the surge protection to be a sudden 340-volt swing and has all the appearance of being a voltage spike that should be suppressed. For a 240v UPS, this worst case switchover results in a sudden 650v voltage swing.
If the surge protection false-triggers, the surge protection suddenly overloads the UPS by several magnitudes beyond its design limits, and can result in the UPS electronics quickly overheating and burning up in seconds. Surge protection is generally only rated to handle brief spikes, but this sudden sustained high current absorption may result in the melting of the plastic power strip casing.
[edit] PDU: Commercial expansion options
When a UPS is used in a commercial environment such as powering rack-mount servers, it is not possible for the UPS to provide all the sockets necessary to support the protected loads. In this case the manufacturer will specifically provide an outlet-expansion option known as the Power Distribution Unit, or PDU. This is typically nothing more than common electrical receptacles and a long power cord in a steel case, with no surge protection at all, but with a high cost due to its special design for rackmount infrastructure. Surge protection is unnecessary in the PDU, since the UPS itself is already designed to provide surge protection.
For large installations with over 30 amperes of output, the UPS may have the option to be wired directly to standard electrical conduit and receptacles, using flex conduit to attach the UPS to the permanent conduit, again avoiding potential surge interaction problems.
[edit] Equipment damage policy limitations
For UPS models with equipment damage insurance policies, the policy is typically valid only if all protected devices were connected directly to the UPS, or if the customer purchases a company-brand PDU or company-brand power strip, as specified in the warranty policy.
[edit] Battery Monitoring Limitations
It is typically difficult to determine the charge capacity of an aging battery, using only simple voltage tests. Capacity declines as a battery ages and the lead plates begin to sulfate and decompose, but if a weak battery is sufficiently charged it will still be able to supply sufficient voltage to appear normal.
Only when the battery has been put under load for an extended period of time, and the battery amperage and voltage measured during the test, is it possible to find the true charge capacity. A weak battery will run properly for a few minutes or seconds and fail suddenly.
A Run-Time Calibration is a special test that is rarely performed, and involves running down a fully-charged battery until it fails. The measured time and voltage/amperage results to create an estimated profile of projected battery life. This profile is how the estimated run-time is calculated based on current UPS wattage load.
However, the profile is rarely updated and becomes incorrect as the battery continues to age. More load testing is needed to verify capacity and update the run-time estimation, which is why some UPS models run a self test every few weeks, to do a quick battery capacity estimation.
Some UPS manufacturers suggest doing a Run-Time Calibration only once a year, because the deep discharging is harmful to the lead plates and accelerates their eventual failure. For longest life the charge should stay near 100% continuously, though without the occasional testing it is not possible to know the battery state at all.
For UPS models that rarely or never run self-tests, the battery life may have declined so severely from the profile that the UPS may fail within seconds of being activated. The UPS may appear to be perfectly normal until the critical moment it is needed, but in that moment as the protected devices suddenly turn off does the truth arise that the battery lost all its capacity weeks or months ago, and the UPS did not report any problems.
[edit] Nonstandard Sinewave Output
The Standby, Line-Interactive, and Online UPS products all contain an electronic inverter to generate alternating current from direct current. Since it is not a mechanical spinning rotor, they can only approximate a true sinousoidal wave. The less-expensive UPS models tend to generate a less-accurate approximation. The lowest cost UPS models tend to produce a very rough square-wave, mid-range UPS models produce a stepped-sine wave, while the highest quality models offer true sinewave output.
In many cases the protected equipment may appear to operate normally on the nonstandard waveforms, but over time may be damaged due to the harmonics of non-sinewave power causing excessive heating of transformers, AC motor windings, and power supply circuitry, for which the protected device was not designed to tolerate.
[edit] Run-Time Capacity Expansion
The least expensive UPS models, and the UPS models built for a specific purpose, are usually not capable of accepting additional battery packs or larger battery packs for extended power protection. To keep the manufacturing costs down, they frequently have no cooling fan, little or no venting for air circulation, and do not provide any form of battery or inverter temperature monitoring. Instead, to prevent overheating, their inverters are designed to only operate as long as the internal battery capacity allows and then shut down before the UPS overheating becomes excessive.
[edit] See also
[edit] Notes
[edit] References
- Part of this article was originally taken from a public-domain entry in Federal Standard 1037C.
- Part of this article was copied with permission from Uninterruptible Solutions.
