UPS (Uninterruptible Power Supply): what they are, what they do and how they are made

An uninterruptable power supply (UPS also said, from English Uninterruptible Power Supply) is a device used to maintain constantly electrically powered in alternating current electrical appliances. It becomes necessary where the electrical equipment may in no case remain without power (eg in public places such as hospitals, power plants etc. ..) a disservice by not creating more or less severe. It is very useful especially in countries where they produce regular and frequent blackouts.

Building technology

Basically it is a device consisting of at least three main parts: a first converter AC / DC (AC converter) which, thanks to a rectifier and a filter, converts the alternating voltage of the electrical network in DC voltage, a battery or more accumulator batteries in which is stored the energy provided by the first inverter and a second inverter DC / AC (AC converter) that draws power from the rectifier or from the battery in case of lack of power supply, provides power to the connected load.

There are groups of continuity of various powers, from small devices for home use (300/400 watts), typically used to power personal computer, up to industrial equipment from several hundred kilowatts. Are in regular production UPS also fed to medium voltage, in autonomous container also containing the batteries, for powers of some tens of megawatts, able to support factories entire start-up of a diesel generator.

A UPS simple (leaving out the part rectifier and batteries), consists of an inverter in which an oscillator generates a square wave signal which then amplified by a battery of transistor fed by the DC voltage makes the necessary power through a transformer for the necessary rise in voltage. UPS advanced have begun to use the most modern components, such as IGBTs, MOSFETs, or more, in order to obtain higher efficiency.

Going further into the scale of complexity, various systems are used to obtain a waveform output which is more like sine wave that is distributed by the electricity companies. This occurs at the level of the oscillator circuit or deitransistor. Capacitors and inductors are used to filter the flow of current to and from the transistor so as to make it more "soft".

You can even produce more sine wave using dual power supply: positive, negative, and ground. A logic circuit takes care of activating the transistor so that switching to alternate in the right way.

All UPS that do not generate a wave perfectly sinusoidal mean that certain loads, such as electric motors (fans, for example), operate less efficiently.

UPS even more sophisticated use a technique called pulse width modulation (in English Pulse Width Modulation, or PWM) with a high frequency carrier: this allows to approximate more closely a sinusoidal function. In the UPS of quality, sine wave output can be even better than that provided input.

 

Supply

The power of the UPS is given by one or more batteries, normally lead. For small groups of continuity using a voltage of 12 volts, while with the increase of the power of the UPS, the manufacturer requires that voltages are used increasingly, often multiple of 12. It can reach and even exceed a series of batteries 20, equivalent to a voltage of 240 volts and more. The number of batteries and then increases as the required power output and the time for which it must supply it. To obtain autonomies longer necessary to have multiple sets of batteries in parallel, so as to increase the capacity of energy storage.

The batteries that are used with the UPS are the so called deep cycle batteries, which, unlike the common lead-acid batteries, bear many deep discharge cycles.

Installation of a UPS must then with IT in the number of batteries adequate to cover the need of power and time. You should also make sure that the periods in which the batteries are recharged are long enough to allow them to then restore energy. To consider carefully the storage temperature, as a battery, must remain at temperatures below 25 degrees, otherwise the decay of life expectancy.

 

Charging the batteries

Many UPS are supplied from the circuit that charges the batteries. This circuit provides a voltage on batteries 13.6 volts per block (block = battery) with a rating that should be limited to 10-20% of the battery capacity mounted, for example, mounting a battery of 150 Ah ( Ampere hour) should recharge it with an amperage between 15 and 30 A, but if we have two parallel series should recharge with a current between 30 and 60 A. The charging circuits are designed in such a way as to reduce the current to values of retention when it detects that the battery is charged.

In some models of UPS charging is carried out in an even more intelligent, by regulating the voltage according to the temperature of the batteries, or by subjecting the batteries to continuous cycles of charge and relaxation. All this in order to improve the performance and durability of the batteries.

 

Operation

There are two main categories of UPS: the on-line and off-line.

The UPS on-line have the advantage of eliminating the induced noise from the network using the double conversion. This type looks like only disadvantage higher consumption than the other types. The rectifier and inverter are therefore always active, this inevitably causes a greater dispersion. In case of power failure, the inverter draws energy from the batteries, which, according to the project, can be placed directly on the continuous or, in this mid-sized machines, interposed SCR switching. Having the double conversion voltage and frequency are always stable. An integral part of the UPS bypass, the static one, also called automatic and manual. The first fully managed by the machine switches the load between the inverter and the network without sag, usually performed by SCR, it becomes important in case of failure both UPS and downstream, that is, the load eg overload and / or short circuit. the manual bypass is precisely operated by an operator, useful in case of failure with possible shutdown of the load and maintenance, in this way it completely excludes the machine ensuring the operation of the load from the network. In this mode the load is not protected in any way. This type of UPS is the best and often it is also the most expensive. The UPS can deliver higher powers of 2, 3 KVA are almost all of this type. There are 2 products that according to the power and philosophy of using or not the output transformer.

The off-line UPS behave slightly differently as they begin to synthesize the wave only a few milliseconds after the blackout, creating a small "hole", lasting only a few milliseconds, the output voltage during which the load is not powered. To overcome this problem of the capacitors are used in output, but not always sufficient to maintain the load voltage. This type of UPS is cheaper, easier to build, often used to power individual computers or users, however, not too delicate, as an added benefit, taking it off the inverter also has a considerable energy saving. Often UPS and small dogs are of this type.

Inside the UPS to generate alternating current inverters are used that draw energy from the batteries. Often generate a sine-wave, similar to the original one but in steps, in a manner similar to the pulse-amplitude modulation, which has the same area of the pure sine wave, and then the same energy. After the drive signal is passed to filters that cant angles making it look more like a sine wave. Typically UPS have two types of outlets. A group connected to the batteries (and inverter) and a group of outlets connected only to the filters, in case of power failure the latter group stops delivering current.

 

Might

The UPS can support only limited loads. The maximum power that can support is indicated in various ways:

• watt

• Volt-Amps (VA)

• Computer volt-amperes (GO)

Where the value more correct to assess the real capacity of the group is the first (watts).

The second value (VA) can be helpful to describe the ability of the group but is more prone to manipulation by the manufacturer of the device to mask the real (lack of) power of the group. In fact UPS manufacturers provide us with the value of the power factor of the UPS output, combined with other data that is able to provide the real power of the Group. Example: 5 kVA power cosφ0, 8 (the most classic), there may actually provide 4000 watts of real power, which then depending on how the load is out of phase with respect to the sine wave output can further demote, then we will have really also a less current available. Please note that the PF that is declared is considered inductive, and normally the power output remains the same up to an inductive load power factor of 0.5; another speech is a capacitive load with power factor that may downgrade the real power of a UPS also 50%.

The third value (GO) and any other method of imagination are used explicitly to mask the real power of the UPS and make it appear larger than life. For example, the GO is typically twice the VA which is in turn typically double the power in watts.

Given that the capacity of a group of continuity in terms of VA and GO can be calculated in different ways, we can have two groups with declared power of 1500 VAI real power but very different, in general, a group of 400 watt is more powerful a 1500-GO.

The wattage is not given in the UPS, not because you want to mask the real power of the same, but because the power varies depending on the load that would we put. That is, if we have a group of 1 kVA UPS (1000 VA) means that we would have in output 1kW (1000W) if the load that would go to apply creates no phase shift sinusoidal, this means that the power factor is equal to 1. So the actual power depends on the load, resistive or inductive. From the mid 2012 have entered the market to cosφ1 ups, or a ups from 20KVA is capable of delivering 20KW regardless of load. In fact, to calculate the power in one stage alternating using the following formula

P = V × I × cos {φ}

Indicates where the generator voltage [Volt], current [Amperes], cos {φ} is the phase shift (which builders UPS can never know what would we put as a load) and indicates the power [Watt ]. The same value (VA) is also given to show the power of the generators, transformers, etc..

 

10/11/2012

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Translated via software

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Source:

Italian version of ReteArchitetti.it