There are 3 main UPS technologies – Offline, Line Interactive and Online Double Conversion – and two different types of inverter – square wave (also called pseudo-sine wave or modified sine wave) and sine-wave.

An Offline UPS provides basic levels of power protection. There is usually a degree of surge suppression incorporated and when the input mains voltage goes out of tolerance (that is too high/low or fails) the UPS inverter starts up and provides power to your equipment. There is a break in the mains supply when this occurs in the region of 10-20 thousands of a second which will generally go unnoticed by the majority of equipment. The inverter in offline UPS is nearly almost always a square wave.

A line interactive UPS is similar to an offline UPS but has the added benefit of voltage regulation. This means that it will reduce the mains voltage if it goes too high (called “buck”) or raise the mains voltage when it goes too low (called “boost”). It will do this without reverting to battery and hence conserve battery life. Since a line interactive UPS has an inline transformer, it also provides in-built filtering and hence a higher degree of power protection. Like the offline types, there is also a break during the transition from mains power to battery power. In higher quality line interactive units, this break may be as low as only 2 to 4 thousandths of a second. Line Interactive UPS Systems come with square wave as well as sine wave inverters.

The online double conversion UPS System is considered by many to provide the highest levels of power protection. The inverter is always on, and provides power to the load. This means that there is no deviation in output voltage and no break if the UPS reverts to battery power. The output waveform in an online UPS System will almost always be a sine-wave, generally of very high quality and can enhance the operation of certain equipment. An online double conversion UPS also has a bypass circuit, which allows power to be continually fed to your load even if the UPS develops a fault or is overloaded.

In choosing your technology you need to consider:

1. The power consumption of your load
2. The type of load you have
3. How critical your load is
4. The environment in which the UPS will sit
5. The required runtime
6. Your budget

Your normal UK socket outlet is rated at 13Amps which means the most power you can draw from a UK outlet is 3KVA, or 3KW. (Power Measurement will be covered in a separate paper). Above this level (for specialist equipment, or hardwired installations) most UPS will be online double conversion technology as the economies of scale start making other technologies non cost effective. Below this level, however all technologies are available, although above around 2KVA, line interactive systems start to become heavy and unwieldy due to the size of transformer that is required.

If your load is made up of computer type power supplies, then both square wave and sine wave products will power this equipment adequately. If your load contains motors, transformers, pumps or other inductive components (on the input power supply) then square wave systems are not suitable and you must opt for a sine-wave system.

Any load that is susceptible to mains disturbances such as in analytical equipment or audio applications should also choose a sine-wave system. Where mains distortions affect equipment performance then you need to opt for online double conversion where a pure sine-wave is always present.

If your load is critical for operation then the increased benefit of online double conversion technology should be used. This provides additional security against all power disturbances as well as the comfort of a fall-back bypass in case a fault develops with the UPS. You may wish to parallel together UPS and again, this can only be achieved with online double conversion UPS. (We’ll cover parallel systems and redundancy in a separate paper). If you have a PC where you are looking for simple battery backup to stop annoyance rebooting or tripping then an offline or line interactive unit would suffice.

Offline and line interactive UPS Systems are generally quiet in operation and do not utilise cooling fans in normal operation (usually). This means they are suitable to be placed in an office or home environment. Online Double Conversion UPS systems require forced cooling and can be quite noisy and therefore tend not to be suitable for use in an office environment.

If you are running your UPS in conjunction with an emergency backup generator then the benefits of online double conversion technology come to the fore, as the UPS will cover for any transition to generator operation and also provide a constant unchanging power source during generator start up and any shifts that may occur in frequency and or voltage during load switching.

Offline UPS tend to be the smallest of all technologies so can be useful to provide emergency ride through for areas where space is at a premium.

Long runtimes on UPS are better handled with online UPS Systems. This is because they are designed to operate continually from the inverter. In which case it does not matter if the input power is from the rectifier or the battery – the operation is the same. Offline and Line Interactive units tend not to be designed for this continuous operation. (Battery selection and runtime will be covered in a separate paper.) It is also unwise to power any equipment with a square wave inverter for any prolonged period of time as you could cause degradation to electronic components. Square wave systems are designed to allow basic computer systems to ride through brief power outages or time to shut down a system in the event of a prolonged power cut.

You may have already made up your mind which technology you need, but let’s take a look at relative costs. Offline UPS systems used to be by far the cheapest. However nowadays an offline UPS and a line interactive square wave unit are typically similar prices. Online Double Conversion used to be far more expensive than other technologies, but now is comparable to a high quality sine-wave line interactive UPS.

With regard to running costs, the offline UPS is the most efficient as in normal operation the input power goes straight through to the load, so the only power loss is to keep the battery float charged and power the UPS logic. Line Interactive units are similar in efficiency but experience more losses due to the transformer required for the buck and boost facility. Online Double Conversion however, has the drawback of being the least efficient of the technologies.

In summary, for simple low cost battery backup then the offline or line interactive square wave systems are suitable. For high grade protection in a quiet environment, or where your load type demands it, opt for a sine-wave line interactive unit. However, if you want the best power solution, then we recommend online double conversion technology as it is compatible with all load types and provides the highest degrees of power protection.

The VIX provides everything you need for a simple low cost power protection solution including surge supression, Voltage Regulation and of course battery backup. The unit is simple to operate and comes complete with PC shutdown software that is connected via USB, so you can rest assured you will not lose any data if you have a power cut whilst you are away from your desk (or busy scrambling around in the dark for a torch).

VIX2060 Uninterruptible Power Supply - Bargain.

We’ve got the IP-PRO-Evolution at under £45 delivered. This will provide you with quality surge protection as well as battery backup allowing you to continue to trade even if the lights go off for a while.

If you’re short of space then what about the OPTI VS575C for £50 delivered? This is an ideal system for a EPOS terminal.

When money’s tight it’s easy to skimp on the power protection and not spend the money, but at these low prices it’s crazy not to.

The first thing you need to know is that battery discharge is non-linear. For those of you who don’t understand the expression I’ll elaborate. A linear expression is one where, for example, you put two in, you get four out. So it follows that if you put three in, you get six out, or you put five in, you get ten out. EG. whatever you put in  you get twice out. In the non-linear world this doesn’t hold true, for example, you put two in, you get four out, but when you put three in, you get nine out etc.. This non-linearity makes the discharge characteristics very difficult to express mathematically.

Luckily, the battery manufacturers provide us with discharge tables that we can look up, but first we need to know some information about the UPS, the load and about the method.

End of Discharge Point

At what point will the UPS switch off? Your normal 12V lead acid battery contains 6 cells each of a nominal voltage of 2V (so you get 6x2V=12V). In practice the voltage is slightly higher than this and reduces as the battery is discharged. It is important not to allow the battery to become too discharged, so the UPS will monitor the cell voltage and cut off when it gets to a predetermined point. This is usually around 1.7V per cell or 10.2V for 12V battery.

UPS Efficiency?

Well, more precisely, the inverter efficiency. The inverter is used to convert the battery DC power into AC power. There will be losses associated with this. The better the inverter, the lower these losses are. If you’re unsure, use a worst case of say, 80% efficient. This means that for every 100W provided to the load, the batteries will need to provide 125W (simply 100/efficiency).

The Load Power Factor

Is the load purely resistive, or does it have a power factor? We’re only interested in the amount of WATTS that are needed.

Amps or Watts Method?

Firstly, there are two methods for calculating runtime, the Amps method, or the Watts per Cell Method. Generally, Watts per Cell is used for short term discharges and Amps is used for long term discharges.

Process

It’s easier to do this with an example, so let us take a standard server type load with a number of ancillary devices. We know from measurement  that the Ampere draw is 7Amps and we have mains voltage of 235V. Our Watts therefore (assuming unity power factor) is 1645W.

Our UPS has an inverter that is rated at 90% efficient, so the amount of power from the battery required to deliver 1645W is 1645/0.9 = 1828Watts.

Our UPS is a 3KVA, 2.1KW UPS that contains 8 batteries connected in series. Each battery is rated at 12V 7.2 Ah.

 I now need to look up the manufacturers data sheet and I find the following for a 7.2Ah battery:

Constant Power Discharge Method 

Our battery load is 1828W and we have 6×8=48 cells. Therefore our Watts per Cell is 38Wpc.

We know the FV (Final Value or End Of Discharge Point) is 1.7Vpc so looking along the Constant Power Discharge Table, we can see that 52Wpc would give 5 minutes, so we will get more than 5 minutes runtime. We can see that 35.16Wpc would give 10 minutes runtime, so we will get less than this.

So the calculated runtime for this example, based on constant power discharge is between 5 and 10 minutes.

 Constant Current Discharge Method

We have a total of 8x12V batteries in series, giving us a string voltage of 96V. We need to deliver 1828W so our Amperage is 1828/96 (from Power=VoltsxAmps, so Amps=Power/Volts). = 19Amps.

Now looking along the table above in the Constant Current Section, with our FV of 1.7, we see that a 26A discharge would give us 5 minutes, so we will get longer than this. A 17.6Amp discharge would give 10 minutes, so we will get less than this.

Therefore using the constant current discharge method we will get between 5 and 10 minutes runtime.

Working it out in reverse – I want 2 hours runtime – how many batteries do I need?

Using Watts Per Cell Method.

From the table under Constant Power Discharge, we can see that for our FV of 1.7, for a 2 hour runtime we need to have a WPC discharge of no more than 6.05. Our load is 1828W, so we need 1828/6.05 = 302 cells, which is 50.35 batteries. The battery requires a 96V string voltage, based on banks of 8, so we will require 6 banks to get close (that is 48 batteries), or 7 banks to be sure (that is 56 batteries).

Using Constant Current Method.

From the table above under Constant Current Discharge, you can see for our FV of 1.7V we need to have no more than a 3.05A discharge from each of our batteries to achieve a 2 hour runtime.

Our total current draw is 1828/96 = 19Amps (1828Watts load/Battery String Voltage = 96V)

Dividing the 19Amps total current by 3.05 gives us the number of strings needed to achieve 2 hours runtime which is 6.24. Obviously we cannot add in a quarter of a string so we need to round up. In this case we require 7 battery strings, or a total of 56 batteries to achieve a runtime of 2 hours.

Alternatively, you could of course opt for higher capacity batteries, and maintain the same number of batteries. The examples above were using 7.2Ah lead acid batteries but there are other choices available.

Now UPS Systems are designed to increase the reliability of connected systems, by providing power protection and back up power. If the UPS System should fail then it should revert to bypass and allow utility mains power through (this is for online double conversion Uninterruptible Power Supplies and particularly three phase systems). It’s unclear what happened in this case, but whatever the outcome the critical systems were left without power and the UPS System is perceived not to have done its job.

Here’s where redundancy is required. Basically, you have one more system than you actually need, therefore if one system should fail, the other can take over without any loss of UPS System support. Should the utility fail for an extended period, then you either need batteries to keep you going (or shut down gracefully) or an external generator to kick in and simulate utility power.

Redundancy is usually expressed as ‘n+1′, which means that if you need ‘n’ UPS Systems to power your load, then you install ‘n+1′. For example, if you have a 100KVA load, you can achieve this with 1x100KVA UPS System. If you want redundancy you will need to use n+1 = 1+1 =2 UPS Systems, i.e. 2x 100KVA UPS Systems. Alternatively, you may have achieved your load by using 2x50KVA UPS Systems in parallel, and redundancy can be achieved by installing an additional 50KVA UPS System i.e. 3x50KVA UPS Systems, which may be more cost effective.

I’ll blog more about this later, as the citizens of Taipei walk to work. 

You can read about the Taipei metro power cut here

Sexy UPS? Is it possible? When people ask what I do for a living, being involved with UPS systems ranks alongside accountancy in terms of the “Wow, Really?” response you are looking for. I met some bloke the other day who worked at McLaren F1 – Wow. He’d just won the drivers championship or something and previously he only made the tea, but interesting nonetheless. Let’s face it UPS aren’t the most exciting of products are they? In fact, a good UPS is just like a good referee – you shouldn’t know its there.

So back to the KR1000J. Well, as far as UPS’ go, it is in fact quite a sexy looking machine (did I just say that – a holiday is long overdue), but in order to spice up the photo shoot we got three sexy blondes to pose with the UPS. Hope you like them:

Sexy UPS? Barbie and the KR1000J

Barbie heaven for the KR1000J

I have a home PC and want to protect it from momentary power glitches and enable me to shut down in the event of a power cut.
You need surge protection and battery backup. As the load is non-critical, a low cost UPS system will be the best option such as the MiniGuard series. The 500VA UPS will be suitable for most home PC’s. Higher performing systems may need the 1000VA UPS.

I have a small business and want to protect our main server from power problems.
Typically the answer would have been sinewave line interactive UPS Systems, but here at UPSMart you can get online double conversion UPS systems at the same sort of money you used to pay for line interactive. For a standalone system I would recommend the KR1000. If you need rackmount capability, opt for the KR1000J. For several systems or longer runtime, try the KR2000 or KR3000.

I have a hospital laboratory piece of equipment and need to protect the system from mains problems especially during the hospital generator test.
Hospital Laboratory equipment is a special case, as well as all analytical equipment, in that they perform sensitive measurements that need a stable electricity supply. Products suitable for this application are all the online UPS systems (VFI systems). For higher levels of protection you will need a power conditioned solution. These products have an isolation transformer which eliminates common mode noise.

I have a piece of lab equipment with a blue commando plug. How can I get a UPS without having to hardwire everything?
What you need is a UPS with pluggable connectivity. Such as the DS6000B Pluugable Online UPS, or the ON3600 or ON7200.

I have a computer room and want to protect against momentary outages.
You need an online UPS system (VFI System) such as the KR6000 or KR1110S that will be wired into your electric circuit supporting the room. Larger rooms with three phase supplies should opt for the KR3320 or KR3330.

I have a computer room and want to have long run times of 30 minutes or more.
You need a long runtime UPS system, such as the KR1110L which uses external batteries to provide the runtime. Larger rooms with three phase supplies should opt for the KR3320 or KR3330. If you do not see the required runtime then contact us for a quotation.

I have a computer room that is small but may be expanding over the next few years. I don’t want to spend all my money now on a system that will be initially operating at low capacity.
You need a Modular UPS System that can be expanded to meet your needs as your business grows.

I need a UPS that will be used near to patients.
You need a medical grade UPS certified to the electrical safety standards for medical applications.

I need a standby power solution giving me hours of runtime and allowing me to run our business during long term power outages.
You need a generator and UPS combination. We don’t sell the generators online as the details are too complex and on a site by site basis. However, we can help, no matter what your requirements. Please contact us to arrange a site visit and quotation.