Uninterruptible Power Supplies (UPS) – facts, fiction and myths PDF Print E-mail
User Rating: / 0
Written by CMR Projects   
Saturday, 24 April 2010 08:08

Not to be confused with a major world-wide courier company an uninterruptible power supply (UPS) is a source of power that includes an inverter, a battery or bank of batteries and an internal charger. The UPS inverter takes DC energy from the battery(s) and normally creates a 230Vac output.


In the 90’s a UPS was considered a “high tech” product and they were only sold to those people who were seriously concerned about reliable power. Typically a 600VA was the “entry level” size and, being transformer based, it was heavy, bulky and noisy, but they were so well designed they are still found working 20-years later! It often required two people to carry one and a 3KVA needed a bakkie to move around! Today a standard 600VA is about the size of a shoe-box, is very quiet and does not use heavy transformers – and they are considerably cheaper. Today UPS's are offered in the range 300VA through to 800KVA, and beyond. Larger systems are also backed up further by a diesel generator.


Unlike inverters that are normally rated in WATTS, UPS’s are rated in KVA. This is a huge area for those sharks in the industry to relieve you of your hard earned money. Typically a 1000VA UPS will only deliver about 800 watts of useful power. Imagine the “quick buck” merchant selling you a 1KVA that he claims is 1KW.


Who uses them - generally users install UPS's on any piece of equipment that requires a continous form of power when the utility power is interrupted. Today 80% of uses are for PC's and servers but they are used extensivley in industry for electronic controllers that require constant and regulated power - PLC's for instance cannot suddenly have the power switched off whilst they are controlling a process.


The "back-up time", often not quoted when buying the UPS, is the amount of time in minutes that the UPS will support the load when the incoming power fails, typically this is 10-15 minutes on a standard UPS but configurations can extend to many hours, it depends on the batteries connected, the load on the the UPS, and the amount of money available in your budget.


Under NO circumstances should a UPS’s EVER be considered to be soley a surge or lightning protection unit.


Some UPS manufacturers:

There are literally hundreds of UPS manufacturers world-wide from the "el cheapo" products, mostly emanating from the Far East, to professional products from Europe and USA (110Vac), if you Google for UPS suppliers you will see what I mean. This is a scary situation for the first time buyer and most of the uninitiated buyers are conned out out their cash. Solution - stick to a recognised make, ask questions of the seller of the warranty and what facilities they have for after sales service and repairs and phone around to find out if those other users are happy with the product. Some recommeneded international manufacturers are Eaton Power (formally MGE), APC, AROS, Liebert, Powerware - but the list goes on. I have been using Eaton (MGE) products since 1989 and have never had any problems with the operation and/or warranty claims or repairs and service.


Some simple definitions:

This whole concept of VA v WATTS in the specifications has come about as in the early days of the UPS development they were only used for PC’s. It is all to do with “power factor”, an issue that I will not cover here save the fact that, as mentioned loosely above VA * 0.8 = WATTS and it is WATTS that provide the “real” power to power your equipment. 1000VA units are often identified as 1KVA and larger units are only rated in KVA, all this is is a short form of 1000.


In South Africa, as with most countries in the world, the frequency of the mains supply is 50Hz (fifty Hertz). That is there are 50 cycles of the waveform per second, the wave form of such from a municipal supply is a pure sinewave. Thus each “cycle” is completed in 1/50th of a second or 20 milli-seconds (abbrev. mS = thousandth of a second)


A/hr, or ampere/hour, is the rating of the battery and is a measure of the current the battery can supply. Typical UPS batteries range from 7A/hr (very common, also often found in security systems) to 35A/hr (larger UPS’s only) but batteries of 102A/hr can be found on UPS’s that (a) have been designed for long back-up times or (b) very large UPS’s.


The actual ampere/hour definition, as found on most Wiki’s for instance, is the measure of the capacity of the battery that it can supply a load current for 1-hour. This is incorrect as a typical 105A/hr battery cannot deliver 105 amps for 1-hour !!! The 1-hour rating of this battery is in fact 50 amps and at 102 amps it will last for just 25-minutes. The correct definition is the amount of current that the battery can deliver over a 20-hour period. In the case of the 105A/hr battery for instance 105 / 20 = +/-5 amps, i.e. one can draw 5 amps from the battery for 20-hours before the battery is considered to be exhausted (flat), typically this is 10.5 Vdc.


Are you still with me here ? Hopefully you are :)


Alternative Energy ?:

This article should not fall into this category in the strict sense of the word as a UPS is not really an alternative energy supply. A UPS is a source of “continuous” power for a critical piece of equipment such as a PC/server, PLC (programmable logic controller) or any equipment that is controlling a process or running a program for such. Thus when the utility power fails you have an element of alternative power, but only for a limited time.


Also in the strict sense of the word a variety of UPS called a “line interactive” does in fact create a break in the supply to the load when the utility power fails, albeit so short that the load is unaffected.


UPS Topologies:

Overall there are three derivatives of UPS namely: “off-line or standby”, “line-interactive” and “on-line”.


Off-Line: A really cheap solution and should be avoided like the plague. Basically this type of UPS offers zero protection to any downstream equipment, save the fact that power is switched through from the internal inverter when an incoming power failure occurs. Traditionally this switch-over period can be 20mS-50mS and thus falls into a range where some equipment will see this as a power failure and crash. The off-line UPS is often sold as an “on-line” device to those unsuspecting buyers who are after a cheap solution. The waveform of the inverter output is typically a modified or stepped sine-wave or indeed a square-wave. The latter is a diabolical wave form and should NOT be used for ANY sensitive equipment as it will cause more problems than the UPS was installed to prevent.


Line-Interactive: As the expression suggests this derivative interacts with the incoming utility line power and produces a constant 230Vac supply for an incoming voltage of the range 184-264Vac (+/-15%). Upon an incoming power failure the internal inverter output is switched to supply the load, the break is typically in 5-20mS. Creating 230Vac from a lower incoming voltage is referred to as a “boost” and the opposite, when the incoming voltage is higher than 230Vac, as a “buck”, i.e. “Buck and Boost”. Most medium quality line-interactive UPS’s produce a modified sine-wave, similar to stand-alone inverters, and these waveforms present little or no effects on the equipment they are protecting.


On-Line: Again as the name suggest this derivative is “on-line” 100% of the time. There are no switchovers or breaks of any nature when the incoming power fails. When this happens ALL that happens is that the internal charger, which is connected to the internal batteries, switches off. Generally this type of UPS is referred to as an “on-line double conversion” unit and normally has the addition of the words “pure sinewave”, as it has this form of output. It is the best, and the most expensive, form of UPS available and should only really be considered for use on very sensitive pieces of equipment. Typical pricing is R4,000.00/KVA.


Battery charger ratings:

In all cases of UPS the internal charger is rated (or should be) for the capacity, in A/hr, of the batteries that the UPS contains. By this I mean that the charger should be rated for 1/10th of the A/hr capacity i.e. 7A/hr = 0.7 amps, 105A/hr = 10 amps. This rate is a guideline but is taken by most technicians as “the norm”.



The internal batteries are normally rated so that the UPS will produce its full-load power for 10-15 minutes but any reduction in the load on the output of the UPS will increase this back-up time.


Also considered as a norm is, after a complete discharge of the batteries, the charger should be capable of recharging the batteries back to 80% of capacity in 8-10 hours.


I trust that I have shed some light on the subject, albeit quite technical in some parts, but if you need any advice my email address is the best form of communication.

Last Updated on Wednesday, 28 April 2010 20:55