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What is an UPS?
A UPS is actually an inverter which changes within less than 2-4 milli seconds to battery converted power during a power failure and has regulated output voltage. These features are ideal for computers for data safety and proper functioning of PCs.
What is an Inverter?
Inverter is an electronic equipment which converts DC power to AC power. As soon as power fails the inverter starts and generates AC power from DC battery power within 20 to 40 milli seconds. Inverters are usually used for lighting tubes, fans, TV's etc and usually does not have regulated output.
What is a Sinewave Home UPS?
Home UPS combines the applications of both Inverter and UPS, which means it can be used for both lighting and PCs. The utility power is supplied in the form of Sinewave which is the best form of power for most of the household equipments, and a Home UPS should ideally have this feature.
What is an OFFLINE or LINE INTERACTIVE ups?
In an OFFLINE UPS, the load is connected to the mains supply which is stabilized directly. The moment the power fails the load will be switched over to the output of the inverter. There is a definite switch over time in Offline UPS which can be between 5ms to 20ms. Upto 20ms, interruption in power will not be felt while using the computer because the SMPS input capacitor, will supply power to the computer.
Line Interactive UPS is a variant at Offline UPS. In a Line Interactive UPS there is only one transformer which, when the mains is present is used for stabilization of output and also delivers power for charging the battery. When power fails the function is reversed and used as INV transformer.
This kind of a design reduces the overall cost and also is more efficient. It must be noted the inverter remains OFF when usable utility power is available.
What is an Online UPS?
An Online UPS works always from a floating DC voltage (of batteries). Batteries are parallely being charged when mains is present so that they do not get discharged at all. An ONLINE UPS has a charger which is called rectifier and an inverter section. So in an Online UPS the change over time is zero. In addition, a true Online UPS should have isolation between input and output and should have pure Sinewave output.
Advantages of ONLINE UPS over LI UPS
What’s Cold start?
When computers are switched on initially it takes too much of power for a short duration (usually for few cycles of supply) and the UPS should be able to give such type of surge power to computers during switch on.
What is an “advanced charger”?
For optimum output, maximum efficiency, healthy performance and long life of the batteries, both the DC voltage of battery and Specific gravity of the acid in it should be built up gradually during charging. Conventional chargers are not capable of this where as a microcontroller or DSP based design with the right logic ensures these conditions and parameters.
MOSFETs, TRANSISTORs, IGBTs and SCR
Transistor is an electronic device used to control voltage and current flow. Mosfets, (Metal Oxide Semiconductor Field Effect Transistor) are much faster than transistors, do not generate much heat, and are smaller and superior. IGBT (Insulated Gate Bipolar Transistors) is a combination of transistor and MOSFET and has all the qualities of a MOSFET. They have an additional advantage of using in high voltages and high power applications.
SCR(Silicon Controlled Rectifier) is used in charger cum rectifier.
What is a power factor corrected UPS?
Power factor of a product decides the efficiency and power consumption of that product. In conventional thyristor charger (rectifier) systems, PF may be normally 0.3 to 0.5 This means that a 5kva UPS will use more than 10kva, in most conditions.
A power factor corrected UPS will have an IGBT Recifier (charger), and PF typically be 0.85 to 0.97 This ensures minimal power usage and losses resulting in high efficiency.
It is a measure of the total performance of a system in %. In UPS we take charger and inverter efficiencies seperately.
Efficiency of a UPS can be defined as total output power divided by total input power multiplied by 100. In UPS systems, the heat generated is called losses. The lesser the heat generated, the more the efficiency. Usually inverter and rectifier efficiency will be more than 80%. In PFC UPS, efficiency will be very high.
Total Harmonic Distortion
Usually ONLINE UPS generates pure sine wave output, OFFLINE UPS generates Quasi Square/ Quasi Sine and Inverter generates Square wave or Sinewave depending on the model.
Total harmonic distortion (THD) is a measure of the purity of the sine wave and the value nearest to zero is the ideal. Usually, UPS systems will have THD less than 5%. fier efficiency will be more than 80%. In PFC UPS, efficiency will be very high.
This feature also is relevant only for Online UPS systems, that too for highly critical load.
There are various applications of computers where high reliability is required and zero down time. Some of the examples are banks, process automation industry etc. The UPS supplied also should be equally reliable and here comes the requirement of redundant systems. The load will be connected to two UPSs of the same capacity, where the output power of the UPSs are synchronized and supplied to the same load. If and when one UPS fails, the other will supply power withour interruption. Here the mean time between failures is absolutely negligible. These types are very costly and are made to order from factory.
Static and Manual Bypass
This again is a feature of Online UPS systems.
Sometimes, it will be required to shutdown the UPS for repair or maintenance work and the load connected may be so critical that the power supply should be available even during such situations.
The manual Bypass or Statuc bypass is used in such cases, where the load can be transferred to mains supply directly, bypassing the UPS, by means of a manual switch or an electronic switch (with zero delay time) respectively.
The static bypass can be unidirectional or bidirectional. When its bydirectional, it automatically switches to UPS supply, when its restored.
3 Phase and Single Phase
In our country we have 3 phase electric distribution system. For low power conumsers a single phase connection is given and for heavy and medium users, 3 phase connection is given.
In UPS, normally, upto 5 KVA input supply to the UPS will be single phase and for higher than 5kva 3 phase input is selected. The output of the UPS can be 3 phase or single phase. This depends on the load structure, but for all computer applications, usually, output will be single phase.
UPS Management software and interface
Normally, this is feature of high end Online UPS systems.
Some UPS systems can be connected to computers by a communication port (RS232) and with the help of a software, the user can monitor the status of the UPS and various readings like current, voltage, battery power level etc. on the screen of the computer itself. The user can even shut down the UPS from his keyboard at the end of the day.
There are many variants to this feature such as monitoring and controlling all the PCs in a network, monitoring a UPS connected to a network of PCs remotely etc.
Simple Network Management Protocol (SNMP) is a very popular protocol used in such networks and the UPS interface and the software should be compatible to connect and work in such environments.
How to select proper battery for UPS Systems
Backup time (BUT) is given by the formula-
BUT IN HRS= VOLTAGE OF BAT * AH OF BAT / WATTAGE OF THE EQUIPMENT
Note : this formula is with the assumption that battery is 100% efficient which may not be the case in reality. So, actually, BUT will be a little less than what we derive using this formula.
Example: 10 nos. of 120AH batteries on a 5KVA UPS
Backup time will be, 120*120/5000 = approximately 3 hrs. (this may practically be, 2.5 hrs.)
For a given backup time, the reverse of this formula could be used to get the AH of the battery.
Automotive batteries Vs Tubular batteries
Automotive batteries are designed for starting the automobiles which require enormous amount of current during starting and afterwards no current requirements and they are called start batteries. A tubular type battery is used where continuous discharge of heavy current for long durations are required.
An automobile battery consists of positive and negative plates and are made up of pasted plates on a lead grid. Whenever the battery charges and discharges there is a density change in the plates. So the plates expand and contracts. If an equipment contracts with a pasted material on it, the active material comes out and it is called shedding. So after many cycles of charging and discharging the main portion of the plates sheds and a 100 AH may become 60 AH battery next year, 40 AH battery next year and like that.
If by any chance this shedding process can be eliminated, we get a long life for the battery. So use the thicker tubular plate batteries. They use a bag to cover it and we have a different type called tubular type battery.
How to calculate the UPS capacity required, based on the requirements of a Customer?
Please refer the Load Chart given elsewhere in this site, decide the load (equipments) that are required to be connected to the UPS, add the total value and then add 10-15% of this value to derive at the suggested UPS Capacity.
Whats the difference between Wattage (W) and VA?
It is a measure of the maximum power handling capacity of the UPS. The Wattage is the actual power whereas the VA is the apparent power. This depends on the nature of the load such as resisitive, inductive, capacitive, switching etc. and varies widely. For practical purposes we can very generally consider 0.8 as the load power factor and calculate the UPS VA rating required for a particular load.
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