Monday, October 30, 2000		On Hardware

Selecting the right UPS

AS the name suggests, Uninterruptible Power Supplies (UPS) are designed to supply uninterrupted and clean power to critical instruments like computers, medical / life support systems and industrial controls. The UPS market is growing in direct proportion to the PC market. Numerous makes and models are available in the market that can easily confuse any prospective buyer.a

In order to select the best UPS, one has to understand the technical details highlighted by the manufacturers to market their product. The first question that comes to the minds of most persons is: Why an UPS and not an ordinary power inverter? The answer lies in the quality of power required by sophisticated equipment that require a stable synchronised clock for continuous processing. Any disturbance in the clock resets the whole operation. Due to a longer transfer rate between the mains and the backup, the backup power is not phase synchronised. There is therefore, a possibility of the system losing clock, resulting in system reset and data loss.

Based on configuration, a UPS can be classified into offline, online or line interactive types. An offline UPS consists of a battery charger, battery bank, inverter and a transfer switch. Under normal conditions, the load is directly supplied from the unconditioned line and battery is charged. In case of power failure, the switch transfers the load from the line to synchronised inverter section. For a smooth transfer, the transfer time is kept to less than five milliseconds.

In an online UPS, power in both normal and backup modes is supplied through an inverter powered through batteries, which are continuously charged by the charger. Of all three types mentioned above, an online UPS provides maximum protection. This is because of isolation from fluctuations in the mains resulting in a clean and regulated power. This kind of a UPS is best suited for networked computers, sophisticated medical and communication equipment.

The third kind – a line interactive UPS consists of a single power converter (which can acts as a battery charger as well as an inverter) and an automatic voltage regulator (AVR). Under normal conditions, the converter acts as rectifier charging the battery. But in times of a power failure, it acts as inverter and supplies power to the load. Due to a poor switchover time, a line interactive UPS is not very reliable and can be used only for stand alone PCs.

Another feature to look for in a UPS is the shape of the output current or in other words – its performance. In a UPS, the inverter shapes the current, depending on the method by which it is controlled. Smaller UPS (mainly offline & line interactive type) uses phase modulation technique, giving a quasi-square output. The online UPS generally use high-frequency Pulse-Width Modulation (PWM) techniques in their inverters, producing a near sinusoidal waveform, thereby minimising losses. However, the quasi-square current in a line interactive type UPS, makes it less efficient as it contains large amount of harmonics that add to losses. Excessive harmonics produce heat and can damage insulation of various components. PWM technique used in online UPS becomes a preferred technique.

The semiconductor power device used in a UPS can either be a MOSFET or IGBTs. MOSFETs being the most popular devices for low power applications and IGBTs in the medium power range. However, some products in the market still continue to use Bipolar Junction Transistors (BJT). The BJT is prone to damage under stressful power conditions. JBTs require a critical driving circuit and are rendered inoperational at high frequencies due to inherent property of the device. The MOSFETs and IGBTs do not require complex driving circuitry and they also offer self-protection under strained conditions. Do not go in for a UPS that uses JBTs.

One of the other major components in a UPS is its magnetics. To keep the pricing down, a UPS is generally built with two iron core transformers. This makes the UPS very bulky. A small ferrite core transformer, which is light and less voluminous, can perform the same task. So, one should go in for a UPS with ferrite core transformer rather than one with an iron core transformer.

Power rating is another important feature to be kept in mind while purchasing a UPS. Computer load can be calculated by adding individual load of the peripherals and load of the PCs. Keep future expansion in mind. If the load exceeds 1kVA, an on-line UPS becomes an ideal choice. Commonly available UPS ratings are 500VA, 625VA, 1kVA, 3kVA and so on.

Look at the supply voltage range of the UPS - wider is better. In simpler terms if the UPS works on mains under extreme voltage conditions, it will involve less uses of batteries. The range of input voltage is dependent on the size of the transformer.

Normally 15-20 minutes of back-up time is generally adequate. But this would solely depend on the power supply in your area. If the back-up time required is more than 2-3 hours, it is advisable to use an UPS backed by external batteries and if possible a standby generator. For UPS working in an environment where a back up of more than half an hour is required, ensure that the UPS has a fast battery charger. Check out the kind of batteries used in the UPS, normally for small UPS use sealed lead-acid batteries with fast chargers.

UPS should generally be protected against short circuit at output, where current may rise to a very high level. Similar protection is also required against high and low voltage at the battery terminals. Additional enquiries should be made for overload capacity (time and % overload that UPS may sustain), total harmonic distortion (indicates losses) and variation in output frequency.

An UPS with indicators and switches on the panel make working easier. See that the UPS also has an alarm indicator. Some UPS can be connected to the PC through a serial port. The connectivity updates the PC about the status of the supply during the back-up mode and all the applications are automatically closed through task manager in the windows, when the UPS batteries get drained.

Finally, physical inspection can detect workmanship defects, sharp edges, scratches etc. Check if the UPS chassis is properly grounded to avoid electric shocks. Don’t forget to check the company’s background such as, service network, ISO certification and after sales support.

— Vishal Verma