Three uninterruptible power supply systems exist – offline, line interactive and online double conversion. These are designated VFD, VI and VFI according to the standard EN62040.
The offline UPS is a standby system that powers the load directly from its input circuitry and only switches to backup power when utility power fails. This type of system is less expensive than a line interactive or double conversion system.
Autotransformer
The autotransformer is an integral part of a UPS system. It is used to supply a percentage of the AC power supply voltage to the secondary load and helps protect against brownouts or surges caused by Undervoltage conditions.
You can also use it to increase the line voltage supply in response to a load surge. In this case, it is called a Variable Autotransformer or variac.
A Variac can be constructed with a single primary winding wrapped around a laminated magnetic core. Still, it can also be built with multiple “tap points” along its length, allowing it to vary the output voltage. When the primary 50/60 Hz power source fails, this kind of transformer is frequently used in low-voltage applications to switch between a battery and line voltage. You can tap it at different points on its winding to provide different voltages.
In a step-up transformer, the AC supply is connected to the fixed terminals A & C, and the load is connected between the two secondary terminals B & C. When the load current (I1) flows, the number of turns in the primary winding N1 between the A & C points exceeds the number of turns in the secondary winding N2, and the turn ratio becomes less than 1, which is the condition for a step-down transformer.
Inverter
Inverters take direct current (DC) from a battery or other DC power source and convert it to alternating current (AC) power. Almost all household appliances and electrical equipment use AC, including computers and cell phones.
When an electric motor or other high-power appliance is involved, an inverter may have a higher capacity rating than the battery in some circumstances. Alternatively, the inverter can be powered by a chain of batteries.
Smart inverters can also respond to changes in the frequency of power produced by the solar system. It helps grid operators stabilize the electricity network against disruptions.
A common inverter topology consists of an onboard microcontroller rapidly switching on and off power MOSFETs at frequencies like 50 kHz. The output of these MOSFETs goes through step-up transformers and a series of capacitors to produce a modified sine wave signal.
The inverter can then be regulated by a pulse width modulation technique, which involves changing the frequency and pulse width of the square-wave pulses that make up the output voltage. Depending on the kind of circuit the inverter is intended for, you can add a filter to either the inverter’s input or output.
The inverter can also be used as a generator, converting DC power to AC power for generating electricity. It is helpful in mega solar power plants or using the energy produced by an electric car.
Battery
The battery in an uninterruptible power supply (UPS) system is an essential component that protects critical electrical equipment. The battery stores energy and delivers it to the inverter when there is no clean electricity from the utility. It allows the inverter to resynthesize clean electricity for input terminal equipment (ITE) use.
Different types of UPS batteries have different life spans and service requirements; some require more maintenance than others. For example, lead acid batteries require a higher cost and have a shorter useful life than lithium-ion batteries, which are becoming increasingly popular for UPS systems.
Lithium-ion batteries are also lighter and less expensive than lead-acid batteries, so they are an option for UPS applications where weight and space are essential. They are typically used in single-phase UPS installations, though you can also use them in three-phase configurations.
Vented lead-acid (VLA) or flooded batteries are the most common type of battery in UPS units. These have a very long design life and are highly reliable. Still, they require additional safety measures, such as separating the battery from other IT equipment and a well-ventilated area to minimize the risk of thermal runaway.
Nickel-cadmium (NiCad) UPS batteries have a very high design life but are more expensive than VRLA batteries and contain toxic materials. They also have more safety concerns, requiring more maintenance than VRLA batteries.
Charger
The UPS is an electronic device that stores energy in a battery and supplies it when the power goes out. It is designed to prevent damage to your computer or other sensitive electronics by protecting against line noise interference, Undervoltage, overvoltage, power sag, and power surge.
It also protects against overheating the batteries, which can cause them to fail prematurely. Your ITE’s power may be cut off automatically when the battery life reaches a certain level, depending on the size and design of your UPS.
Most modern UPSes consist of several smaller units that are easily combined to provide capacity, redundancy or both. The system can also be configured to use multiple batteries, increasing battery longevity by minimizing the number of charged and discharged batteries.
Batteries are charged by an electronic device that converts incoming utility AC electricity to DC power for charging the batteries, then converting it back to AC for powering the equipment. Generally, the charger is connected directly to the batteries for maximum reliability, but the batteries may be attached to an inverter if necessary.
The charger uses a voltage divider to regulate the current and a programming resistor to accept a fixed DC to charge the batteries. The voltage on the program resistor (PROG) is monitored and set by the LTC1734 to ensure that the charging current is always at a specified value when the charger enters a constant current mode. If the PROG pin is pulled above the 2.15V voltage or allowed to float, the charger enters a manual shut-down mode and charging stops.
