An Uninterruptible Power Supply (UPS) consists of a battery, an inverter, and a rectifier circuit. The grid-supplied-AC is rectified to direct current (DC) to charge the battery when the grid is operational.
The battery supplies DC to the inverter to power the AC load for as long as the battery charge is maintained at a minimum state of charge (SOC).
A UPS is a special type of inverter where the inverter circuit always works on converting the battery-supplied DC to power a fixed AC load that cannot tolerate power interruptions. As the UPS inverter is always on, there is no switching time when the grid AC used to charge the battery is interrupted.
The AC-power supply to the UPS is used to maintain the battery state of charge at a sufficient level to keep the inverter operational. It is true to say that a UPS is a special type of inverter system. There are three types of UPS systems:
- Online UPS
- Offline UPS
- Line Interactive UPS
Let’s look at the various definitions more closely and when a UPS can be used as an inverter.
The Definitions Of An Inverter And A UPS
An inverter is designed to convert direct current (DC) from a DC source such as a battery or solar panel to alternating current (AC) to power office, workshop, or household appliances and devices.
In a grid-tied power supply system, the inverter will sense when the grid power (AC) is interrupted and switch on the inverter to power the AC-powered devices. The time between the grid-AC-outage and the inverter-supplied AC turning on is critical to some appliances.
Computer systems cannot sustain power interruptions for even a micro-second as they will lose all data in their operational memory system. Such systems need an uninterrupted power supply of constant voltage and operational frequency.
For this reason, electrical engineers devised a special combination of power supply components that will provide a constant and uninterrupted AC power supply, allowing the system to remain running when the AC-power grid goes down for as long as the source of direct current is sufficient.
What is a UPS?
An Uninterruptable Power Supply (UPS) is a device that continually supplies AC power from an inverter that converts battery supplied DC power to AC for as long as the battery bank state of charge remains sufficient.
When the grid power is operational, the grid AC is converted to DC by a rectifier circuit in the UPS to charge the battery and maintain the battery state of charge optimally. The inverter, battery, and rectifier combination is called a UPS.
The UPS must have sufficient backup power capacity to allow appliances such as computer systems or life support systems to continue operating until the grid power or a backup power generator comes online.
What are An Online UPS?
An Online UPS supplies power to an AC load during normal grid power using the built-in inverter to convert the battery DC supply to power the AC load.
The rectifier circuit in the UPS converts the grid AC to DC to charge the battery. The UPS serves as a filter between the grid AC, and the AC is needed for critical power devices.
There is no switching when the grid power is interrupted, as the UPS inverter will continue to function for as long as the UPS battery has sufficient charge. The UPS supplies a pure sine wave AC which enhances the performance and longevity of the connected AC devices.
An Online UPS generates a lot of heat and thus requires large heat sinks and heat dissipation to withstand being operational full-time. These factors make Online UPS expensive.
What Are Offline UPS?
An Offline UPS is connected to the AC load but has a bypass circuit that allows the AC-powered device to be powered directly from the AC grid when it is operational. The Offline UPS uses some grid AC to charge a battery via a rectifier circuit.
The Offline UPS will sense the presence of the AC grid power and will switch to battery DC converted to AC when it detects a disruption in the grid power supply. Offline UPS inverters typically require between five and ten microseconds to make this switch.
The battery will stop charging as there is no longer grid AC available, and the inverter will draw DC from the battery to convert to AC needed to power the AC load. The UPS remains offline when the grid AC is available and only activates when the grid power is interrupted, hence “Offline UPS.”
An Offline UPS is more efficient as it does not draw power when the AC grid is active, thus making it run cooler and not needing large heatsinks and is thus a lot cheaper than Online UPS systems.
Offline UPS system cannot be used with AC-loads that cannot tolerate any voltage disruptions such as computers.
What Is Line Interactive UPS?
Line-Interactive UPS systems are based on the best features of Online and Offline UPS systems. When the AC power grid is active, the Line-Interactive UPS will act as an Online UPS and use the grid-AC to charge the battery while allowing the grid AC to power the AC loads.
When the AC-grid power is interrupted, the Line-Interactive UPS will require less than five microseconds to switch to the battery-supplied DC to power the AC loads. This type of UPS is more sensitive to grid fluctuations and will react more quickly than Offline UPS systems.
Line-Interactive UPS systems are ideal in areas where the grid-AC is not often disrupted but has frequent voltage fluctuations. The UPS provides much better voltage filtering capabilities and very low switching times.
Line-Interactive UPS systems are more expensive than Offline UPS systems but significantly cheaper than Online UPS.
When To Use An Inverter Or A UPS
The off-grid power system will require the use of an inverter. By design, an off-grid power supply system uses solar, wind, or hydropower generation to charge a large backup battery bank from which AC and DC loads can be powered.
An inverter will be required to convert the battery-supplied direct current (DC) to power the alternating current (AC) loads.
An offline UPS system will be ideal for powering small AC devices that require a constant and stable AC power supply for grid-tied homes.
The domestic appliances that require a constant power supply are fridges and freezers, lights and air-conditioners. These types of devices are not sensitive to switching.
For offices running computer systems and offline communication systems or offline or line-interactive UPS systems will suffice.
The lights, computers, and cooling systems are the most important AC loads that need power during grid outages.
For banks, hospitals, or security installations where no power interruptions or voltage fluctuations can be tolerated by the computers, cooling systems, communications systems, or security infrastructure requires an Online UPS to function.
Depending on your power needs, you can design your UPS by determining the amount of backup power needed and the size of the battery bank to store enough energy to power an inverter to which your AC loads can be connected.
Your system can be designed for complete off-grid or grid-tied applications with an online or offline switching arrangement.
The critical assessment needed is which AC-powered devices you need to power, how long, and whether they can cope with switching.