Energy can be stored in different ways. Rechargeable batteries are the most common. For simplicity the examples and illustrations in this article will be based on that technology. However, kinetic energy can also be stored in heavy, rotating flywheels or energy can be stored as fuel.
What are the different types of UPSes?
The most commonly used type of UPS is also the most effective, generally called a full-time or full double conversion UPS. For any UPS, incoming utility power is alternating current (AC), which is also what is required by most information technology equipment (ITE).
Batteries, on the other hand, are direct current (DC) devices, so all battery-type UPSes must convert — or rectify — the incoming AC power to DC to charge the batteries. The UPS must also still deliver AC to the ITE, so DC power must be converted back to AC through a device known as an inverter.
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In a double conversion UPS, power flows continuously through the rectifier and then through the inverter to the ITE. The output voltage and frequency are completely isolated from, and independent of, the input voltage and frequency. They can even be completely different than the input, so this system is technically classified as voltage and frequency independent (VFI).
Voltage and frequency independent: Figure 1 below shows a VFI system in normal operation. Anomalies in the input power are dealt with in two ways. A surge suppression device (SPD) absorbs particularly bad voltage spikes. These can be caused by lightning strikes on power lines, large motors as used on elevators or medical electronics equipment; welders or numerous other sources. But even the smallest variations, including voltage sags or brownouts never make it through a VFI UPS to the output.
Batteries are excellent electrical shock absorbers, but they also maintain a steady and constant voltage to the inverter, which completely resynthesizes the voltage and current so that the power delivered to the ITE is clean and steady. Connecting air conditioners or other motors to the UPS serving the ITE could contaminate this clean output power, so it’s not recommended.
Note the bypass circuit going around the UPS. We’ll address that later.
The battery is always in the circuit in normal operation, delivering small amounts of power when needed, such as during brownouts, so there isn’t the slightest interruption in output power.
When utility power fails, as shown in Figure 2 below, the battery continues to deliver stored energy to the inverter, which continues to deliver clean power to the ITE. When utility power is restored, power flows back through the rectifier, feeds the inverter and recharges the batteries.
UPS static and maintenance bypass: UPSes are not uninterruptible. They are electrical or mechanical devices, so they not only require routine maintenance, but also are subject to component failures. For these reasons, all UPS systems have a built-in bypass to route incoming power around the system and directly to the ITE when necessary.
The high-quality SPD is still in the circuit but is only slightly better than running your home electronics on a power strip with surge protection. It won’t stop utility power interruptions or deal with voltage sags or brownouts. If the UPS fails, the bypass operates immediately as a static switch.
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