Why control systems need backup power
A motor can usually tolerate a brief loss of supply and restart. A controller often cannot. When the power to a controller, network switch or instrument disappears for even a fraction of a second, the equipment can reset, lose its state, or cause an unexpected restart of a process. Operator screens go blank, and data may be lost or corrupted. For critical processes, an interruption of the controls can be more disruptive than the interruption of the machine itself.
An uninterruptible power supply, or UPS, bridges that gap. It supplies power from stored energy, usually batteries, when the normal supply fails or misbehaves, for long enough to ride through the event or shut down in an orderly way.
This article is a general overview. Selection and installation should be done by qualified personnel following manufacturer instructions and local codes.
What a UPS does
A UPS typically has a charger and rectifier section, a battery, and an inverter that produces alternating current from the battery when needed. Depending on the design, it also conditions the power by smoothing out sags, spikes and noise. It is not a long-term source. Battery systems generally support minutes, not hours, unless sized specifically for longer. Standby generators, if available, takes over for extended outages, and the UPS covers the gap while it starts.
Main UPS types
Offline or standby. The load runs from the mains supply directly, and the UPS switches to its inverter when the supply fails. There is a short transfer time. These are simple and economical, and are suited to less sensitive loads.
Line-interactive. Similar to standby, but includes voltage regulation so it can correct moderate high or low voltage without using the battery. This reduces battery wear in places with unstable supply.
Online or double conversion. The incoming power is always converted to DC and then back to AC, so the load is isolated from the supply and runs from the inverter at all times. There is no transfer time. These provide the cleanest power and are widely used for critical loads, at the cost of complexity and some added losses.
DC UPS and DC power systems. Many control systems run on 24 V DC. Dedicated DC supplies with battery modules, sometimes called DC UPS units, can back up that control voltage directly, avoiding conversions. Plants often combine a regulated DC power supply with a battery or capacitor-based buffer module. Larger substations and switchgear use battery and charger systems for protection and control power.
What to protect
A common approach is to rank loads by how serious an interruption would be.
- Programmable logic controllers and their I/O, particularly where a restart would create a hazard or a lengthy recovery.
- Distributed control system and SCADA servers, historians and operator stations.
- Network switches and industrial networking equipment that link the controllers.
- Safety systems and emergency shutdown logic, which usually follow specific design rules.
- Critical instruments and analyzers.
- Communications, alarms and emergency lighting, which have their own requirements.
Variable speed drives are also normally left off. Large motors, heaters and other heavy loads are normally not put on a UPS. Doing so would require a large unit and offers little benefit.
Sizing and runtime basics
Sizing begins with listing the loads and their power draw, often in watts and VA, and adding margin for growth and for the efficiency of the UPS. The required runtime follows from the intended purpose: ride through a short dip, allow a generator to start, or support a controlled shutdown. A longer runtime means a larger battery.
Capacity ratings assume certain conditions, and the usable runtime depends on the battery’s age, temperature and load. As an illustration with hypothetical numbers, a unit with a nominal runtime of 10 minutes at half load might give noticeably less when fully loaded or when the battery has aged. The manufacturer’s runtime curves are the reliable reference.
Batteries
Batteries are the part of a UPS that most often fail. Common types include sealed lead-acid and lithium-based batteries, and each has its own life expectancy, temperature sensitivity and replacement considerations. Heat shortens battery life considerably, so placement and ventilation matter. Because failure is often invisible until the moment of need, batteries are typically tested and replaced on a planned schedule rather than waiting for a fault.
Installation points
- Place the UPS where it has adequate cooling and clearance, and where staff can reach it for service.
- Provide a way to bypass it safely for maintenance, as the manufacturer describes.
- Coordinate its output protection with downstream breakers so a fault on one branch does not collapse the whole supply.
- Consider how it interacts with generators, which can behave differently from the utility supply and may require compatibility checks.
- Label what the UPS feeds so that people know what depends on it.
Maintenance and testing
Battery and charger checks fit naturally into predictive maintenance records, and plant maintenance data can show when capacity is falling.
Practical routines include checking alarms and status, inspecting for heat, swelling or leaks, testing transfer and runtime under controlled conditions, replacing batteries on schedule and keeping firmware and settings documented. Testing should be planned so that it does not risk the very loads that the unit protects.
Safety
A UPS contains stored energy and can keep its output live even with the input switched off. Work on it, including battery handling, should be done only by trained personnel who follow the manufacturer’s manuals, lockout tagout procedures and local regulations.