Using buffer modules for increased system up-time
07Apr
Figure 1: TDK-Lambda DBM20 DIN rail mount buffer module
Short interruptions or dips in the AC input supply voltage, although rare and whatever their cause, can result in an industrial manufacturing facility temporarily stopping production, impacting profitability. Buffer modules may be used for increased system up-time.
Although some machinery may tolerate a loss of power due to their mechanical inertia, they are often reliant on digital controls and programming. An interruption of a tenth of a second (100 milliseconds) may cause a reboot of an onboard computer, taking several minutes, and even require manual intervention to resume operation for safety reasons. For example, a robotic arm would not want to be left in an undesirable state or position.
Common causes of power interruptions are lightning strikes on high voltage power lines, snowstorms, falling or moving tree branches during strong winds, or even animals. In rural areas, the risk is higher as power lines are more likely to be located above ground.
AC-DC power supplies provide the regulated voltages and currents needed for electrical and electronic equipment to function. They can withstand an AC power loss for only several thousandths of a second (milliseconds) before the output voltage drops to zero.
Figure 2: Buffer module connection
Preventing equipment shutdown
Back-up generators are suitable for long term power interruptions which may last several hours or even days. These will take several minutes to come online, however.
Computer systems are often backed up by an uninterruptable power source (UPS). They utilise batteries to store the back-up energy, requiring periodic preventative maintenance schedules and occasional battery replacement. As they continually operate, they consume more energy, cost more to operate, and add considerable cost to the system.
Figure 3: No buffer module across the power supply output
The great majority of interruptions and dips in the AC supply occur for less than 0.5 seconds (500ms). As a standard AC-DC power supply may only provide hold-up to 50ms, if it is not fully loaded, additional energy storage is required.
Buffer modules
As the name implies, a buffer module “buffers” the load from short interruptions in the AC source. The load could be a digital process control board, relays, contactors, or motors. Energy is stored within the module in electrolytic capacitors, like those found inside an AC-DC power supply. DIN rail mount buffer modules are the most popular (Figure 1), although open frame printed circuit board (PCB) construction units are also available.
Figure 4: Buffer module across the power supply output
The buffer module is connected across the output of the power supply, as shown in Figure 2.
If the AC power is briefly interrupted, the AC-DC power supply output will begin to drop as its internal energy is depleted. At that point, the buffer module then provides power to the load. When the AC returns, a controlled amount of current will flow from the AC-DC power supply into the module to recharge it.
The timing diagram in Figure 3 shows the impact of an AC loss without a buffer module, where time T is the power supply’s hold-up time (typically 20 to 50ms).
Figure 5: Providing power to the load for an extended time
When a buffer module is connected across the power supply output (Figure 4), voltage is always present at the load during the AC interruption. During “T”, the power supply delivers the power to the load, and during “Tb”, the buffer module provides power until the AC recovers.
In the event of an extended AC loss, the buffer module can be used to power the load for a longer time, allowing the equipment to shut down safely or to store vital process information for a faster restart when the AC power is restored (Figure 5).
The time (Tb) is stated on the manufacturer’s buffer module datasheet at a specified power level. For example, the 20A rated TDK-Lambda DBM20 provides an additional 250ms at a power level of 448W. When operated at lower power / current levels, the hold-up increases, see Figure 6.
Figure 6: DBM20 buffer time vs load current
For a load current of 5A, the buffer time (Tb) will be approximately 1 second. A load current of 2.5A will provide 2 seconds hold-up. Buffer modules can be connected in parallel to further extend the hold-up time. Note, the curves labelled as “typical” include the tolerance in the electrolytic capacitors.
As the buffer module operates independently of the power supply and the end equipment, it can be added after installation. No AC wiring is required, and no batteries are used, requiring maintenance or replacement. If a particular machine location is found to be more prone to short AC interruptions, a field retrofit may be possible, providing there is sufficient space on the DIN rail for mounting, of course.
More on TDK-Lambda may be found here.
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