Siemens 24 V nonstop User Manual
© Siemens 2020 1
24 V nonstop –
even upon
power failure
The right UPS system for every
application –
summary of advantages and
disadvantages
A reliable power supply is essential for guaranteeing the productivity of automated
plants and machines. PLCs, sensors and actuators are usually provided with
24 V DC from a switched-mode power supply. Modern power supply units such as
SITOP offer a maximum degree of security for the supply. However, they are not
invulnerable to longer power supply failures. Critical applications therefore require
upgrading to an uninterruptible power supply. But which UPS system is the right
one, and what must be considered when dimensioning?
Application Note | 24 V nonstop – even upon power failure
© Siemens 2020 2
AC or DC UPS?
In order to provide protection against power failure, an uninterruptible power supply can be used on the AC or DC
side. The advantage of an AC UPS is that it provides buffering of all electrical consumers, e.g. also of AC drives. However, an AC UPS is more expensive than a DC UPS. If it is permissible with the application to only buffer the 24 V side in
the event of a power failure, a DC UPS is quite definitely the
more economical solution. On the one hand, the powers required are usually smaller, resulting in smaller dimensioning
of the DC UPS, and on the other hand an AC UPS is always
more expensive because of its increasingly complex design.
Furthermore, the total efficiency is significantly better with
a DC UPS. This is because conversion of the battery voltage
into an AC voltage and the repeated transformation into the
required 24 V DC voltage are unnecessary.
With the DC UPS, the energy is provided where it is required, namely directly on the consumer without "loss-making detours".
The right solution for every application
No other manufacturer of power supplies provides such a
comprehensive range of units for safeguarding a 24 V DC
supply like Siemens does. The range extends from a simple
buffer module up to the multi-function DC UPS.
24V DC power supply from SITOP can be combined with
A
3 different solutions for 24V buffering:
Additionally to the above 3 solutions, the PSU8600 offers
its own solution for buffering ranging from millisecond to
hours.
Figure 1: Buffering the 24V control circuit with the SITOP PSU100S
power supply, SITOP DC-UPS UPS1600 and 2 battery modules SITOP
UPS1100. The DC-UPS is fully integrated via PROFITNET in the
automation system.
Note:
The SITOP PSU8600 system power supply and its buffering
technologies merit its own application note. For more information, please see www.siemens.com/sitop-psu8600
for more information.
Application Note | 24 V nonstop – even upon power failure
© Siemens 2020 3
Bridging brief power failures
When power supply conditions are unstable, for example
in low-meshed network infrastructures, brief power failures may occur occasionally or even frequently as a result
load transfers in the network, for example. Problems following such interruptions with non-buffered power supplies are the long ramp-up times and initialization of the
automation system or involved drives. It is already possible
to significantly increase the plant availability by using a
buffer module for bridging such brief interruptions of up
to 10 seconds. The buffer module is simply connected in
parallel with a 24 V SITOP power supply. The electrolytic
capacitors supply up to 40 A which supports the power
supply even in the event of an overload.
Protection of plant status upon power failure
In applications where a plant is to be switched off in the
event of a power failure with retention of the last plant
status, extended bridging of the power failure is required.
Such requirements are typical for PC-based automation,
visualization, or archiving of operating data. Recording of
the failure, saving of the plant status, as well as controlled
shutdown of the PC require bridging in the minute range.
Comparatively high buffer reserves are required in such a
scenario by powerful industrial PCs, especially when a
large panel has to continue to operate during the shutdown. High buffer capacities are also required by actuators
which have to be driven into an end position or processes
in which plant components must continue to be powered
until the power supply is restored. This is the case, for example, when measured data is being recorded or a communications link must be maintained. Uninterruptible
power supplies (UPS) are required in such situation
SITOP power supplies with 24 V output voltage can be upgraded into a fully-fledged UPS. Two different UPS concepts are available corresponding to the mentioned requirements. They mainly differ in the type of energy store.
One of them is based on lead or lithium batteries, the
other on double-layer capacitors. All DC UPS modules have
the same basic functionality with comprehensive monitoring functions and signaling contacts and are available with
a USB interface. The battery based SITOP UPS1600 is also
available with an Industrial Ethernet/PROFINET interface
and communicates also over OPC UA.
The free software tool SITOP Manager provides simple integration into PC-based automation solutions. They support further processing of status messages, safe shutdown, and correct restarting of the system. The UPS1600
with IE/PN interface can be configured via the TIA Portal or
integrated in TIA (Totally Integrated Automation) with
STEP 7 and WinCC
The energy storage makes the difference
Whether the capacitor-based or the battery-based concept
is right for the respective application depends on the respective demands. If long buffer times are required, the
UPS with batteries is the best choice. They can supply energy for hours depending on the current requirements.
Battery modules for the UPS1600 are available with capacities ranging from 1.2 Ah to 12 Ah. Connection in parallel
allows for a flexible combination for the required capacity,
up to 72 Ah. A UPS1100 battery module is equipped with
electronics with specific parameters as well as for recording the current operating data that are read by the
UPS1600 UPS module via a two-wire cable (Energy Storage
Link).
The UPS modules are available with rated output currents
of 10 A, 20 A and 40 A. They also offer high overload capability and can supply three times the rated current for 30
ms and 1.5 times the rated current for 5 s per minute.
In many cases, a plant can be brought into a safe status
within minutes, and thus minimize the effects of a power
failure. Many advantages for such time requirements are
provided by the SITOP UPS500 based on double-layer capacitors. These are also referred to as ultracaps, supercaps
or super capacitors because of their high energy density.
The innovative UPS for installation in control cabinets consists of a basic unit with energy storage of 2.5 or 5 kWs,
and delivers an output current of up to 15 A. Add-on modules of 5 kWs each permit configurations with up to 20
kWs.
The type of energy storage is not only critical for the buffer
time, is also decisive for the possible applications of the
two types of SITOP UPS systems.
Available capacity of lead batteries is highly dependent
on the temperature
Lead batteries are extremely temperature-sensitive, since
the charging and discharging processes of a battery are
the result of an electrochemical reaction. Aging depends
on the electrolyte used (sulfuric acid) and the plates serving as poles (lead and lead oxide) and is highly temperature-dependent. Temperatures higher by 10 K reduce the
service life by half. With an ambient temperature of 40 °C,
for example, the service life is therefore only 1/4 of that
at the rated operating temperature of 20 °C. A lead battery with a service life of 4 years at the rated conditions
must therefore be replaced after only one year when used
at 40 °C.
Special batteries with an increased thermal stability can
also be used as an alternative to conventional lead batteries, but these are also more expensive. SITOP offers, for
example, a high-temperature battery with pure lead
plates for use at temperatures from -40 to +60 °C.
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