Supplying critical DC loads safely, increased system availability, confined spaces, and tough ambient conditions – system designers and operators are faced with a variety of challenges. New DC UPS models from the Trio Power series offer interesting solutions – a power supply unit, switching unit, and battery charger have been combined in one device.
An uninterruptible power supply, or UPS for short, is used wherever mains fluctuations or mains failures may occur. The aim is to prevent the failure of production systems caused by short-term interruptions to the power supply. In the event of prolonged mains failure, the UPS should ensure that a safe system state is entered. In addition, uncontrolled crashes on control systems such as industrial PCs should be avoided in order to prevent data loss and other damage.
Structure of a UPS system
The UPS system for DC loads generally consists of at least three components. First, the existing AC mains voltage is converted into a 24 V DC voltage by a power supply unit. This voltage now needs to be buffered. To do this, a dedicated DC UPS is used which combines two functions. Firstly, it acts as a switch which switches to the load supply provided by a connected energy storage device in the event of mains failure. Secondly, the UPS contains a charge controller which recharges the connected energy storage device during mains operation and ensures that it is charged and ready. The third component is the energy storage device that is connected to the UPS.
Wiring effort and space required
This proven, modular structure presents advantages as well as disadvantages. The biggest drawback is the large amount of space it requires on the DIN rail. For example, a modern 24 V DC power supply unit with 5 A output power has an overall width of around 35 to 50 mm. Then you need to factor in the DC UPS for the same performance class, which has an almost identical overall width. This results in a maximum overall width of almost 100 mm. Then we have the energy storage device, which usually takes up the most space. For example, to buffer 5 A for around 20 minutes, an energy storage device with a nominal capacity of 3.4 Ah for 24 V DC is required. This has an overall width of approximately 85 mm. This means that a total of 18.5 cm is already required on the DIN rail.
Another aspect that planners need to consider for this common structure of a UPS system for DC loads is the selection of suitable power supply unit and UPS combinations. In the example above, a 5 A power supply unit and 5 A UPS were selected. But what does this actually mean? Both the UPS and the power supply unit can deliver a maximum output current of 5 A under all operating conditions. So does this mean that a 5 A load can be connected and reliably supplied in both mains and battery operation? The short answer for mains operation is no.
The charging current has not been taken into consideration thus far. The UPS needs this current in addition to the load current in order to recharge the energy storage device. Assuming an additional charging current of 1.5 A, a correspondingly larger power supply unit must be selected. With some sort of reserve, e.g., to compensate for losses in the system, this would require a power supply unit with an output power of 7 A. However, the next most common size is usually 10 A. This means that a power supply unit that is twice as powerful has been selected to ensure a load current of just 5 A. This results in higher initial costs and takes up even more space.
However, the modular structure does have advantages. With this option, it is easy to distinguish between critical and non-critical loads. Critical loads must continue to be supplied in the event of mains failure, such as in the case of a controller. Non-critical loads, on the other hand, do not have to be supplied in the event of mains failure. This is why uncritical consumers are connected directly to the power supply unit output, while the relevant consumers are connected to the UPS output first.
Practical example for an elevator controller
In numerous applications, however, a complete structure, such as a control cabinet, needs to be supplied – which is where the problem of having limited space to work with really becomes apparent. One example for the use of a UPS is the supply of an elevator controller. Here, in the event of mains failure, the operator or even the emergency services must be notified by means of remote signaling. The power consumption of the required consumers is a maximum of 4 A, and the required buffer time exceeds 5 minutes. Furthermore, special normative requirements also apply to these types of applications – the connected energy storage device must be actively monitored, for example.
For these types of application, Phoenix Contact offers a practical solution in the form of second generation Trio Power uninterruptible power supplies. Combining a power supply unit with wide-range input and integrated UPS, the devices are particularly suitable for applications where space is very limited and all connected loads needs to be buffered.
In our elevator application, an output current of 4 A is required. The ideal solution here is the Trio Power UPS with 5 A nominal output power, which has a 20% reserve to support extension. With an overall width of just 60 mm, the compact device also provides a maximum charging current of 1.5 A internally on a permanent basis in addition to the load current. The user can therefore be confident that the 5 A device has sufficient reserves to charge the energy storage device. In addition, they can select the device that best suits the load requirements of their application. No further consideration needs to be given to the charging current.
In order to achieve the required buffer time, an energy storage device with 1.3 Ah simply needs to be connected to the UPS using two cables. The UPS system is then ready for use.
Active battery monitoring can also be easily implemented using UPS devices from the Trio Power series. They feature an alarm signal contact and an LED indicator, which are activated, for example, if the maximum permissible recharging time has been exceeded and the battery is potentially faulty. In addition to error evaluation via signal contacts, a PC can also be connected via USB. The UPS system can then be conveniently monitored via the UPS-CONF software, which is available free of charge.
Compact devices with useful functions
The new UPS devices from the Trio series really showcase their advantages in confined spaces. The combined power supply unit and UPS not only saves space, but also simplifies planning. This is particularly the case for applications that only require loads to be buffered. Despite their compact design, the devices deliver a very high level of performance and are tailored to the requirements of numerous fields of application.
This article is written by Felix Schulte, product marketing, at Phoenix Contact Power Supplies GmbH