When designing industrial automation systems, selecting a switch mode power supply very often comes down to two parameters – the output voltage and the required power. In practice, however, it is the operating conditions, and not the power alone, that most often determine the reliability of the entire system.
Most failures do not result from a faulty power supply
If the PLC, the input and output modules and the remaining loads require 24 VDC and together draw about 120 W, choosing a power supply rated at 150 or 240 W seems obvious. This approach works well in many standard applications.
In industrial practice, a power supply often operates in a tightly enclosed control cabinet, at an elevated temperature, surrounded by power electronics and other heat-emitting devices. It also happens that power is available only between two phases of a 400 VAC network, or that the loads draw a current during start-up that considerably exceeds the rated value.
Under such conditions, two power supplies of the same power rating may behave completely differently. One will operate without any problems for many years. The other will cause random restarts of the PLC, tripping of protection devices, or hard-to-diagnose interruptions in the operation of the entire machine.
Our experience shows that the cause of such problems is most often not a defect in the power supply itself, but rather its improper selection for the operating conditions.
A good power supply is not just about adequate power
In conversations with machine builders, we are often asked: “Which 24 V power supply will be best for our application?” There is no single universal answer. A different question is far more important: “Under what conditions will this power supply operate?”
It is the operating environment that determines which parameters should be taken into account during the design. Most often we analyse:
- the available supply voltage,
- the nature of the loads,
- the ambient temperature,
- the possibility of overloads occurring,
- the requirements regarding diagnostics,
- the mounting location within the control cabinet.
Only after answering these questions can you deliberately select the appropriate type of power supply. The Breve range includes, among others, two series of switch mode power supplies mounted on the DIN rail, developed with different application requirements in mind:
- KWR – designed to operate with a wide input voltage range,
- KDR – designed for applications requiring high resistance to overloads and high efficiency.
Although both devices look similar on the outside, their application is completely different.
Problem no. 1 – there is no neutral conductor in the control cabinet
This situation is far more common than it might seem. In new industrial facilities, power substations, wastewater treatment plants or infrastructure installations, designers very often have only the phase-to-phase voltage of 400 VAC available. A neutral conductor is not always provided.
In practice, this means that a classic power supply with an input range of 85…264 VAC cannot be connected directly to the existing installation. Most often we encounter three solutions to this problem.
First solution – providing a neutral conductor
Technically, this is a correct solution. In practice, however, it often means the need to rebuild the installation, carry out additional measurements, and increase the cost of the investment. It is also not always possible in existing facilities.
Second solution – using a transformer
The second solution is to use a step-down transformer. This makes it possible to use a standard 230 VAC power supply. It must be remembered, however, that a transformer:
- takes up additional space,
- increases the weight of the entire distribution board.
In small control cabinets, every centimetre of space can matter.
Third solution – using a power supply with a wide input voltage range
In many cases, the simplest solution is to use a power supply adapted to operate directly with the phase-to-phase voltage. The KWR series was designed precisely for such applications.
The input voltage range is 200-500 VAC and 230-700 VDC, thanks to which the power supply can be connected directly between two phases of a three-phase network without using an additional transformer. These parameters correspond to WDR-class solutions used in industrial applications.
For the designer, this means above all a simpler design and a smaller number of components in the control cabinet.
A practical example
Let us imagine the modernisation of a pumping station. Only 3×400 VAC is available in the existing distribution board. The design provides for the installation of:
- a PLC,
- an HMI panel,
- input and output modules,
- an industrial Ethernet switch,
- several relays.
The total demand is about 180 W. At first glance, any 240 W power supply will suffice. Once installation begins, however, it turns out that there is no neutral conductor in the distribution board – only three phases are available.
In such a situation, using a standard power supply would require rebuilding the installation or adding a transformer. Using the KWR series makes it possible to power the device directly from two phases, simplifying the design and reducing the number of components. This is one of the examples in which the right selection of a power supply makes it possible to avoid costly changes even before the installation is commissioned.
Problem no. 2 – the temperature inside the cabinet is considerably higher than assumed
This is one of the most frequently overlooked parameters during design. In technical documentation, most measurements are carried out at an ambient temperature of about 25°C. Meanwhile, the interior of a working control cabinet looks completely different. It contains, among others:
- variable frequency drives,
- servo drives,
- contactors,
- soft starters,
- communication modules,
- PLCs,
- safety relays.
Each of these devices dissipates heat. After a few hours of operation, the air temperature inside the cabinet can be even a dozen or several dozen degrees higher than the ambient temperature. It is not unusual to reach values of 45°C, 55°C, and in some applications even over 60°C. Under such conditions, a power supply operates completely differently than during laboratory tests.
Why do manufacturers publish a derating diagram?
In the datasheets of most professional switch mode power supplies, you can find a diagram showing the permissible load as a function of ambient temperature. Some designers treat it as additional information. In reality, it is one of the most important characteristics of the device.
Every switch mode power supply has a defined ability to dissipate heat. If the temperature of the electronic components exceeds the permissible level, their service life begins to fall rapidly. This is why manufacturers apply so-called derating, that is, the gradual reduction of the permissible power as the ambient temperature rises. Such characteristics are also found in the technical documentation of professional DIN power supplies, including models with increased environmental resistance.
A practical tip from a Breve engineer
Never select a power supply based solely on the power of the loads. If the device is going to operate at a temperature exceeding 50°C, always check the manufacturer’s derating diagram and the recommended mounting clearances. A few extra centimetres of free space above and below the power supply can have a greater impact on its service life than increasing the power by one size.
Summary
It is worth starting the selection of a switch mode power supply for a control cabinet by analysing the actual operating conditions, and not solely the output voltage and the required power. The supply voltage range, the ambient temperature, the nature of the loads, and the way the operation of the power supply is monitored all have a direct impact on the reliability of the entire system.
Already at the design stage, it is worth answering a few simple questions:
- Is a neutral conductor available in the installation?
- Do the loads draw a large current during start-up?
- How high will the temperature inside the cabinet be?
- Should the system monitor the state of the power supply? (in part 2)
- Is expansion of the installation planned in the future? (in part 2)
The answers to these questions very often lead to the choice of a solution that will ensure many years of failure-free operation of the device. The Breve range includes both power supplies of the KWR series, intended for applications requiring a wide input voltage range, and the KDR series, designed with applications requiring high resistance to overloads and high efficiency in mind. This makes it possible to select a solution matching the actual requirements of the project, instead of being guided solely by the rated power.
