Designing control cabinets that save as much space as possible can be a challenge for plant operators working in fieldbus-based automation. Fieldbus installations help optimize space in electrical cabinets, since intermediate wiring is not required, and each field cable supplied from the cabinet connects up to 20 field devices to the control technology. The number of segments that can be installed per control point cabinet plays a key role in planning, because more segments per cabinet means fewer cabinets in the supply room and significant cost savings during construction. These factors contribute to savings for investors, planners, and plant operators. In order to achieve this goal, three factors have to be taken into account:
1) Heat Generation during Operation
When installing an industrial control cabinet, system planners not only have to consider the available space and cabling, but also the power dissipation of installed components and the heat build-up inside the cabinet. If excessive heat is generated, active cooling systems such as fans and air conditioning systems must be used. For a more cost-effective alternative, modules with low heat dissipation should be used.
This can be illustrated by comparing two Pepperl+Fuchs products.
Here you can see the two Power Hubs used in a Pepperl+Fuchs case study.
In a case study that was conducted (Fig. 2), the power dissipation from the selected auxiliary power supply was 300 W.
In this table, you can see two case studies (A and B) with different basic requirements.
The FieldConnex® high-density Power Hub (now phased out) has a maximum power dissipation of 2.54 W per segment, while the latest compact Power Hub converts only 2.37 W per segment into heat. In the case study, a cabinet was able to use the 300 W of heat capacity available to supply 118 segments of the high-density Power Hub and 126 segments of the compact Power Hub. In this calculation example, an outdoor temperature of 40 °C is assumed, where the indoor temperature could be as high as 52 °C.
Table 3 shows that 52 segments of the compact Power Hub can be installed with a heat capacity of 124 W.
2) The Auxiliary Power Supply
The number of connectable fieldbus power supplies depends on the maximum current provided by the auxiliary power supply. Case study B demonstrates that the current limits the number of segments the most—in this case, a maximum of 112 segments.
The maximum power supply current limits the number of segments to 112.
3) Fieldbus Power Supply Size
Of course, the size of the fieldbus power supply also plays a role in construction planning, though automation cabinets generally offer sufficient mounting space for today’s compact electronics. So this factor is less important than heat generation and the auxiliary power supply, which influence the number of segments more than the size of the fieldbus power supply.
An additional benefit: a slightly lower power dissipation per segment means a small but noticeable reduction in the costs for electrical energy in the continuous operation of a system. With 1200 segments, a 0.5 W lower power loss per segment means a savings of more than 5 MWh per year.
Heat generation, auxiliary power supply, and the size of the fieldbus power supply are three important factors to consider when planning both new and upgraded plants. During the planning phase, it is important to calculate how the heat dissipation from the selected modules will affect the temperature of a control cabinet without overloading the existing cooling technology. On the other hand, the cabinet’s auxiliary power supply limits the potential number of segments. The last factor that plays a role is the size of the fieldbus power supply, which is not usually significant because of the compact design of today's electronics.