Centralised Versus Decentralised Installation

Monday, June 29, 2015
Row of drives

When selecting frequency converters for your installation, you often have the choice between centralized installation, whereby the frequency converters are installed together in one or more control cabinets, and decentralized installation, whereby the frequency converters are installed in the plant, each close to the motor that the drive is controlling. Both options have their advantages and disadvantages.







Centralized Installation

The most common form of installation is beyond doubt centralized installation of frequency converters in control cabinets. The advantages of centralized control cabinet technology lie, above all, in the protected installation of the units and centralized access to them for power, control, maintenance, and fault analysis.

With installation in the control cabinet, the primary aspect that must be taken into account is heat management, not only of the units but also of the whole installation. As a result of the heat dissipation in the control cabinet, additional cooling of the control cabinet may be necessary.

Depending on the frequency converter manufacturer’s mounting regulations, minimum distances must be maintained above and below the unit and between the unit and adjacent components. For better heat removal, direct mounting on the rear wall of the control cabinet is recommended. Some manufacturers also specify minimum distances between the individual units. It is however, preferable to mount the units side-by-side if possible in order to utilize mounting surface area effectively.

A disadvantage of centralized installation in some cases is the long cable lengths to the motors. While the use of shielded cables definitely reduces the RFI effects of the motor cable, these effects are not completely eliminated.

Decentralized Installation

As an alternative to centralized installation, a decentralized approach to the layout of a facility can also be chosen. Here the frequency converter is located very close to or directly on the motor.

Motor cable lengths are thereby reduced to a minimum. In addition, decentralized installation offers advantages in fault detection, since the relationship between the controllers and their associated motors is easy to see. In decentralized configurations, a Fieldbus is usually used to control the drives.

When planning a decentralized installation, factors such as ambient temperatures, mains voltage drops, limited motor cable lengths, etc. must be taken into account. Important factors such as these are often overlooked in the high-level design of engineering projects.

For example, not only the decentralized units but also the supply cables must be suitable for the installation environment. For instance, the Fieldbus cable must be suitable for a harsher environment and sometimes also of a flexible type. In addition, installation of units in inaccessible locations should be avoided in order to ensure quick access for servicing.

Another major consideration is the segmentation of a decentralized network. For economic reasons, it is beneficial to combine units into groups or segments. Careful consideration must be given to determining which segments require other segments for their operation, and which segments can, must, may, or should continue to operate autonomously. For example, if certain chemical processes cannot be interrupted, the failure of a lower-level segment must not be allowed to disrupt important segments.

Finally, the expertise that is necessary for the installation of a decentralized network should not be underestimated. In addition to knowledge of the fieldbus systems used, the technician must be aware of the structure (what happens to the total system if an individual unit fails) and the ambient conditions of a decentralized network and must be able to estimate these effects.

Although decentralized units are always more expensive than centralized units, well conceived decentralization concepts can achieve savings of around 25% compared to centralized systems. The potential for savings in the installation arises from reduced cable lengths and from using equipment modules that have already been built and tested by the machine manufacturer or supplier.