Centralised versus Decentralised Installation

Centralised versus Decentralised Installation banner

Without a doubt the most popular form of installation is centralised installation of VSDs in control cabinets. The advantages of centralised control cabinet technology lie, above all, in the protected installation of the units and centralised 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 VSD 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 utilise mounting surface area effectively.

A disadvantage of centralised 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.

As an alternative to centralised installation, a decentralised approach to the lay-out of a facility can also be chosen. Here the VSD is located very close to or directly on the motor.

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

When planning a decentralised installation, factors such as ambient temperatures, main voltage drops, the 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 decentralised units but also the supply cables must be suitable for the installation environment. For instance the field-bus cable must be suitable for a harsher environment and sometimes also of the 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 decentralised 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 decentralised 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 decentralised network and must be able to estimate these effects.

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

The following examples illustrate the basic procedure for selecting a VSD in the design process. Here the data sheet reproduced below is used for the selection process. The VLT AutomationDrive FC 302 is selected as a VSD that can operate with a 150m shielded cable.

P11KP15KP18KP22K
HONOHONOHONOHONO
Output Current
Continuous (380-440 V) [A] 24 32 32 37.5 37.5 44 44 61
Intermittent (380-440 V) [A] 38.4 35.2 51.2 41.3 60 48.4 70.4 67.1
Continuous (441-500 V) [A] 21 27 27 34 34 40 40 52
Intermittent (441-500 V) [A] 33.6 29.7 43.2 37.4 54.4 44 64 57.2
Output Power
Continuous (400 V) [KVA] 16.6 22.2 26 30.5 42.3
Continuous (460 V) [KVA] 21.5 27.1 31.9 41.4
Typical shaft output [kW] 11 15 18.5 22.0 30.0
Max. Input Current
Continuous (380-440 V) [A] 22 29 34 40 55
Intermittent (380-440 V) [A] 35.2 31.9 46.4 37.4 54.4 44 64 60.5
Continuous (441-500 V) [A] 19 25 31 36 47
Intermittent (441-500 V) [A] 30.4 27.5 40 34.1 49.6 39.6 57.6 51.7
Estimated power loss at rated max. load [W] 291 392 379 465 444 525 547 739
Efficiency 0.98
Max. cable size (mm²) ([AWG²]) 16(6) 35 (2)
Max. pre-fuses [A] 63 80

Output Current

P11KP15KP18KP22K
HONOHONOHONOHONO
Continuous (380-440 V) [A]
24 32 32 37.5 37.5 44 44 61
Intermittent (380-440 V) [A]
38.4 35.2 51.2 41.3 60 48.4 70.4 67.1
Continuous (441-500 V) [A]
21 27 27 34 34 40 40 52
Intermittent (441-500 V) [A]
33.6 29.7 43.2 37.4 54.4 44 64 57.2

Output Power

P11KP15KP18KP22K
HONOHONOHONOHONO
Continuous (400 V) Continuous (400 V) [KVA]
16.6 22.2 26 30.5 42.3
Continuous (460 V) [KVA]
21.5 27.1 31.9 41.4
Typical shaft output [kW]
11 15 18.5 22.0 30.0

Max. Input Current

P11KP15KP18KP22K
HONOHONOHONOHONO
Continuous (380-440 V) [A]
22 29 34 40 55
Intermittent (380-440 V) [A]
35.2 31.9 46.4 37.4 54.4 44 64 60.5
Continuous (441-500 V) [A]
19 25 31 36 47
Intermittent (441-500 V) [A]
30.4 27.5 40 34.1 49.6 39.6 57.6 51.7
Estimated power loss at rated max. load [W]
291 392 379 465 444 525 547 739

Efficiency

P11KP15KP18KP22K
HONOHONOHONOHONO
Max. cable size (mm²) ([AWG²])
16(6) 35 (2)
Max. pre-fuses [A]
63 80

If you have any questions about which drives to use for your application, or which installation type would be the most ideal, contact us on info@nicontrols.com or call +44 0333 270 8511. You can also visit our variable speed drive selection page which enables you to filter by elements such as power, input phase, supply voltage, I/O, comms and more.

Find this interesting? Share it