A frequency converter controls the speed of the 400 V asynchronous motor that drives a conveyor. You can mount it in a central electrical cabinet or directly next to the motor, on the conveyor. The decision rule is simple: on long routes with many low-power drives, field converters usually come out cheaper and easier to extend. For large drives, dense zones or a controlled environment, the cabinet remains the safe choice. Most real installations combine the two.
This article explains the difference in terms of total cost, downtime and expansion, for the person who signs off the budget of a conveyor system and does not need to parameterize a converter personally.
What each architecture actually is
The cabinet converter (centralized) sits in an electrical panel, usually in a technical room or next to the line. From there, one motor cable runs to every drive, sometimes tens of metres long. The cabinet gives protection, cooling and access to all equipment in a single place.
The field converter (decentralized) is a compact unit mounted either on the motor or on a bracket right beside it. Only power supply and a communication bus reach it. The unit has IP55 to IP66 protection, so it handles dust, vibration and, in some versions, water jets.
The motor is the same in both cases: asynchronous, three-phase, 400 V. Only the place where the electronics live changes, and nearly everything else follows from that.
The comparison, criterion by criterion
Installation cost
In a cabinet layout, each motor has its own cable, often shielded, routed through trays to the panel. On a route hundreds of metres long, the cables, trays and pulling labour can weigh more than the converters themselves. NORD, for example, presents exactly this as the case for decentralized solutions: less cabinet space, lower wiring cost and simpler maintenance.
The field converter costs more per unit. The gain comes from cabling: one power line and one data line, instead of a motor cable for every drive.
Commissioning and expansion
A conveyor with field converters is commissioned zone by zone. Mount, connect, parameterize, move on. When you add a section two years later, you add a unit instead of redrawing the cabinet.
With a central cabinet, expansion soon hits the physical limit of the panel: free space, available power, spare outputs. Often it means a new cabinet or a rebuild of the existing one.
Maintenance and downtime
Here the arguments run both ways.
A field converter is swapped on site with a preconfigured unit, and many models include a local service switch, so a technician can isolate the drive safely without hunting for the cabinet. The stop usually affects one zone.
The cabinet keeps everything in a clean, dry, accessible environment, with one place to check. But a power or cooling fault in the cabinet can stop several conveyors at once.
Environment and protection
The cabinet shields electronics from dust, moisture and vibration, provided it is well ventilated. Heat is also its weak point: a closed cabinet full of converters needs cooling, sometimes air conditioning.
A field converter lives in the conveyor's environment. Siemens, for instance, quotes protection up to IP66 and an operating range of -30 to 55 °C for the SINAMICS G115D, which covers cold storage. The IP class is chosen per zone, not for the whole installation.
Power range
Field converters cover low and medium power well, which is typical for belt and roller conveyors. The manufacturers' documentation shows the order of magnitude:
- Siemens SINAMICS G115D: 0.37 to 7.5 kW wall-mounted, 0.37 to 4 kW motor-mounted.
- NORD NORDAC ON and ON+: 0.37 to 3.7 kW. NORDAC LINK: 0.37 to 7.5 kW. NORDAC FLEX: 0.25 to 22 kW.
- SEW-EURODRIVE MOVIMOT flexible: 0.55 to 7.5 kW, depending on motor type.
Above these powers, or when you need regenerative braking, precise positioning or multiple axes, the cabinet takes over. NORD, for example, positions its NORDAC PRO range for cabinet installation, with a multi-protocol Ethernet interface and a multi-encoder interface.
Energy and electromagnetic compatibility
Long motor cables need filters and careful shielding so they do not disturb the rest of the installation. A converter next to the motor removes the problem at the source. As for consumption, the real saving comes not from the architecture but from how you control zones: a conveyor that stops when it carries nothing uses less. Zone logic is easier to build with field drives, but it can be done with a cabinet too.
Safety and diagnostics
Safety functions such as STO (Safe Torque Off) exist in both layouts. Siemens lists STO at SIL2 level for the G115D, through Profisafe. The practical difference is diagnostics: field drives usually report the state of each unit over the bus, straight into the control system (WCS), which shortens the search for a fault.
Total cost, not unit price
A budget built on converter price almost always favours the cabinet. A budget built on total installed cost changes the picture: cables, trays, installation labour, cabinet space and cooling, commissioning time, then the cost of future expansions.
There is no universal percentage. The result depends on route length, the number of drives and their power, which is why ZEDlog does not quote figures without project data. What you can ask any integrator for is the calculation in both variants, on your layout, with the same assumptions.
Choosing by project type
Long, spread-out route (collection, transport between zones): field converters, almost certainly. Cabling is the dominant factor.
Compact, dense area (grouped workstations, a sorter with many drives close together): the cabinet can be cheaper and easier to supervise.
Harsh environment (cold, dust, moisture): field, with the IP class matched to the zone, or a well air-conditioned cabinet, depending on space and access.
Expansion or retrofit of an existing installation: field, because you add zones without touching the cabinet.
High power or regenerative braking: cabinet.
An installation that will be reconfigured often, for example in a 3PL warehouse: field, because moving a drive does not need new cables back to the panel.
The hybrid is the reality
Almost no serious system is pure either way. A typical installation has field drives on the long routes and a cabinet for the large zones or the sorter. What matters is that both speak the same communication language and answer to one control system.
Three common mistakes
First: choosing the architecture by unit price before the layout exists. The numbers change completely once the real route length appears.
Second: the same protection class everywhere. A dry dispatch zone does not need the same housing as a cold room, and the difference shows in the price.
Third: ignoring expansion. A cabinet sized strictly for today becomes a problem at the first added zone. With a few spare outputs, or with field drives, the problem does not arise.
A third path, outside this article
The comparison above assumes 400 V asynchronous motors. For modular conveyors with short zones, there are also low-voltage drives (24 or 48 V) built into the roller, which need no separate converter. The principle is explained in the conveyor systems guide, in the section on driven rollers (MDR). It is worth considering when the conveyor has many short accumulation zones.
How the three manufacturers' ecosystems look
Siemens, NORD and SEW-EURODRIVE apply the same idea under different names. We mention them as examples of the technology, not as a recommendation.
- Siemens: SINAMICS G115D, a distributed system designed for horizontal conveyors, in motor-mounted or wall-mounted versions, commissioned from the TIA portal or over Wi-Fi.
- NORD: NORDAC LINK, ON and ON+ for field drives, NORDAC PRO for the cabinet. NORDAC ON is designed for asynchronous motors, ON+ for IE5+ synchronous ones.
- SEW-EURODRIVE: MOVIMOT flexible, a decentralized converter you can mount close to the motor, with asynchronous or synchronous motors.
Product names belong to their owners. ZEDlog is not affiliated with these manufacturers, and the data above comes from their public documentation.
What to ask your integrator
Before the quote, ask for written answers to five questions:
- What total motor cable length results in the cabinet variant, compared with field drives?
- How many drives exceed the maximum power of a field converter?
- Where are the spare parts kept, and how long does replacing a drive take?
- How does the installation expand in two years, in each variant?
- How does each drive's diagnostics reach the control system?
An integrator who answers these five with figures on your layout helps you decide. One who answers only with a brand recommendation does not. If you are preparing a project, our conveyor solutions start from exactly this kind of analysis, before any equipment is chosen.
Sources: Siemens SINAMICS G115D, NORD DRIVESYSTEMS, drive electronics, SEW-EURODRIVE MOVIMOT flexible.
