Panel Building - Progress in panels


Over the last few decades, control panels have changed beyond all recognition and this process of evolution shows no sign of stopping, or even of slowing down. Steve Rickard of Moeller Electric looks at the developments that can be expected in the not-too-distant future

Strange though it may seem to today's engineers, it's only a few decades ago that most control panels had no electronic devices in them at all. In those days, even the most complex of control schemes was implemented using electromechanical relays - sometimes hundreds of them in a single panel - pneumatic timers and other arcane electromechanical devices like stepping switches and uniselectors.

These panels actually worked, but they were huge, expensive, unreliable and almost impossible to modify or upgrade. Small wonder then that when the first programmable controllers appeared on the scene in the early 1970s, they were received with enthusiasm. At a stroke, they dramatically reduced size and physical complexity of control panels and, best of all, they made modifications easy.

Those early PLCs were still expensive and not particularly easy to program. For these reasons, their use was initially confined to larger projects. PLCs soon, however, became available in smaller, less expensive versions that were simple to program, making them an economical choice in smaller applications. Nevertheless, until quite recently the very smallest applications - typically those that might be implemented with less than a dozen conventional relays - remained uneconomic for programmable control.

The introduction of intelligent relays such as Moeller's easy Relay, has addressed this issue. These tiny programmable relays have most of the facilities of a low-end PLC, but are very small, very inexpensive and very easy to program.

In addition to their relay-replacement functions, they usually offer a range of extra facilities, such as real time clocks, retentive memory to preserve system status information even when the power is off and, in the more advanced versions, communications facilities. These intelligent devices have effectively taken over the last stronghold of control systems based on conventional relays and, as a result, these systems have all but disappeared in new applications.

The adoption of programmable devices vastly decreased the amount of control wiring needed in panels, but it did nothing to simplify the power sections of the panels and, in particular, the motor starters that are such a central feature of most control panels remained just as complex as ever to implement.

The first step to improving this situation was a move away from the conventional - even old-fashioned - UK starter design that is based on three fuses, a contactor and an overload. By replacing the fuses and the overload with a motor protection circuit breaker, the number of components in a DOL starter is reduced from five to two, which instantly cuts the mounting and wiring time for the starter components.

And that's not all. Because the connections between the contactor and the motor protection circuit breaker are standardised, it is possible to arrange for these components either to connect directly to one another, or to be linked by a simple tool-less plug-in connector. Both methods simplify the mounting of the starters, and both eliminate most of the wiring that would have been needed with a conventional starter.

The tool-less connector has the additional benefit that removal of the connector provides secure and positive isolation for the starter, which is a very useful aid during fault finding and commissioning. The best of the connector-based systems, such as those in the Moeller xStart range, also go beyond simple DOL starters, and make it equally easy and convenient to fabricate reversing and star-delta starters.

Another area of recent progress in relation to motor starters is the growing availability of electronically controlled coil systems. These provide much better control over the closing stroke of the contactor, thereby increasing its life because of reduced mechanical stresses. From the panel building point of view, however, they have another advantage.
 Since they reduce the coil current needed to close the contactor, they make it possible to switch even quite large contactors directly from low-power PLC outputs. This means that inexpensive high-density output modules can be used in the PLC, and also that the need for interposing relays, which always add complexity and cost to a control panel, is eliminated.
Of course, despite all of these improvements to the motor starters themselves, it is still necessary to provide power to those starters. The standardised size of starters based on motor protection circuit breakers has, however, made it possible to design convenient prefabricated busbar systems on which the starters can be directly mounted. Such an arrangement eliminates most of the power wiring, and makes modifications to the panel much easier.

Having considered motor starters in some depth, let's return to the control circuitry associated with panels. Here, the biggest development to date has, in fact, been outside the panel, where conventional field wiring has, in all but the smallest systems, been replaced by fieldbus connections.

Besides reducing the amount of costly field wiring needed by a very large factor, this change also provides the field installation with flexibility to match that of the PLC-based control system. No longer is it necessary, if changes are needed, to resort to expensive and inconvenient re-wiring for field devices. Instead, all that's usually needed is to connect any new field devices to the fieldbus and adjust the PLC program to accommodate them.
The widespread use of fieldbus systems has also brought about significant changes within the control panel. Gone are the banks and banks of screw terminals that used to be needed for the connection of conventional field wiring, and instead there are just a few fieldbus connectors. In addition, the PLCs no longer have racks and racks of I/O cards, just one or two fieldbus masters. The result is panels that are smaller, less expensive, easier to wire, easier to install and easier to maintain.

As we've seen, most areas of control panel design have undergone huge changes in recent times but, until now at least, one area has remained almost untouched. This is the control wiring within the panel, for example, between the motor starters and the programmable controller or intelligent relay.

This last bastion of conventional wiring is, however, about to fall, with the introduction of Moeller's SmartWire, the first in a new generation of smart panel wiring systems. The objective of smart panel wiring is to provide all of the benefits of a fieldbus system within the panel itself. Indeed, one way of looking at it is to consider it as a fieldbus system that is optimised for panel use.

With smart wiring, the ordinary control wiring between the motor starters and the PLC is replaced by daisy-chain style connections that use simple pre-fabricated cables with plug-in connectors. Not only does this reduce the amount of wiring and eliminate most of the I/O modules needed on the PLC, it also dramatically cuts the wiring time, and makes it virtually impossible to make wiring mistakes.

Further, smart wiring brings a high degree of flexibility. If it's necessary to add another starter, for example, all that's needed is to mount it and plug it in to the smart wiring system - usually the work of a few minutes.

Other important benefits of smart wiring, at least in the Moeller Electric implementation, are that it is completely self-addressing, so setting it up is a trivial task, and that it works with ordinary motor starters. To use the system, it's simply necessary to add a SmartWire interface to the starter in the same way that an ordinary auxiliary contact block would be added.

With the introduction of smart wiring, virtually every aspect of control panel design and construction has been given an overhaul, resulting in panels that are smaller, more cost effective, faster to produce and much more versatile. But what of the future?
One fairly obvious speculation is that smart panel wiring systems will expand in scope. At present, they are mostly limited to motor starter connections, but there's no real doubt that their scope will soon be extended to cover other devices such as pushbuttons and indicator lamps.

Also, given the close affinity between smart panel wiring and fieldbus systems, it's perfectly probable that a single system will evolve that efficiently and cost-effectively embraces the applications areas of both.

Power switching devices such as contactors are unlikely to see major changes unless dramatic developments are made in the materials from which they are fabricated. This is because they have already reached the limits of size and performance that can be achieved with present-day materials.

One possible development, however, is growth in the use of vacuum contactors for low voltage switching, especially in high power applications. Already widely used in MV applications, vacuum contactors are more compact than their air-break counterparts and deliver much better service in arduous applications. Their relatively high cost limits their adoption at present, but this may well change in the not too distant future as manufacturing volumes increase.

To those of us who have worked in the control gear industry for a long period, the changes in control panel design seem to have happened quite slowly. And so they have, but the cumulative effect is little short of amazing. Just about the only resemblance between a modern control panel and one built in, for example, the 1960s, is the enclosure!

The changes to date have been truly beneficial, delivering reduced cost, better performance, vastly enhanced reliability and much greater flexibility. One thing is for sure - the developments won't stop here. I've given some hints for the future but to stay up to date and get the maximum benefit from new developments, the best advice is to stay in close touch with your favourite controlgear suppliers and, in particular, watch out for their news announcements!