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Innovative Improvements in Interfacing

Virtually every modern automation system is based on programmable controllers or similar electronic devices. However, the cost of interfacing electronic devices with large loads like motors is significant. Steve Rickard, Product Manager at Moeller Electric explains how a new type of contactor cuts these interfacing costs and also provides major technical benefits.

Programmable controllers (PLCs), intelligent relays and their ilk are undoubtedly very successful in combining economy with power and flexibility, making them an almost universal choice as the basis for automation systems of every kind. They do, however, have one important shortcoming: they can't control motors or other high-power loads directly.

The standard solution is straightforward: use the output from the PLC to switch a contactor, and use the contactor to control the motor or other load. In fact, this arrangement is so commonplace that there are probably few engineers who give it much thought.

But let's delve a little deeper into this PLC/load interface. For a start, is it best to use contactors with AC coils or a DC coils? There's no doubt that DC output modules for PLCs are usually less expensive than their AC equivalents. Indeed, some PLCs, particularly compact non-expandable types, don't even have an AC output option.

This doesn't mean, however, that DC contactors are necessarily the best choice. Standard DC contactors cost more than AC contactors of the same size and, in many cases, they're physically larger, and they tend to have shorter operating lives. In addition, the conventional DC coil systems used in contactors are relatively inefficient, which means not only that they waste energy, but also that they generate undesirable quantities of heat.

As a workaround for these problems, interposing relays are sometimes used. These are typically small DC-operated relays which are switched by the PLC outputs, and which, in turn, switch the contactor coil circuits. This means that the PLC can use inexpensive DC outputs, but that the contactors can be AC types.

At first, this sounds like the best of both worlds, but in practice it is not a particularly attractive approach. The cost of the interposing relays adds to the overall cost of the control system, as does the cost of fitting and wiring them. In addition, space has to be found for the relays in the control panel.

The use of interposing relays makes the design and construction of the control system more complicated, as well as reducing overall reliability as additional electromechanical components have been introduced.

With these considerations firmly in mind, Moeller Electric set about devising a better way of interfacing PLCs and similar devices with large loads. The result is the new xStart range of contactors which are specifically designed interfacing applications. The key innovation is that coil circuits of these novel products incorporate a sophisticated electronic controller.

This means that xStart contactors are DC operated, yet they provide all of the benefits of conventional AC contactors, as well as some unique additional benefits. The new contactors are, for example, completely free from the objectionably humming often produced by AC devices, making them very attractive for use in building automation and medical applications.

Small size is another feature of the xStart contactors, for AC3 ratings up to 15kW all types are just 45mm wide – exactly the same size as the Moeller Electric's equivalent AC contactors.

Electronic control means that the coil current can be continuously optimised according to the state of the contactor. A high initial current ensures that it closes cleanly, while a controlled reduction of the current during the closing cycle virtually eliminates contact bounce which, in conventional contactors, can cause arcing and unnecessarily rapid erosion of the main contacts.

The controlled closing sequence also reduces the mechanical wear and tear on the contacts and on the magnet assembly, enhancing reliability and extending the life of the contactor.

One of the most important features of the Moeller design, however, is that when the contactor is fully closed, the electronic controller reduces the voltage applied to the coil to around 10% of its nominal value. This is easily sufficient to ensure that the contactor remains properly sealed, but the power used by the coil typically falls to 0.5W, which means that energy used and heat produced are both minimised.

The low power requirements of the coil with the contactor in the closed state also have other benefits. For example, there is no risk of overheating when the contactors are mounted side by side without gaps between them. Valuable panel space can, therefore, be saved.

In addition, the most cost-effective DC output cards for PLCs are usually high-density types with perhaps 16 or 32 output channels. However, the outputs on these cards cannot all be simultaneously fully loaded, as this would cause the card to overheat.

This limits the usefulness of high-density output cards for switching conventional contactors but, with the new xStart devices, the situation is different. These contactors draw peak current only momentarily as they close, and the holding current after closing is so small that the heating effect on the card can be neglected.

Finally, it's easy to forget that the current used to hold the contactors closed comes not direct from the mains, but from a power supply. The low holding current requirement of the new contactors means that smaller, less costly power supplies can be used, which again have the additional benefit of requiring less panel space.

Electronic coil control makes it possible to produce contactors which operate reliably and safely with a very wide range of voltages applied to the coil circuit. The EN 60947-1 standard requires that all contactors operate correctly with voltages between 85% and 110% of the rated coil voltage. Moeller Electric's xStart contactors, however, achieve a range of 70% to 120%.

This is a particularly useful feature in systems which are battery powered, and those which involve long cable runs.

The mechanical design of the new xStart contactors is, in many ways, similar to that of a conventional AC contactor. As a consequence, many of the contactor components which are manufactured in huge quantities for the company's AC product range can be used in the new devices. This helps to keep manufacturing costs and, therefore, selling prices low.

With electronic technology already built into the contactors, it becomes easy and inexpensive to add other useful features. For example, a suppression function is provided which prevents the coil circuit from generating voltage transients when it is switched off. Unlike many of the more usual suppression arrangements, however, it does not delay contactor drop out.

Protection against polarity reversal is also provided, which prevents the contactor from being operated if it is incorrectly wired.

For interfacing large loads with PLCs and similar electronic devices, contactors with electronically controlled coils provide many important benefits. They allow the use of cost-effective high-density output cards without the complication of using interposing relays. They save energy and reduce the amount of heat produced in the control panel, they help to save panel space, and they combine long operating lives with high reliability.

In short, "electronic" contactors are exactly the right solution for interfacing applications in automation systems that are built around electronics!

This page last updated: 9 December 2005