Drives & controls - Making redundancy redundant

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Modern control systems are invariably designed with safety as a prime requirement. Often this can add significantly to the cost, particularly where high-power contactors have to be duplicated to provide redundant operation. Fortunately, there's now a better approach, as Moeller Electric's Steve Rickard explains

Almost every control panel incorporates motor starters and there can be no doubt about it, when a safety relay operates or an emergency stop button is pressed, the motors controlled by those starters have to stop.

It's easy enough, of course, to design the control circuits to behave in this way, but what happens if there is a component failure? In particular, what happens if a contactor has welded closed? The answer is that, unless further measures have been taken, the motor controlled by the welded contactor will continue to run.

Clearly, this very dangerous state of affairs cannot be tolerated, particularly as welding is a relative common failure mode for contactors, especially those that have reached the end of their service lives.

The usual solution is to incorporate two identical contactors in series in the starter circuit which meets the requirements of Safety Category 3 and 4. The risk of both contactors welding simultaneously is unlikely, so at least one of the contactors will always open when required to do so, and the motor will be safely stopped.

While this approach of using redundant contactors is effective and widely used, it does have several drawbacks. The first is cost. While small contactors are relatively inexpensive, their larger counterparts certainly are not. So, with a 100kW drive, for example, using a second redundant contactor adds significantly to the overall cost of the control system. The next drawback is panel space. High current contactors take up a lot of panel space, and not only is panel space expensive, there is often insufficient room available on site to allow large panels to be accommodated. Finally, duplicated contactors increases the amount of heat generated within the panel, because of the losses from the coil and main contact circuits.

To address these issues, Moeller Electric has pioneered the development and introduction of a new type of control component - the contactor monitoring relay, a device which is both compact and inexpensive.

The principle of operation of this innovative device is easy to state. It simply compares the state of the main contacts of a contactor with the voltage that's being applied to the coil. If the coil is de-energised but the main contacts are still closed, the output relay of the contactor monitoring device operates, opening a set of contacts that can, for example, be used to trip an undervoltage release on the circuit breaker protecting the motor circuit. In this way, it's clear that, even if the contactor welds, the motor will still be brought to a stop safely. The need to use a duplicate contactor to ensure safe operation is, therefore, eliminated.

While the principle of operation of the contactor monitoring device may be simple to state, however, designing a practical product is a little more challenging. For example, what is the best way to monitor the main contacts of the contactor?

The solution adopted by Moeller Electric is for the monitoring device to look at the state of an auxiliary contact on the contactor. Not all auxiliaries are guaranteed to accurately reflect the state of the main contacts, however, especially when the contactor is faulty. The auxiliary used for this function must, therefore, meet the requirements for a mirror contact, as defined in the IEC EN 60947 Annex F.

Essentially, these requirements state that a mirror contact on a contactor is a normally closed contact that can only ever close if all of the main contacts have opened. All N/C contacts on DILM and DILH contactors from Moeller Electric meet this requirement. That's not quite the end of the story, however, as we need to consider what happens if the auxiliary contact itself welds closed. This is very unlikely, but by no means impossible. The solution here is to compare the state of the N/C auxiliary with an N/O auxiliary in the same contact block. Provided that the contacts are positively driven - a condition once again met by the standard auxiliaries used with Moeller Electric contactors - they can never be closed at the same time unless a fault has occurred.

By monitoring a positively driven N/O auxiliary as well as the N/C mirror contact, the contactor monitoring relay can, therefore, immediately detect problems with the mirror contact and generate an output to trip the drive.

As this discussion shows, the characteristics of the contactor and of its auxiliary contacts have a critical bearing on the operation of the contact monitoring device. For this reason, Moeller Electric provides details of approved product combinations that will ensure the appropriate levels of safety are achieved.

With these combinations, the operation of the contactor, the contactor monitoring device and the motor protective circuit breaker complies with the requirements of EN ISO 13849 for Performance Level e, provided the number of switching operations of the contactor does not exceed 350,400 per year, and that the number of switching operations of the protective device does not exceed 1,095 per year.

In the calculation of the overall performance level for the control system, however, the safety components upstream of the motor starter also have to be taken into account. When this is done, the result is that, with the aid of the contactor monitoring device, control systems that overall meet the requirements for Performance Level d can be readily implemented.

Small, easy to use and cost effective, contactor monitoring devices are a very attractive and convenient alternative to the adoption of redundant contactor designs in control systems. They save money and space, as well as reducing the amount of heat generated within the panel, all without compromising the level of safety achieved by the overall system.

With contactor monitoring devices offering so many benefits, surely it has to be time to make redundant contactors redundant?