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So, LV designers, what’s wrong with vacuum?01 July 2009In low-voltage (LV) applications, controlgear designers are curiously reluctant to specify vacuum contactors, despite the many and important benefits that they have to offer. Steve Rickard believes it’s time for a change in attitude towards a technology seemingly neglected by the LV community When it comes to medium-voltage equipment, vacuum contactors are commonplace, but how often do you see them in low-voltage panels? The answer is very rarely. In fact, many controlgear designers specify air-break contactors almost automatically, without giving a thought to whether vacuum devices might be a better choice for the application.
Why is this? One reason is undoubtedly familiarity. Designers of LV control panels are used to working with air-break contactors, but vacuum types are something of an unknown. I believe, however, that there is no reason for undue caution; vacuum contactors are just as easy to specify and use as their air-break counterparts. There are no hidden traps for the unwary.
Another reason often given for shunning vacuum contactors is price, and it has to be said that, rating for rating, they are considerably more expensive than air-break products. That’s not quite the whole story, however, and as we shall see, in appropriate applications the lifetime cost of a panel using vacuum contactors can be much lower than that of an apparently equivalent panel fitted with air-break contactors.
To see why I’m so enthusiastic about the virtues of vacuum contactors let’s start by taking a look at the types of applications where they score. Typically, these involve large loads – in excess of 250kW or so – and severe switching duties, such as AC4, which involve regular inching, plugging and/or reversing. Examples are heavy industrial hoists, crushers for material recycling and bow thrusters in ships. Applications with category AC-1 include wind generation, large heating outputs and induction welding in the steel industry.
In such applications, air-break contactors are prone to failure and invariably have short operating lives. Watch the contactor in operation, and it’s easy to get a clue to the root cause of the trouble. Spectacular arcing is the order of the day and, while this may be interesting to observe, it’s not doing any good for the life of the contactor.
Here comes the science. The real problem is the way that air-break contactors interrupt heavy currents. As the contacts part, an arc forms which is fed by ionised gases. The arc starts at a tiny point on one of the contacts, and ends at a similar tiny point on the other. A particular characteristic of arcs struck in air is that these points don’t move – they remain fixed until the arc is extinguished. Thus the contact surfaces at these points reach very high temperatures and many even melt, resulting in rapid erosion and, in bad cases, even welded contacts.
And it gets worse! If we’re lucky, the arc will be extinguished the first time the load current passes through zero but, more often than not and particularly in severe applications, the arc re-ignites and is not extinguished until the second current zero after the switching operation. When this happens, the arcing time and, therefore, the potential for damage to the contact is at least doubled.
Let’s make no bones about it, vacuum contactors also produce switching arcs, but the way they behave is very different. First of all, instead of a single arc there are multiple arcs between the contacts and, crucially, the start and end points of these arcs are constantly in motion. As a consequence, no local hotspots are produced on the contact surfaces, so the rate of contact erosion is very much lower than in air-break contactors.
It’s also worth noting that the arc in a vacuum contactor is always extinguished at the first current zero, which means that the length of time during which contact erosion can take place is kept to a minimum.
The differences in the way arcs behave mean that, in applications involving severe switching duties, vacuum contactors will always outperform their air-break counterparts by a very wide margin in terms of electrical life. In itself, this is enough for me to encourage designers to look favourably on the use of vacuum contactors but, in fact, there is whole range of additional benefits that should also be taken into account.
Consider, for example, the locations in which contactors with severe switching duties are likely to be used. Many, such as on board ship or in heavy industrial installations, will involve very poor environmental conditions that can rapidly have a detrimental effect on switchgear. With air-break contactors, the contacts are exposed to the poor environment, but with vacuum devices, the contacts are completely sealed within the vacuum bottles. They are, therefore, totally protected against contamination, tarnishing and other forms of degradation.
The sealed construction of the vacuum bottles also has another benefit: it prevents arc by-products from escaping. This means that the possibility of flashover to surrounding metalwork during switching operations is eliminated. As a result, clearance distances can be reduced, so panels that use vacuum contactors can be made much smaller than similar panels based on air-break devices. In some situations, such as on-board ship, this can be a very significant advantage.
The best of modern vacuum contactors incorporate electronically controlled coil mechanisms. Not only do these ensure that the force on the contacts is optimised throughout the whole of the closing stroke, thereby preventing noisy operation and contact bounce, they are also much more energy efficient than conventional coil circuits.
A question of cost
I would like to suggest that, in appropriate applications, the technical benefits of vacuum contactors over air-break devices are convincing but what about cost? It is important to take into account operating life. If, in a demanding application, an air-break contactor has to be replaced several times to equal the life of a vacuum contactor, the cost equation quickly shifts in favour of vacuum. And that doesn’t take the cost of downtime into account.
So, designers of LV control systems, when you’re next called upon to produce a panel for a particularly demanding application, don’t forget that vacuum contactors can provide an efficient and cost effective way of addressing the many challenges you will face.
Steve Rickard is with Moeller Electric
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