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Staying cool the liquid way

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Comparatively few specifiers or users of variable speed drives give serious consideration to liquid-cooled products yet, in appropriate applications, they offer many valuable benefits. Stephen Takhar of Vacon explains

The latest ac variable speed drives are remarkably efficient, with figures of around 98% being reached by the best models. Nevertheless, that still means losses of 2%, and those losses inevitably appear in the form of heat. This means, for example, that a typical 250 kW drive will generate about 5 kW of heat while it is running. That’s a lot of heat, and arrangements have to be made for it to be dissipated.


These arrangements most often take the form of forced air cooling, with one or more fans blowing air over the drive’s heatsinks. In many applications, this arrangement is perfectly satisfactory, but that’s by no means always the case. The hot air from the drive may, for example, lead to an unwanted build up of heat within the room where the drive is installed, especially as equipment rooms are almost always small and cramped.

This heat build up can be a big problem. The EN 61439 standard for type-tested switchgear specifies that the 24-hour average temperature of the room in which the equipment is installed must be below 35ºC, and that the maximum temperature must be below 40ºC. With a drive producing 5 kW of heat, as in the example above, it can be difficult to stay below these limits without making extensive use of air-conditioning equipment, which is often costly to install and always costly to run.
In addition, it’s no secret that big heatsinks and big fans are an essential feature of large air-cooled drives, which means that the drives take up a lot of space. The high levels of noise generated by the fans can also be objectionable in some applications, such as when an industrial plant that operates round the clock is sited close to a residential area.

There’s also the perennial problem of dirt being sucked into the drive by the fans, which means that filters have to fitted and regularly maintained. Once again, this is a major issue – a common cause of drive downtime is blocked filters that, in a well-designed drive, cause the over-temperature protection to operate, and in a poorly designed drive lead to the failure of expensive power modules.  Finally, it’s very difficult to produce an air-cooled drive that has the sort of high ingress-protection (IP) rating needed for reliable operation in areas where high levels of dirt, dust and damp are present.

Liquid-cooled drives provide solutions for all of these problems. First of all, the amount of heat dissipated directly to the air from a liquid-cooled drive is very small – usually around 0.1% to 0.15% of the drive rating. This means that our fully loaded 250 kW drive is likely to be putting out less than 375 W of heat to the surrounding air, which is for all practical purposes negligible. Not only does this mean that there is unlikely to be a need for additional air conditioning, it also means the users of the drives have much more freedom about where they can be installed, as there is much less need to take into account provisions for ventilation and heat removal.

Next come reduced noise levels. Since they have no need to push large volumes of air over heatsinks, liquid-cooled drives are much quieter in operation than their air-cooled counterparts with a similar rating. For example, Vacon’s 400 kW 690 V liquid cooled drive is 20 dBA quieter than its air-cooled equivalent. It’s also 68% smaller, which is a very important factor in many applications, especially where existing plant is being upgraded to take advantage of the energy efficiency gains made possible by AC variable speed drives, but space is limited for the installation of new equipment.

And, of course, liquid cooled drives can readily be produced in versions that are, for all practical purposes, sealed. Achieving an ingress protection rating of IP54 or higher is no problem at all, which means liquid-cooled drives will perform reliably even in the toughest and dirtiest of environments. This makes them an ideal choice for demanding applications in, for example, the marine and offshore, pulp and paper, mining and metal, and renewable energy sectors.

With liquid-cooled drives offering so many benefits in such a wide range of applications, the question that has to be asked is why are they are not more widely used? The reasons usually given are that liquid-cooled drives are hard to get hold of, they’re expensive to buy and they’re difficult to install and work with. In fairness, these comments may once have been accurate, but today they are most certainly not. To find out why, let’s look at each of them in turn.

These days, liquid-cooled drives are, in fact, very easy to source. Vacon, for example, offers a comprehensive range with power ratings from 7.5 kW to 5 MW. True, they are unlikely to be available off-the-shelf at the local distributor, but all except the very largest sizes are nevertheless standard products that can be delivered in just a few days.

Now let’s look at costs. It is fair to say that the initial investment required for liquid-cooled AC drives is in many cases slightly more than for air-cooled drives, because of the more advanced technology used and because of the additional costs associated with pipework and heat exchangers.

But on the other side of the coin, there is no requirement to provide additional air conditioning capacity or extra ventilation for the areas in which the drives are used. This means that the running costs associated with the air conditioning and/or ventilation are also eliminated. Further, the freedom to site the drive almost anywhere often means that shorter cable runs can be used, which, in these days of astronomical copper prices, can translate into very significant savings.

According to extensive evaluation work carried out by Vacon across a range of applications, these factors mean typically mean that the total cumulative costs for installing and running a single 630 kW liquid-cooled drive will, after three years, be the same as those for air-cooled drive. Thereafter, the liquid-cooled drive will actually save the user money. In the case of larger drives, the break-even period is even shorter – with drives of 1 MW or more, it is usually less than a year.

But what about smaller drives? Yes, liquid-cooled versions will cost somewhat more to install than their air-cooled counterparts, but they may still be a very cost-effective choice if the alternative is to make major changes to the plant in order to accommodate air-cooled drives, which invariably require much more space, and/or to install additional ventilation and air-conditioning equipment.

Finally, let’s look at ease of installation and use. The liquid-cooled drives themselves are no more difficult to install than air-cooled types, but they do need an external heat exchanger that, of course, has to be installed, along with the pipework to connect the drive to the heat exchanger. These are not exactly major engineering challenges, however; the work involved is no more complex than that needed to fit an ordinary two-part air conditioning unit.

And, when it comes to setting up and using the drive, the difference between air-cooled and liquid-cooled units is virtually non-existent for products from the same manufacturer. It’s also worth noting that modern liquid-cooled drives are every bit as versatile as air-cooled types. The best, for example, are fully compatible not only with standard induction motors, but also with the new high-efficiency permanent-magnet motors that are now rapidly growing in popularity.

As we’ve seen, in many applications, liquid-cooled variable speed drives are a very attractive option. They save space, they simplify cooling requirements for the areas in which they are installed, and ultimately they can even save money. They’re readily available, they’re easy to use, and expert suppliers like Vacon are more than ready to provide all of the support needed by even the most trepidatious of first-time users.

It is, therefore, time for users and specifiers alike to cast aside the old image of liquid-cooled drives as exotic beasts that are only worth considering for the most specialised of uses, and to start seeing them instead as an invaluable option that’s capable of delivering valuable benefits in all of those slightly more than usually demanding variable speed ac drive applications.

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