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Variable speed drives generate heat – that's a fact of life – and usually that heat is unwanted. But what’s the best way to deal with it? Alan Baird, country manager UK and Ireland at Danfoss Drives looks at the options.

Heat, when it’s not required, is always a problem, and the best solution, in an ideal world, would always be to eliminate it at the source. But that’s not always possible. Consider, for example, variable speed drives (VSDs). There are no VSDs in existance that do not produce heat while operating and it’s very unlikely that one will be developed anytime soon. That doesn’t mean, however, as all VSDs are equal in this respect.

The amount of heat produced by a VSD is inversely proportional to its efficiency – hence, the more efficient the VSD, the less heat it will generate. And, since high efficiency also reduces running costs, the message is clear: always start by choosing the most efficient VSD available that will suit the application in hand. 

Except in the smallest sizes, modern VSDs can be expected to be up to 98% efficient, so only 2% of the energy they use is converted to waste heat. That’s an impressive performance nevertheless, and with large drives it can still amount to a lot of heat. 

Consider, for example, a 450kW drive as might be found in many industrial applications. This will generate 9kW of heat, which is quite enough to be problematic! And if several VSDs of this size are in use close to each other, the problem increases proportionally.

What’s to be done? If the VSD (or VSDs) have a high IP rating such as IP54,  that can be used without an additional enclosure, the most usual answer is to provide air conditioning for the room in which they’re installed. If they are of a lower IP rating such as IP20  for instance, which have to be installed in a control panel, the panel will need to be force ventilated, which means fitting fans and filters. And the room in which the control panel is installed will still need air conditioning.

Actually, if only a few small drives are involved, these solutions will most likely be perfectly adequate. With big drives or multiple drives, however, the cost of running the air conditioning system continuously to remove the heat quickly becomes prohibitive for a number of reasons, including the running cost and additional noise generated. And, for panel mounted drives, there’s the extra cost of the energy to run the forced ventilation fans to be considered, not to mention the cost of the air filters, which will need to be changed periodically.

Surely there has to be a better solution? In fact, there are two solutions, as we shall see. The first is back-channel cooling, which is available as an option for all Danfoss VLT high power drives. In these VSDs, the power devices are mounted on efficient aluminium heatsinks, which are arranged so they shield the control electronics from the heat produced by the power devices. The heatsinks are also designed so that they form a continuous channel at the back of the drive, over which cooling air can flow.

VSDs with back channel cooling are arranged typically with the aid of a simple and low cost ducting system, so that they draw in cool air from outside the building in which they are fitted, pass the air through the back channel formed by the heatsinks, and then discharge the warmed air into the environment. As a result, heat corresponding to only around 0.3% of the drive rating is released into the room where the drive is installed. With our earlier example of a 450kW drive, the heat released is significantly reduced from 9kW to 1.35kW, which is far more manageable. 

With this reduced heat release, air conditioning may not be needed at all and, in those cases where it is still necessary, the running costs will be very much lower than they would be if conventional drives had been used. When back-channel cooled VSDs are mounted in control panels, forced ventilation is rarely – if ever – needed. In addition, the ducting kits supplied for use with these drives ensure that the IP rating of the panel itself is never compromised.

Back-channel cooled VSDs are an ideal solution in many applications, but particularly where space is limited or where it would be difficult to install ducting, another option is available: liquid-cooled VSDs. These work on the principle of a simple heat exchanger. 

Liquid is circulated around the heatsinks that power electronic devices, such as Insulated-Gate Bipolar Transistors (IGBTs) and other heat generating components are mounted, from which it removes the heat. The liquid is then routed via pipes to a radiator where it loses the heat to the environment. The benefits are that the pipework is small and easy to accommodate and that, if necessary, the radiator can be mounted at a considerable distance from the drive. With this arrangement, as with the back-channel air-cooled drives, very little heat is released into the room in which the drives are installed.

The best liquid-cooled drives are those which have been designed specifically for this form of cooling, rather than simply being air-cooled types that have been adapted for liquid cooling. In fact, dedicated liquid-cooled drives offer additional benefits. They are physically smaller than air-cooled drives with the same power rating. 

Their small size and reduced weight make liquid-cooled VSDs a popular choice for marine applications, such as the control of bow thrusters and propulsion motors. More recently, they are increasingly finding applications on land.

All VSDs generate heat, but as we’ve seen, dealing with this heat doesn’t have to be a problem. With smaller drives – provided that high efficiency types are chosen – special measures are unlikely to be necessary. For high power types, back-channel cooling provides an effective solution where access can be gained to the outside air reasonably easily, while liquid-cooling provides a very versatile solution for more challenging applications. 

So, whatever the VSD and whatever the application, there’s no need to feel the heat – a cool strategy is always available!