Advances in circuit design and in digital circuits have improved the efficiency of power electronics – an old class AB audio amplifier had an efficiency of about 50percent whereas its modern successor has an efficiency approaching 95percent.

The main reason for this improvement is that most modern power electronics, including drives for ac motors, utilise semiconductors in a switched mode – they are either on or off. As the power dissipated by a power semiconductor is proportional to the current through it times the voltage across it this means that the losses are low as either the current or the voltage will be very low.

The power loss can still reach appreciable values; even if the efficiency of a drive can be as high as 98percent this means that a 1MW drive at this efficiency still has losses of about 20kW. This is sufficient to heat a normal house.

These losses have to be cooled away in order to keep the internal temperature of a drive within reasonable limits set by the ambient temperature, drive losses and the temperature rating of the components.

New semiconductor technologies will allow much higher semiconductor temperatures than allowed today – creating intense spot heat sources and requiring new cooling concepts. These new technologies will continue to drive the development of drives.

Normally the losses are cooled away using forced air cooling. The heat producing semiconductors are mounted on an aluminum heat sink (copper in rare cases) with fins on the opposite side.

A separate fan blows air over these fins, transferring the heat to the ambient air. This solution is used in the vast majority of drives. It works well, is robust and reliable as well as simple. Air is obviously available everywhere. In some instances the air required for the cooling has to be ducted to the drive and this ductwork can occupy significant space, especially if space is at a premium.

A large drive requires up to 2000m

The highly efficient and densely finned cooling elements used in Vacon’s drive ranges allow a high power density design. In spite of the fact that a large volume of air is required for the cooling the drives are compact both as IP21 and as IP54 versions – both have the same dimensions. This is mainly because the design uses a cooling channel concept where the ambient air is not allowed to enter into the drive; it is used to cool the cooling fins protruding into the airstream.

For drives with a relatively low power it is possible to create a cooling based purely on convection – they do not require fans. This is possible to achieve in a reasonable physical size up to a power level of about

5.5–7.5kW. This cooling system allows the creation of drives with an extremely high degree of protection – up to IP65. Drives of this kind, for example the X4 or X5 ranges are very suitable for harsh environments or in environments where long fibres can clog the heatsink fins, such as in textile industry. The enclosure also allows the use of high pressure water for washdown purposes, which is required in the food and beverage industry. The high degree of protection also protects against corrosive chemicals in the environment. A similar protection is given by conformally coating the electronics, or, alternatively ducting clean air to it – although this may not always be realistic.

A related type of cooling system is the so called cold plate drive. In this type the finned heatsink is replaced by a simple, smooth aluminum plate, designed to be mounted on a smooth surface that will maintain the temperature within the drive within limits. Typically such surfaces are either air- or liquid-cooled parts of a machine. The solution eliminates the need of a separate drive cooling system and makes the whole machine more compact.

For the ultimate in power density and small size a pure liquid-cooled design is used. The power semiconductors are directly mounted on a heatsink where liquid is circulated. This design prevents the liquid from coming into direct contact with the high voltages on the semiconductors, allowing the use of normal drinking water for the cooling. In many cases glycol is added to the cooling liquid, reducing its heat carrying capacity but increasing the resistance to cold and inhibiting the growth of biological organisms in the cooling water circuit. As the heatsink is aluminum, no brass or copper tubing should be used, as a small particle of copper or brass in direct contact with the aluminum will cause corrosion. Compared with a forced

air-cooled drive a liquid-cooled drive may be up to 70percent smaller in volume, leading to significant space savings in for example ships or offshore installations. More space is saved as there is no need to duct large amounts of air to and from the drive. The heat can be transferred by the liquid either to a place where it can be used or to some external sink – for example the ocean.

Vacon can supply drives of all the described types, in the power range 0.2kW to 5MW.