Variable speed ac drives to change compressor industry

Paul Boughton

Timo Kasi explains how variable speed ac drives and new innovations can change the compressor business.

Compressed air is an essential utility in many processes and manufacturing operations. However, generating the high volumes of air required can have a significant environmental impact. Since energy costs account for 80-90 per cent of a compressor's total cost over its lifetime, energy consumption is a real focus for product development. With the advancement of power electronics and control software, standard variable speed ac drive products can at present handle more and more demanding applications and different types of motors. For instance, most high-speed compressors that incorporate ac drives technologies use permanent magnet synchronous motors due to their efficiency qualities; however, generally these motors can be costly.

Previously it has been problematic, in contrast to the PM motor, to design a motor that is based on standard ac induction technology capable of producing both high speed and high power simultaneously. Now this has also been achieved. From the ac drives perspective, controlling a motor of this type is a challenge. The challenges are generally related to high switching frequencies and/or output frequencies. These demands require from the ac drive a robust design in the hardware and software without sacrificing the overall drive efficiency.

In the compressor industry, Vacon has been partnering compressor manufacturer CompAir for more than six years. CompAir's compression assembly, having only one moving part spinning in a magnetic field, forms the heart of the new compressor design. Its rotor and direct-driven compression elements are levitated by active electromagnetic bearings to allow them to reach speeds of up to 60000rpm. This means that the compressor has no gearbox, no oil, no contact and no wear. Consequently, the compressor has a much lower environmental impact than conventional compressors.

A variable speed ac drive is a standard, key component in the CompAir's centrifugal compression assembly. Centrifugal compressors require high speeds in order for their aero components to be highly efficient - and they have always achieved these speeds via a gearbox. Now, the ac drive forms an electronic gearbox for the compressor and, in conjunction with the high-speed motor, allows it to attain these speeds without the need for a mechanical gearbox. No gearbox means no oil, which is an additional benefit for customers looking for oil-free compression. No gearbox and the very compact size of the ac drive also means that the package is about half the weight and physical size of equivalent compressors, so it can be easily sited in existing plant rooms.

Other 300kW compressors typically have noise levels around 74dB (A) and above, whereas the new design of a compressor is housed in a modular acoustic canopy, resulting in the lowest noise levels in its class at just 69dB (A). This allows the compressor to be installed at the point of use and not in a separate room.

Manufacturing and production are shifting more towards oil-free processes. Oil-free compressors are used in processes where purity and clean air is of utmost importance, eg in food processing and pharmaceutical industries or in certain applications where there cannot be any oil in the air.

The ac drive allows the compressor to run at variable speeds, which is unique for a centrifugal compressor. Variable speed operation matches the compressor flow to plant demand with great efficiency.

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Timo Kasi is Vice President, R&D, with Vacon Plc, Finland.