SKF's magnetic bearing technology is part of the variable speed drive solutions that are seeing screw compressors replaced by centrifugal turbo-machines in air and chiller compressor applications.

This technology shift aims to deliver lower operating costs in applications such as air conditioning, industrial refrigeration and the oil and gas industry SKF said that the benefits include: lower energy consumption; higher reliability; no bearing component wear; virtually maintenance free; and smaller machine footprint.

The technology is based on high speed direct drives, coupling the compressor impeller to the drive shaft and using magnetic bearings as the support bearings.

As there is no metal-to-metal contact when magnetic bearings are in operation, there is almost no bearing friction generated.

Magnetic bearings are oil-free and can operate safely at very high speeds through control systems developed by SKF.

A magnetic bearing system has three main elements: bearing stator and rotor to apply electro magnetic forces to levitate the shaft; position sensors to identify the position of the shaft in five axes; and controller and control algorithms to control the current sent to the magnetic bearing stator to keep the shaft at a pre-defined position.

The magnetic bearing stator consists of a stack of laminations with copper coils around it to form a series of north and south poles.

A current is supplied to each coil to produce an attractive force that levitates the shaft inside the bearing.

The controller controls the current applied to the coils by monitoring the position signal from the positioning sensors in order to keep the shaft at the desired position throughout the operating range of the machine.

Usually there is 0.2mm to 0.5mm air gap between the rotor and stator depending on the application.

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