Over the past 18 months, Durban-based compressor original-equipment manufacturer Airstream Compressors has supplied 15 rotary-screw air compressor (SCR) permanent magnet (PM) driven compressors across South Africa, saving clients between 30% and 40% on energy costs.

Airstream Compressors has adopted the use of PM-drive technology in the manufacture of its compressors to minimise the energy needs of the compressors. The technology needs a low start-up current, equal to that of a rated motor current. This eliminates start-up spikes and reduces the impact on electricity supply and costs.

Fixed-speed compressors need a set pressure band to work and avoid the frequent starting and stopping of the compressor. SCR PM driven compressors automatically adjust the inverter frequency and motor speed according to the air consumption requirements of a client’s operation to maintain a constant pressure.

The rotor of a PM drive contains several permanent magnets. There are no copper windings that require excitation as are found in standard alternating current motors.

The PM drive uses special neodymium iron boron (NdFeB) magnetic material – the latest technology available, comprising rare-earth permanent magnetic material. The highly magnetic properties and high efficiency of NdFeB material enables the motor to run at low revolutions every minute.

A challenge when using the PM drive is the removal of heat in the compressor so that it does not lose its magnetic properties. This can be overcome through air cooling and water cooling, although the most appropriate for this application is oil cooling.

“Using oil cooling for the PM motor requires only a small space. This cooling method has good heat radiation and is more cost effective than air and water cooling, as it uses the compressor’s existing oil cooling system to cool the motor as well,” says Airstream Compressors technical manager Marius Janse van Rensburg.

“If you calculate a client’s energy use over 8 000 h/y, the PM-drive technology can save our clients collectively more than R2-million a year in electricity costs,” he notes.