Inverter driven PM motors offer multiple benefits

03 June 2010

Brazilian textile manufacturer, Buettner has reduced its annual power consumption by 33% and increased its machine utilisation by 80% thanks to a retrofit involving the replacement of standard induction motors with permanent magnet machines. The company initially purchased three WMagnet motors from WEG in order to assess their performance on its ring spinning machines, and now intends to fit them on all 45 of them over the next three years.

Previously, these machines were driven by standard induction motors via two-speed gearboxes. Each speed change required the machine to be stopped – a process that resulted in substantial losses in production time.

Motor speed is now controlled electronically using frequency inverters, eliminating the need for manual gear changes and introducing a desirable soft start function, which reduces machine wear and tear. The variation of motor speed with constant torque and energy savings has improved both the reliability and the performance of Buettner’s production line.

A PM motor is considerably smaller than its induction motor equivalent and thus provides more torque per volume and a significant weight saving. Moreover, the PM motor is at least one frame size smaller than the induction motor, and in some cases it can be two frame sizes lower. The cooling requirement is correspondingly reduced, so there’s also less noise. Add to this the space gained by eliminating a gearbox from the power train, and you have a relatively simple retrofit that brings multiple benefits.

According to WEG, its WMagnet motors have lower Joule losses in the rotor, compared with conventional squirrel cage induction motors. As Joule losses account for a significant portion of the total losses in induction motors, replacing the squirrel cage construction with magnets leads to big efficiency gains – easily exceeding the requirements of new harmonised IE grading standard (IE2 and IE3) that is set to replace the current voluntary ‘Eff’ labelling scheme this year.