Low-noise fan drives

Noise behav­iour is a crit­ical feature of fan drives. Fans are used in a wide variety of different devices and instal­la­tions that have indi­vidual struc­ture-borne vibra­tion char­ac­ter­is­tics. The usual proce­dure for avoiding reso­nance by matching the natural oscil­la­tion and exci­ta­tion is often not effec­tive for these vari­a­ble­speed drives. Secondary measures such as mechan­ical decou­pling are device-specific and incur extra costs. There­fore, this appli­ca­tion calls for a solu­tion with robust noise reduc­tion and low vibra­tion exci­ta­tion, which provides good results in various specific appli­ca­tions.

Three-phase asyn­chro­nous motors work with a sinu­soidal power supply with low vibra­tion and noise. Prob­lems can occur in speed setting due to current harmonics caused by the speed controller. Contin­uous and low-noise speed setting perfor­mance requires oper­a­tion while connected to a well-adjusted frequency inverter. In this case, the EC motor is the prefer­able solu­tion from an economic perspec­tive. Though the motor’s vibra­tory behav­iour is not always optimal due to the design prin­ciple, ebm-papst considers this an oppor­tu­nity to develop partic­u­larly low-noise EC drives.

For EC motors, we distin­guish the causes of noise and vibra­tion exci­ta­tion between alter­nating forces caused by perma­nent magnets and those caused by elec­tro­mag­netic effects. For example, the slots of the stator cause a vari­able magnetic air gap perme­ance which, in conjunc­tion with the perma­nent magnet field exci­ta­tion, gener­ates torque fluc­tu­a­tions known as cogging torque. The commu­ta­tion of the currents in the wind­ings can also cause elec­tro­mag­net­i­cally induced fluc­tu­a­tion. In addi­tion to the torque fluc­tu­a­tion, the forces acting on the rotor in a radial direc­tion are also of great impor­tance.

The motor and commu­ta­tion elec­tronics like­wise have an effect on the noise behav­iour. ebm-papst develops and produces both drive compo­nents as one unit — an impor­tant require­ment for an optimal drive system. Thanks to a bene­fi­cial combi­na­tion of the number of slots and poles and a special sheet steel and magnet geom­etry, the cogging torque of the ebm-papst motors is less than 0.5 percent of the rated torque. Conven­tional motors have cogging torque between 5 and 8 percent. In addi­tion, a method of current commu­ta­tion is used that is specially opti­mised for fans. This provides an ideal compro­mise between low vibra­tion and high effi­ciency.

Opti­mising the motor and commu­ta­tion elec­tronics together makes it possible to improve the vibra­tion and noise behav­iour of our EC motors dramat­i­cally. Today’s drives provide excel­lent results, even in sensi­tive appli­ca­tions. Normally, no addi­tional specific measures are required, decreasing the appli­ca­tion effort substan­tially.

The noise compar­ison shows a three-phase asyn­chro­nous motor with speed that has been changed via phase-angle control (a) and using a contin­u­ously vari­able trans­former with the ideal sinu­soidal power supply (b). For compar­ison, the curve of a conven­tional EC motor (c) is shown. The current Green­Tech EC motors (d) attain values compa­rable to the three-phase asyn­chro­nous motor in its most favourable oper­ating case.