Introduction

Acoustic noise of electrical machines includes the contribution of electromagnetic, aerodynamic and mechanical sources. This article reviews the sources of mechanical noise in electric machines.

Mechanical noise sources in electric machines

Mechanical noise depends on the bearing type:

• journal bearing / sleeve bearings
• fluid bearings / oil film bearings
• ball bearings / roller bearings

Journal bearings noise is due to sliding surface, lubrification faults, and manufacturing imperfections.

Floating bearings noise is due to instability of the oil film (oil whirl).

Ball bearings noise is due to manufacturing imperfections, dirt and lubrification faults, resonance of outer ring, assembly faults and rotor shaft resonances.

Mechanical noise depends on and can be modified by operating speed, mechanical load, temperature and misalignement.

Mechanical noise frequencies in electric machines

Oil whip instability can occur at subsynchronous frequency 0.1fR, 0.25fR.
Contact rub between rotor cage and ball bearings can create subsynchronous excitations at 0.5fR.

Ball bearing excitations occur at frequencies multiple of:

• ball passing frequency
• ball passing frequency on outer raceway
• ball passing frequency on inner raceway
• ball rotational frequency

Mechanical noise mitigation in electric machines

• Modification of damaged ball bearing
• Use of chemical additives
• Use of vibration absorption or vibration isolation device
• Modification of rotating speed
• Use of alignment tools when mounting the motor
• Application of axial pre-load by means of coil springs
• Addition of elastic damping elements in bearing housing
• Application of shield or seal to prevent dirt from entering the bearing
• Dynamic rotor balancing

Use in Manatee software

Manatee software aims at calculating acoustic noise and vibrations due to magnetic forces in electrical machines.