Electric motors inherently generate a low-pitched hum during operation due to the rotational forces at work. However, any deviation from this norm—such as unusual noises or vibrations—may signal an internal issue that requires attention or immediate servicing like motor rewinding in Singapore. Although diagnosing the source of abnormal motor noise can be challenging, a systematic approach can help narrow down potential causes and identify effective remedies.
It is important to note that if the noise stems from a design flaw or manufacturing defect, a straightforward solution might be impractical or even unattainable. The following discussion explores the primary sources of motor noise and outlines strategies to reduce or eliminate them.
1. Windage Noise
Windage noise is predominantly produced by turbulent airflow around high-speed rotating components. This noise is typically broadband or encompasses a wide range of frequencies, lacks any distinct pure-tone or sinusoidal waveform, and is often the largest contributor to overall motor noise. In larger open-enclosure motors, the fan action of the rotor bars—not the cooling fins or dedicated fans—is usually responsible for most windage noise. Additionally, motors equipped with radial vent ducts may exhibit a “siren” effect, a pure-tone noise above 1,000 Hz caused by abrupt interruptions in the airflow.
Reduction Strategies
- Minimise Obstructions: Since windage noise arises from turbulent airflow at obstructions near the rotating parts, reducing these obstructions is key.
- Optimise Fan Design: In totally enclosed, fan-cooled motors, opting for a smaller or alternative fan design can lower noise levels—especially in unidirectional applications. However, keep in mind that doing so may reduce cooling efficiency and potentially shorten the life of lubricants and windings as well as potentially require taking everything apart via electric motor overhauling.
- Adjust Clearance and Blade Spacing: Increasing the gap between the fan and stationary components, or arranging the fan blades asymmetrically, can help reduce the noise generated at the fan-blade frequency.
2. Magnetic Noise
Magnetic noise—also known as electrical or electromagnetic noise—is produced by mechanical forces generated through the attraction and repulsion of magnetised parts within the alternating magnetic field of the motor. This phenomenon creates vibrations typically at twice line frequency (resulting in a characteristic hum) and only occurs while the motor is energised. The cessation of noise when power is removed is a strong indicator of a magnetic source.
Magnetic noise is generally the second most significant noise source in 2-pole and 4-pole motors, while it often becomes the primary concern in motors with six or more poles. Slow-speed motors, in particular, are more susceptible due to their stator’s lower back-iron depth, smaller air gaps, and potential eccentricities in housing fits or bearing tolerances.
Types of Magnetic Noise
- Slip Noise: This intermittent, low-volume noise is associated with rotor slip and is more pronounced under load. It often points to irregularities in the squirrel-cage rotor—such as an open rotor bar or end ring. In such cases, replacing the rotor may be the most effective solution.
- Unequal Air Gap Noise: An uneven air gap creates unbalanced magnetic forces, potentially deforming the stator, rotor, or frame and resulting in electromagnetic noise. Operating the motor at a reduced voltage can serve as a diagnostic test; if the noise diminishes, the issue likely lies with air gap uniformity.
Reduction Strategies
One common method to mitigate magnetic noise is by skewing the rotor slots. While there is no definitive guideline on the optimal skew, ensuring that at least one rotor or stator slot (whichever is fewer) is skewed can substantially reduce magnetic noise without significantly impairing motor performance.
3. Mechanical Noise
Mechanical noise in motors arises from physical interactions such as friction, loose components, and bearing issues. Common sources include:
- Loose Stator Core: A stator core that is not securely fixed can cause a persistent buzzing sound. In motors with rolled steel frames, tapping the frame with a mallet while the motor is operating can reveal if a loose stator core is the culprit—changes in the noise level upon tapping indicate a poor frame-to-core fit.
- Bearing Noise: Bearings are often a major source of mechanical noise. Problems such as eccentricity, poor surface finish, nonuniform rolling elements, or rattling retainers can produce noise in the 100–300 Hz range. While such noise is generally low in amplitude, it may excite other components’ natural frequencies, leading to damaging vibrations.
- Component Friction: Physical rubbing between internal components—such as between the rotor and stator or between fans and covers—also contributes to mechanical noise. Ensuring proper alignment and restoring rotor-stator concentricity are crucial steps in addressing friction-related issues.
Reduction Strategies for Bearing Noise
One effective method to reduce bearing noise is to preload the bearings axially using a wavy-spring washer. This technique applies a consistent force on the outer race of the bearing (typically at the non-drive end), eliminating internal clearance and ensuring that each rolling element follows the same path. The result is a reduction in rattling noise and high-frequency vibration. However, excessive preload can introduce low-frequency noise and potentially lead to overheating, so careful calibration is essential to avoid bearing failure.
Conclusion
Addressing motor noise involves a comprehensive evaluation of various factors. By systematically diagnosing the noise source and implementing targeted corrective measures, organisations can significantly reduce the overall noise levels of their motor. Still, it is important to note that there may be times when inherent design limitations or manufacturing anomalies may prevent a complete resolution of noise issues.
