Tom Bishop, P.E.
EASA Technical Support Specialist

Determining the source of noise in a motor is often much more challenging than correcting it. However, a methodical approach to investigating the noise can narrow down the possible causes and therefore make it easier to resolve the noise issue. There is a caveat. If the cause of the noise is due to something in the motor design, that is, a manufacturing defect or anomaly, a solution may not be possible or practical.

There are three primary sources of noise in a motor: magnetic, mechanical and windage. We will discuss the causes and characteristics of each and provide guidance in dealing with reducing or eliminating the noise associated with them.

Magnetic Noise
Magnetic noise, also termed “electromagnetic noise” or “electrical noise,” is the result of mechanical forces (e.g., pressure) when magnetized parts of a motor are attracted and repelled from each other by an alternating magnetic field. The alternating magnetic field excites vibration and noise at twice line frequency (e.g., hum). Since the field only alternates when the motor is energized, that yields a useful clue: If the noise ceases immediately when the power is removed, the source is magnetic.

Magnetic noise is typically the second-highest source of noise for 2- and 4-pole motors (windage is the first) and can be the first one for motors with six or more poles. The main reason for this is the stator back-iron has less depth in slow speed (greater number of poles) than higher speed (lower number of poles) cores (see Figure 1). This makes the back iron more susceptible to deformation and results in vibration at a higher magnitude with smaller forces. Slower speed (6 or more poles) motors are more susceptible to higher noise levels due to smaller air gaps and the effects of eccentricity due to out-of-tolerance bearing and housing fits.

If magnetic noise is the main source of motor noise, the overall noise tends to increase when load is applied (see Table 1). Usually, the difference in overall noise level between no load and full load conditions is small for 2- and 4-pole motors but can be significant for motors with six or more poles.

Making the air gap as large as possible during the design process (provided the power factor remains acceptable) reduces magnetic noise. Similarly, decreasing the air gap flux density by using a longer core decreases the magnetic forces arising from air gap variations and generally improves the power factor. The magnetic force associated with the air gap flux density is proportional to the square of the air gap flux density. Thus, a relatively small change in air gap flux density can result in a much larger change in magnetic pull. 

The service center can redesign for a lower flux level if power capability can be reduced. Another step that may help reduce magnetic noise is simply to dip and bake the stator.

Returning to considerations during the motor design process, the use of closed slots never causes an increase in noise. For this reason, rotors with a closed slot configuration are preferred. Similarly, random-wound stators should utilize semi-closed rather than open slot configurations. Further, the width of the slot opening should be kept at a minimum. Keep this in mind if you ever contemplate making the slot opening wider for ease of wire insertion.

Another design issue is slip noise, which is a low-frequency beating of higher frequency components. Because it is intermittent, it may be objectionable even though its level is relatively low. Being a function of slip, it is more noticeable under load, with the frequency varying directly with slip. The noise is often associated with a defect in the uniformity of the squirrel-cage rotor, in which case a new rotor is required. Note that an open rotor bar or end ring could also result in slip noise.

Skewing of either the stator or rotor (typical for a motor) slots reduces magnetic noise. However, there is no consensus on the optimum amount of skew. Further, the noise produced by varying amounts of skew cannot be calculated accurately. It is most often suggested that the rotor be skewed at least one rotor or stator slot, whichever has fewer slots. Skewing less than one slot is not effective in appreciably reducing magnetic noise, and larger skews generally sacrifice motor performance.

Some winding configurations can lead to objectionable magnetic noise. If the winding has half as many circuits as poles, using extra-long jumpers can reduce magnetic noise. An example would be a 3-circuit connection for a 6-pole winding, which would use 1-10 jumpers to reduce noise. Unbalanced fractional slot windings can result in unbalanced line currents and twice line frequency vibrations, with accompanying noise. Also, an error in grouping could result in unequal coils per circuit, which in turn creates circulating currents and magnetic noise.

Another source for magnetic noise is the stator-rotor slot combination. Potentially noisy combinations include those that result in stator slots minus rotor slots that are +/-1, +/- 2, poles +/- 1, or +/- 2. Table 2 provides combinations applicable to some windings between 2 and 48 poles. Note: Skewing, mentioned above, can be used to reduce magnetic noise. Thus, if the rotor (or stator) is skewed circumferentially at least one stator or rotor slot, whichever is shorter, the slot combination is not expected to be a concern.

An unequal air gap causes unbalanced magnetic pull, which can cause higher magnetic forces in the direction of minimum air gap, as shown in Figure 2. This leads to structural deformation of the stator, rotor and frame and to electromagnetic noise. Running the motor at reduced voltage is a simple diagnostic tool. For example, if the motor is noisy at full-rated voltage and sounds fine at half of rated voltage, focus on the airgap. Check for issues such as a mis-machined housing or an eccentric rotor.

Causes of an unequal air gap are:

• Eccentric rotor
• Eccentric stator
• Bent shaft
• Shaft journals machined out of concentricity with rotor body
• Bearing housings (or sleeve bearings) not concentric
• End bracket to stator fit not concentric
• Distorted frame

The effect of manufacturing variations on magnetic noise is more significant in slower speed motors than with 2-pole motors. The reason is that the air gap of 2-pole motors is much greater than those with four or more poles, making the margin for error much smaller for the slower speed motors. For example, the air gap for a 22″ (560 mm) OD stator for a 2-pole motor could be 0.055″ (1.4 mm), while the air gap for a 22″ (560 mm) OD stator for a 6-pole motor could be 0.022″ (0.55 mm).

A method that helps reduce magnetic noise in form coil motor windings is to use magnetic wedges in the stator slots. The magnetic wedges reduce the tangential forces being applied to the teeth.

Mechanical Noise
All noise is mechanical in origin and occurs when waves of pressure are transmitted through either air, liquids, or solid materials. Noise frequency components within the range of human hearing are generally between 20 Hz and 20 kHz.  

The audible component of magnetic noise in a motor is the result of mechanical pressure produced when magnetic material parts are attracted and repelled from one another due to an alternating magnetic field. Additionally, since the motor has a rotating internal part (rotor), unbalance forces are transferred to the motor frame as noise.

A stator core that is loose in the frame will cause a buzzing noise. If the motor has a rolled steel frame, you can verify that the core is loose by tapping the outside of the steel frame (shell) with a mallet while the motor is running. If the impact of the tapping deforms the frame to core fit, the noise level will change or may even cease. Strictly speaking, the source of the noise is magnetic; thus, the noise will also cease when the power is removed.

Physical rubbing between components within the motor is another source of mechanical noise. Possible causes include damaged or worn bearings, external fan contact with the cover, internal fan contacting an air deflector and rotor contact with the stator. These issues can be corrected by replacing noisy bearings, ensuring fans are properly located and making certain that rotor-stator concentricity is within tolerance. Machines with brushes can produce noise associated with the brushes sliding on the collector rings or commutator or sliding over a high bar or bars in a commutator.

Bearings are a frequent source of mechanical noise in a motor. In addition to rubbing, which was mentioned above, we will expand on other bearing noise topics.

Excessive noise in rolling element bearings may be caused by nonuniform balls or rollers, poor surface finish, ball or roller retainer rattle and eccentricity. These result in impact noise or resonance excitation of the bearing housings, air deflectors (baffles) and other parts that are relatively efficient at radiating noise.

The mechanical noise from some of these sources is distinct enough to be easily identified. The following are some examples:

• Brinelling is characterized by a low-pitched noise
• Dirt in the bearings causes a shrill noise
• Ball or roller skidding at low temperatures with insufficient lubrication results in a high-frequency noise
• An intermittent popping noise is often due to the grease. 

A noise in the frequency range of 100 to 300 Hz may be caused by the passage of the balls or rollers and is a characteristic of rolling element bearings. This is generally a low amplitude noise and not otherwise physically detrimental unless it excites the natural frequencies of any motor parts and results in damaging vibration.

One of the best methods to reduce and dampen bearing noise is the use of a wavy-spring (wave) washer to preload the bearings axially. The washer acts as a spring to exert a force, usually on the outer race of an axially free ball bearing (typically the non-drive end). The force between the axially free and the locked bearing eliminates the internal clearances, causing each ball to follow the same raceway path on each bearing.

Preloading bearings thereby decreases noise due to balls rattling within the raceway and cage, decreases the generation of high-frequency vibration and improves dynamic balance by removing bearing looseness. However, too much bearing preload produces low-frequency noise and possible overheating of the bearings.

Friction noise in a bearing arises from insufficient lubrication between two sliding surfaces. It is caused by high impact vibration that results from rapid intermittent contacting of the surfaces. Sleeve bearings exhibit this phenomenon when there is insufficient oil film, whereas rolling element bearings commonly experience sliding contact due to a lack of preload. The noise where the point of contact occurs is high frequency, with a sound like that of hissing air. When the impact vibration is transmitted to a resonant part inside the motor, the sound can be described as a screech. 

Any structural part of a motor may become a source of air-borne noise if it is excited with sufficient energy at its natural frequency. For example, rotational unbalance itself may not emit audible air-borne noise, but it may act as an energy source for vibration, which is then transmitted through the support structure. The vibration is then converted to air-borne sound waves at the resonant component. Thus, the vibrating part sets the air into motion, making it appear as if it were the noise source. If the vibrating part is an air deflector or drip cover, or similar component, noise deadening or damping material can often be applied to change vibratory motion into heat energy by making use of the internal friction of the material. An example of this method of noise reduction would be the use of room temperature vulcanizing RTV silicone between an air deflector and an end bracket.

The emission of air-borne noise created within the motor can be reduced by using sound-absorbing materials. These are porous materials that absorb the energy from sound waves passing into their pores and convert it to heat energy. The absorption capability of this type of material increases with its density, tightness or pore structure, and thickness. If possible, the barrier should completely enclose the source. A potential drawback to using sound-absorbing material is that it may restrict airflow or heat transfer, thus increasing motor temperature.

Windage Noise
Windage noise typically accounts for the majority of all noise from an electric motor. Windage noise is most significant in high-speed (e.g., 2- and 4-pole) motors. It is caused by the presence of obstructions in the vicinity of the rotating part that moves air and creates turbulence. Therefore, to reduce windage noise, obstructions to the cooling airflow pattern must be minimized. Windage noise differs from most motor noise sources in that it is created in the airstream rather than in the motor parts. In most cases, it is broadband (a wide range of frequencies) noise with essentially no significant pure-tone (sinusoidal waveform) components. 

Most of the windage noise of larger open enclosure motors comes from the fan action of the rotor bars, not the cooling fans or fins. Because of this, a reduction in cooling fan diameter probably won’t result in a significant  reduction  in  noise, but it could result in a significant reduction in cooling airflow. Also, large open motors may exhibit airflow induced vibration that can be reduced by changing the number of fan blades.

Large open motors with radial vent ducts through the rotor and stator may produce pure tone components of airflow noise that are highly irritating. The frequency is typically above 1000 Hz, and the noise is often termed the siren effect. The noise itself is produced by the sudden interruption of the airflow exiting the radial ducts in the rotor. Offsetting of rotor ducts with respect to stator ducts can be beneficial in reducing noise levels. 

On totally enclosed fan-cooled (TEFC) motors, an external fan-diameter reduction, or changing the type of fan, especially in unidirectional applications is very effective at reducing noise.  The primary drawback is that any fan or vent path reduction will cause the motor to run hotter, and thereby reduce winding and lubricant life. The fan-blade-frequency noise level of a TEFC motor can also be reduced by increasing the clearance between the fan and stationary parts; or by using a nonsymmetrical spacing of the fan blades.  

Air flowing around or against surfaces produces turbulence, a potential source of objectionable noise. From a design and manufacturing perspective, the following are some considerations for avoiding windage noise issues:

• Eliminate sharp edges and burrs on all parts in contact with the airstream
• Use a short, streamlined airflow path
• Keep the cross-section of the airflow path as large as possible for as much of its length as possible
• Minimize abrupt changes in airflow direction
• Keep boundary surfaces smooth
• Provide gradual changes in airflow path cross section.

When dealing with a windage noise issue, use the above points to evaluate the design of the airflow circuit in the motor.

Categories: Technical topics, AC motors, DC motors, Troubleshooting & failure analysis

Tags: Magnetic wedges Noise

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