If you’ve been experiencing issues with magnetic noise in your electric motors, don’t worry – you’re not alone.
This type of noise can be caused by a variety of factors, and it’s not always easy to determine the root cause.
In this blog post, we’ll discuss the causes of magnetic noise in electric motors and provide some solutions that you can try. Let’s get started!
All noise originates from mechanical forces that propagate pressure waves through air, liquid or solid materials, and the frequency of noise in the human hearing range is usually between 20 Hz and 20 kHz. Magnetic noise in motors, also called “electromagnetic” or “electrical” noise, is caused by mechanical forces (e.g. pressure) generated by the attractive and repulsive forces of magnetized parts in their alternating magnetic fields. In most cases, this type of noise can be eliminated or reduced by proper design and manufacturing of electric motors.
However, in some cases the source of the noise cannot be eliminated without redesigning the motor, and in these cases it is necessary to take steps to suppress or isolate the noise. The following are some common causes of magnetic noise in electric motors, and some possible solutions:
Unbalance or misalignment of the rotor and/or stator.
Solution: This type of noise can often be eliminated by proper balancing and alignment of the rotor and stator.
Winding unbalance or poor insulation.
Solution: This type of noise can often be eliminated by proper insulation and balance of the windings.
Stray magnetic fields from other equipment or currents in the wiring.
Solution: This type of noise can often be eliminated by proper shielding and filtering of stray magnetic fields.
Faulty bearings or improperly seated shafts.
Solution: This type of noise can often be eliminated by proper lubrication and maintenance of the bearings.
In addition to these common causes, there are other factors that can contribute to magnetic noise in electric motors.
For example, high temperatures can cause the insulation on windings to break down, resulting in noise.
Environmental conditions such as humidity and dust can also cause electrical noise.
Magnetic noise in motors can be caused by a variety of factors, including unbalance or misalignment of the rotor and/or stator, winding unbalance or poor insulation, stray magnetic fields from other equipment or currents in the wiring, faulty bearings or improperly seated shafts, and high temperatures.
Environmental conditions such as humidity and dust can also cause electrical noise.
The alternating magnetic field excites vibrations and noise at twice the linear frequency (e.g., hum) only when the motor is energized, and if the noise stops immediately after power is removed, the source is magnetic noise.
Magnetic noise is usually the second largest source of noise for two and four pole motors (wind resistance is the first).
maybe the primary source of noise for six or more pole motors.
This is mainly due to the fact that the depth of stator residual silicon steel pieces in low-speed cores is smaller than the stator depth in high-speed cores with fewer poles (see Figure 1) and residual silicon steel pieces in 2-pole and 6-pole stator cores, which makes them more susceptible to deformation and produces greater amplitude vibration due to smaller forces. Low-speed motors with six or more poles are prone to higher noise levels due to smaller air gaps and the eccentric effect of super poor bearing and housing fits.
If magnetic noise is its primary source, the overall noise of the motor increases when a load is applied.
Typically, the difference in total noise level at no load and full load is small for two- and four-pole motors, but can be significant for motors with six or more poles.
Motor designers manage magnetic noise by making the air gap as large as possible (while maintaining an acceptable power factor), they can reduce magnetic forces caused by air gap variations, and they can reduce air gap flux density by using longer cores, which generally improves power factor.
Another consideration is that closed slots do not lead to increased magnetic noise, which explains why designers prefer closed-slot rotors, and they also prefer semi-closed slots with minimal openings for random-winding stators, even though wider slot openings make windings easier to insert.
A related form of magnetic noise is sloshing noise, a relatively low-volume, low-frequency, high-frequency component of beating that can be objectionable because it is intermittent.
As a function of sloshing, which is more pronounced under load, the frequency varies directly with sloshing.
Causes may include open rotor bars or end rings, but the slipping noise is usually associated with a uniformity defect in the rotor, and the remedy is a new rotor.
There are several solutions that you can try if you’re experiencing magnetic noise in your electric motors:
-If the noise stops immediately after power is removed, the source is most likely magnetic noise. You can try to reduce the magnetic force by making the air gap as large as possible (while maintaining an acceptable power factor).
You can also reduce air gap flux density by using longer cores, which generally improves power factor.
-If the noise is its primary source, the overall noise of the motor increases when a load is applied.
You can try to manage magnetic noise by making the air gap as large as possible (while maintaining an acceptable power factor), reducing magnetic forces.
-Another consideration is that closed slots do not lead to increased magnetic noise. You can try to use closed-slot rotors, and you may also prefer semi-closed slots with minimal openings for random-winding stators.
-A related form of magnetic noise is sloshing noise. As a function of sloshing, which is more pronounced under load, the frequency varies directly with sloshing. Causes may include open rotor bars or end rings, but the slipping noise is usually associated with a uniformity defect in the rotor, and the remedy is a new rotor.
If you’re still experiencing issues with magnetic noise in your electric motors after trying these solutions, you may need to bring in a professional.
Contact your local motor shop for assistance.
Tilting of the rotor slots can reduce magnetic noise
but there is no agreement on the optimum number of slots to tilt, or even an accurate method of calculating their effect on the noise generated. A common recommendation is to tilt the rotor with at least one rotor or stator slot (whichever has fewer slots); any smaller deviation will not significantly reduce magnetic noise, and larger deviations will usually reduce motor performance.
Magnetic noise can also be reduced by using a motor with fewer poles. The higher the number of poles, the more quickly the magnetic field alternates and the greater the noise generated. If a motor has to be operated at a high speed to meet its performance requirements, reducing the number of poles will usually quiet it down without significantly affecting its output.
Finally, adding insulation to the motor housing can help reduce magnetic noise. The thicker the insulation, the better it will work, but there is a limit to how much noise can be eliminated in this way. In some cases it may be necessary to add additional shielding around the motor to completely eliminate magnetic noise.
Here are a few tips on how to reduce magnetic noise in electric motors:
-Tilt the rotor slots
-Use a motor with fewer poles
-Add insulation to the motor housing
-Add shielding around the motor.
Uneven air gaps can cause unbalanced magnetic pull and stronger magnetic forces in the direction of the smallest air gap, which can distort the stator, rotor and frame while creating electromagnetic noise, and running the motor at reduced voltage is an easy diagnostic tool.
lux leakage is another common cause of magnetic noise. Leakage can be reduced by using a better-quality magnet material, increasing the number of turns in the coil, or using a ferrite core.
Ferrite cores are specifically designed to reduce flux leakage and are usually made from a ceramic compound.
They are relatively cheap and easy to install, solux leakage can be reduced by using a better-quality magnet material, increasing the number of turns in the coil, or using a ferrite core.
Ferrite cores are specifically designed to reduce flux leakage and are usually made from a ceramic compound.
For example, if the motor makes noise at full voltage but sounds good at half the rated voltage, look for air gaps and problems such as improperly machined housings or off-core rotors.
Causes of uneven air gap include: off-core rotor off-core stator bent shaft journal machining and rotor body not centered bearing box (or sleeve bearing) not centered end bracket and stator fit not centered deformed housing.
Compared with two-pole motors, manufacturing differences have a greater impact on the magnetic noise of low-speed motors, this is because the air gap of four-pole or multi-pole motors is much smaller than two-pole motors, which makes them much smaller margin of error. consequently, the manufacturing quality of four-pole or multi-pole motors is more critical.
Magnetic noise can also be caused by stray flux leakage paths in the winding, which can be minimized by using better insulation and tighter winding clearances.
Conclusion.
Identifying the source of noise in a motor is often more challenging than correcting it, and a methodical approach to investigation can narrow the possibilities and make problem solving easier.
If the noise is caused by some factor in the motor design, such as a manufacturing defect or anomaly, the solution is to find the cause of the major magnetic noise generation in the motor and to select the appropriate method for reducing or eliminating them.
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