Noise sources of the motor

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Noise sources of motors: When a motor is in operation, various types of noise are generated simultaneously. Different noises originate from different components of the motor, and these include: ① aerodynamic noise, ② electromagnetic noise, ③ mechanical noise and bearing noise, ④ brush noise. Depending on the motor type, structural design, and operating speed, the main sources of noise also vary. For high-speed motors, the main source of noise is aerodynamic noise. In motors operating at medium and low speeds, electromagnetic noise and bearing noise are more noticeable. (i) Aerodynamic noise: The aerodynamic noise of motors consists of two main components: eddy current aerodynamic noise and fluting noise. Eddy current noise is primarily caused by the eddies in the cooling air generated by the rotor and fan, with these eddies alternating on the rotating surface. Its spectral range is wide. Flute noise is generated by compressed air or by air passing over fixed obstacles, that is, the \"whistling effect.\" The flute noise in motors is mainly caused by radial ventilation channels; the vortex effects of the cooling air are not directly related to this phenomenon. These two types of noise can be addressed separately. Open-type high-speed motors produce the most aerodynamic noise. Aerodynamic noise is inevitable; however, it can be reduced by lowering the speed around the rotor, decreasing the surface area of the motor rotor, and increasing the smoothness of the rotor’s surface. The main cause of whistling noise is the friction between the blades of fans and other components and the airflow, or the uniform division of the airflow across the rotor components. For example, the radial ventilation grooves between the stator and rotor of an asynchronous motor actually function as \"alarms\" that produce whistling noise. This noise decreases as the gap between the rotor components and the fixed components increases. Using blades that are not spaced at equal intervals, as well as increasing the gap between the stator and rotor, are effective methods for reducing whistling noise. (ii) Electromagnetic noise: Electromagnetic noise is caused by the temporal and spatial variations in magnetic forces, as well as by the interactions between various components of the motor. When the motor is operating, there is a gap magnetic field in the air gap between the stator and the rotor; this is a rotating force wave, and the electromagnetic force generated is alternating. In addition to the main flux, the gap magnetic field also contains many harmonic components, whose frequencies are often multiples of the number of pole pairs. Therefore, electromagnetic noise is not only caused by the main flux at twice the supply frequency, but also primarily by the higher-frequency noise generated by the harmonic fluxes. When an asynchronous motor is in operation, the air-gap magnetic field is a rotating force wave that exerts magnetic pulling forces on both the stator and the rotor, causing them to deform and vibrate periodically. This generates sound waves that are emitted into the surrounding space, resulting in noise. ①Causes of electromagnetic noise in asynchronous motors: 1. The air-gap magnetic field is a rotating force wave; its radial force waves cause the stator and rotor to undergo radial deformation and periodic vibration, resulting in electromagnetic noise. 2. In the air-gap magnetic field, in addition to the fundamental component of the power supply, there are also higher-order harmonic components. The radial force waves generated by these components act on both the stator and the rotor, causing them to undergo radial deformation and periodic vibration, which results in electromagnetic noise. 3. The deformation of the stator core due to harmonics of different orders has distinct natural frequencies. When the radial force wave of the local magnetic field approaches one of the core’s natural frequencies, resonance occurs; even if the amplitude of the radial force wave is not large, it can still cause the core to deform, resulting in periodic vibrations and significant noise. ②The causes of noise in DC motors: The electromagnetic noise generated by DC motors is also caused by the magnetic field in the air gap – primarily harmonic magnetic fields – which leads to deformation of the poles and the stator as well as periodic vibrations. The frequencies of this electromagnetic noise are mainly the electrical pole frequency and the electrical pole rotation frequency. ③Reasons for electromagnetic noise in synchronous motors: Electromagnetic noise in synchronous motors is caused by harmonic components in the air-gap magnetic field, with frequencies at ④. Other reasons for electromagnetic noise: 1. When the core becomes saturated, it flattens the top of the sine-shaped magnetic field distribution, increasing the component of third harmonics and thereby raising the level of electromagnetic noise. 2. The effect of open slots: Both the stator and rotor slots are open, and the air-gap magnetic resistance changes and fluctuates as rotation occurs. Many \"slot-opening waves\" appear in the air-gap magnetic field, which are related to the size of the air gap and the slot openings; the smaller the air gap and the wider the slot openings, the greater the amplitude of these waves. 3. When the natural frequency of the core is low, significant electromagnetic noise is generated during startup; however, during normal operation, this noise level decreases. If the core is not properly compressed, the vibration of the core laminations and the ventilation grooves will lead to an increase in electromagnetic noise. 4. Flux oscillation generates noise. In DC motors, electromagnetic noise arises due to improper selection of the electrical pole pitch and the compensation winding pitch, as well as an inappropriate match between the main pole arc width and the electrical pole pitch. This noise is primarily caused by the lateral oscillation of flux on the surfaces of the electrical poles and pole shoes, as well as by the lateral vibration of flux at the edges of the pole shoes on the surface of the electrical poles. It is related to the motor’s load and speed; this type of noise can manifest as a loud humming sound. In low-speed motors, the open slots in the stator can produce a sound similar to that of hammering. 5. Dynamic air-gap eccentricity: This phenomenon is caused by the rotor’s elliptical shape, bending of the shaft, and misalignment between the core and the bearings. As a result, one side of the air gap becomes larger while the other side becomes smaller, leading to periodic variations in magnetic resistance along the circumference. This causes the fundamental magnetic flux to have an additional harmonic component. As the rotor rotates, the position of this eccentricity keeps changing, which generates noise due to the unbalanced magnetic field at the rotational frequency. 6. Higher harmonics in the air-gap magnetic field caused by other factors can also generate noise, such as a non-sinusoidal magnetic field distribution resulting from the winding arrangement, pulsating components in thyristor power supplies, and harmonic components in the power grid. 7. Fault conditions of the motor cause electromagnetic noise. In synchronous motors, short circuits may occur in the pole windings; in asynchronous motors, the field windings may break; in high-resistance DC motors, short circuits can arise between the armature and the pole windings; in AC motors, the core packing may be loose, and the assembly air gaps may be uneven. ㈢ Mechanical noise and bearing noise ① Mechanical noise: The noise generated by rotating motors is mainly mechanical noise, and it occurs most frequently in large, high-speed motors. Poor rotor dynamic balance is a common cause of mechanical noise. The frequency is the same as the rotational frequency; when assembly and installation are poor, and the natural frequencies of the stator and rotor components are close to the rotational frequency, resonance occurs, resulting in noise. Component vibration noise is also a common type of mechanical noise; when a motor is equipped with an end-capped fan shroud, the fan shroud generates noise as it vibrates along with the motor. Terminal boxes, covers, maintenance window covers, latching mechanisms, and the like can all generate structural vibration and noise. The vibration of the rotor and the bearings is transmitted to the baseplate and foundation through the end covers; when the rigidity of the end covers is low, they vibrate and generate noise as a result of being excited. ②Bearing noise: The motors use two types of bearings: rolling bearings and sliding bearings. Sliding bearings produce less noise, while rolling bearings generate more noise; the noise level is higher at high speeds compared to low speeds. When a rolling bearing is in operation, the rolling elements move relative to the inner and outer rings as well as the cage; friction and impacts on the working surfaces generate bearing noise. This noise can be divided into noise originating from the bearing itself and structural vibration noise resulting from the assembly of the bearing. The noise generated by the bearing itself includes crushing sounds, damage-related noises, grinding sounds, rolling sounds, cage-related noises, and dust-related noises. Under assembly conditions, the noise generated by bearings is a humming sound. The cause of this bearing noise lies in manufacturing tolerances, precision, assembly gaps, as well as damage to the working surfaces and electrochemical corrosion that occur during transportation, installation, and operation, all of which lead to irregular impacts during the operation of the bearings and thus generate noise. Bearing noise is noise distributed over a wide range of 1–20 KHZ, fluctuating over time and amplified by the motor’s end covers. Most bearing noise occurs within the 1–5 KHZ frequency range, and it sounds like a hissing noise when heard with an electronic stethoscope. Medium-sized induction motors equipped with roller bearings have bearing noise as the primary source of noise; this is broadband noise ranging from 2 to 5 KHZ. The humming noise of bearings under assembly conditions falls within the frequency range of 100–500 Hz; it is a vibration noise generated by the retaining springs of the rolling bearings as a result of the thrust exerted by the rotor and end covers. As the precision of these components continues to improve, slight axial movement is inevitable during the operation of the motor. When the bearings are not properly lubricated and have defects or faults, bearing noise increases significantly due to mechanical friction and impacts; the characteristic frequency of the bearings appears, and this frequency depends on the geometric dimensions of the rolling elements as well as the rotational speed. ㈣ Brush noise: In motors equipped with slip rings or commutators, brush noise is inevitable. There are three reasons for this noise: ① Friction noise – Friction and resulting noise are inevitable due to the sliding contact between the brushes and the slip rings or commutators. The level of this noise depends on the surface condition of the slip rings or commutators, the friction coefficient of the brushes, the absolute humidity of the air, and the pressure applied to the brushes. Under good conditions of the oxide film and brush operation, the friction noise is very low, and there is little wear on the brushes, slip rings, and commutator. When the oxide film or the condition of brush operation is poor, the friction noise increases. Especially when the air is dry and an adequate oxide film cannot be formed on the surface of the commutator, the brushes begin to vibrate, which is a sign of poor lubrication. In such cases, the brushes generate high-frequency frictional vibrations that produce noises such as squealing or screeching. If this situation persists for a long time, it can lead to the destruction of the brushes, the detachment of brush holders, and the breakage of the brush clamps. Therefore, measures must be taken to improve the lubrication of the commutator. The friction noise has a relatively high frequency and a wide frequency range, and its frequency is not closely related to the speed of rotation. ②Impact noise: There is a mica groove between all the commutator segments. Due to deformation of the commutator, some of these segments may bulge or dip. The grooving and chamfering processes for these mica grooves are not optimal. As the motor rotates, the brushes strike the leading edges of the commutator segments; the periodic impact between the brushes and the commutator segments causes the brushes to move radially within their holders, resulting in periodic vibrations of the brushes and the brush holder. This leads to brush noise, which often consists of multiple single-frequency components corresponding to the number of commutator segments. Impact noise is generally below 10 KHZ; the noise level increases when the commutator is deformed or when its surface finish is poor. ③Spark noise: Sparks that are generated during the contact and conduction between the brush and the slip ring in the commutator can also produce noise. This noise is actually the sound of arc discharge; within the acceptable range of sparks, the noise level is quite low. Spark noise increases as the magnitude of the commutation sparks grows. Dangerous, explosive sparks and intense, prolonged sparks (classified as level 3 sparks) often come accompanied by a cracking sound of discharge.