Noise is a big problem in the application of DC motors. How to reduce the noise of DC motors is also the key research object of various DC motor manufacturers. For example, micro DC motors are fine, and there is almost no noise in products. Large-scale industrial DC motors The noise is almost unbearable.
01 Air Noise
Large DC motors will be equipped with fans. This is the main source of air noise. It is generated by air flow. The size and shape of the fan, the speed of the DC motor and the air path all determine the size of the noise. We can look at a fundamental frequency (Fv) relationship formula for air noise
Fv=Nn/60(Hz)
N: number of fan blades;
n: The speed of the DC motor.
That is to say, the larger the diameter of the fan, the greater the air noise of the DC motor. It can be assumed that reducing the diameter of the fan by 10% can reduce the noise by 2-3dB. When the gap between the fan blade edge and the ventilation chamber is too small, a whistle-like noise will be produced, which is sharp and harsh. The unreasonable shape and structure of the fan blades will cause air vortex noise. In addition, the rigidity of the fan is not enough, and the impact of the airflow will generate vibration and noise.
This kind of noise only occurs in industrial DC motors. According to the causes of air noise, a series of measures are adopted to reduce air noise, such as reasonable design to improve the fan structure and blade shape to avoid the generation of eddy currents. Minimize the diameter of the fan as much as possible to ensure smooth air passage and reduce air impact friction. In addition, sound insulation can also be used to reduce the noise of the DC motor, and sound-absorbing materials can be placed near the radial air duct of the stator. There is also a simple method to surround the DC motor with steel plates or wooden boards. According to tests, it can be reduced by about 20 decibels. Noise, but this is not friendly to the heat dissipation of the DC motor, and will increase the space occupied.
02 Mechanical noise
The mechanical noise of the DC motor is mainly the noise of the friction between the carbon brush and the commutator (no brushless DC motor), bearing noise and rotor imbalance noise.
Carbon brush structure TF DC motor
Disassembly drawing of TF DC motor
(1) In the DC motor, except that the brushless DC motor does not have brushes and commutators, there are these two things. Their constant friction with each other will generate noise, and they are like those using semi-plastic commutators. , the surface roundness is not good, the friction noise will be larger. The friction noise frequency (fk) of carbon brush and commutator is publicized as follows: fk=k(n/60)(Hz)
k is the number of commutator segments. Another point is that the structure of the carbon brush holder is not firm, which will also increase the noise. I believe everyone understands the principle. Therefore, to reduce this noise, it is necessary to control the roundness of the commutator. Ensure a good surface finish and use a solid carbon brush holder to reduce noise, or use a brushless DC motor, which naturally does not have such noise without brushes and commutators.
(2) The bearing noise is caused by the ripples, pits and roughness of the inner and outer rings of the DC motor bearing. After experiments, the noise sound pressure level is proportional to the product of the corrugation height and the number of corrugations on the rolling surface. In addition, the size of the radial clearance also affects the noise. Reducing the radial clearance can reduce the noise. However, the bearing with small radial clearance needs to be used in the casing and end cover with high concentricity between the two bearing chambers, and The requirements for the coaxiality of the rotor are increased. Of course, the quality of the lubricating grease is also one of the reasons. In addition, the installation error of the DC motor bearing will also increase the noise. If the installation error of the bearing exceeds a certain critical value, the bearing noise will increase sharply, and the critical angle will increase with the decrease of the bearing radial clearance. decrease.
In order to reduce the bearing noise of the DC motor, the processing quality of the bearing is very important. Of course, the choice of lubricating grease should not be ambiguous, which can reduce the noise in the DC motor very well.
(3) The solution to the unbalanced noise of the rotor is that the rotor of the DC motor should be strictly checked for dynamic balance to reduce the amount of rotor unbalance.
03 Electromagnetic noise
The alternating electromagnetic force in the air gap between the stator and rotor of the DC motor will cause vibration and noise of the stator and rotor of the motor. Since the air-gap magnetic field not only has fundamental waves but also a series of higher harmonics, the interaction of these magnetic fields will generate periodic forces, and both fundamental waves and higher harmonic electromagnetic forces will cause vibration and noise.
The frequency distribution of electromagnetic sound is mostly between 100-4000Hz. The magnitude of the vibration and noise intensity is related to the magnitude of the electromagnetic force and the stiffness of the stator and rotor. When the electromagnetic force that excites vibration matches the natural frequency of the vibrating component, resonance will occur, and vibration and noise will increase significantly. The electromagnetic force has a radial component and a tangential component. The radial component of the electromagnetic force plays a major role in causing the vibration and noise of the motor. It causes the stator core to generate radial vibration, and the noise generated by the radial vibration is the main component of the electromagnetic noise of the motor. When using odd-slot rotor punching, the slot-induced noise becomes the most important part of the electromagnetic noise. During the operation of the motor, the rotor core of the odd-numbered slot is periodically subjected to the change of the unilateral magnetic pulling force.
According to the above figure (1), the upper magnetic pole arc covers three rotor slots, while the lower magnetic pole arc covers only two rotor slots. At this time, the upper magnetic pulling force is large, and the lower magnetic pulling force is small, which makes the stator core move upward. the trend of. When the rotor rotates by half a slot pitch, as shown in Figure (2), the lower magnetic pole arc covers three rotor slots, while the upper magnetic pole chord only covers two rotor slots, and the magnetic tension at this time starts from With the change, the lower magnetic pulling force is large, and the upper magnetic pulling force is small, so the stator core has a tendency to move downward. Therefore, during the rotation of the rotor, the stator core vibrates periodically up and down. In the same way, the rotor is subjected to a periodically changing unilateral magnetic pull, which causes the rotor to vibrate.
When a double-slot rotor is used, the above situation does not occur, but when the rotor rotates, the slot position changes, causing a pulsed magnetic field in the air gap, and may also cause vibration.
In electromagnetic noise, in addition to the noise generated by the above reasons, due to the high-order harmonic components in the current, harmonic magnetic fields are generated in the stator and rotor gas, and uneven torque will also be generated, causing vibration and noise.
Since the electromagnetic noise does not account for a small part in the total noise of the motor, measures are often taken to reduce the electromagnetic noise in the design and manufacture of the motor. However, when there are too high requirements for limiting noise (such as air-conditioning water pump motors and other occasions that are used for indoor electrical and noise requirements are relatively high), and when air noise and mechanical noise have been effectively suppressed, the rotor slope can be used. Slots, increasing the stator and rotor air gap and reducing the magnetic flux density and other measures to reduce electromagnetic noise.
