Electrical noise occurs
The motor continuously rotates through the electrical rectification generated by the brush and the commutator. That is, the high voltage generated by the reverse current flow, the spark that occurs at this time, is also the main cause of electrical abnormalities.
When the motor is stopped from the state to the start of rotation, although the current is transitional, it is still larger than the normal state, and the electric noise becomes large.
This is caused by the follow-up instability of the sliding contact portion of the commutator and the brush or the overload current.
An insulating film is formed on the sliding surface of the commutator and the brush, and the electrical contact is unstable.
Measures to reduce electrical noise To reduce electrical noise, capacitors and choke coils can be installed at the motor terminals. Generally, the method of eliminating the spark voltage is to install near the rotor of the source, and the cost of this method is low. The performance is also high.
Can be installed inside the motor to eliminate the noise components, such as the installation of ring varistor (D / V), ring capacitors, rubber ring resistors (RRR), chip capacitors approach.异 High frequency range noise has effect
Can be installed outside the motor to eliminate the noise components, such as external capacitors (electrolytic type, ceramic type), choke. ⇒Effective for low-frequency range noise
As countermeasures to reduce noise, there are 1 or 2 used alone, and 1 and 2 are used in combination, and 1 and 2 have a large effect. Types of electrical noise The interference noise of electromagnetic waves can be broadly divided into the following two types, and they can be regarded as interference waves to measure the noise.
*Sometimes the frequency range will vary according to specifications
Type Bit Definition Frequency* Line Noise (Conduction Conductor Terminal Voltage) dBμV Power Cable,
The 0.15~30MHz radiated noise (radiation noise electric field strength) emitted from the connecting cable is emitted in the air and becomes a radio wave.
The interference noise 30~1000MHz
*Sometimes the frequency range varies depending on the specifications.
Type Definition 1 Brush friction noise of brush brush and commutator 2 axis vibration noise of shaft and bearing 3 Gap sound (GORO, arpeggio) Particularly harsh sound in the second item 4 Resonance sound only to a certain extent Resonance sound in the rotation (frequency) area 5 axial vibration sound (KOTSU sound, collision sound) Back to the rotor in the axial vibration, movement of the squeaking sound emitted 6 wind chimes sound of the motor built cooling fan Wind cut
The above-mentioned 3 to 6 are typical examples of abnormal sounds, and there are various other abnormal sounds that are judged as "grating noise." Measurement of mechanical noise Measurement of mechanical noise uses a noise meter to physically detect the noise level (the size of the sound).
It is generally measured in terms of frequency compensation (auditory compensation) called "JIS-A characteristics" and the unit is dB (RMS). In the measurement, conditions that should be defined include [voltage, number of revolutions, load, side pressure, motor positioning, microphones position, background noise, etc.].
[example of delivery specification book]
Mechanical noise: 48dB or less (RMS)
The JIS-A characteristic level at a distance of 10 cm from the output shaft-side mounting surface of the motor was measured with a voltage of 3 V and no load and the shaft was in a horizontal position. The background noise must be below 26dB (RMS).
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