- When a motor runs, the commutator switches the direction of the electricity that flows in the windings. Though the system keeps the motor running, occasional spark occurs between brushes and commutator at the timing of the commutation. The spark is one of the causes of the electrical noise.
- Especially when the motor starts from its stalled position, comparably higher current, or a stall current, flows into the windings. Higher current usually causes higher noise.
- Also the similar noise happens when (1) brushes stays unstable on the surface of the commutator, (2) the input to the motor is much higher than expected (3) and so on.
- Other factor may be an insulation created on the commutator surface, which results in unstable flow of the electricity.
Various Cause of the Electrical Noise
How to Reduce the Electrical Noise
An electrical noise could be reduced by fitting a capacitor or a choke coil to a motor terminal part. However in order to reduce spark or voltage generated by spark, these parts are to be fixed to armature, which enables to eliminate the noise more effectively; fixing parts closer to the armature sometimes contributes to the lower cost policy.
Typical methods are;
- A disk varistor (D/V), a disk capacitor, a rubber ring resistor (RRR), and/or a chip capacitor to the inside of the motor ⇒Effective for higher frequency band.
- Electrolytic or ceramic capacitors and/or the choke coil to the outside of the motor ⇒Effective for lower frequency band.
The methods 1 and 2 can work. However, the combination of 1 and 2 is often the most effective way to reduce the electrical noise.
Types of the Electrical Noise
Electromagnetic interference(EMI) are classified into the following 2 types.
|dBμV||Noise that travels through
power cables and connection cables.
|0.15 - 30MHz|
|dBμV/m||Noise that is radiated from the source to the air
and causes interference in TV and a radio.
|30 - 1000MHz|
- Definition of the frequency range may vary depending on standards concerned.
Types of the Mechanical Noise
|1||Brush Noise||Frictional sound caused by brushes and commutator.|
|2||Shaft Sound||Frictional sound caused by shaft and bearings.|
|3||Clearance Sound||Specifically grating sound out of 2.|
|4||Resonance||Resonance that occurs at the specific speed range only.|
|5||Thrust Sound||Tapping noise caused by the thrust move of the rotor.|
|6||Wind||Wind that occurs in a motor especially with a built-in cooling fan.|
- Usually, the noise comes from multiple sources. To identify the root cause and specify the frequency, FFT analyzer is commonly used.
Measurement of Mechanical Noise
In measuring a mechanical noise, the noise level (loudness) is physically measured by a noise meter.
Calculation will be made to convert to JIS-A characteristics ( dB with RMS ).
In the measurement, the following factors in/out of the motors may affect to the test result, therefore should be specified strictly before the measurement.
- Voltage to be applied to the test motor.
- Speed (r/min) of the test motor.
- Mechanical load onto the test motor shaft.
- Radial load onto the test motor shaft.
- Positioning of the test motor.
- Positioning of the microphone.
- Background noise.
Example of the Specification
Mechanical noise: 48 dB (RMS)max.
JIS-A characteristic level measured at a point, which is located at a distance of 10cm from a motor-output-shaft-side mounting surface under the following conditions: voltage: 3V, no load, and a shaft horizontal position.
Background noise shall be 26 dB (RMS)max.