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    Electrical Machines

    Electrical and Mechanical Noise in Motors

    • Categories Electrical Machines, DC Motor, Generation, Single Phase Motors, Synchronous Motor, Three Phase Induction Motor
    electrical noise motor

    Two types of noises occur in an electric motor, one is electrical noise and the other is mechanical noise.

    The causes of remedial treatments for both are discussed in this article.

    Table of Contents

    • Electrical Noise in Motor
      • How to Reduce Electrical Noise
    • Types of Electrical Noise
    • Types of Mechanical Noise
      • Measurement of Mechanical Noise
      • Related Articles

    Electrical Noise in Motor

    The following are the various causes of electrical noise in a motor.

    85989896bae16633e04c89157e074bddWhen a motor runs, the commutator switches the direction of the electricity that flows in the windings. Though the system keeps the motor running, the occasional spark occurs between brushes and commutator at the timing of the commutation. The spark is one of the causes of 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, a similar noise happens when

    1. brushes stay unstable on the surface of the commutator,
    2. the input to the motor is much higher than expected and so on.

    Other factors may be insulation created on the commutator surface, which results in the unstable flow of the electricity.

    How to Reduce 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 a spark, these parts are to be fixed to the 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; 
    1. 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 the higher frequency band.
    2. Electrolytic or ceramic capacitors and/or the choke coil to the outside of the motor ⇒Effective for the lower frequency band.
    Methods 1 and 2 can work. However, the combination of 1 and 2 is often the most effective way to reduce electrical noise. 

    Types of Electrical Noise

    Electromagnetic interference(EMI) are classified into the following 2 types. 
    1. Line Noise (Conductive Noise)
    2. Radiation Noise (Radiation Noise)
    Type
    Unit
    Definition
    Frequency*
    Line Noise
    (conductive noise)
    dBμV
    Noise that travels through
    power cables and connection cables.
    0.15 – 30MHz
    Radiation Noise
    (radio wave)
    dBμV/m
    Noise that is radiated from the source to the air
    and causes interference in TV and a radio.
    30 – 1000MHz

    Types of Mechanical Noise

    The following are the different types of mechanical noises.
    1. Brush Noise
    2. Shaft Sound
    3. Clearance Sound
    4. Resonance
    5. Thrust Sound
    6. Wind
    Type
    Definition
    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.

    The 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 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.
    • The positioning of the test motor.
    • The positioning of the microphone
    • Background noise.

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      2 Comments

    1. Stency
      June 21, 2019
      Reply

      Nice article. Keep up the good work.

    2. Frank
      August 16, 2021
      Reply

      The Source of this article is:
      https://www.mabuchi-motor.com/product/knowledge/performance/noise.html

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