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Noise in Electric Motors – Causes and Treatments

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Two type of noises occurs in an electric motor, one is electrical noise and the other is mechanical noise. The causes an remedial treatments for both are discussed in this article.

Various Cause of the Electrical Noise

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

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

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