Noise Suppression Filter Guide

Featured products! Case Study of Onboard Motor Noise Countermeasure with PLT10HH


Previously, we introduced the common mode choke coil for strong electric currents. This time we follow up with a case study of an onboard motor noise countermeasure with the PLT10HH.

*An earlier article introducing the PLT10HH can be found at the following URL:


To make automobiles more energy-efficient and lightweight, an increasing number of their components have become electrically driven.. For this reason, electric motors are used not only for traditional devices like windshield wipers, but also for power steering, fuel pumps and so on. Some DC motors use brushes to rectify the current, but because such brushes are repeatedly contacting and separating from commutators, sharp fluctuations in electric current cause noise and spark noise (Fig. 2). If such noise flows through the power line to other parts, it could have an adverse effect. While it also depends on motor speed, this noise can range widely in frequency from hundreds of kHz to hundreds of MHz, making it necessary to take adequate noise countermeasures.

Fig. 1: Why there is noise in a brush motor

Figure 2 shows a model of how noise is generated in a brush motor and illustrates key points of a basic method to counter the noise. Basically, there are two kinds of noise: differential mode noise, which is conducted similarly to a power current, and common mode noise that is conducted parallel to two power lines. Effective noise countermeasures must therefore be taken for each type. The use of a capacitor is a relatively easy and inexpensive countermeasure. A countermeasure for differential mode noise uses an across-the-line capacitor (X capacitor) that connects across two power lines. A countermeasure for common mode noise uses a line-bypass capacitor (Y capacitor) that creates a connection between both lines and a chassis ground. If the capacitor alone is not sufficient to suppress noise, the addition of a common mode choke coil can effectively suppress common mode noise that is otherwise difficult to overcome, and it is possible to take measures against even often problematic high-frequency noise.

<Cause of noise in brush motor and countermeasures>

Spark noise occurring between the brush and commutator is the cause. Noise includes both normal mode and common mode noise and occurs over a wide frequency range.

<Key points to noise countermeasures>

(1) Taking measures inside the motor (close to the noise source) is effective.
(2) Using an X capacitor and Y capacitor together is effective against both normal mode 
and common mode noise

Fig. 2: Cause of noise in brush motor and countermeasures

Example of motor noise countermeasure experiment

Figure 3 shows the  results from an actual experiment with a measure against power-window motor noise.
The results show that a noise countermeasure using only capacitors (across-the-line and line-bypass capacitors) is adequately effective against frequencies lower than 10 MHz, but the effect is not sufficient against frequencies greater than 30 MHz and this measure would not pass CISPR Part 25 Class 4. When a PLT10HH common mode choke coil is added to capacitors, noises of 30 MHz and above are suppressed and CISPR Part 25 Class 4 is successfully cleared. With power line noise, the differential mode is dominant at low frequencies and the common mode at high frequencies, so a common mode choke coil that suppresses common mode noise is effective and high-frequency noises can be suppressed. Additionally, because there are strong electric currents in power lines, when a strong current is passing through a regular inductor, inductance declines because of magnetic saturation and noise is less effectively suppressed. However, with its internal structure, the common mode choke coil makes magnetic saturation unlikely to occur so the noise suppression measure tends to maintain its effectiveness. In this sense also, it is efficient to use a common mode choke coil for power lines.

Fig. 3: Example of motor noise countermeasure

The combination of a common mode choke coil with an X capacitor and Y capacitor passes Class 4.

The PLT10HH common mode choke coil used here supports rated currents of 6 A to a maximum of 10 A and is designed to support various motor applications in which there is a large electric current during startup. It also has a highly reliable design so it can be used in severe environments such as in automobiles. Of course it can also be used for regular electronic devices, making it suitable for countering noise in a wide range of motor power lines.


Person in charge: Murata Manufacturing Co., Ltd.  Yasuhiro Mitsuya

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