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How can the electromagnetic emissions of brushed DC electric motor be minimized

2022/02/16 > Back

DC motors are not only widely used in home appliances, but also can be found in factories, it has derived serious electromagnetic interference problems, through the power line to conduct outwards, almost all home appliances without power line filters can pass the CNS13783-1 standard.

The main sources of electromagnetic interference are:
1. The brush system (arcing because of inductivity and brush vibrations)
2. The PWM amplifier (Pulse Width Modulation)

By installing the recommended power line filter (link) on the 230 VAC power supply side brings a significant reduction in the noise level in the 148kHz ~ 30MHz band. With the EMC filter, the drive meets the requirements of the standard for this frequency band. As for the 30MHz ~ 300MHz high-frequency radiated electromagnetic interference, Neither a common-mode nor a differential-mode filter can effectively suppress EMI, mainly because the radiation generated by the spark of the DC motor commutator is too strong, it is beyond the conductive EMI filter function, and it is better to install the high-frequency reactance components such as magnetic rings or choke to solve. As for general home appliances, the installation of medium/high-frequency conductive common-mode filters has the effect of suppressing the 30MHz ~ 300MHz frequency band because the high-frequency radiation is not serious.

Some researches has shown that the amplitude of the motor phase current fluctuations has an impact on the magnitude of the conducted disturbances in the 148kHz ~ 30MHz band. The measured disturbances also show the effects of motor phase commutation. Therefore, it is important to reduce the amplitude of the current fluctuations and to properly conduct the commutation of the phases.

To solve the problem of conductive electromagnetic interference of DC motors, the design criteria we proposed are as below:
1. To suppress EMI from 150KHz ~ 30MHz, the common mode filter can be displaced, and the differential mode filter should be installed to the main one on the AC side.
2. Choosing 15uH differential mode inductor. The turning point (on the graph of testing) should be taken below 150KHz to smoothly turn the voltage interference curve.
3. In order to match with 15uH inductors and to close the corner frequency 150KHz, the use of capacitor specifications 0.047uf, 0.056uf, 0.068uf, 0.082uf, 0.1uf, 0.15uf, 0.22uf are strongly recommended, then, decide which combination to use according to the performance and cost.
4. Install with a Y cap on rectifier side for protecting the diode.

Data sources: bsmi.gov.tw; semanticscholar.org