EMI/RFI interference is either radiated or conducted in wide frequency range from several hundred hertz to several gigahertz.
Radiated noise occurs when voltage is applied at varying levels to the wiring. To keep the radiations confined in the motor housing, several precautions should be taken by the manufacturers of Brush DC Motors. The most important is the material used for the motor housing, which should be metal, as well as a metal cap (not plastic) on top of it. When the cap is made of plastic, the user needs to cover it with a metal shield (that may be a metallized PCB).
When EMI/RFI is conducted, the noise generated travels along the electrical power leads and is then radiated. Shielding is ineffectual against conducted noise, so filtering is required with a separate device.
Traditional common mode filtering approaches include low pass filters comprised of capacitors that pass signals with a frequency lower than a selected cutoff frequency and attenuate signals with frequencies higher than the cutoff frequency.
Among the options for OEMs are 2-capacitor differential, 3-capacitor (one X-cap and 2 Y-caps), feed-through filters, common mode chokes, LC filters, or combinations of these.