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            Electromagnetic Interference of Permanent Magnet D

            Electromagnetic interference of PMDC motors
            This essay discusses EMI of brush type PMDC motors only.
            Sources of EMI in PMDC motors
            a. There are sparks between commutator and brushes when the rotor is commutating. Such sparks are actually charged particles formed under strong electric field. With their electromagnetic emission, they emit continuous electromagnetic interference. The frequency ranges from 150KHz to 1GHz, falling exactly in the frequency range of RF. So we called it RF interference.
            b. Short term electromagnetic pulse interference when the motor starts, stalls or the load varies. It is a kind of intermittent interference. In practical application very few customers care about it. We will not discuss it here.
            Theoretically frequency of EMI ranges from 0 to 30GHz. It is impossible to study the EMI at all frequency in this range. What we care about is the frequency range that has influence to our daily life. For PMDC motors used in general applications such as home appliance, power tool, automotive products and office automation, the most concerned frequency range is 150KHz to 1GHz. Standards like IEC, EN, GB and FCC all refer to the EMI in this range.
            Standards update over time. In 3rd edition of IEC/CISPR25, EMI frequency range for automotive electric products is extended to 150KHz to 2.5GHz.

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