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  1. #1
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    Best PWM frequency for DC motor driver

    Hi
    I am designing a DC motor dirver. I use IRFP150 power mosfet. what is the best PWM frequency to reduce dissipation in my device?

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  2. #2
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    Re: Best PWM frequency for DC motor driver

    This depends a lot on the power switch element and the
    DC motor attributes. Your armature inductance will be one
    concern, you need ripple current to be sensible (maybe
    10% of full load amps) and you can get that from worst
    case max line voltage and motor inductance (though I'd
    guess maybe this is modal too, what's the worst case
    minimum inductance found at?).

    Then you have the tradeoff between conduction losses
    and switching losses which for any DC load and device
    type (IGBT, MOSFET, bipolar, GaN) have a "sweet spot".
    IGBTs like to run slow but are the winner at very high
    bus voltage and current (less so at low voltage where
    their saturation voltage eats up efficiency). MOSFETs
    as you indicate just have a fairly fixed Qgg per cycle,
    and a fairly fixed conduction loss per cycle (Iavg*V*ton)
    and you can play with that.

    I recommend making an -in-efficiency oriented spreadsheet
    where each of the loss terms is individually figured. This
    is a useful design tool, unlike rollup efficiency numbers.



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  3. #3
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    Re: Best PWM frequency for DC motor driver

    Hi,

    I've heared that high frequencies may increase wear out of the brushes inside the motor.
    The company slowed down switching frequency in the audible range. Some kHz.
    I don't have exact or more reliable informations.

    Klaus



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  4. #4
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    Re: Best PWM frequency for DC motor driver

    Back when inverter drives were the New New Thing there
    was some hoopla about "inverter duty" motors. Supposedly
    the HF components of the drive waveforms were too much
    for standard winding insulation (varnish) and the capacitive
    currents could cause damage. Now I'm told all of the main
    motor mfrs are just doing better construction because they
    have to assume someone will use inverter drive even on
    "dumb ol' 60Hz 3-phase motors".

    But I think this is less about fSW and more about the
    switching edge rates, where the real high harmonic content
    is.



  5. #5
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    Re: Best PWM frequency for DC motor driver

    If you can it might be a good idea to add some filtering to remove the PWM components if it is an old motor to remove the effects that have been highlighted.
    Please don't send me PMs asking for direct help as this somewhat defeats the object of posting to a board like this.
    Any posts are simply an opinion and are not to be taken as professional advice. You are responsible for your own actions.



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