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Re: [SeattleRobotics] Bypass capacitors for DC motors -- How to select

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  • Pete Miles
    Hello Virginia, or should I say Nick For the most part, bypass capacitors have almost no effect on motor performance. The capacitors are use to reduce the
    Message 1 of 3 , Jul 31, 2006
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      Hello Virginia, or should I say Nick

      For the most part, bypass capacitors have almost no effect on motor performance. The
      capacitors are use to reduce the electrical noise that comes out of the motor that might have
      an adverse effect on your microcontroller or remote control system.

      Pete


      ----- Original Message -----
      From: "Nick Johnson" <uvasux@...>
      To: <SeattleRobotics@yahoogroups.com>
      Sent: Monday, July 31, 2006 10:01 AM
      Subject: [SeattleRobotics] Bypass capacitors for DC motors -- How to select


      Hello from Virginia!

      I've what I think is a pretty simple question, and if anyone could
      help, I'd really appreciate it.

      Suppose I'm controlling the speed of a DC motor with a PWM signal.
      I've read online somewhere that I should connect a "small" capacitor
      across the motors terminals to improve the life of the motor. So, how
      do I select the sizing of that capacitor?

      My intuition tells me that I should treat the motor as an inductor,
      and select a capacitor so that the time constant of the LC network is
      equal to the period of the PWM signal... But, even if that's the case,
      I'm not sure how to measure the inductance of the motor. Any ideas?


      Thanks,
      Nick Johnson
      http://uvasux.googlepages.com


      Visit the SRS Website at http://www.seattlerobotics.org
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    • Triffid Hunter
      ... The capacitor s purpose is to supress voltage spikes caused when one of the motor s coils is disconnected by the commutator before they can radiate from
      Message 2 of 3 , Aug 1, 2006
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        Nick Johnson wrote:
        > Hello from Virginia!
        >
        > I've what I think is a pretty simple question, and if anyone could
        > help, I'd really appreciate it.
        >
        > Suppose I'm controlling the speed of a DC motor with a PWM signal.
        > I've read online somewhere that I should connect a "small" capacitor
        > across the motors terminals to improve the life of the motor. So, how
        > do I select the sizing of that capacitor?
        >
        > My intuition tells me that I should treat the motor as an inductor,
        > and select a capacitor so that the time constant of the LC network is
        > equal to the period of the PWM signal... But, even if that's the case,
        > I'm not sure how to measure the inductance of the motor. Any ideas?
        >
        >
        > Thanks,
        > Nick Johnson
        > http://uvasux.googlepages.com

        The capacitor's purpose is to supress voltage spikes caused when one of the
        motor's coils is disconnected by the commutator before they can radiate from
        the motor's leads and disrupt signals elsewhere.

        The most important property of the capacitor is that it can respond to very
        high frequencies, so disc ceramics are quite suitable. Anywhere between 5nF
        and 50nF seems to work.

        Connecting a capacitor from each lead to the motor's case can help sometimes
        too, but not if you've grounded the case since then the caps just conduct
        noise into your ground plane.

        The capacitors on my robot's motor are actually inside the gearbox housing. I
        couldn't physically put them any closer to the motor without disassembling it
        and putting them directly onto the brushes. In some of the pics of my robot
        (at http://members.westnet.com.au/triffid_hunter/robotpics/ ) you can see
        where two capacitors' leads are connected to the motor's case.
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