Help with motor noise Please

Discussion in 'General Electronics Chat' started by chico1st, Mar 12, 2010.

  1. chico1st

    Thread Starter New Member

    Mar 12, 2010
    2
    0
    I have an issue with motor noise on one of my projects.
    I am running the motor through an H bridge and controlling it with a PWM at 250 Hz (tried 4 KHz same thing). Its a 6V motor that we run in various directions using the HBridge.

    When the motor is on, our analogue sensors signal has large noise spikes (see picture):
    [​IMG]

    We have 10nF ceramic caps on the + and - of the motor to ground.

    But they didnt seem to do anything. Anyone have an idea on what we should do? I saw the 10nF cap number going around a lot for spike reduction.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Post your schematic.

    You usually have to use a low-pass filter (or more than one) to reduce/eliminate the noise spikes.
     
  3. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Divide and conquer: is the noise inherent to the motor? If you can, put it on a simple, clean source of power. This will help you decide whether your control circuitry is causing the noise problem or not.
     
  4. Duane P Wetick

    Active Member

    Apr 23, 2009
    408
    19
    First a LC mains filter for the PWM control is a must have device. Next, mount the filter and PWM control in a mild steel box w/ conductive gasket for the door. Add 2-pass ferrites to each output wire before exiting the box. A double shielded cable to the motor is last. This should kill the noisemakers.

    Cheers, DPW [ Everything has limitations...and I hate limitations.]
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    In addition to the schematic that has already been requested, I would ask you to take a few digital pictures and post them here also. It could help us spot a weakness in the layout of your system.

    hgmjr
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If it's a brushed motor being driven by PWM at any speed, there will be more noise than a punk band concert in a library. :eek: ;)
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    I think you meant a low-pass LC filter or low-pass RC filter for the current monitoring?

    A simple toroidal ferrite would kill EM interference back-feeding to the mains; were the power connections wrapped through the toroid 10 times or so.

    That might do it; but it's getting a tad pricey, too. Have you priced that sort of thing lately?
     
  8. chico1st

    Thread Starter New Member

    Mar 12, 2010
    2
    0
    ok all the info is very helpful.
    Sorry I cant post a schematic right now but I will.

    I know its not just an unclean power source. If I just have the motors running near my sensors from a different supply and holding them in my hand not on our PCB the sensors still get noise.

    It is brushed DC under PWM, but the noise exists even without the PWM.

    How do i make multiple RC filters? My signal is a step every 1ms (like a stair case) so I guess I would want the cutoff just over 1 KHz, then do i just put like 4 of those rc filters in parrallel... series makes more sense to me.
     
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