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Alternatives to varistors as overload protection at DC motor

  1. Jun 24, 2016 #1
    Alternatives to varistors as overload protection at DC motor

    DC Motor here implies 12V linear actuator w/DC Motor that consumes 2.8A under loading condition. Pretty costly equipment.

    I'm using a L298N motor driver connected in parallel to run the linear actuator. The motor driver is run by a 12V 10Ah lead acid battery. An electrical engineer advised me to use varistors 'MOVs' instead of 10A blade fuses for overload protection at my linear actuator. He specified that I select a varistor rated at 1.5x12V=18V rms and 1000A surge. I've been able to source only 130V rated Varistors in my locality/country. Is there any other alternative cheap way to ensure surge protection of my costly linear actuator? Are the fuses engh?
    From what I've read varistors start to conduct current only above the DC rated voltage.
    L298N connected in parallel: hb_l298.jpg

    Varistor working:
    VaristorCharacteristicCurve.gif
    10A blade fuse:
    http://s1097.photobucket.com/user/wlwltd/media/10_AMP_BLADE_FUSE_STANDARD_2.jpg.html?t=1324466963
     
  2. jcsd
  3. Jun 25, 2016 #2

    CWatters

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    What sort of faults are you trying to protect against?

    An 18V 1000A varistor would imply you are concerned about over voltage - perhaps due to a failure of the lead acid battery charger?

    If you are concerned about something stalling/blocking the linear actuator/DC Motor causing excess current draw then I think a fuse is a better option. However there are Varistors intended for use as automatically resettable fuses...

    http://www.littelfuse.com/products/resettable-ptcs.aspx

    http://www.littelfuse.com/~/media/e..._dc_application_varistor_design_guide.pdf.pdf

    Perhaps read up on the different types of varistor. There are some you put in parallel with the load and these clamp or absorb over voltage transients. There are others you put in series with the load that act like automatically resettable fuses and protect against over current faults.
     
    Last edited: Jun 25, 2016
  4. Jun 25, 2016 #3

    Baluncore

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    That advice is not applicable to your situation.

    The advice from CWatters to use a current limiting device is good. Resettable fuses are also marketed as “polyswitch” or “polyfuse” because they are based on a polymer. https://en.wikipedia.org/wiki/Resettable_fuse

    If PWM control of the DC motor was being used then a current sense resistor on pins 1 and 15 of the L298 could detect load current and reduce duty cycle to limit maximum current.
     
  5. Jun 25, 2016 #4
    Thanks for the replies. FYI I managed to only get hold of 130V clamping voltage varistors. Nothing in the range of 18V is available. So, I'm thinking of connecting series-opposing 15V zener diodes instead. Is that okay?

    CWatters- "If you are concerned about something stalling/blocking the linear actuator/DC Motor causing excess current draw then I think a fuse is a better option."

    My DC Motor (Linear actuator) already has limit switches inside of them. So, this would mean that everytime my actuator shaft reaches it end limits, my motor no longer draws any more current (mechanical switches). Nevertheless I'm going to use a 10A fuse in series.

    "An 18V 1000A varistor would imply you are concerned about over voltage - perhaps due to a failure of the lead acid battery charger?"

    My lead acid battery is getting charged by solar power( regulated by a charge controller ). This battery in-turn powers my actuator dc motor. I'm concerned about over voltage from motor switch-off and the collapsing magnetic field that could spike voltage. .

    Baluncore- "If PWM control of the DC motor was being used then a current sense resistor on pins 1 and 15 of the L298 could detect load current and reduce duty cycle to limit maximum current."

    Great idea Sir. I'll see if I can use these pins for ordinary motor switch on/off also. I don't use PWM speed control, nevertheless.....

    Unfortunately, I've got to import polyfuses & 18V varistors. Lead time and money are constraints. I'm thinking of going with the zener diode idea/sense pins idea/try and rip-off spare ECU circuits to search for polyfuses.
     
  6. Jun 26, 2016 #5

    Baluncore

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    That is quite unnecessary as maximum supply voltage should not be a problem.
    The overload problem you fear is motor current when the motor is stalled.
     
  7. Jun 26, 2016 #6

    jim hardy

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    i guess D2 through D5 direct motor surges to the power supply rails ?
    If somebody is worried that's inadequate

    One of these might be reassuring
    http://www.littelfuse.com/~/media/e.../littelfuse_tvs_diode_1_5ke_datasheet.pdf.pdf
    or if you're protecting against lightning (1000 amps? sounds impractical to me)
    here's a 548 amp 20 volt.....
    http://www.littelfuse.com/~/media/e.../littelfuse_tvs_diode_20kpa_datasheet.pdf.pdf


    and their whole line of them
    http://www.littelfuse.com/products/tvs-diodes.aspx
    i'd say pick one higher than your power supply tolerance but below absolute maximum voltage rating of the device
     
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