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Dry Run protection

Discussion in 'General Plant Topics' started by scottward, Apr 10, 2010.

  1. scottward

    scottward Guru Class Expert

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    Hi Guys,

    Just wondering if there is anybody on here that knows a bit about 'thermal protection' for circulation pumps?

    I have an Ocean Runner OR-2500 pump which has 'thermal protection'. I am trying to understand exactly how this works.

    According to the handbook that came with it, if the impellor gets stuck, this thermal protection mechanism will kick in.

    So, I understand that if the impellor gets stuck the pump will start to heat up and the mechanism will kick in; but what exacly does the mechanism do when it kicks in? I assume that the pump will simply stop running, and will only resume running when the power is turned off and back on again? Or do you have to wait several minutes for it reset itself after turning the power off?

    Along the same lines, if the pump were to run dry, shouldn't the thermal protection mechanishm also kick in?

    I read something in another forum saying something about the thermal protection mechanism only working for jammed impellors but not for dry run - I don't understand the difference? Surely the designers of these pumps would forsee dry running as a likelyhood and therefore build in protection mechanisms?

    Scott.
     
  2. Biollante

    Biollante Lifetime Charter Member
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    Winding Thermostat

    Hi Scott,

    I have never owned or even seen an Ocean Runner OR-2500 pump, but thermal protection comes in a several forms.

    Since there is no reset button, it is probably a winding thermostat, which is a snap-action, bi-metallic, temperature actuated switche installed in the connection end turns of the motor winding. The purpose is to cut the power to the motor upon excessive winding temperatures. Thermostats are the simplest and least expensive of the protective devices. :)

    If the impeller is stuck, this thermal protection mechanism simply cuts the power and when the temperature drops into the acceptable range restores power.

    The reason thermal protection mechanism only works for jammed impellers and not for dry run is that the damage to the impeller and shaft before the windings over heat. This is why I am pretty sure the thermal protection is not Resistance Temperature Detectors (RTD’s) running into a Wheatstone bridge that would give temperature and current protection.

    I hope this helps. :)

    Biollante
     
    #2 Biollante, Apr 11, 2010
    Last edited by a moderator: Apr 11, 2010
  3. scottward

    scottward Guru Class Expert

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    Hi Biollante,

    Thanks for your reply.

    I don't understand though. :)

    If the pump is running dry, wouldn't it also just get hot?

    Scott.
     
  4. shoggoth43

    shoggoth43 Lifetime Charter Member
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    Probably not. The worst possible situation for any motor is a locked rotor. This is the maximum possible current draw. ANY movement of the rotor will draw less current than this failure mode. So if we take a locked rotor and apply the current it may draw a few amps, but the wiring and thermister or whatever thermal control is in the circuit will take that full load and trip. If the pump is working hard ( restriction/gravel in there whatever ) the pump will pull more power and may trip. If we free spin the motor in air, it's moving full tilt and isn't going to draw much more power. What WILL happen most likely is that there won't be any lubrication on the motor's parts and the friction in there will cause it to heat up. The problem is that the heat coming from that is not likely anywhere near the coils or bits where the thermal overload protection is. By the time that heat transfers through the pump, you're likely toast. Not to mention that once you get friction and heating in the bearings and such, you probably already have damage. More heat, more friction, more damage. It's more likely the damage will lock the rotor first and then the thermal cutout will trip. Mag drive type designs can probably get away with this a bit more in that any damage will just trash the volute and impeller and leave the motor fine leading to a rebuild vs. a trash and replace.

    If you want run dry protection ( I'm trying really hard not to picture antiperspirant commercials... ) use a float switch and put the pump at the lowest point in the system. Any water level below where the float trips will cut power to the pump. Be aware that these can fail too, so you'll want to test those periodically as well.

    Bottom line, don't run your pumps dry. Just don't do it.

    -
    S


     
  5. scottward

    scottward Guru Class Expert

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    Cool, thanks Shoggoth43, that's cleared it up for me.
     
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