Advantage of using two wires on the negative of a DC motor

Discussion in 'General Electronics Chat' started by Mashly, Oct 23, 2014.

  1. Mashly

    Thread Starter Member

    Apr 4, 2014
    33
    1
    Hi

    Recently I have been asked to set up a system controlling a 24V fuel pump. The schematic has one wire running from the voltage source to the +ve and two wires running back from the -ve. Supposedly this has been done to improve the output of the motor, but my instinct says that the systems performance will be limited by the one wire in.

    Can someone explain why using two wires on one side of the pump would help?

    Thanks,

    Matt
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    How much current does the pump draw when running?

    What is the Wire Gauge of the wires?

    How long are the wire runs?
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Is this a fuel-immersed pump, as in automotive tank etc?
    What brand-make?
    Are you sure one is not a frame earth ground?
    Max.
     
  4. Mashly

    Thread Starter Member

    Apr 4, 2014
    33
    1
    I don't really have to many specifics at the moment as it is just the preliminary stages. I was more interested in the idea of doing this rather than the specifics of the actual project. All I know is it is a fuel pump for a turbo prop engine on a four engined plane. I understand if you would need to know specifics.

    I thought that one could be a frame ground but it is not. There is one wire going to the +ve and two from the -ve back to the battery. Apparently the thinking behind it is that it gives a better return to ground due to less lose in the cooper. I would have thought this would be negligible with a 20m cable with what looks like quite thick cable.
     
  5. #12

    Expert

    Nov 30, 2010
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    I was going to tell you all about remote sensing power supplies, but you don't have one. You just have 2 fat cables to ground. For now, trust the designer. Put it back the way you found it.

    As a guess, I'd say the designer found that (2) 20 mm wires weren't quite good enough to carry that much current, but (4) was overkill, so he just doubled the ground cable and called that, "good enough". And remember, I'm guessing. You are only going to find out with some good measurements performed with good meters because you are looking for start surges and the voltage error they develop in the power wires. That''s difficult enough in a 70 degree hangar in California. Let us know how it turns out at 40 below zero. :eek:
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Both conductors carry the same current so it does not seem logical to have two different size conductors for the same load?
    Max.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Was involved in a situation where a mechanic thought current just went "to" the device and was only on the positive lead.. So he replaced an old frayed return wire with some 22 AWG cable they had laying around.. Which of course broke and the only "path" to ground for this device was the shifter cable and due to the current flow through it it melted the plastic insulation on the sheath around the shifter cable and locked it into place.. Luckily this happened during startup and they just couldn't shift out of park.
     
  8. #12

    Expert

    Nov 30, 2010
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    Nobody said there were different size conductors. I'm thinking the designer couldn't buy aircraft quality connectors for anything larger that 20 mm, so he just doubled the ground wire. Suppose you can afford a 3 volt loss during a start surge but the 20 mm cables cost 2 volts each. Double one of the cables and you have 2 volts +1 volt = 3 volts.
     
  9. Mashly

    Thread Starter Member

    Apr 4, 2014
    33
    1
    Yes #12 my thoughts as well, I am just going to trust them on this one.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    Same guess that was in my mind as I read the question. Another idea: On a car, the sending unit for the fuel gauge is also at the fuel pump assembly. It uses little power compared to the pump but could warrant adding another return wire.
     
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