Current Limiter In Starter Circuit

Discussion in 'Instruments / Avionics / Electrical System' started by HomeBuilt101, Apr 1, 2016.

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  1. Apr 1, 2016 #1

    HomeBuilt101

    HomeBuilt101

    HomeBuilt101

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    I am running a pusher with two Optima batteries... one in the nose and one in the back under the back seat. I will have a Bus Tie located at the back battery box. To reduce weight, I want the 2 AWG cable that connects the nose battery to the Bus Tie to serve both as the conductor used to connect the two batteries and also the conductor that the second alternator uses to get the power to the front battery so therefore this 2 AWG cable will be hot all of the time including starting because the front battery will need to carry the start loads.

    Since it is such a long run I want to protect the wire with a current limiter however I do not want to design in protection that might fail during the time when the back battery is dead and the front battery is starting the engine.

    Any advice would be greatly appreciated!!!

    Bill Hunter
     
  2. Apr 1, 2016 #2

    Dan Thomas

    Dan Thomas

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    Maybe an industrial fuse of whatever amperage is suitable? They come in pretty big sizes. Determine the current capacity of that cable and fuse it appropriately.

    Or just install contactors at strategic places so you can shut the system down if it shorts.
     
  3. Apr 1, 2016 #3

    djschwartz

    djschwartz

    djschwartz

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    You have other issues to consider. If the two batteries are simply connected in parallel and one battery goes dead the other battery will attempt to charge it and go dead itself quite quickly. I would install a diode battery isolator in the charging circuit and then contactors controlled by switches in the cockpit to allow you to control which battery will be used to start the engine and run the electrical system. That will also allow you to isolate the dead battery so it doesn't pull down your entire electrical system.

    There are no practical "current limiters" available off the shelf for such an application. You have protection devices such as fuses or breakers that will prevent fires, but they do not limit the current, they simply disconnect the protected circuit if to much current is drawn.

    Dave
     
  4. Apr 2, 2016 #4

    goldrush

    goldrush

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    As suggested contactors, although requiring manual intervention, probably best.
    If you go down the simple fuse route, and are not aware, auto "Mega fuses" and insulated holders are available in sizes from 60 to 500 amps.


    You may find this info on starter currents from the Aero Lithium site of help in deciding sizes etc...
    ............
    List of starter amp readings from various aircraft engines:
    PISTON Battery needed
    HKS 700E – 60Hpwr – 125amps 225A (4.5 Ah unit)
    Rotax 582 – 65 Hp – 137A 225A (4.5 Ah unit)
    Rotax 503 – 52 Hp – 153A 225A (4.5 Ah unit)
    Rotax 912 & above – 240A 400A (8 Ah unit)
    Hirth 2704 – 55 Hp – 167A 225A (4.5 Ah unit)
    Cont. O-190 – 75Hp – 175A 360A
    Lyc. I O – 360-200HP – 350A 440A
    Lyc. I O 540, Cont. 470 / 550 800A
    Cont. 550 – 24V 400A (8 Ah unit)
     
  5. Apr 2, 2016 #5

    Joe Fisher

    Joe Fisher

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    My thought is to have separate master switches for each of the battery. Each battery would have a master solenoid other wise the systems would be paralleled. You could have a volt meter and a push button for each battery and you could check the condition of each battery before start and not turn on a dead battery.
     
  6. Apr 2, 2016 #6

    kent Ashton

    kent Ashton

    kent Ashton

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    Usually in a pusher, these big AWG 2 wires are not fused. The usual setup for pusher with a battery in the nose is to have a short run from the battery to a master solenoid also in the nose, a longer run from the master solenoid to a starter solenoid on the firewall, and a short run to the starter. The starter circuit is only engaged momentarily but for starting, you want all the juice you can get at the starter, otherwise we could use AWG 4 or smaller to and from the nose.

    If a current limiter limits the current to the starter, the airplane might be hard to start. These AWG 2 are not fused because other than starting, they don't carry much amperage. To get them to burn up, you'd probably have to have a dead short from the positive to negative wire. There is generally no path for that and a stuck starter won't do it. The big wires are connected firmly at the solenoids and secured in the airplane so that they should never rub together. If you got the indications of a dead short (smoke, sparking) or a run-on starter, you would turn off the master switch which deenergizes the master solenoid.

    I use two batteries. I have two master switches, two master solenoids and a crossover solenoid that I turn on for starting that puts the batteries in parallel for more amps. I turn the crossfeed solenoid off after start so I have independent batteries powering the two EIs.
     
  7. Apr 2, 2016 #7

    kent Ashton

    kent Ashton

    kent Ashton

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    Usually in a pusher, these big AWG 2 wires are not fused. The usual setup for pusher with a battery in the nose is to have a short run from the battery to a master solenoid also in the nose, a longer run from the master solenoid to a starter solenoid on the firewall, and a short run to the starter. The starter circuit is only engaged momentarily but for starting, you want all the juice you can get at the starter, otherwise we could use AWG 4 or smaller to and from the nose.

    If a current limiter limits the current to the starter, the airplane might be hard to start. These AWG 2 are not fused because other than starting, they don't carry much amperage. To get them to burn up, you'd probably have to have a dead short from the positive to negative wire. There is generally no path for that and a stuck starter won't do it. The big wires are connected firmly at the solenoids and secured in the airplane so that they should never rub together. If you got the indications of a dead short (smoke, sparking) or a run-on starter, you would turn off the master switch which deenergizes the master solenoid.

    I use two batteries. I have two master switches, two master solenoids and a crossover solenoid that I turn on for starting that puts the batteries in parallel for more amps. I turn the crossfeed solenoid off after start so I have independent batteries powering the two EIs.
     
  8. Apr 3, 2016 #8

    AdrianS

    AdrianS

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    As mentioned above, mega fuses are available up to 400 amp :
    Littelfuse Mobile Home

    Be aware that when one of these fuses blows, there is a lot of heat generated, and possibly sparks - it should be used in an appropriate holder (spark-proof holders are available for marine engine room use etc.)
     

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