Automobile starter solenoids

Discussion in 'General Electronics Chat' started by strantor, May 7, 2012.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    A while back I was working on my controller for a big DC permanent magnet motor. That project has fizzled out for the time being, but one thing I was considering at the time was making an H-bridge out of automobile starter solenoids and providing PWM via a single array of high powered low side mosfets instead of 4 arrays. While working on a family member's car I encountered this drawing:
    [​IMG]
    According to this (I don't know if it should be taken literally or not) the start signal engages only half of the solenoid coil, after which the solenoid closes and the remainder of the coil is powered through it's contact. This also should result in the solenoid latching closed IMO, which doesn't happen. So I'm a little confused about how the solenoid works, and if this drawing is flubbed or not.

    1. If I wanted to use solenoids as I described above, is it possible to bring the negative side out the coil out so that it's not a chassis ground anymore?

    2. If I wanted to use solenoids as I described above, when I apply voltage across it, will I be applying voltage across the entire coil or only half of it?

    3. Is one side of the coil in fact internally connected to one side of the solenoid contact, and if so, why does it not latch?

    4. How much current can a starter solenoid contact sustain for long periods of time? (keep in mind, it would not be switching heavy loads a whole lot, just remaining closed)

    5. How much current does a starter solenoid draw?
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
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    The biggest problem I can see with your plan is that automotive starter solenoids are designed for very short duty cycles. Read this as just a few seconds on with a very long wait before being energized again.
     
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  3. Pencil

    Active Member

    Dec 8, 2009
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    This may be what the schematic you posted is
    trying to show. See attached.

    Edit: Instead of starter "solenoid", I think you want a starter "relay".
    The relay mounted on the firewall of some automobiles is sometimes
    called a "solenoid". The solenoid showed in your diagram may be the
    device mounted on the starter that mechanically engages the starter
    as well as making the electrical connection to the starter motor.

    I also believe BillB is correct about the duty cycle rating of starter relays and starter solenoids.
     
    Last edited: May 7, 2012
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  4. gerty

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  5. strantor

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    200 continuous, possible spikes @ 400A. 72 or 96V, probably 72V.
     
  6. strantor

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    a quick tour around the internet reveals that 4 properly rated solenoids will cost more than the entire intended application. I am throwing in the towel. Maybe some custom DIY switchgear....
     
  7. strantor

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    I'm referring to what's Traditionally called the solenoid - the thing on the starter. The relays on the firewall are nowhere near beefy enough.
     
  8. WBahn

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    Mar 31, 2012
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    When I was in high school I put a dual battery system in my Bronco and wanted to have separable electrical systems, one for the vehicle and one for the additional radios and lights I was putting in. So I mounted four starter solenoids on the fenders and they worked fine... for a couple of days before they failed. A few trips around town later I had discovered the battery isolators used in RV applications, which looked just like a starter solenoid. They were noticeably more expensive than the solenoids, but then Bronco solenoids are cheap. I put four of them in there and it worked great for the next couple decades.

    One concern I would have about using solenoids (or battery isolators) is the switching time involved. These aren't designed to switch real fast and if you are using them in an H-bridge with a PWM signal, that signal would probably have to be at a very low fundamental in order for it to work; perhaps too low for the motor to filter out the harmonics adequately.
     
  9. strantor

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    I wasn't planning to PWM the solenoids. Just energize them in one direction or the other, and leave them there. PWM would be through them (as they remain closed) via a giant mosfet.



    [​IMG]
     
  10. BillB3857

    Senior Member

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    Exactly how much current will your motor draw? Standard automotive "Ice Cube" relays can handle 30A.
     
  11. strantor

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    ~200A continuous, possibly up to 400A bursts
     
  12. dataman19

    Member

    Dec 26, 2009
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    So why exactly are you hung up on starter solenoids and contactors?
    ..
    You can use a DPST or even a DPDT Relay with 10Amps and 25Amp contacts. Using a protector diode across the relay coil windings will help protect it while switching in/out.
    ..
    Robots people have used this method for decades. If you are trying to control PWM controllers, you only have to incorporate transistor switches to control the PWMs.
    ...
    As for the cost of a heavy duty contactor (A contactor looks like a starter solenoid, but is rated for continuous duty - Even Auto/Truck Solenoids that say they are heavy duty, are still only limited cycle duration rated).
    ..
    Dave
     
  13. gerty

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    [QUOTEYou can use a DPST or even a DPDT Relay with 10Amps and 25Amp contacts.][/QUOTE]

    What he needs

    Not even close!!
     
  14. dataman19

    Member

    Dec 26, 2009
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    My Bad....
    ..
    Still there are 200A to 400A Solid State Contactors that can be had for around $100+.
    ..
    But what concerns me most here is that you would need a 1" thick Conductor at that amp rating... The ampacity of wire is finite, exceed it and you have a lot of voltage drop and wasted energy in heat.
    ...
    Dave
    Phoenix, AZ
     
  15. cork_ie

    Member

    Oct 8, 2011
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    You seem to have misunderstood how the solenoid works .
    Basically there are two windings in the solenoid , both of them simultaneously powered by the starter switch. They are called the "Pull in" & "Hold in" windings The "pull in" winding is series grounded through the motor and will only draw current until such time as the solenoid contacts close. These windings are generally of low resistance, draw 30A or more and will quickly burn out if connected for more than 10 secs or so. The "hold in" winding is connected to ground permanently and will hold the solenoid engaged as long as the switch is engaged. They normally only draw about 5-7 A Depending on the size of the starter motor. What you can get are "Tail lift" solenoids - designed for operating tail lift motors on delivery trucks and many other applications too. They have only one continuously rated winding and can easily handle 300A. I hope this is of help to you.
     
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  16. Smoke_Maker

    Active Member

    Sep 24, 2007
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    I think what would work for you is a reversing contactor form a golf cart, the 72volt carts has the reversing built into the controller but they have a power contactor that will handle your needs.
     
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