Starter motor as a main motor?

Discussion in 'Hybrid' started by mitchellleary, Jul 23, 2015.

  1. mitchellleary
    Joined: Jul 2015
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    mitchellleary Junior Member

    Hi all,

    I'm currently working on a project that I hope someone will be able to help me out with.
    The project is to create an autonomous small motor-boat and I'm currently searching for the right motor for the job(I don't need any advice currently about the solar or battery element of the build as I already have this planned and tested).

    The part I need help with from someone who has electric expertise is this;

    - As my boat is very small and light, I will only need 2 x 2.5kW(3.5hp) 12V motors to drive surface-drive props. I have researched many systems and available motors for this purpose and they are coming out very expensive(for me) at around £3,500 each.
    I then carried on researching to find a cheaper option and I came across some marine starter motors that were exactly the same output as the dedicated motors. So they are 2.5kW 12V93.5hp) electric marine starter motors but cost around £90 each!! That's a big difference.

    My questions is, would the starter motor perform the same task equally as well as a dedicated motor? I have discussed with a few close colleagues who say that really there should be no reason why they shouldn't perform the same task but I wanted to get some feedback from the knowledgable people on this sight.

    Thanks for your time and I hope someone can come up with a definitive answer.

    Mitch
     
  2. PAR
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    PAR Yacht Designer/Builder

    One of the primary specs when looking at motors in general is it's duty cycle. Starter motors are intermittent duty. What you'll want is a continuous duty motor, unless you don't plan to use the starter motors for a very long duration. There may also be a thrust bearing/washer need as well. I don't know how long you can get a starter motor to run, without damage, but it's certainly a cheap way to test the viability of a concept.
     
  3. mitchellleary
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    mitchellleary Junior Member

    Hi PAR,

    Thanks so much for that reply. That's exactly what I was looking for.

    That was also my concern that the durability of a starter motor over long period wouldn't be good enough. I like the idea of using it as a concept tester! That is very useful.

    Can I ask you PAR if you have any recommended suppliers of cheaper motors than the £3,500 Drivemasters that I've found that have the same spec (2.5kW, 12V)?
    I would be extremely appreciative if you know of anywhere.

    Thanks again!
     
  4. PAR
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    PAR Yacht Designer/Builder

    I use a few electrical suppliers, but they're not going to have what I think you're looking for, though I'm sure they can order one for me. 3 1/2 HP DC motors aren't all that common and I'd probably have to do the same searching you're beginning. I can get great prices on a replacement motor for a compressor though.
     
  5. Petros
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    Petros Senior Member

    starter motors will over heat is ran for more than about one min. they are not designed for continuous operation, but short duration high output. I am not an expert on motors, but I have spent a lot of time messing with auto engines, machinery and similar devices. Motors are specifically designed for their intended usage.

    You need a motor designed for continuous operation as PAR points out. You can not find an electric low cost trolling motor? they are made in the thousands in third world countries for use on fishing skiffs.
     
  6. kerosene
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    kerosene Senior Member

    several kw motors at 12V is a bad plan. You can get a lot more hp from cheap motors (with less losses) when you up the voltage.
    Also I don't quite get the combo of low voltage, 5kw, autonomous and solar.
    5kw @ 12V = 400 Amps, that is some serious cables you need...
    @ 48V it would be 100A which is way more reasonable.
     
  7. CDK
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    CDK retired engineer

    Look at sites like robotmarketplace.com for less expensive DC motors.

    A starter motor is OK if you plan trips of no more than 3 minutes. That is about the time it takes for a starter motor to reach combustion temperature. These motors have field windings instead of magnets and a large air gap that makes them cheap to manufacture. The efficiency is approx. 60% so for every KW at the shaft there is 400 Watts of heat, to be absorbed by the motor body in the absence of a fan.
     
  8. whitepointer23

    whitepointer23 Previous Member

    Starter motors have bushs not roller bearings and that cannot be good for running long periods.
     
  9. mitchellleary
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    mitchellleary Junior Member

    Thanks Petros!

    Your advice is much appreciated. I'm going to look into suppliers in the third world as this is a good call that I hadn't explored.

     
  10. mitchellleary
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    mitchellleary Junior Member

    Hi Kerosene,

    This is a really invaluable insight. I've sent you a private message as I'd like to get your thoughts on a few other points.

    I really appreciate your input.

     
  11. mitchellleary
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    mitchellleary Junior Member

    Another excellent call CDK. I hadn't come across Robot Marketplace before, have run my eyes over it and it looks very good. I'm going to explore it and hope to find what I'm looking for.

    Thanks!

     
  12. mitchellleary
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    mitchellleary Junior Member

    Thanks all who have advised on my post. I've got to say that this forum have some very knowledgable people who are more than happy to get involved in helping other people. That's nice and refreshing to see.

    If there is any other feedback or input, I look forward to receiving it.
     
  13. PAR
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    PAR Yacht Designer/Builder

    When looking for an odd part or to retask something, I go to an outfit near me called Skycraft (> skycraftsurplus.com <). The online catalog doesn't have half of what their store does and you can always find something you've never seen before.
     
  14. blisspacket
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    blisspacket Junior Member


  15. tom28571
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    tom28571 Senior Member

    A multi KW DC motor run on 12V is a really bad idea. As Kerosene said, the cable and source losses are hard to get rid of. Don't know about the power available, but motors used in aircraft for stuff like landing gear and control surfaces run at much higher voltage and are available from surplus equipment dealers. Some of these may be continuous rated.

    Motors from wrecked hybrid cars are also a possibility.
     
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