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Prop shaft alternator conundrums

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by SkipperColin, Aug 21, 2017.

  1. SkipperColin

    SkipperColin New Member

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    Hi all, I'm hoping that you can help with some design ideas.

    I have a sailing boat that I have fitted a Lucas Marine 45 amp alternator to the prop shaft.

    Here are some of the specs.

    The prop shaft spins @ about 200rpm when sailing.

    I have a pulley ratio of 9:1 so get approx 1800rpm at the alternator

    This all works well giving between 6-10amps at 12 volts depending on boat sailing speed.

    However my problem is that when motoring I have a shaft rotation of about 2500rpm so close to 18500rpm at the prop alternator.

    I do not want to use the prop alternator when motoring so have a switch on the field wire so the field is open circuit.

    However I think I am blowing diodes and possibly the field is self exciting when in open circuit due to the high rotation speeds, but I'm guessing.

    My question is, how do I ensure the alternator is 'off' with the field wire switched open and not self exciting

    Secondly should I fit bigger diodes to cope with the open circuit voltages at the higher speeds. If so could you give me a spec for the diodes I should use in the bridge rectifier.


    If you can think of anything else that is may have missed i'd be grateful

    Hope you can help. This is a great project for us and means we are entirely green when under sail as we can generate enough power to mean the boat is energy independent.

    Many thanks
    Colin and Nichola
    Yacht Emerald
    www.yachtemerald.com
     
  2. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    What does the alternator power that can't stay connected when operating at high speed?
    And, yes most alternators are not designed to run at high speed without being connected to a battery for the same reasons you are finding.
     
  3. JimB

    JimB Super Moderator Most Helpful Member

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    I have just had a look at your website, looks like an interesting "journey" that you are on.

    Regarding your alternator, I am not sure that I understand where you think the problem is.

    Is it:
    1 Overspeeding the alternator?
    2 What to do with all the electrical energy generated when motoring?
    3 Something else.

    My ideas:
    1 An obvious solution would be to have a mechanical clutch which could be disengaged when running the engine.
    Does the engine have its own alternator to re-charge the starting battery?

    2 Does the normal alternator controller not take care of this?
    When it detects that the battery voltage is consistent with it being fully charged, it turns down the charge current appropriately.

    3 I seem to be making more questions than answers here.
    How many alternators?
    How many batteries?
    Are the batteries selectable with a big switch, so that they may be used individually or in parallel?

    JimB
     
  4. dave

    Dave New Member

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  5. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Prop rpm 2500, 9:1 gearing, = 22500 alternator rpm by my reckoning. That's a lot of unnecessary wear on the alternator if it's not being used. I'm with Jim: a clutch to disengage the alternator seems a better solution.
     
  6. SkipperColin

    SkipperColin New Member

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    Hi all, thanks for the replies.
    I'll try and answer the questions posed.
    1. The alternator is a Lucas Marine A127mt rated at 45amps.
    2. The house bank is 675AH @ 12 volt.
    3. We are an ocean going sailing boat so we use the prop alternator whilst under sail to charge our house bank. (Then we can keep up with demand of things such as radar, nav lights, autopilot, fridge, freezer etc)
    4. When motoring we have a high output alternator (115amps) which takes care of charging house bank, with clever microprocessor controls. However we do not motor purely to charge batteries if at all possible.
    5. We do not want to run prop alternator at such high revolutions so are trying to find a way to 'turn it off' when motoring

    6. I did think of finding something like a 12v clutch in a breakers yard from car aircon pump or similar but I am concerned the power drain to engage the clutch starts to negate the energy generated especially at slow sailing speed


    Finally I like the idea of a mechanical clutch, but have no idea where to source one or what it would even look like! With a mechanical clutch I could also consider upgrading the alternator to a pma that then does not need the 2 amps of field excitation.

    So I guess the question moves from electrical to mechanical engineering! Help!
     
  7. JimB

    JimB Super Moderator Most Helpful Member

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    I had in mind a simple dog clutch, a bit like some of these:

    http://www.industrialclutch.com/361-dog-clutch

    Depending on how the alternator was coupled to the propeller shaft, a guess that some mechanical work and fabrication would be necessary to mount the thing.

    JimB
     
  8. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    Simply rigging the alternator up to have a spring loaded belt tensioning mechanism with a lever that lets you loosen the tension enough to drop the belt off the prop shaft would be the simplest way to go.
     
    • Agree Agree x 1
  9. SkipperColin

    SkipperColin New Member

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    Wow thanks guys.
    I understand the idea of a dog clutch now. I'd simply put the alternator pulley on one side of clutch and the alternator or pma on the other side. That way I could set the tension on prop shaft/alternator pulley belt correctly (so that it does not wear out my cutless bearings) and just engage the clutch when I want it to run. I guess as long as the spec is correct to allow one side to spin @ 20000 rpm then it should work.

    I'm not sure how the spring tensioner would work though. The prop shaft has a 12" v pulley on it and the alternator has a 1.5" ish pulley on the alternator shaft. I would think that the belt is likely to fall off the prop shaft pulley at 20000rpm but again I don't understand how the spring tensioners work!
    Thanks for your help guys. I certainly have some things to consider. Mind you in wish I could have left the engineering as is and just put bigger diodes in the rectifier! That would cost a lot less!
     
  10. Les Jones

    Les Jones Well-Known Member

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    I think (Providing the alternator is rated for 20000 RPM) that breaking the connection to one of the slip rings with a switch and placing a small load on the output should solve the problem. Car alternators I have seen produce almost no output when the rotating part is not energised as there is very little residual magnetism. I also suspect that even without breaking the feed to the field that the regulator would maintain the voltage at about 14.5 volts. (Even if there was enough residual magnetism to start the alternator working.) When I have tried testing alternators out of a car they would not start giving an output without initially connecting them to a battery to initially energise the field coil.

    Les.

    PS I like the pictures on your website. They reminded me of the times I visited many of those places in Greece and Turkey. (I was in Crete in June and will be in Rhodes in October.)
     
  11. SkipperColin

    SkipperColin New Member

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    Here is a photo of the setup. You can see the alternator on the engine bed in the foreground and the 12" pulley (aluminium) on the prop shaft by the flexible coupling.
     
  12. SkipperColin

    SkipperColin New Member

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    Attached Files:

  13. SkipperColin

    SkipperColin New Member

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    Hi Les, I think I understand that you mean just breaking one of the battery connections? So I could put a 100amp switch on the positive side of the battery feed from the alternator, leave it open then switch on field and battery lead when using the alternator while sailing. Therefore it is all open circuit while motoring?

    Not sure what sort of load you mean though or where to put it?

    Another thing I thought about was maybe trying something with the sense wire on the prop alternator.

    Many thanks for the help. We loved Rhodes.

    I've tried to post a photo of the alternator setup but not sure if it worked!
    Best wishes Colin
     
    Last edited: Aug 21, 2017
  14. Les Jones

    Les Jones Well-Known Member

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    Hi Colin,
    To insert a picture you first have to upload the file (Upload a File button at the right under where you type the text.) Then click the Thumbnail or Full image button.
    Image016.jpg This is from clicking the thumbnail button (You will probably recognise this picture. The Samaria gorge in Crete. Taken in 1978)

    I was thinking of a load such as a car headlight bulb or equivalent high power resistor. You would have to make sure that the alternator was stopped before opening the switch. If you did not do this it would continue to output power. Is there just one battery on the boat or do you have one for starting the engine and one for lighting etc ? I think the clutch idea would be a better solution as there would be less ware on the alternator brushes and bearings. You can get magnetic clutches but I have only seen them which engage when power is applied to them. One that disengaged when power was applied would be better for your application.

    Les.
     
  15. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    Given your picture you just replace the tensioning brace on the top of the alternator with a heavy spring. Push the alternator in and take the belt off and go.
     
  16. dr pepper

    dr pepper Well-Known Member

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    If you can verify the alty can take that kind of Rpm to prevent the thing commign apart, this was common with the old dc dynamo's.
     
  17. SkipperColin

    SkipperColin New Member

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    Hi there everyone, thanks again for the input. The alternator can take the revs, it has been in (and working!) For about 4 years and 5000nm. However I think some of the diode pack have died, as it is cycling on and off when trying to regulate. It also appears to switch on even though the field current circuit is broken.

    So I am looking for a way to turn the alternator off even though it is rotating.

    The best way so far would appear to be with a manual mechanical clutch.

    The problem with taking the belt on and off with spring tensioner is where to store the belt when motoring. It could wrap on the shaft and put us in trouble very quickly.

    Does anyone else have any other ideas on how to solve the problem?

    Many thanks
    Colin & Nichola
    Emerald
     
  18. Musicmanager

    Musicmanager Active Member

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    Good Morning Colin

    We have a commercial laundry business here in the UK which uses a number of Canadian built machines which offer a number of motion variations after the motor.
    They use a series of Dog clutches to offer these combinations .. ..

    dog-clutch-71350734.jpg

    If the gear wheels were changed for pulleys ... .. ? The locating teeth that the dog collar locks on to is a bolt on plate, not cast into the gear, so it should be possible, although I would imagine there would be some cost involved.

    S


     
  19. SkipperColin

    SkipperColin New Member

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    Thanks for the idea. I like the thought of a manual dog clutch. However there would have to be quite a bit of fabrication I think.
    Jabsco make a manual clutch bilge/water pump which could be adapted very neatly, but it is an eye watering £800 new!

    https://www.jabscoshop.com/marine/p...size-foot-mounted-with-bsp-threaded-ports.htm



    I like the idea of a manual clutch because I could fit a missouri wind pma which would allow me to generate 10-15amps under sail.

    I now have to work out how on earth to source a good quailty lever operated manual clutch assembly otherwise the project becomes too expensive.


    Many thanks for the ideas, keep them coming please.

    Best wishes
    Colin & Nichola
     
  20. Musicmanager

    Musicmanager Active Member

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    Hi Colin

    Yes, I imagine the costs will be significant, but then, so will replacing a damaged alternator .. .. .

    A quick look found this https://www.aliexpress.com/item/The...114.search0104.8.4.0HVne2&transAbTest=ae803_3

    which is probably not man enough for your needs .. ... and also this ... http://www.industrialclutch.com/361-dog-clutch which is at the other end of the £££ signs ..

    the mechanism on our machines is nowhere near as sophisticated, so I would expect there to be a range between the two extremes.

    Happy hunting .. .. ..

    S
     
  21. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    If that the case you alternator is already damaged and need replacing to which I would change it out to a larger capacity commercial unit that can operate at a lower RPM.

    I use the larger 22SI case type units for our farm machinery that can putout 140+ amps at 3600 RPM and can top 30 - 40 amps at less than 1000 RPM
    They run about $150 at Napa
    https://www.napaonline.com/en/p/WIL90014297

    With one of those you could put a larger pulley on it and reduce the alternator speed to a point you don't need to unhook it when running your engine.

    Unlikely.

    A lot of early farm a machinery used that method to uncouple drives for certain things and never had problems with it.

    At worse just add a couple pieces of stiff wire some place to work as hangers to hold it up of the shaft.
     
    Last edited: Aug 22, 2017

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