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Can an alternator be to big?

Discussion in 'Boat Electronics' started by Glenn Ashmore, Aug 1, 2003.

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  1. Put another way, is a large alternator operating at 60% of capacity more
    or less efficient than a smaller one operating at close to maximum output?

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  2. I am avoiding side load all together. Welded up a frame in heavy 6x2
    channel that bolts tothe stringers in front of the engine with a short
    1.5" diameter shaft on double 1.5" pillow blocks. The Niehoff and Cat
    watermaker pump mount on the bracket with all the V-belts and pulleys on
    the shaft. It is connected to the damper plate with an Aquadrive.
    (Basicaly a very short CV joint) The engine can wobble all it wants
    without any side load.

    I am still a little worried about the total horsepower coming off the
    front though. I am figuring I will need about 9HP with the alternator
    and the pump running flat out. The Yanmar installation manual does not
    give limits for this kind of mount. It is more concerned with side load
    on directly attatched drive pulleys.

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  3. Larry

    Larry Guest

    What worries me on off-engine powered equipment is the belt tugging
    away on the engine to that one side. On Geoff's old boat, you could
    see the engine vibrating sideways as the freon pump went over TDC each
    cylinder, then releasing as the piston went down slackening the load.
    It's GOT to pull the engine mounts and wear them that way. This is
    not an issue where the load is mounted directly on the engine and not
    pulling against the engine mounts. Engine mounted, however, needs a
    little weight and balancing, too. I see too many engines, especially
    in outfitted sailboats with limited space, that have too many addons
    bolted to ONE SIDE of the engine, unbalancing the weight distribution
    on the engine mounts, which again must be bending with that load.

    Either way, I'd think it would be hard on shaft alignment, worse after
    time, wearing out those shaft bearings and cutlass bearing.



    Larry W4CSC

    "No, NO, Mr Spock! I said beam me down a WRENCH,
    not a WENCH! KIRK OUT!"
     
  4. Larry

    Larry Guest

    Dual belts, if you got the space, are a good idea, anyways....no
    matter what the ratings are.




    Larry W4CSC

    "No, NO, Mr Spock! I said beam me down a WRENCH,
    not a WENCH! KIRK OUT!"
     
  5. What I am doing is kind of hard to visualize. The only places I have
    seen is it on an Arodyne 46 and a USCG MLB. The heavy auxillaries are
    mounted off the engine. Nothing but torque reaches the engine. It can
    put stress on the engine mounts but compared to the propulsion torque it
    is minor.

    It will be about 6 months before I can set the engine and the
    auxillaries but then I will post some pictures and you can see what I mean.

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  6. It is not a rule of thumb. It is an engineering fact. All the belt
    drive makers have web sites and they all have pages about sizing belts.
    I used the Martin sheave formulas and double checked on the Gates Belt
    site. "A" and "AX" belts which are normally used on boat engines have a
    maximum of about 4 HP each at the speeds and pully sizes we use. (6-7"
    drive at 1500-3000 RPM and 2.5" to 3" driven at 3200-7000 RPM)

    There IS a rule of thumb involved though. Counting efficiency losses
    and the ever present safety factor, the rule is that an alternator
    requires an average of 1 HP per 25 amps of output at 13.8V. So when you
    get past about 90-100 amps a single AX belt just can't handle the power.


    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  7. It is more a matter of balance. The immediate problem is that I
    suddenly have a surplus of alternators. I was planning to use the big
    270 amp hot rated Niehoff that I picked up off a fire engine that got
    crushed by a falling wall. It needs about 10 HP at full output. It is
    to be mounted off engine through a CV joint arangement to eliminate side
    loads but when I add the Cat pump for the watermaker it runs the maximum
    HP off the front of the engine up to about 12HP max. That is getting a
    bit much even with the CV joint so when I got offered a 200 amp (cold
    rated) Leece Neville fot $140 I bought it.

    Now I am wondering which should be the primary and which the spare. For
    all practical purposes the 750 amp bank is only going to take 175 amps
    max and more like no more than 150 amps most of the time so the question
    was will I be wasting enengy by swinging the heavier alternator? I
    believe I agree with you that the answer is probably not. I think I
    will mount the Niehoff and keep the LN in reserve.

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  8. Probably because of the limited power available. The torque required to
    turn the alternator is governed by the current in the field coil. The
    regulator sees only the vlotage going to the battery and has no idea how
    much power is avilable to turn the alternator. When the regulator sees
    power is required it loads up the field coil to increase output. The
    bigger the alternator, the more torque is required so the wind can't
    spin the larger one as fast.

    The power that an alternator puts out relative to RPM is a curve that
    starts off steeply and levels out. Optimum RPM is about at the point
    where the curve starts to level out. Even with specially designed low
    speed alternators that point is more than 2,000 RPM. The wind generator
    designers want to get the RPM as far up the steep part of the curve as
    fast as possible with the available power. It is much better to get 60%
    of capacity out of a 30 amp alternator than 10% out of a 100 amp
    alternator.

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
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