Electric Motor Questions

Discussion in 'Functional Gear & Equipment' started by birddseedd, May 31, 2016.

  1. Tevin

    Tevin Monkey+++

    I'm curious as to where the 3/5 rule comes from and how/why this figure came to be. I've never heard of this.

    I was taught that "horsepower" is a fixed value. It does not change according to the source of the power.

    There may be differences in efficiency, losses through gears & transmissions, etc., but 1 HP = 550 foot-pounds = 745 watts no matter where it comes from.

    It's been a long time since I've been in technical college, but I'm certain the laws of physics have not changed.
  2. BTPost

    BTPost Stumpy Old Fart Snow Monkey Moderator

    If one were really interested in Electric Drive Propulsion in these modern times, one would be looking for a Three Phase AC Motor, and a Solid State Frequency Drive, to convert the DC Battery Voltage to power the Motor.... This gives one complete control over the RPMs, Torque, and Direction of Rotation, of the Motor, as well as about the best conversion Efficency around... 60HP FreqDrives are "Off the Shelf" Devices, these days...
  3. VisuTrac

    VisuTrac Ваша мать носит военные ботинки Site Supporter+++

    Yes, horsepower = horse power but it's how they are rated. IIRC, Typically a gas engine or internal combustion engine is rated at peak HP whereas an electric motor is rated at continuous load. So it's not really comparing apples to apples with respect to horsepower so that is where the 3/5th thing comes into play.

    It's not a hard a firm law, but for back of the envelope calculations .. you may see it thrown about when comparing IC engines to electric motors.
    techsar and Tevin like this.
  4. Tevin

    Tevin Monkey+++

    Car wheels have very low friction compared to boat hulls. Once a car is up to speed, it does not require much input energy to maintain that speed (momentum). This is why cars get the worst gas milage when they are accelerating. It takes a lot of energy to overcome the friction from a stop. Cars also pick up momentum going down hill that will partially offset the energy needed from the engine to pull the car up the next hill.

    Boats on the other hand have a great deal of friction between the hull and the water, which in turn means they have very little momentum. Once a boat is up to speed, it will not cruise along with little input energy like a car. You will still need to feed the engine lot of fuel to overcome the friction and maintain that speed. If you cut the engine on a boat, it will come to a stop very quickly compared to doing the same in a car. And of course, boats do not have the benefit of going downhill.

    This is why you see boats that are not particularly fast even with 100-200 HP engines.

    It has nothing to do with gas vs. electric or anything like that. The amount of energy needed to maintain momentum in a given mass under identical conditions is the same no matter what the source of energy is.
    Last edited: Jun 1, 2016
    3cyl and kellory like this.
  5. VisuTrac

    VisuTrac Ваша мать носит военные ботинки Site Supporter+++

    You could always go with hydrofoils and get the hull out of the water and use the electric motors to drive a water jet pump.
    don't know what the efficiency of a water jet vs a shaft and prop is but .. it'll be faster

    Last edited: Jun 1, 2016
    BTPost likes this.
  6. BTPost

    BTPost Stumpy Old Fart Snow Monkey Moderator

    I suspect that it would take more that 60 HP, for that vessel....
    VisuTrac likes this.
  7. VisuTrac

    VisuTrac Ваша мать носит военные ботинки Site Supporter+++

    I'd bet you are right!
  8. VisuTrac

    VisuTrac Ваша мать носит военные ботинки Site Supporter+++

    My question is, do you need a pilots license for this one?
  9. HK_User

    HK_User A Productive Monkey is a Happy Monkey

    What size boat?
    What Speed?
    How long of a run time and do you plan on running at night.
    Fork lift Batteries, when being worked, last about 8 hours and then need to be recharged about 12 hours.
  10. Tempstar

    Tempstar Praeclarum Site Supporter+

    About that 3/5 thing. I know when engineering water pumps that the opposite is true, as electrics deliver torque constantly while engines only deliver the max torque during the power stroke. This will even show as ripples on the scope when testing an engine on a water brake, showing up as valleys in the peak HP curve, whereas electric motors are constant on DC, with slight ripples on AC units during the zero voltage crossing. Just my observations...
    Tevin likes this.
  11. AD1

    AD1 Monkey+++

    On my lake(electric only) we have several 20+foot pontoon boats with Ray motors on them. You wount ski behind it but with 2 on a 25 foot boat it moves it at around 10 knots. 60 volt system is ~270lbs thrust

    Ray Electric Outboard :: Motors

  12. 3cyl

    3cyl Monkey++

    golden motor makes up to 50 hp electric outboards
  13. ghrit

    ghrit Bad company Administrator Founding Member

    When you start getting into sea going vessel of any particular size, think diesel electric drives. Like this -
    That lays out one of a number of schemes.

    The most (to me) interesting scheme is rotating motor pods that are used in place of rudders. Check out page 20 of this brochure. The most practical is probably gas turbine driven alternators feeding variable frequency motors. This is NOT yet personal sized transport, believe it, but maybe some day --
    Cruisin Sloth, VisuTrac and Tevin like this.
  14. VisuTrac

    VisuTrac Ваша мать носит военные ботинки Site Supporter+++

    I'm going to stick with a canoe and a paddle.
  15. azrancher

    azrancher Monkey +++

    Math, oh crap, I knew this was going to catch up to me sometime, is this like 2x+Y = who knows what?

    I give up, please just put me out of my misery.

  16. ghrit

    ghrit Bad company Administrator Founding Member

  17. arleigh

    arleigh Goophy monkey

    Power is relative to the load of energy.
    2 stroke engines have more power than 4 stroke engines .
    The power is during the moment that shaft actually has ignition ignited power.
    Electric motors have power constantly .
    The 2 stroke has power only on the ignition every other direction the piston moves .
    The 4 stroke only gets power every 4th stroke.
    Pathetic when you think about it in terms of power.
    But more economical in terms of fuel versus work, IF THE WORK DOES NOT DEMND THE CONSTANT ECCELERATION MODE.
    The 4 stroke relies on it's momentum from the previous firing to make it to the next 3 strokes away.
    So in things like boats the 2 stroke is stronger than the 4 stroke comparing cubic centimeters of piston displacement.
    Take that same 2 stroke engine and add a cam shaft and valves to make it a 4 stroke and you've cut the power in half, or more.
    I worked on air compressors as well and the same principle exists .
    An electric motor driven air compressor is the hardest work a motor can do , and usually the motor gives out because it cannot compensate for over load .
    I made a pretty good living on folks that insisted on using extension cords.
    Typically it takes an engine usually 4 stroke , with twice the HP rating or better to drive the compressor pump ,but it's advantage is that it has a governor that helps it to compensate for extended loads where an electric might over heat and trip the breaker.
    A pump rated for 3 HP electric, usually takes a 6 HP gas engine.
    Unfortunately now days popular stores manufacturers lie about true horse power, and us the figures on motor starting load amps, not full load running amps and there is a big difference.
    Some have actually refrained from listing on the motor it's amperage at all .
    If they can't give you strait answer in advertising, chances are they are lying about every thing else too .
    All electric motors are not the same .
    Most house hold and industrial motors have a start winding and run windings ,the start windings are disengaged after reaching a certain RPM or in some cases resistance .
    That start winding pulls 3-4 times the amperage the running winding works on.
    There are also split phase and 3 phase , star and delta wound motors, brush started and brush run motors.
    Due to the size requirements in certain situations the brush run motor is stronger , where is the larger motors for other wise non size critical applications can be significantly larger. the draw back is service. Brush run motors require more service , and due to the brushes wear tends to create a very dirty environment with in the motor.
    To the OP . there are electric boats/users out there and it would be prudent to contact them for current info on their experiences .
    A friend of mine had a tour service with an electric on Big Bear Lake California. I'll see if I can get some info from Him if he's still in business.
    captain John's fawn harbor marina Big bear lake California.
    Last edited: Jun 6, 2016
  18. oil pan 4

    oil pan 4 Monkey+++

    I have an air compressor I built out of a 2.5hp 240v electric motor.
    It draws up to 22 amps at shut off. My cord of choice is a NEMA10-30 dryer cord.This is my reserve compressor so it doesn't come on a lot or run continuously. Or the motor would burn up. Full load on that motor is 21 amps. The continuous rating is 2.5hp, 8 amps in a 120'F environment. And it is powerfactor corrected, just like all my machinery.

    Arleigh speaks the truth. If you have a single cylinder gas motor and you want to power some kind of air or water pump or generator with it, normally you size the engine horsepower to be about double the expected load.
    This is why for my next generator build, I am stepping up to a V-twin. I might be able to use a 13 to 15 horse single cylinder and it should work good enough but I don't like gray areas or risk playing with maybe good enough. I think I am going to step it up to a 22 to 25 horse V-twin and remove all doubt and I should have almost unlimited power.
  19. arleigh

    arleigh Goophy monkey

    Something to consider.
    Though I have my gas in 55 gallon drums full to the brim, and with stabilizer fuel still will age but much slower.
    Along with gas engine generators, I also have a 6500 watt single cylinder diesel Yan mar commercial generator.
    I also have 5 hp engine/automotive alternator units as well for addressing the battery system of my solar design, I also have a 600 watt wind mill as well.
    My point is, not all my eggs are in one basket .
    If/when the economy dumps there will be a very high demand for power, particularly till the food in refrigerators is consumed .
    This, among other things, is a bartering tool I will/can implement with the neighbors.
    Nothing is free, and the consequences for not preparing, COST SOMETHING, even if it's labor.
    I have dipped into my fuels and replenished them several times, both out of need and practicality , having tractors both needing diesel and gas as well as transportation needs.
    My generators I rarely need to work, but they are exercised regularly to keep the fuels fresh in their systems.
    I just acquired some aluminized plastic ,I plan to make faraday jackets for each generator amongst other things, to keep them covered while not in use. Also keeps critters out .
    If these jackets are painted something innocuous a passing raider might ignore them.
    It's a margin of protection, even if it's not guaranteed ,it's worth the effort.
    Most every with electronics is stored in a steel building any way, that or in steel cabinets .
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