Electrical motor maths: 25HP=8HP?

Discussion in 'Propulsion' started by voltage, Mar 3, 2009.

  1. voltage
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    voltage New Member

  2. kistinie
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    kistinie Hybrid corsair

    Resulting trust per HP ?

    10KW electric gives 150N trust
    Fuel ?
    Who knows the HP for 150N trust with fuel engine ?
     
  3. Alik
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    Alik Senior Member

    Trust or bollard pull? Not same.

    Movie seems good manipulation with numbers... to get result required for supplier :)
     
  4. mydauphin
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    mydauphin Senior Member

    The advantage of electric is you get torque from zero rpm up, diesel needs to be operating at torque range of lets say 1500 rpm. So at 800 rpm 8hp electric has same torque and can drive same propeller as 25hp diesel.
    But of course set both at same rpm and the diesel will give you same hp as electric hp all things being equal. But remember not all engines, motors, batteries or transmission are same. Neither are propellers or boats... that is why there are engineers and experimentation. Don't believe what people are trying to sell, Physics still rules our lives.
     
  5. Alik
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    Alik Senior Member

    Yes, true. Diesel is not efficient at idle speed, needs about 30% of max RMP to keep it running. The advantage of electrical motor is better use of power at idle speeds, but who compares propulsion/power at idle speeds??? Every comparision should be fair.
     
  6. Guest625101138

    Guest625101138 Previous Member

    There is no maths in the clip. He is providing limited factual information with no numbers. Basically little more than nonsense.

    Most diesel have ratings given at the shaft. You have to determine what allowance there is in this for auxiliaries but these losses are typically small unless you have oversize alternator for example.

    The diesel will be able to deliver peak power at close to peak rpm. The peak torque will be a tad higher than the rated torque but at lower rpm. If the load torque exceeds the peak torque then the diesel speed will collapse. Combustion is poor and there will be lots of black smoke.

    An electric motor has a continuous rating based on how well it is cooled. Ratings are normally given in open air. However an electric motor can exceed its continuous torque and power rating by two or three fold. So if you have an electric motor rated at 10kW there is no problem getting 20kW from it if the power supply is up to it. It will deliver this power until it overheats. If the load torque exceeds the rated torque the motor will just draw more current until it trips or hits current limit. Most of the low end EV motors have a current limit around 2X rated current.

    So a diesel has quite a narrow range of operation while an electric motor has a wider range.

    Some of the high powered electric vehicles being built do not bother with gear changes because the motor has the torque to spin the wheels in the normal gear that allows top speed.

    This is a good comparison of electric and IC torque:
    http://www.youtube.com/watch?v=pm6gD6r3-cw&feature=related
    The electric car has rated power less than half of the IC.

    There are a few electric dragsters that take advantage of modern batteries and electronics. My favourite is Bill Dube's bike:
    http://www.youtube.com/watch?v=GDHJNG2PngQ&feature=related

    Rick W
     
  7. Joakim
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    Joakim Senior Member

    Since we are discussing marine applications it is important to remember that a propeller has a lot of slip during acceleration from zero. Thus the engine never needs to work hard at low rpm. Here is the curves for my engine: http://www.yanmarmarine.com/uploads/products/pdf/GM_YM/1GM10_TechData.pdf Note that the maximum torque is almost constant (16-18 Nm) between 1800 and 3600 rpm. With any decently chosen propeller the engine will take well over 1800 rpm during bollard pull or rapid acceleration from zero. Thus the diesel will always give about the maximum torque when needed in marine applications (this is not true for fast planning boats). The only problem with low torque at idle is that it takes a while to accelerate the engine from 800 to over 2000 rpm where the thrust starts to be good.

    This is very different for cars, since the wheels are not supposed to slip thus you basically start from zero rpm, which is OK for an electrical engine, but bad news for a diesel. Thus the good torque at low rpm is a clear benefit for a car engine.

    How does the torque (and thus efficiency) of a typical marine electrical motor behave at low rpm? I'm guessing it is relative to current, thus efficiency is very low at low rpm. Is that so?

    Joakim
     
  8. jehardiman
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    jehardiman Senior Member

    Electric motors have thier maximum torque at stall and minimum torque at sync speed (the shape of the torque curve varys for each type but they are all similiar in this..see this site http://www.reliance.com/mtr/mtrthrmn.htm). The motor is rated for torque, and therefor HP, at a given speed. Normally, this rating is based upon continious operation and is a winding thermal or circular mil amp limit. It is important to notice that EM torque increases as rpm drops and torque decreases as RPM rises forcing a flat or negative HP curve.

    ICE's have thier maximum torque at about 60-70% of thier maximum RPM, and it remains fairly constant as RPM drops to some minimum idle. Therefor ICE's carry thier HP lower into the RPM curve and have more HP available at high rpm.

    Because the powering curve is exponential, ICE's are better suited to marine propulsion because their HP curve follows the powering curve. EM are not as well suited because thier HP curve cuts across the powering curve as RPM imcreases. This is why you need expensive motor controllers for marine propulsion, becuase you need to change the % of sync speed to RPM in the motor controler (either with MG-TG sets or solid state frequency generators).

    All in all, an electric prime mover is heavier, larger in volume, and more complicated than an ICE. What you pay for in the light weight and size is slightly less efficency, but most of that can be recovered and usualy is in large plants.
     
  9. drmiller100
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    drmiller100 Junior Member

    the lecture is nonsense with his definition of "designed horsepower".
    further, I'm a little interested how the magic electricity gets created to run the electric motors across a large lake.
     
  10. Guest625101138

    Guest625101138 Previous Member

    Torque is proportional to current. My Mars PMSM, using the 4-quadrant Kelly controller, takes 260mA from a 24V (6W) to turn over at minimum speed. So no load loses are very low. Not too shabby for a motor/controlle combination able to deliver 9kW.

    The only additional losses at full torque are copper losses. The winding resistance is 10mohm. So at peak current of 200A you need 400W to produce 27Nm of torque.

    By comparison a Yanmar 2YM20 operating to spec will deliver 32Nm once it gets to 1800rpm and pulls 6000W to achieve it. This might give you an idea of the comparative pulling power. This is why electric dragsters do so well against IC motors. Instant on; with twice rated torque at zero rpm no problem.

    Rick W
     
  11. Joakim
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    Joakim Senior Member

    Where is this 200A? How much is taken from the batteries? 200A * 24 V = 4800 W. Or is there a boost circuit that creates that 200 A * 2 V = 400 W only using about 20 A at the 24 V batteries?

    Joakim
     
  12. marshmat
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    marshmat Senior Member

    The 400 W is the loss due to the resistance of the motor winding: R*I^2, for 10 milliohm resistance and 200 amp current.
    If Rick is using a 24 V battery, you are correct that 200 A at 24 V is 4800 W.
     
  13. Guest625101138

    Guest625101138 Previous Member

    Joakim & Matt
    These controllers all use switching mode DC-DC link. Losses are very low. I gather they use either MOSFETs or IGBTs but I have not opened either of my controllers to actually see what they have inside:
    http://www.irf.com/technical-info/whitepaper/choosewisely.pdf

    I have a 200A variable frequency variable current TIG welder and have had it open. The small size of the electronics to handle the current is impressive. My Kelly controllers are likely to have even smaller components given it is 120A continuous.

    Most portable devices like computers and phones use switching mode power supply for battery charging. It gives much greater flexibility with input voltage so the same power supply can be used worldwide. The output voltage does not change irrespective of the input voltage. Also the power losses are almost independent of input voltage.

    Rick W
     
  14. Joakim
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    Joakim Senior Member

    Yes I'm well aware of switching mode power supplies and I have even designed and built them, but only for <1A currents.

    You said your system can provide 9 kW. At 120 A that would be 75 V, thus much over the supply of 24 V. This would require a buck-boost system, which is able to both increase and decrease the voltage.

    I have some doubts about the efficiency of the controller designed for >>24 V 120 A at 2 V 200 A. They don't typically do well on such a wide voltage range and most likely they are optimized for the higher power outputs.

    Is your controller one of these: http://www.kellycontroller.com/mot/KellyPMUserManual.pdf ? After fast reading it looks like a PWM controller NOT a switching mode one. That means it only switches on/of the voltage you have and thus it takes the full 200 A from 24 V (or whatever batteries you have) when using the full torque and can not provide 9 kW unless you have much more than 24 V batteries. Or did I miss something?

    Joakim
     

  15. Guest625101138

    Guest625101138 Previous Member

    Joakam
    I have a Mars PMSM with a regenerative Kelly controller. I checked the spec per attached. It is the KBL48210. The peak current is 200A. The continuous current is 100A not 120A.

    PWM is one form of switching mode operation. All of these controllers use energy storage in the switching cycle so the power output is not much less than the power input. That is the big feature of switching mode. The current is multiplied in relation to the voltage reduction ratio. Effectively an interchange of magnetic and electric energy similar to a transformer. It is only at full voltage where the battery current equals the motor current or its 2-phase equivalent given that they are 3-phase motors.

    My intention is to use a supply voltage between 40 to 50V. I was intending to use 4 x 12V VRLA batteries but lithium are looking more promising. I may end up buying a 10S lithium battery of about 6000mA rating and 20C power rating so I can do more serious testing with my test outboard. My current 12Ah VRLA batteries limit output to about 270W:
    http://www.boatdesign.net/forums/at...288392-electric-boat-data-drive_leg_test1.wmv

    Rick W
     

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