diesel electric

Discussion in 'Hybrid' started by whitepointer23, Oct 4, 2015.

  1. whitepointer23

    whitepointer23 Previous Member

    What is the approximate relationship between the generator and propulsion motor. . Say the electric motor is 10 hp, how large will the generator be that powers it. In kva and the diesel hp , hope this makes sense.
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Small generators are about 80% efficient at most. Then there will be some power loss through cabling and the electric motor itself.
     
  3. whitepointer23

    whitepointer23 Previous Member

    Thanks gonzo. So a 7.5kw motor would at least a 12 kva generator ?. Some of the diesel gensets have a 3 phase 415v 20 amp outlet. Is that better than a single phase 240v.
     
  4. DCockey
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    DCockey Senior Member

    What type of generator, motor and control system?
     
  5. whitepointer23

    whitepointer23 Previous Member

    Don't know. On ebay there are diesel gensets fairly cheap and i thought they might be good for a diesel electric drive in a yacht.
     
  6. pdwiley
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    pdwiley Senior Member

    Why? You are stacking inefficiencies and introducing more points of failure at each step of energy conversion.

    The *last* thing I'd put anywhere near salt water is one of those cheap Chinese diesel gen-sets. I don't know what their IP rating is, but it's going to be piss-poor. Also - you have seen the burgeoning sales of aftermarket voltage regulators? There's a reason for those sales....

    PDW
     
  7. FAST FRED
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    FAST FRED Senior Member

    The only reason for claimed "efficiency" is a genset can work harder pushing the boat as it heavily charges a batt bank.

    The instant the bank is full , it pushes the boat.

    The only saving is the difference in efficiency of a loafing gen set and a fully loaded one.

    Perhaps 5% , for an awful load of weight and complexity.
     
  8. groper
    Joined: Jun 2011
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    groper Senior Member

    This may look like a simple problem at first, but it's not for several reasons. Firstly, your asking the wrong question. The KVA output of the generator is not the main concern. The maximum current the generator can deliver is the limiting factor in this scenario. Read on;

    Electric induction motors generally run at 80%-85% efficiency.

    As they are also an inductive load, they generally cause a lagging power factor of around 0.8 give or take depending on the individual motor.

    The reason generators are rated in KVA rather than kW, is because the manufacturer doesn't know the power factor of the load you connect to it.

    For close approximation you can assume a power factor of 0.8 for induction motors. To work out full load line current for 3 phase motor, do the following;

    mechanical kW /0.8 efficiency = electrical kW.

    Line current = elec kW / (sqrt3 x line voltage x power factor)

    So if you have a 12.5kw 415v 3 phase motor, then it will take 15243.9 electrical watts.

    From a 3 phase power supply, it will draw a line current of 15243.9/ (sqrt3 * 415*0.8)
    = 26.84 amps @ 415v. - this is all assuming motor is delta connected of course.

    KVA taken is Electrical watts / pf = 15243.9 /0.8 = 19.055kva .

    This is not the end of the story however... If you look at a 20kva generator to run this load, you need to look at the name plate and ensure it has a continuous current rating in excess of the max line current the motor will take. You also need a bit of headroom above this as the lagging power factor will cause a significant voltage drop in the genset so the actual current ends up higher than this. How much higher depends on the quality of the voltage regulation within the genset, typically they should be better than 5% but you have to check the nameplates.

    Now the last part is the starting current. Not such a huge problem if you soft start the motor on a variable speed drive as I would imagine you would need for yacht propulsion. If the motor was started direct on line, it could draw up to 8 times the running current - which this genset could not supply. It would most likely trip the over current protection etc. assuming you have a VSD for this motor, just add the VSD efficiency loss on top of what we already worked out, and then you know the required generator current output. Remember , it's the current which generates heat, and therefore it's KVA is largely irrelevant. What you need is the ability to continuously deliver the required current- this should always be printed on the nameplate of th generator.
     
  9. whitepointer23

    whitepointer23 Previous Member

    Thanks for all the replys. Sounds like the gensets are normal chinese quality and the system would be to inefficient anyway. The reason i asked is because like a hydraulic system you can place the powerpack where you want it. I know the question has probably been asked before but it is an interesting concept .
     
  10. whitepointer23

    whitepointer23 Previous Member

    Are you away from the storm fred. Showed some pretty severe weather on the news tonight.
     
  11. powerabout
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    powerabout Senior Member

    hydraulics not very efficient either but yes gives you a choice of where the power is like DE.
    and at a guess HP for HP the hydraulic pump is way smaller than an alternator to drive the same prop. ( as is the hyd motor versus elec motor and by the nature of the beast waterproof)
     
  12. groper
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    groper Senior Member

    Well that's not entirely true either.... You can improve th efficiency with good quality Brushless DC motors, some of which can achieve ~95% efficiency. And for poor factor on AC equipment you can correct it via capacitor banks etc. There is still the losses accociated with the controller and I^2r losses, but as a whole it's pretty good. Still not as good as a direct drive via gearbox tho... It all depends on what floats your boat :)
     
  13. powerabout
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    powerabout Senior Member

    I notice 750kw engines make 500kw gen sets, so there is a big loss not to mention the weight.
     
  14. groper
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    groper Senior Member

    I've never seen a genset with an output given in kW. Th reason is as I stated before, they cannot do that because they don't know the power factor of the load your connecting to it. The reasons you may see a 750kw prime mover on a 500kva alternator , could be many depending on what type of alternator it is and what it was designed for, what types of loads would be connected to it, how the loads are switched in and out, and what type of prime mover was running it etc.

    You can also reverse calculate alternators in the same fashion as electric motors as they are very similar beasts. If the alternator was top quality, you should see efficiencies well over 90% , probably over 95%. If the alternator is poorly designed and constructed with poor magnetic circuit losses, low copper density, high i^2r loss etc then it will waste energy no different to a poor quality motor....

    If you have a 1000kva genset, and connect it to an electric motor which has a power factor of 0.7, then the most kW you will b able to deliver is 1000*0.7=700kw. If you improve the power factor closer to unity (1) then you get closer to the 1000kw deliverable. The power in and power out will be identical minus the magnetic losses and heat and friction etc which on a well designed machine will be close to 95% efficient. You won't get that on a cheap little Chinese junker tho...
     

  15. Joakim
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    Joakim Senior Member

    E.g. MAN claims 90-92% overall efficiency from diesel engine to electric SHP: http://marine.man.eu/docs/libraries...diesel-electric-drives-guideline.pdf?sfvrsn=0

    So good efficiency is possible, but not with cheap parts. BLDC motors aren't that expencive and you can find ones capable of 90+% efficiency both as a generator and as a motor. With those 80+% efficiency from diesel to electric SHP should be possible.
     
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