AC or DC alternator

Discussion in 'OnBoard Electronics & Controls' started by simon, Feb 18, 2010.

  1. simon
    Joined: May 2002
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    simon Senior Member

    Good day,

    What are the advantages of using an AC alternator (like the dynawatt, http://www.leab.ch/dynawatt/wEnglish/produkte_dienstleistungen/dynawatt_4000.php?navanchor=2110000 ) over using one or several big size DC alternators (12V or 24V)? What would be the setup for charging the batteries and feeding the AC circuit? I have seen the Victron Multiplus (http://www.victronenergy.com/upload/documents/whitepaper - variable speed generator - rev.pdf) which will deal with the load-spreading between the batteries and the generator. The AC-Alternators seem to be used quite often on canal-boats.

    The idea is to avoid installing a separate generator, but still have sufficient AC to operate a washing machine / dryer. The main engine is a 1402 Kubota with 28hp. What other options are there besides the VW, Dometic or Dynawatt AC alternators, and also with a smaller pricetag.

    Thanks for the feedback

    Simon
     
  2. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    From what I understand, the Dynawatt and related systems are not the kind of AC generator we're generally familiar with- they are essentially a 250 V three-phase generator, coupled with a remotely mounted rectifier/inverter circuit that converts 250 V variable-frequency three-phase to 230 V 50 Hz single-phase. (Anyone who's worked on these- please correct me if I don't have this quite right.)

    Such a system may end up being cheaper and more compact than a big DC alternator, remote rectifier, battery pack and big inverter- if, that is, the vast majority of your loads are AC and you tend to run the engine whenever you need power.

    The Victron Multiplus units combine a synchronous inverter, battery charger, and a heap of sophisticated control electronics, the intent being to automatically take care of changes in power sourcing and demand. If your AC demand goes above what your generator or shorepower can provide, the Victron kicks in with extra power from your battery. If your AC demand is low, it'll switch to charging your battery from whatever source is available.

    I haven't managed to get my hands on one of these Victrons (yet....) but have heard many favourable reports of how they hold up in day-to-day use. If your boat is heavy on AC loads and is often on shorepower or running a generator of some kind, they make a lot of sense.

    If your AC loads are going to be less than a kilowatt or so per circuit, conventional pure-sine inverters running off the battery will likely be cheaper. In this case, all AC loads would be inverter-driven, the alternator would be a big DC unit at 24 or 48 volts, and the shorepower circuit would be connected only to the standalone battery charger.

    For larger, intermittent AC loads, something like the Dynawatt might make sense (it'd be a great choice for service trucks, for example). On a boat, I'd only consider it if my AC loads were going to be consistently high and I knew I'd be running the engine most of the time anyway. If it's just a few occasionally used things (such as your washer/dryer) that you need the higher power for, a standalone genset is probably the way to go.
     
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  3. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    British canal boat usually has 2 alternators one on the starting battery and another on the house battery at 400=600 AH . The start alternator can be connected to help the house alternator in some systems by auto or manual switching ...you will not charge at ove 25 A per 100 ah and thats how you size your alternator ...Most use a big inverter for 240v ac ( central heating pump /tv /fridge etc ) and connect to the canal side supply or run a seperate generator to do the washing ...have a look at a british canal boat forum though I do find them to be a bit anal retentive ....you will also find restrictions on when you can run your engine/generator hence the 12 /inverter system .
     
  4. Landlubber
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    Landlubber Senior Member

    The big advantage with the Victron units is that you may be able to run a smaller conventional AC genset, the start up excess loads are handled by the Inverter.....many 5KVa gensets can run a decent AC system by using the occassional kick in from the batteries (via the Victron), it is the start up current that needs the high amperage usually, not the running of the equipment. The standard genset can handle up to 2x the run current for a few seconds, but if two units try at the same time, it goes into overload, and suts down. The Victron saves this from occurring, thus allowing a smaller genset than would be required without it.
     
  5. CDK
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    CDK retired engineer

    By definition an alternator is a device producing alternating current, whether or not that is subsequently rectified into direct current is irrelevant.

    If the device generates direct current, we call that a dynamo and if we want to cover the whole group or do not know the nature of the current, we call it a generator.

    That makes the question in post #1 unanswerable.
     
  6. pistnbroke
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    pistnbroke I try

    Nit quite CDK

    An alternator gernerates AC and turns it to DC with a diode pack if DC is required ...
    A Dynamo generates AC and turns it to DC with a commutator
    There has never been a rotary generator that makes DC without rectification.
     
  7. CDK
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    CDK retired engineer

    No pistbroke, that is nitpicking and you know it.

    A dynamo is a DC device, both when used to generate electricity or as an electric motor. There is no terminal from which you can take AC, the commutator is an integral part of the device.
    An alternator can be a self contained unit without diodes or field control circuitry and still be recognized as an alternator.

    That the power is generated by windings passing NS and SN magnetic fields is the principle of all conversions between mechanical and electrical energy.
     
  8. jonr
    Joined: Sep 2008
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    jonr Senior Member

    The choice is between two types of inverters. A grid-tie inverter will sync to another source of AC power and add to it. This can be useful if you have another source - such as a too small 120/240V generator or limited shore power. A regular inverter does not have this feature.

    One could also use the engine to drive a 120/240V 50/60hz generator head. It would require a fixed engine rpm while you are using it but would be cost effective.
     
  9. simon
    Joined: May 2002
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    simon Senior Member

    If I need 3 kW over a time of 1 - 2 hours, is the 230V alternator a valid alternative to a generator, not wanting to have an separate generator to maintain on board? This would mean that I would not need a big inverter, which will be used only when the engine is running anyway.

    Thanks

    Simon
     
  10. jonr
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    jonr Senior Member

  11. Landlubber
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    Landlubber Senior Member

    Simon,

    "The idea is to avoid installing a separate generator, but still have sufficient AC to operate a washing machine / dryer"

    ........the washer side is not so hard to do, it is the dryer that will use the Watts up. Any element running fdrom generated power is the heavy load side of the system, it will exceed and maintain the maximum the little units can handle.
     
  12. pistnbroke
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    pistnbroke I try

    your engine driven 3 kw system is a good alternative to the seperate generator (cost ignored ) BUT it will only give you 240v when you run the main engine so in my opinion you will need another source of 240v for tv laptop etc etc when the engine does not run..sounds like some doublepole isolating switch or dual wiring . dont neglect to give full consideration to your charging of the 12v house bank .....
     
  13. simon
    Joined: May 2002
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    simon Senior Member

    Thanks for the feedback.

    I have got a 12v battery bank, which is charged by solarpanels, windgenerator, the 12v alternator and the 230v alternator via a charger.

    There is an inverter installed for 240V appliances with less consumption, like microwave, computer etc.

    With the engine running and the 230V alternator excited, I will have AC power for the bigger appliances, like washer/dryer and watermaker, etc.

    I have found:

    http://www.vw-m.de/index.php?id=284 for 3.770,00 €

    http://www.leab.ch/dynawatt/wEnglish/produkte_dienstleistungen/dynawatt_4000.php?navanchor=2110000 for £2,308

    http://www.dometic.com/enuk/Europe/...pment/Generators/product/?productdataid=67399 for ????

    Are there any other strap-on and belt driven alternators that can be fitted to the engine? What other solutions are there? Like using a hydraulic pump and motor to run the generator. There are a lot of cheaper belt-driven generators, like the one jonr mentions. How could they be integrated?

    How do I ensure that the engine revs are high enough to provide sufficient output?

    Cheers

    Simon
     
  14. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    I dont think the revs are a problem its just a question of what speed you want to run your main engine at and then fit the right pulley ratio....I noted that there are two types of generator .First sort are constant speed 1500 rpm like the FuXing .......Second type used electronics to produce the 50 hertz .even with varying speed and load .like the ones above..so you need to be sure what you want .keeping the engine at constant rpm say 1000 rpm and a 1:1.5 ratio pulley system may be difficult ( if using a diesel engine keeping rpm constant with varying load should not be difficult as the system has a govener built in) .
    You may have to knock the belt off for normal running or the generator my spin too fast ....clearly the variable speed types would be better but probably more expensive .Note they use a poly V belt so you need to have one on your engine to match . Standard V belt only good for 1 kw ..carefull selection and think it through are required
     

  15. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    dont forget your propulsion diesel has a variable speed govener and for that job you need a constant speed govener like a gen set has
     
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