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80 volt DC generator

Discussion in 'General Electronics Discussion' started by xalu, Sep 11, 2010.

Which method would you choose to produce 80 volts at 100 amps(must be mobile)

  1. 30v 500a Sepex DC generator with modification

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  2. AC 8kw Generator ( with AC to DC conversion)

    3 vote(s)
    100.0%
  3. None of the above

    0 vote(s)
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  1. xalu

    xalu

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    Sep 10, 2010
    Hi all,

    I am new to the forum but I suspect I will be around here often. I am looking to build a dc generator with an 80-90 volt output at a constant 100 amps. So a continuous 8kw generator of DC current. The thing is, I am having a horrible time finding something in this range for DC generation.

    So I have a couple of options about what to do.

    1)I have an aircraft J&H sepex generator rated at 30v 500a. I haven't tested its generation yet but I suspect that the above is a maximum and not a constant voltage.
    I would need to up the voltage to 80 in order for it to be useful. I was wondering if anyone has a thought on this? I still need to connect an Internal Combustion engine to it to generate the power. So that involves some more complication. Although I do have the motor ready (14.5 hp)

    Or

    2) I purchase an 8kw cont. AC generator and modify it to allow for DC current at the correct amperage. A rectifier could be used?

    Which would you choose if any? Any pointers on how would be great.

    -James
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Imho you could even do it with a 100A 12V car alternator. Car alternators have a built-in regulator that make them deliver a constant voltage. You'll need to bypass this.
    Aircrafts usually apply a 24V system (= 27.5V charging voltage) and I'm quite sure the (30V) generator you have there is capable of delivering 80V if properly controlled.
    Alternators & generators have one winding (the field) that needs to be fed a small current, and another winding (stator/armature) that then produces a large current.
    If you feed a constant current to the field then you get a constant current out of the other winding, with an off-load voltage depending on RPM (there is a maximum rating).
    A 12V unit can produce 24V if it's run at double the design RPM, so you just need to feed a specific current to the field and run it at a specific RPM to get what you want.
    Generators have a brushed commutator that automatically rectifies the generated current, while alternators needs a semiconductor bridge rectifier to get DC out of it.
    There is another thread somewhere on this forum that describes how to set up and control a welding generator (which I believe is what you're trying to make here).
     
  3. trobbins

    trobbins

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    Jun 15, 2010
    Why do you want 80-90VDC? Why will it be a constant 100A current draw load? Why does it have to be a genset? Do you mind having extra parts - eg. a transformer?
     
  4. xalu

    xalu

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    Sep 10, 2010
    I am making a range extender for my electric beetle(car) :)

    I need it to be portable and somewhat compact. So I can fit it in the "Trunk" of the car.

    That's why I was thinking of a home generator so it would be relatively compact.

    I'll look around for the link. What if the aircraft generator can't run at double rpm? Is it still possible to up the voltage?
     
    Last edited: Sep 11, 2010
  5. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Ok, I think you'll want a fairly constant voltage then, with some current limiting of course. The trunk (being in front?) may present a safe exhaust challenge though.
    That aircraft generator has a substantial current overhead, so I think it should be able to deliver a substantial overvoltage at 100A with not too much of an overspeed.
     
  6. xalu

    xalu

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    Sep 10, 2010
    Yea, the exhuast is an issue. My other idea was to use a trailer. The issue with that is that its going to make driving a bit ridiculous. I could fabricate a custom exhuast that goes out of the bottom to the side.

    What what I need to do to get the voltage right? I figure if I just find the correct RPM for the motor I can go from there. But what I don't understand is how to get the voltage (which is qouted at 30) to 80v.
     
  7. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    The more voltage (or current) applied to the field winding the more voltage you get out of the armature winding at a given RPM. Both RPM & field determines the voltage.
    The field is obviously made to be exited with up to 30V. Applying 12V and turning it at a certain RPM will produce a certain off-load voltage. The rest is mostly calculations.
    I'm not too familiar with (these) generators so I'll need a lot of informationt to give an educated guess. Is the generator shunt, compound, or series wound for instance?
    What are the windings resistances, and are their terminals available for reconfiguration? At what RPM can it deliver 500A, & what is its maximum allowed RPM.
    Edit: reading up on these beasts it seems that at the rated voltage (30V) the field iron is pretty much saturated. After that RPM increase may be the only way to increase output voltage. There may not be enough headroom in that respect to get it up to 90V in your case.
     
    Last edited: Sep 11, 2010
  8. trobbins

    trobbins

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    Jun 15, 2010
    If you have an AC mains charger specifically for your battery bank/technology - and it's a switchmode type (ie. relatively light weight) - then you could use a much more common AC genset along with the charger - and the output voltage would be regulated to suit.
     
  9. xalu

    xalu

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    Sep 10, 2010
    yes, but the charger is only rated for 1.5 kw/h. So it won't be very useful when I will be using 150 watts/mile

    That was my original worry.

    Is it illogical then to use an ac generator? I saw a bridge rectifier on ebay for a decent price, can that be used to convert the generators power to DC?
     
    Last edited: Sep 12, 2010
  10. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Alternators have a much larger RPM reserve than generators which is why using a geared-up car alternator instead may be a viable choice.
    I imagine a mains AC alternator can be rectified just as well as the car alternators are.
    The (bad) power factor you'll get and the low voltage you need (64V AC) may force you to choose an over-rated unit though. It's the Amp's that counts, not kW's.
    A 110V Y-connected unit reconfigured to delta would be as perfect as you can get it in terms of voltage and VA rating however.
    That rectifier looks nice, but at these current levels I'd bolt on ring terminals instead of using push-on flat-blade contacts.
     
  11. iss407

    iss407

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    Sep 13, 2010
    I am new to this forum as well. I have been looking at doing the same thing. I want to do something inexpensive. I was wondering about taking an off-the-shelf AC generator and re-wiring the field to prodoce the voltage/current I want. I assume that some kind of regulator in the generator adjusts the field to produce 120VAC at the output. I need 120V DC for my car. If it has a 240 with a center tap I can rectify it with two diodes. I would add a cap to smooth it a little, but otherwise I want to dump right into the battery pack. The regulator would need to adjust the field for max current output of the generator up to a cut-off voltage (max charge voltage for the batteries) and then adjust the field to keep that voltage.

    Is this possible? I'd like to be able to switch the normal regulator back in so I could use the generator to run my house if the power goes out. No need to waste a perfectly good generator if I'm not using it with my car at the time, right?

    Thanks in advance for any info.

    -- Paul
     
  12. trobbins

    trobbins

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    Jun 15, 2010
    The problem with modifying a genset, or making your own genset regulator, especially with such a hoped for high power rating - is that you run a very high risk (given your asumed inexperience with the topic) of inadvertantly cooking your batteries, your dc motors, and any regulator you may have for your dc motor - all very expensive items I suggest.
     
  13. iss407

    iss407

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    Sep 13, 2010
    A simple independent circuit with a relay can protect the batteries from over-voltage. I don't need the generator to charge them to max, just to push through the initial bulk-charge stage. If the car is driving then all of the generator current will go straight to the motor controller most of the time. A controller that can handle 500 amps won't be phased by anything a 10KW or so generator can push out.
     
  14. trobbins

    trobbins

    83
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    Jun 15, 2010
    Why not use a motor controller designed for regenerative breaking, and use it in reverse as the charger - when powered from a motor acting as alternator. If you use the same motor as used for traction, then you are always carrying around a spare! If you use an equivalent, but lower power rated controller then again you are carrying around a spare limp-home controller!
     
  15. iss407

    iss407

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    Sep 13, 2010
    That is a very interesting idea. I hadn't thought of it. I'm trying to go cheap though and if I can get a 10kw portable generator for less than $1000 if I shop around. I'd pay that for just the regen controller.

    I've thought of using the back end of a motorcycle as a pusher at the back of the car. I'd only engage it on the highway. I could hook up a little something to lift it off of the ground when I put the car in reverse.
     
  16. trobbins

    trobbins

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    Jun 15, 2010
    It's an interesting human trait to pay lots for the base equipment but then go into frugal mode with anything extra - rather than budget more at the start to get what they want - everyone does it!
     
  17. iss407

    iss407

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    Sep 13, 2010
    I built my care on a shoestring using a lot of unconventional ideas and a lot of work. It runs well and I'm happy with it. I'm the kind of guy would would rather put in 3 times the work to do it my way than to do what everyone else has done because it is easier. I like a challange. In fact, if someone says it'll never work, I start thinking about it more and more. I love to prove the naysayers wrong.

    So I have been in frugal mode all along. All the battery cables are old welding cables saved from the scrapyard. I milled the 3/4" aluminum adapter plate with a router. I even used the existing throttle body and added a poteniometer to it.
     
  18. xalu

    xalu

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    Sep 10, 2010
    iss407, if you would like I can send some links your way I have collected on the subject. PM your email if you'd like.

    I talked to several people about hooking a genset up to the batteries and it is a viable option. It has been done and works. As long as the current of the batteries never exceeds the overcharged limit which shouldn't happen since the motor takes the load like iss407 said. It should be a simple set up as far as switch is concerned.

    A generator producing 120v at 8000watts is producing just over 66amps. You should be able to use a rectifier to switch it to DC. You have a great advantage at that voltage.

    I would prefer to stick with a generator already created..but where do I get my hands on a mains AC alternator ? Tried doing a search and couldn't find one.

    For my purposes can the voltage be dropped to 80v by changing the RPM's or the field voltage? or can something be designed externally to drop the voltage and raise the amps? My controller can handle a max surge of 90volts. I'd hate to have to upgrade when this voltage is fine for my cars weight.

    Here is a link to a car that used a setup that worked until the ICE ran out of gas.
    http://visforvoltage.org/forum/8677-series-hybrid-generator-control#comment-51916
     
  19. xalu

    xalu

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    Sep 10, 2010
    I came across these http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=300466120521&actionType=singinformore

    Two of them at around 3000rpm would give me a good amount of amps(240) for acceleration and also the perfect voltage. Does anyone know if there would be an issue with running two generators off of one engine? I am thinking of using belts in a triangle pattern. I have a vertical engine, so I imagined having them mounted vertical as well. Just wanted to get some input before I delve into it.

    Any tips would be great :)

    The best part about these is they only weight 20 pounds. Which is great for weight concerns.
     
  20. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    The Manta being a brushed PM motor the only way to control it is by RPM. You need to spin them at 6000 RPM to get 83V, and at 6350 RPM to produce 100A @ 83V.
    Both triangular or flat (one belt to each side) may work well. The flat way puts no stress on the crankshaft though. There are belt dimension calculators out there.
     
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