DC Generators

sawmill
sawmill Solar Expert Posts: 93 ✭✭✭
in Off Grid Solar & Battery Systems #1
It's always with interest to read all the information on this forum concerning back-up generators for battery charging and stand by use. I have always been curious as to the lack of small DC generators. It seems with a DC charging source one step would be eliminated in the charging routinue. Rather than use the inverter for charging from an AC source which decreases some of the available AC output from the inverter, a direct DC source would not take away the AC capacity.

I feel sure there are reasons for the lack of DC generators but would like to hear what others think on this question.
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: DC Generators

    I'd say it's because DC generators aren't what most people need. The big demand for generator power is for 120 VAC, so that is what is built and sold.

    As an aside, all generators are actually AC regardless of their wiring output. The DC version simply rectifies the AC out, whether through semiconductors or the brush placement on commutator. You can't swing opposing magnetic poles past a wire loop and get anything but alternating current.

    Now someone will come in and tell me I'm wrong. Too bad for them.
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #3
    Re: DC Generators

    Many small generators do have an aux DC 12 v output but its usually only about 10 to 20a.. But for most people thats adequate to charge a vehicle battery or 100 to 200 ah deep cycle battery.
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  • peakbagger
    peakbagger Solar Expert Posts: 341 ✭✭✭
    #4
    Re: DC Generators

    I have the pieces of a diesel 12 volt generator that came from one of the portable highways signs before they swtiched over to solar. It has a petter diesel and even is set up for pull start (but has a electric starter)

    I have heard arguments in the past that the DC generators are thw wya to go for solar but I think it was just easier to come up with a car generator and a lawn mower motor. If you look around in the Home Power archives there are couple of articles on building your own.
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #5
    Re: DC Generators

    Vehicle alternators up to 350A and 12V are easily obtained and if connected to a small 10hp diesel engine would be more than adequate for most uses .
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #6
    Re: DC Generators
    Cariboocoot wrote: »
    IYou can't swing opposing magnetic poles past a wire loop and get anything but alternating current.

    Now someone will come in and tell me I'm wrong. Too bad for them.

    Too bad for me then, and also it does not really use a "wire loop", but the Faraday Hompolar generator (http://en.wikipedia.org/wiki/Homopolar_generator) is really fascinating. Not used a lot though. :-)
    Capable of producing a surge current of millions of amps DC (very low voltage, incredibly short time!) just from flywheel effect.
    SMA SB 3000, old BP panels.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #7
    Re: DC Generators

    I can remember back in the late 1950s my old man was a printer and I was just a tyke, they had a 4 foot high DC generator attached to a large AC motor. The DC was used to variable speed control the all the large printing presses in the building.

    The other thing they had was a large arc light to burn offset plates. Man that thing was bright. Still back then most of the plates were copper engravings. He took me to the local newspaper where they had Linotype machines and a bunch of guys actually typed on those machines to make lead type set into racks. They then took the racks down to layout the pages of the paper.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #8
    Re: DC Generators
    Cariboocoot wrote: »
    As an aside, all generators are actually AC regardless of their wiring output. The DC version simply rectifies the AC out, whether through semiconductors or the brush placement on commutator. You can't swing opposing magnetic poles past a wire loop and get anything but alternating current.

    Now someone will come in and tell me I'm wrong. Too bad for them.

    i think i'll address your comment in bold as you can get pulsed dc from a generator if using a split slip ring for the brushes to go over and this will act much like what one might see with full wave rectifying. the action of rectification, as is also the case for a split ring, does not smooth out the frequency component, but merely prevents the opposing polarity. filtering is what brings this ripple voltage down close enough that we'd use it as a steady dc voltage source.

    i do have to clarify that pulsed dc is not technically ac because it never dips below 0v for a negative component, but it can be considered as ripple voltage or varying dc as well as pulsed dc and it does have a frequency.

    you are correct only in that there isn't a steady dc voltage from any generator without it being filtered.
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  • sawmill
    sawmill Solar Expert Posts: 93 ✭✭✭
    #9
    Re: DC Generators

    Several years ago I rigged a 24v alternator to a 1 cylinder diesel for emergency charging duties. This was less than satisfactory, once the battery bank reached 27.4v, the internal regulator would stop the charging. Of course one could load the bank enough to keep the voltage below 27v and the alternator would continue charging but you would not gain any meaningful charging on the batteries. While this is fine for automotive applications, was not good with deep cycle batteries. 29 to 30 volts is needed for bulk charging.

    If the internal regulator is bypassed there doesn't appear an easy (cheap) way to control voltage from an alternator. With a permanent magnet alternator I wonder if one could control the voltage by RPM's once bulk was reached?
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  • frankson
    frankson Registered Users Posts: 22 ✭
    #10
    Re: DC Generators

    I use a www.alten-dc.com 4.6hp HATZ air cooled diesel driving 200 amp alternator for
    back-up to quick bulk charge 1020 amp AGM battery bank if/when 1264 watt solar can't
    keep up with demand. So far, rarely ever needed, but when it is, less than 2 qts of fuel
    and under two hours of run time gets me from low of 50% SOC back up above 85% SOC.
    There's much, much cheaper DIY set ups than Alten, but I had gotten a deal on mine.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #11
    Re: DC Generators

    Balmar makes some interesting alternator products (probably not cheap as they are designed for marine use).

    Here is a programmable regulator that will fit Ford type alternators PDF Download for 24 volt version (I think).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • 2manytoyz
    2manytoyz Solar Expert Posts: 373 ✭✭✭
    #12
    Re: DC Generators

    I considered building my own. The Epicenter website has parts/plans to build one: http://theepicenter.com/tow02077.html

    But then I end up with a single purpose piece of hardware. It would have a noisy motor that gulps fuel, and needs to run at a high RPM to make the full rated power of the alternator.

    Ultimately, I bought the Yamaha generator. Supplies 8A DC via the built-in charger, or 20A @ 120VAC. The DC feature is a "nice to have", since it takes up no extra room. But it's too small to be very effective for recharging a large battery bank. So when I need a larger DC charging capability, I connect the generator to a 75 Amp Iota 3 stage charger. The charge rate tapers off fairly quickly, and since the generator engine only runs as fast as the load demands, it slows down - conserving fuel, and greatly reduces the audio signature.

    dscn6134.jpg

    dscn4134-1.jpg

    iota75a.jpg

    If the weather cooporates, I can get almost 50 Amps of charging via solar too. The Iota charger works nicely with the solar charge controllers, giving a combined charge rate over 100 Amps. About the only time I need to use the generator/Iota charger is if we've had a couple days of poor weather, or we've had a major outage and I've run all my critical loads (2 fridges, lights, TV, fan, computers, CCTV system, etc) via the battery bank/inverters.
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