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Thread: Generator Speed

  1. #1
    genmake is offline Plastic
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    Default Generator Speed

    Hello,

    I am wondering why small 900 RPM generators are so uncommon nowadays. Is there still someone who manufactures them? 50KW, three phase,120/208 volt, 60 Hz.

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    DaveKamp is offline Titanium
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    Default My guess...

    Well, my guess is that there just aren't that many 'small' prime movers that favor the 900rpm speed. You need to get pretty high up in the power category before 900rpm becomes a reasonable speed, particularly in diesels. Aside from mainline locomotive diesels (way above 50kw) , most everything else reaches beneficial performance at the 1200+ speed. Not many folks will go to a big single-cylinder or oilfield-type engine for 50kw @ 900rpm...

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    A_Pmech's Avatar
    A_Pmech is offline Stainless
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    Probably not much of a market for a generator of that size or lower RPM. They're big and heavy, offering longevity over high specific power of more portable generators.

    900RPM or less suggests continuous stationary duty, AKA "off the grid". Most people try to build where they can hook up to the grid. Those in the field temporarily can't afford to haul around the weight and size of a 900 RPM generator of that size.

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    HelicalCut is offline Stainless
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    It is due to engine design. modern generators are direct drive. Engine designers aim for 1250 to 1500 feet per minute average piston speed at their engines peak efficiency. Most diesel engine have stroke to bore ratios of 1.1 to 1.5
    Turbocharging has increased the power output for a given bore size.
    Engine makers like economies of scale meaning that they may make 1 2 and 4 cylinder versions of a small bore engine then 6 cylinder versions of a larger engine then 8 12 and 16 cylinder versions of still larger bore engines.

    The mechanical implications of these factors are that for an engine to have a bore size big enough to need a 900 rpm operating speed to stay at optimum piston speed the power is 1000 HP + for the smallest engine.

    Where I used to work all the 1000 Kva machines were driven 1000 rpm V12's (50 hz) Old 1000 Kva machines were 750 or 600 rpm

    Low speed generators are larger and more expensive to produce

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    RC99's Avatar
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    Helical Cut, how did they get 50HZ when running the alternators so slow???

    I have an old non working 30KVA alternator here that ran at 1000rpm but made 50Hz...

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    "how did they get 50HZ when running the alternators so slow?"

    For 60 Hz, the relationships of poles to rpm are as follows"

    2-pole -- 3600 rpm

    4-pole -- 1800 rpm

    6-pole -- 1200 rpm

    8-pole -- 900 rpm

    12 pole -- 600 rpm

    Etcetera.

    For 50 Hz, apply a 50/60 (5/6) ratio.

    For unitized generator sets -- prime-mover directly coupled to the generator (usually an alternator) -- you usually want to operate the prime-mover at its best specific fuel consumption speed, which, for 60 Hz, and a 5,000 HP EMD 20V-710 engine, may be 900 rpm.

    Smaller engines can, of course, operate at a higher speed.

    Just as larger engines can, of course, operate at a lower speed.

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    RC99's Avatar
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    But this alternator puts out 50Hz @ 1000 rpm, it is from the pre WW2 era and thus has no electronics for the regulator...

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    HelicalCut is offline Stainless
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    .RC a 1000 rpm 50hz alternator will have 6 field poles so there are six north south field transitions per rev. Every cycle has 2 north south field transitions.

    Electronics doesn't come into it, the field transitions are caused by the relative motion of the field poles past the armature / stator poles.

    FWIW many automotive alternators have 12 poles and can easily make 400 hz 3 phase power by running them at 4000 rpm..

  9. #9
    genmake is offline Plastic
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    Is it possible to get a 400 Hz generator that is designed for 200- RPM and replace the AVR if necessary and then through a gear reduction spin the generator at the necessary 300 RPM to generate 60Hz?

  10. #10
    DaveKamp is offline Titanium
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    Default Anything's possible...

    But not necessarily always efficient, practical, or probable.

    AC alternators (casually referred to as 'Generators") are electromagnetic animals of reciprocating polarity, and contain polepieces and windings with capacitive and inductive reactance. This means they're characteristically somewhat resonant.

    In 'English', this means that they LIKE to run at the pace for which they were designed. You can over, or underspeed one to get the frequency you want (since output frequency is armature speed and pole-quantity), but you may find that at some frequency OTHER than the design freq... it doesn't perform with as much chutzpah.

    Disabling or modifying the AVR circuitry really doesn't give you anything different... the AVR MAY be a resonant-device, and you may run 'out of range' for it to be effective, but that's not what determines frequency or shaft-speed... it simply controls field intensity to keep output VOLTAGE within a certain range. While the AVR is doing it's thing, the machine's GOVERNOR is doing it's part... to try to maintain frequency spot-on. Unfortunately, altering the shaft speed DOES have an effect on voltage, and the AVR will "TRY" to correct it (just like the governor will TRY to correct engine speed).

    There's quite a few variables involved... and the guys with slide-rules in pocket-protectors definately took a ride through the mill to get it right for the original intended application.

    If you try to change it, don't be surprised if something gets kinda hot.

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    HelicalCut is offline Stainless
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    Is it possible to get a 400 Hz generator that is designed for 200- RPM and replace the AVR if necessary and then through a gear reduction spin the generator at the necessary 300 RPM to generate 60Hz?
    Yes but why? the output will be well down. A properly designed AVR will only ramp up the field excitation to the rated value, any more will risk cooking the field windings and will only raise the voltage slightly, at 60 Hz the output voltage will be less than 15% of rated (assuming 5% impedance) at rated load current.

    If you were determined enough to try this then using the 400 Hz to drive a VFD to make 60 Hz would make more sense or back in days of old a cycloconvertor run direct off the 400 hz could make 60 hz. A trip to an aviation or computer wrecker (mainframes) will provide all manner of goodies designed to run off 400 Hz.

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    RC99's Avatar
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    Quote Originally Posted by HelicalCut View Post
    .RC a 1000 rpm 50hz alternator will have 6 field poles so there are six north south field transitions per rev. Every cycle has 2 north south field transitions.
    But then shouldn't it run at 750 rpm like a 6 pole induction motor does?? Or is it an 8 pole that runs at 750 rpm (1/4 of 3000) and six pole 1000 rpm seeing 1/3 of 3000 (2 pole speed) is 1000?

    I wonder if you can buy controllers for these old alternators...

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    HelicalCut is offline Stainless
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    But then shouldn't it run at 750 rpm like a 6 pole induction motor does
    Most 6 pole induction motors run at 950 RPM remember a generator is a synchronous machine induction motors are asynchronous an have slip FWIW many 8 pole motors run at around 710 rpm.

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    genmake is offline Plastic
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    This thread is about where to find 900 RPM generators. If there is a general question about the relationship between excitor poles and frequency please start a new thread. Thank You.

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    RC99's Avatar
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    Thanks HelicalCut.

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    James Rogers's Avatar
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    Quote Originally Posted by genmake View Post
    Hello,

    I am wondering why small 900 RPM generators are so uncommon nowadays. Is there still someone who manufactures them? 50KW, three phase,120/208 volt, 60 Hz.
    Looks like you wanted to know why they are so uncommon and that is what this discussion is about ie: why.

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