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  1. #1
    James Rogers's Avatar
    James Rogers is offline Aluminum
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    Default 3 phase motor used as generator

    This might be the wrong place to post this but I didn't see anyplace else for a question of this nature.

    We just went through a 4 day run without any electricity and I just wondered if a 3 phase motor attached to a gasoline motor would produce power? I use a 3 phase motor in a RPC capacity and produce power on one leg by running it on 2 legs to get 3 phase for the mill and lathe so, I figure it might work. If it is run with an outside source of power would it produce power on all legs? Would it need run caps?

    I ask because I have a couple of Model A motors that I could use as a dedicated power source for a generator if this would work. I could machine a groove in the flywheel and use it as a pulley for a belt drive and gear the motor to run at or about 1750 RPM.

  2. #2
    johnoder's Avatar
    johnoder is online now Diamond
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    I just wondered if a 3 phase motor attached to a gasoline motor would produce power?
    Yes but there is a catch.

    It has to be connected to power and rotated at slightly over synchronous speed.

    Some wind generators do this.

    Tens of thousands of makers and users of alternators would be using ordinary motors if this was an easy or effective solution


    John Oder

  3. #3
    minder is offline Stainless
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    Unfortunately you need armature excitation, an alternator uses a DC field on the armature fed by slip rings.
    The RPC gets rotor excitation from the single phase on the other two windings.
    Minder.

  4. #4
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    Default 3 phase motor used as generator

    Hi
    No it will not act as an alternator.
    To generate electricity you have to move wires through an electromagnetic field, or move the field past the wires..
    There is no such field in an ordinary three phase or single phase AC motor which is just being spun with a petrol or diesel motor.
    In a real alternator, the windings that generate the Voltages are stationary, and the magnetic field is produced by pushing DC current into a field coil on the armature .
    When this armature is spun by means of the petrol motor, the magnetic field produced by its coil cuts the stationary windings and hey presto it generates electricity.
    Because both North and South poles of the armature pass the stationary coils, an alternating current is generated.

    This is exactly how a car alternator works. The three phases produced are rectified to DC via the diode block, and the regulator switches the armature field on and off so that the output voltage never goes above 14.2 volts, so as not to boil the battery.
    The current from the car battery is fed to the armature field coil via carbon brushes on slip rings.
    Wind generators usually have permanent magnet fields so that there is no need for a supply voltage. Again the coils in which the voltage is generated is stationary, and the permanent magnet field is spun by the wind driven propeller

  5. #5
    CalG is online now Titanium
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    Davycrocket

    There is often a bit of residual magnetism present in an induction motor armature. Sometimes enough to initiate "self excitation" of the stationary windings, then the entire inductance/ reactance phenomenon takes palace, and "Viola" AC voltage and current output.

    It remains that an inductance motor is a very unreliable AC generator source!
    A reliable excitation field is needed.

    Look at Win-Power pages for good ideas! I have a nice little 5 KVA device by that manufacturer. Perfect size, to small for just about any need, It should last a lifetime ;-)

    CalG
    Last edited by CalG; 12-22-2009 at 08:05 AM. Reason: spell much?

  6. #6
    davycrocket's Avatar
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    Default 3 phase motor used as generator

    Whoops, sorry Minder
    I wasn't elaborating on your post.
    You must have put yours in whilst I was still typing mine.

    But we are both singing from the same Hymn sheet. !

  7. #7
    Jraef's Avatar
    Jraef is online now Stainless
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    There are people who have successfully used induction motors as "island mode" AC generators (island mode means not connected to the grid). To do it, you have to set up capacitors that basically boost the residual magnetism in the motor to a level that can excite the windings. There are plans and papers written on this available at various wind power sites. It's tricky though, and a lot of it depends on the residual magnetism of a particular motor, something you cannot specify at purchase. So essentially, it's a bit hit-and-miss as to whether the motor you want to use is capable of doing the job, and there is no real way to know in advance without experimenting. The other pitfall is that they are extremely sensitive to overloads and can stop generating all of a sudden if subjected to one. I tried my hand at doing one with an old 3/4HP motor and some random starting caps I had laying around. I got it to work, but I plugged in a 500W heater to test it out and it overloaded, so the voltage dropped down to 10V; basically the amount it could generate from the residual magnetism. I admit to not taking much time to make it work more reliably, I just wanted to see if it could be done. It could, but it was a lot of bother for a little bit of juice.

    There are other plans and papers I have seen on modifying AC induction motors by machining out the rotors and embedding permanent magnets, something the average homeowner is unable to do. But within this group, that might be a workable solution.

    All in all though, it's probably cheaper and easier to just buy an alternator, or better yet, buy a backup generator. Clean, quiet, reliable.

  8. #8
    James Rogers's Avatar
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    Thanks guys, I guess I'll get one of the 10,000 watt powerheads from HF and belt drive it off one of the extra motors I have.

    BTW, we are off the power grid again today. On for 24 hrs and back off.

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