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  1. #1
    OzHeat is offline Junior Member
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    My problem is that I want 415V 3ph power for my lathe which is a 3.75/7.5Hp constant torque 2 speed motor.

    I have tried using a 3ph motor as a rotary convertor but as I have only single phase 240V going to the motor I cannot get enough volts out without the use of a big step-up transformer.

    As I understand it I can use an 3 ph induction motor as a generator.
    My Idea was to use either a electric or petrol motor to turn the 3ph motor used as a generator.
    Would this idea work and how much drive power would be needed to drive the generator?

    Would a 3Hp 2800RPM single phase motor be enough power to drive a 10Hp 1440RPM generator if it was geared at 1:2?

    Cheers Andy.

  2. #2
    GregSY is offline Diamond
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    No - power is power, the gearing won't help a bit.

    Additionally, since the lathe motor will pull several times its full load current during the starting period, you would require a much larger generator than the motor. In other word, you might need a 40 or 45 HP generator to start the 7.5HP motor.

    Lastly, since the induction motor on the lathe has to pull its excitation from the power source, and
    the power source in this case would be an induction generator, which cannot provide reactive power, you could not even run if you somewhow did get it to start.

  3. #3
    FarmallMan is offline Senior Member
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    Andy, your best bet (if you choose to use a RPC) is to oversize the RPC a little bit, and get a step up transformer. The stepup transformer will raise the voltage from 220 to 415. You will probably need at least a 10HP RPC to overcome any losses in the transformer, and have enough gumption to fire the lathe.

    I have no experience using an induction motor as a generator. I was under the impression that this would not work. However, I may be mistaken.

    Have you considered using a VFD? I, personally, don't know that much about them, but there are a lot of intellegent guys on the board that can tell you whether it would work or not.

    Nick

  4. #4
    creamium is offline Member
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    Hey Ozheat, If your motor is star wound you can take it to a motor rewinders and have them connect it internally in to a delta configuration. It will then run on 240V but take twice the current!

  5. #5
    OzHeat is offline Junior Member
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    Thanks for the replies and suggestions.
    Please excuse me if I don't explain the process corectly or use the correct terms as I ain't a sparky or electronics tech, I'm just a dunny diver with a intrest in electronics as a hobby.

    However, I did try some experiments with a 2.2Kw 2880RPM 3ph 440V star motor and a small petrol engine.

    The engine drove the 2.2Kw motor with a 2:1 drive arrangement.
    There was a 1uf 1000v capacitor connected accross one of the legs and a variable power supply to act as a start circuit which is only used to start the generator process.

    I connected 3 100W incandesant light globes to the 3 phases.

    With the engine at idle I switched in the start circuit, I was then answered by 3 dimly lit globes. I measured 230VAC@~22Hz between the the centre tap and each leg.

    Now that I got some action, I throttled up the engine and each of the globes were brightly glowing then when I got to 230VAC@40Hz all 3 globes blew almost simutainusly.

    I'm not sure why the globes blew but that was the end of my experiment as I didn't have anymore globes left that I had 3 off of the same wattages.


    PS. To answer the suggestions that were given.
    Big step-up transformers ~ 6KV are rare and if found verrry expensive if found, so by using a 3ph 440VAC motor I do not need to use a step-up transformer.
    The use of a VFD or rotary converter both would still need a step-up transformer.
    As to rewiring the motor, it is a 2 speed constant torque motor so it isn't wired internally like a single speed motor.
    I had a quick enquirery about rewinding the motor and was given an approx price of close to 3/4 of what I paid for the whole lathe.
    Above all that my single phase power to the house would be overloaded if I try to load another 25-30A out of it.

    Cheers Andy.

  6. #6
    bnelson is offline Senior Member
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    This is a very interesting experiment.

    According to what I know of induction motors, you should NOT be able to use them as a generator of three phase by simply applying power to the shaft.

    It appears you did have three phase there.

    Did you connect the bulbs phase-to-phase, or phase-to-center? What is the rated voltage of the bulbs?

    If the bulbs are rated 240 volts and you connected them phase-to-phase, the bulbs probably saw 415 volts and promptly self destructed.

    Somebody needs to explain to me how driving an induction motor as a motor-generator can work. Everything I know about induction motors says it can't work.

  7. #7
    jim rozen is offline Diamond
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    Bnelson, what he did is commonly know about
    in certain circles. One can indeed make an
    induction motor (non-wound rotor) behave as
    a power source in certain applications.

    There was a web site kicking around (don't have
    the URL handy) that detailed a ham operator's
    work in this area. Basically he was trying
    to cook up some inexpensive field day rigs for
    folks to use, and figured out that if you put
    some capacitence on the windings of an induction
    motor, and don't load them until they are up
    to speed and putting voltage on the buss, they
    will put out electrical power once loaded.

    He had to flash one of the windings on some of
    them to get them to light off, so apparently
    residual magnetism in the motor is an issue.

    If he was using 120 volt or 240 lamps, that
    would of course explain the big flash(es). [img]smile.gif[/img]

    For the original poster: Most folks will use
    the lower voltage converter, and then use an
    open delta setup with two transformers to make
    the 415 that you need. It's only a matter of
    finding out who has surplus transformers that
    are a) nearby to you and b) inexpensive enough.

    Other thought: motor re-wind.

    Jim

  8. #8
    Mike.CO is offline Aluminum
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    Curious, what was the exciter value, vac or vdc? I've re-read this a couple of times but want to confirm, I think it's implied you disconnected the exciter and it continued to generate? I suspect this is very similar in operation to an automotive alternator, which is an AC generator with diodes, and I know it needs 12V to get started. But I can't remember what happens if you completely disconnect the battery. I do suspect the regulation is at risk and it'll for sure drop out if you load it too much without the exciter.
    I-want-to-know-more...
    Mike

  9. #9
    bnelson is offline Senior Member
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    . . . and figured out that if you put
    some capacitence on the windings of an induction
    motor, and don't load them until they are up
    to speed and putting voltage on the buss, they
    will put out electrical power once loaded.
    Jim, that might explain why large converters I build with heavy sheaves on the idler can illuminate my delta-connected pilot lamp bank (7.5 watts each bulb) for as much as two whole seconds AFTER I switch off the power. The lamps' brilliance seems related to the decelerating flywheel speed.

  10. #10
    jim rozen is offline Diamond
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    For those who are interested, here is that
    link to the guy who made generators out of
    induction motors:

    http://www.qsl.net/ns8o/Induction_Generator.html

    Enjoy!

    Jim

  11. #11
    GregSY is offline Diamond
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    I've seen that link as well Jim.

    The key to it all is the type of load you want to drive; light bulbs are essentially 100% resistive devices that have no inductance. That's the key. They don't need to be excited.

    Since most of us want to drive induction motors, I don't see an induction gen as much of a path.

    It's certainly better than nothing, but if I were going to go through the trouble (time /effort/money) of making a generator, I'd want it to be able to drive more than light bulbs.

  12. #12
    bnelson is offline Senior Member
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    Thanks, Jim for that link.

    I think I see how it works.

  13. #13
    peterh5322's Avatar
    peterh5322 is offline Diamond
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    "Induction generators" have been discussed in the alternate power forums for quite a while.

    Obviously, these folks are concerned with conservation, and are using these devices for mainly lighting loads.

    It's pretty hard to generate true "power" (meaning, the ability to deliver energy to inductive, resistive, or capacitive loads, and with essentially equal capability and efficiency) using an induction machine.

    OTOH, it is fairly easy to do so using a separately excited, wound rotor machine, which is what all electric utilities use for their generators.

  14. #14
    countryman is offline Plastic
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    I own a small (1.7 kVA) gen set that uses a generator of this type. It is also called "asynchronous generator".
    It is essential to have the right size of capacitor to create a resonant circuit for the desired frequency (50 or 60 Hz), and regulate the drive to the nominal rpm of the generator (e.g. 3000rpm for 50 Hz)
    The units that are built for generator use have some specialized attributes to obtain good results, but basically any inductive motor can be a generator!

    My 1.7 kVa unit can drive a 2.0 kw angle grinder. It will run, of course may not perform its full power. But it starts perfectly, without soft start circuitry!

    Larger generators have wound rotors and a regulator circuit. Asynchronous generators may be a cheap and maintenance free solution for small power plants (alternative energy) that run in parallel to an existing grid!
    countryman

  15. #15
    oldironhead is offline Plastic
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    I recall that some years ago, a magazine called The Mother Earth News had an article on using an induction motor in a very small scale hydroelectric plant. The motor was simply hooked into the existing utility power grid and it would turn, and draw power from the utility grid. When the motor was spun to a speed above its slip speed by being driven by the water wheel or pelton wheel, current flowed out of the motor into the power grid. Power meters were installed and verified that this worked quite well. An induction motor will readily serve as a generator when its load is predictable and constant.

  16. #16
    GregSY is offline Diamond
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    Oldironhead, that works only because the induction generator is able to take its excitation from the grid that it is connected to. Disconnect it from the grid and it falls flat. Better than nothing, but it doesn't allow the hippie living off Mother Nature to fully extend his middle finger towards The Man.

    Whether the load is predictable or constant doesn't matter to the generator, it may help the driver by making it easier to maintain something close to the desired RPM.

  17. #17
    Thermo1 is offline Senior Member
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    Induction generators have been around for a long time, and not just for "alternative energy". I am aware of one hydro plant built by the electric company that used an induction generator. Basically, any induction motor with a source of reactive power (power that is at 90 degrees to the regular power that flows through a light bulb, or resistive power) can generate electric power. This has a lot of advantages, such as no synchronizing equipment. If you have a 3 phase line to provide the reactive power, driving a big electric motor with a big enough gas engine should do the job. The power line will also keep things more or less on frequency. Without the power line, it is sometimes possible to use capacitors to provide the excitition, but the amount of capacitance will vary with the load. Getting the right amount of excititation is probably a chancy thing. I use about a 2 hp motor as a phase convertor for the 3/4 hp motor on my drill press. Everyting was wired up for 220 volts and I had 110 volts in my shop. The simple fix was to just get a surplus transformer, to boost the 110 volts to 220. That works with a 3/4 hp motor and a 20 amp wall outlet. One will have to figure the input to a phase changer for a larger motor. The Grainger catalog shows that 15kw, 480/220 volt transformer for 3 phase will cost about $1200 US. Your cost will vary, but that should give you an order of magnitude. It worked for me. If I had had to buy transformer new, I would have checked with the transformer manufacturer.

    Thermo1

  18. #18
    GregSY is offline Diamond
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    Thermo, you're missing the point; getting a source of reactive power is very difficult, especially as the size of the generator increases in size.

    You CAN get it from the grid, but understand that you'll be paying someone somewhere for that reactive power - it's not pennies. Hydro projects are a little off-beat as their power source could be considered 'free' - but in the end if you are selling power you want to get as much as you can from your equipment and using induction generators will cost you due to lower output.

    Why would you want to use an induction generator when you can use a synchronous generator that has better output and higher efficiency? Modern excitation/synchronization controls are extremely reliable.

    Capacitors are no practical solution; they are expensive, take up space and cabling, have a finite life, and are not easily tuned to the load demands. Plus, they, too have to get their power from somewhere.

    Driving a big motor with a big gas engine? Very inefficient, high dollars per KW cost, maybe the worst of all.

    The bottom line in power production is, as usual, dollars. Use an induction generator and at the end of a year you'd fall over dead from shock in seeing the difference in cost.

  19. #19
    Thermo1 is offline Senior Member
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    I agree, bottom line in power production is almost always dollars, but I may not have stated it too clearly. The way I do economics, the bottom line is the sum of capital costs, plus the present worth of the revenue stream, less the present worth of the operating costs. Case in point, that little hydro plant, which was originally built with an induction generator because it was simple and cheap. They got about 50 years out of it, but the people who proposed to rebuild it wanted to build the turbine out of stainless, and install a variable frequency drive to synchronize it to the power line. Needless to say, there was nowhere near enough revenue to pay off the cost.

    With regard to OzHeat's problem, how does he run a 7.5 hp 3 phase, 415 volt motor off 240 volts, I suggest he get about a 15 hp motor to use as a phase convertor, and run it off the power line. If he gets a 415 volt motor, he will need a transformer from 240 to 415 volts. If he gets a 240 volt motor, he will have to get 2 transformers from 240 to 415 volts, each at 86.6% capacity (open delta), or each of 3 @ 1/3 of the total motor rating. The voltage out of an induction phase convertor is a bit unbalanced, and I would not be one to tempt fate by using the unbalanced delta connection.

    I have spent some time trying to try to quantify how big an induction phase changer has to be but cannot seem to find anything in my references, but will keep looking, as I have seen something in the past.

    Both the phase changer and the drive motor for my drill press are NEMA motors. The drive motor is 3/4 hp, and the phase changer is either 1 hp or 2 (I bought the Delta 17" drill press secondhand). I am running it off a 1 kw transformer, which has to introduce some reactance. Still, it works. As will all phase changers, you have to kick start it with your foot to get it started, and the phase changer slows down when the drill press motor starts, but it works with a fairly small phase convertor.

    Based on that one piece of experience, OzHeat will need a motor at least twice the rating of the motor he is trying to start, and needs transformer rating larger than the nameplate rating of the phase convertor. These are approximate ratings. Your actual required ratings will vary. Make sure you have enough power coming into your house to start the big motor. Some people put a small "pony motor" in to start the big motor. You will have to design that.

    OzHeat will have to work out the economics. Not to prejudge, but it seems to me if one can get secondhand motors and transformers, the cost of getting power to the lathe will not be unreasonable. The lights in his house will dim when this thing starts.

    Two practical notes: if he gets a 15 hp motor, for example, it will weigh about 200 pounds. If he has a choice of speeds, get 2880 rpm, as the higher speed motors have more stored energy than the lower speed motors, and from my experience, some energy is coming out of the phase convertor when the main motor starts.

    thermo1

  20. #20
    Thermo1 is offline Senior Member
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    While this article may have been referrenced in another post, I am including it here to give OzHeat a bit more information: Home Shop Machinist, November/December 2001, page 26, induction motors and rotary phase converters, Bob Swinney.

    In researching this, I find phase convertors have been around for quite some time, and were used to drive some large electric locomotives in the 1920s.

    thermo1

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