3 phase generator driven by single phase motor
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    Default 3 phase generator driven by single phase motor

    Hi all,
    I've been wondering if it's possible to drive a 3-phase generator with a suitably sized single phase motor, to provide 3-phase for a shop? I'm thinking a ''motor-generator'' set up, but to get 3 phase from single phase. I remember reading the Monarch 10EE had a motor generator, where a 3 phase motor turned a DC generator that powered the DC spindle motor.
    Is it possible to buy 3-phase generators on their own, without them attached to an engine, and spin them at rated speed? I realize this would be complicated with a single phase motor, and possibly require mechanical transmission to spin the generator at correct speed to have standard line frequency out of the generator, but say you could do that. Is this done anywhere commercially? Has anyone done such a thing? I'm thinking if you have a powerful enough single phase motor, and you could spin the generator at the right speed, you'd have a much quieter 3 phase generator that you could keep inside the shop, and power moderate loads with it (say 3 hp motors?)

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    Yes its called a rotary phase converter.

    Sent from my Phone 2 using Tapatalk

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    Unless I'm mistaken, aren't generators different from a 3-phase motor used in an RPC?

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    It does work but...60% eff and 58 cps output....A lot of inrush amps... and a real big motor taking a lot of amps...not worth the money and time...Phil

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    A generator is different from a motor in that the generator has source of excitation. This is typically a small dc generator attached to the generator main shaft. The problem (among others) with this is that a driving motor will not operate at synchronous line frequency of 60 cycles/sec. Sometimes the motor is belted to the generator to correct for this or the driving motor could be a synchronous motor.Another consideration is that single phase motors are hard to find after a certain hp (7 1/2, 10, 15?). If you need more hp than available with an MG set then the driving source has to be a gas engine or similar.

    Most find that this approach is simply not practical.

    Tom

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    In the late 1960s and early 1970s I worked in a factory that produced high voltage, low current transformers. They were employed in electrostatic air cleaners, photo copiers and other applications. The company sold to manufacturers whose products were used in both 60Hz and 50Hz markets so they needed a way to test the 50Hz transformers with 50Hz input. The way that that was done was by using a standard 60Hz induction motor to drive a 50Hz generator.

    The same strategy could be used to turn a 3-phase generator at the correct speed using a single phase motor. The driving motor would need to have the horsepower rating equal to the desired horsepower rating of the 3-phase output plus a margin to account for losses in the driving motor, the generator, and the coupling mechanism (if not direct drive). I have thought about doing this occasionally but I haven't yet begun a search for a suitable 3-phase generator.

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    I have been looking at this company that does exactly what you are thinking of. They have up to 100 horsepower single phase motor that rus a suitable size 3 phase gen head. Smallest single phase motor they do is 30hp. Claims mid 90% efficient. I want to remember I was told the 50 hp genny was about $17k, I wanted a price on 30 hp but it was not an off the shelf item. Should be a little cheaper is what I was told though.

    Large Horsepower Single Phase Motors Up to 100 Horsepower!

    One reason I like these is shop work load is totally isolated from house load. At least one Phase perfect failure has ruined all electrical components in the nearby house sharing the same service, documented on here in B&A Precision thread:
    B&A Precision

    There are capacitors to help start the drive motor, I wonder if the failure of those could cause electrical damage elsewhere on the same supply line? Like the phase perfect failure...

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    Tom,
    thanks for the input. I figured that would be the case, the driving motor size on single phase seems to be the limiting factor. But If I can put something together that will start and run up to a 3hp (2.2KW) motor I would be a very happy camper. If I can find a suitable generator and figure out how to drive it at the right speed with a single phase motor (say a 5 hp motor?), I think that would be a real nice, quiet set up, and I'll have real 3 phase. NO VFD, no RPC, no ripping machine control panels apart to rig the power source (not that I'm afraid of doing that, it's just nice to have real 3 phase and hook up any old factory machine I might drag home, and use it the way it was intended, and feed the motors/controls what they were designed to eat

    Don,
    Are you saying a normal asynchronous 3-phase induction motor powered off 60 hz (running at it's expected asynchronous speed, slip accounted for) was driving a generator, slower than the speed that would give 60 hz (synchronous speed),and it gave 50 hz? If yes, was the generator putting out exactly 50 hz, or just something close to it for test purposes?

    Rob,
    Thanks for that link. Everything they've listed about their big single phase motors seems so favorable, i.e starting current, power factor etc. Unfortunately even the 30 hp unit is a bit beyond what I had in mind. It weighs 950 lb. I would require a step up transformer (not a problem really) to be able to start it on my 240V 100A single phase service
    which is the norm in the UK.
    I saw a 15 hp motor listed on there (as limited quantities) just a minutes ago, but can't find it now.

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    Quote Originally Posted by Aryan165 View Post
    Are you saying a normal asynchronous 3-phase induction motor powered off 60 hz (running at it's expected asynchronous speed [...]
    If I recall correctly, the 60Hz motor was coupled to the 50Hz generator by a belt and pulley arrangement. I suspect that the pulleys were sized to allow the generator to be driven at the correct speed to yield 50Hz output.

    The military MPE 802a gensets might be a source for a suitable 3-phase generator. These units are rated at 5KW (about 6.5 HP) and need to be driven at 1800 rpm to produce 60Hz output.

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    Just get the 3phase generator head up to speed with a pony motor and connect it to the line throuh some oven heating elements, or something more professional. Then short out the resistor. You will get 3 phase out of it good enough to run an induction motor as large as the 3 phase gemerator is, without capacitors, ut you can add them to boost teh voltage of the third leg anyways.

    Exitation value is much less critical than i thought. Like +/-20% doesnt change the power factor or current much, but i had a rotor without much saliancy. A traditional generator head may be different.

    You will need an intelegent motor controller to disconnect it in the case of a supply side brown out causing the motor to lose synchronization.

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    A prior employer had the same setup, a commercially made one, used to test electronic products for sale in the rest of the world.

    Yes, a pulley size difference to make up the speed. They may have used a synchronous motor, I do not recall.

    Other similar systems do. Later, a client had a very large setup like that to do HVAC testing for the offshore market, and had a synchronous motor larger than a 55 gallon drum. They used it to replace rented diesel gensets.

    And, I saw a setup that a fellow had in his shop which he used to test 3 phase motors. That was not 50 Hz, but 60, he modified motors for various purposes, and that was his power source, since his shop had only single phase. It was much smaller, the generator was only a 6kVA or so.

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    If you are hell bent to build one start life with a Salient Pole Synchronous motor/ Generator, with the salient poles around the rotor it will start like a standard induction motor but with less tork. When it get close to synchronous speed excite the rotor poles with DC. You will need to have starting caps to get the motor turning but once up to speed it will generate true 3 phase at 60hz for single phase. Might be in the 80% eff range but Salient Pole Synchronous motor/ generators are quite rare in small sizes. In the early days they were used as stabilizers for long 3 phase lines for voltage and power-factor....Phil

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    These were common technology used in large mainframe computer installations in the late 20th century. They were used for power isolation purposes. These used large 3 phase synchronous motors, enormous flywheels driving proper 3 phase generators will electronic voltage regulators. It works really, really well and the efficiency isn't that bad. This could also be done with single phase synchronous motors. If I had to build one today, I would use a 3 phase motor with a solid state electronic drive. The electronic drive I would acquire from a scrapped UPS. A motor generator with large flywheel is a very good, reliable solution that will provide a dedicated isolated 3 phase system that could be used to power everything in your shop, not just one machine. Cost wise it would not be practical if all components were bought new, but used stuff could make this project very practical.

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    Quote Originally Posted by Don Kinzer View Post
    If I recall correctly, the 60Hz motor was coupled to the 50Hz generator by a belt and pulley arrangement. I suspect that the pulleys were sized to allow the generator to be driven at the correct speed to yield 50Hz output.
    HGR gets these in all the time.

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    Why not get a 3-phase generator, and run it like a RPC idler ?

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    You could convert a single phase generator or a 3 phase one to a motor. You would need a means for starting, but once you got up to speed, it would lock in. The 3 phase one might be easier because you could use starting and running capacitors like an RPC.

    Bill

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    Problem running a 3 phase generator with a single phase motor is that no motor runs at a constant speed. All AC induction motors run at a synchronous speed dependent on the number of poles therefore a 4 pole motor will run at 1800RPM when not under load and will eventually lower the RPM to the name plate speed when name plate HP rating is reached. If there is a constant load then suitable speed adjustments can be made but if not similar to running a machine shop a governor of some type would be required like a VFD. If no governor is used when operating something like a lathe the spindle speed will be high before taking a cut, the cut load will now cause the generator driving motor to slow to the slip speed of the required HP load changing the generator output frequency therefore lowering the lathe motor speed. That is a normal occurrence with any AC motor running off the line where frequency don't change but adding a change in frequency as load increases will increase speed change. Probably the only way to avoid the problem is to use a driving motor much larger than the needed HP if some sort of governor is not used. As mentioned single phase motors larger than 7-1/2 or 10HP are not readily available also these sizes are quite costly. I'm sure someone somewhere is making large single phase motors but I bet the price is quite high not to mention Amp draw. A 100HP single phase motor was mentioned, I'm wondering what the 240VAC Amp. draw on that monster might be.

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    Quote Originally Posted by Froneck View Post
    A 100HP single phase motor was mentioned, I'm wondering what the 240VAC Amp. draw on that monster might be.
    The single phase motors I linked to above are 230/460v for 30& 40hp and 50 to 100hp are 460 only. 100hp has FLA of 170. Specs are only 2x load for starting. Here is spec sheet for the motors:
    100 HP Single-Phase Motor

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    Quote Originally Posted by Aryan165 View Post
    Tom,
    thanks for the input. I figured that would be the case, the driving motor size on single phase seems to be the limiting factor. But If I can put something together that will start and run up to a 3hp (2.2KW) motor I would be a very happy camper.
    You many be re-inventing the wheel. You want something that looks like utility 3 phase power, and you are willing to go to considerable lenghts to get it. To the
    point where you would install two pieces or rotating machinery (three if you count the exciter) which will require a fair bit of tuning to get it right, and some specialized
    control circuitry to run it, as you will not have this running 24-7-365 probably.

    1) apparently utility three phase power is unavailable to you in your location. A common problem for US residences.

    2) most home shops in the US, given the limitation of (1) above use a rotary phase converter which is only one piece of rotating machinery.

    For a 3 hp max motor, you proabably want to investigate an idler motor for a setup like this in the 10 hp range. Sounds daunting but be
    aware that :

    a) used three phase motors of this size are commonly available as surplus, inexpensively. (your excited three phase alternator, combined with a 5 hp single phase drive
    motor, is a really tough item to find. Inexpensively that is. Good luck if you do fine one)

    b) the power drawn by the idler, when not drivling a load motor, is much less than the nameplate rating on the idler.

    c) for an appx 10 hp idler motor you probably want to start it spinning with a smaller single phase motor before energizing it. This reduces the inrush current.

    d) a setup like this provides three phase power that is phase-locked to the utilty power, at the correct frequency, by the nature of the idler motor. Once the idler motor
    is turning at near syncronous speed, utility power is applied acoss one winding. The motor then continues to run on its own when the start motor is disconnected.
    The rotor in the idler motor is now excited and provides the rotating magnetic field. The rotating field sweeps across the other two two windings and creates
    the third phase at the unexcited terminal. The two utility wires, plus the manufactured phase from that terminal, provide a very close approximation to "real" three
    phase power.

    e) the larger the idler is, in relation to the load motor, the better it works. If the idler motor is around twice the size of the largest load motor, sometimes some
    special tuning is needed to be sure the manufactured phase is within ten percent of the utility voltage. This is done with capacitors. If the idler is three times the
    size of the load motor (10 vs 3 in your case) often this is not needed.

    There's a lot of information about this around. Sounds complicated, it's not. A 5 hp idler that runs 1 hp machines:



    How good is the power from this converter? Actual measurement, machine with load motor attached and running:



    Entire converter built for the cost of the wiring devices, and some assorted timbers and hinges. The idler motor was free.

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    Companies like Northern Tool and Equipment, Northern Tool - Quality Tools for Serious Work, sell generators for mounting to tractor PTO's. Might also find used equipment from HGR, Used Machinery & Industrial Equipment | HGR Industrial Surplus.

    Tom

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