DFIG with 3ph rotor and 1ph stator

Discussion in 'General Electronics Chat' started by wvuengr04, May 28, 2015.

  1. wvuengr04

    Thread Starter New Member

    May 28, 2015
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    Can an induction machine be designed with a single phase stator and a wound 3phase rotor?

    I have been interested in small/home made wind power for many years. There are lots of folks building fairly powerful turbines from scratch. (hand carved wooden blades, axial flux permanent magnet generators). But, you need a pricey inverter to take that DC and put it back on the grid. There is at least one company that will sell you parts for a system that uses a standard induction motor for a generator and a very simple controller w/ contactor to tie it to the grid. There are lots of companies selling expensive versions of the permanent magnet variety. A few selling non DIY induction motor turbines with gear boxes etc. I think that an induction generator with a wound rotor that is controlled by an off the shelf frequency drive would be a useful addition to the above arena. The goal would be a system that could be connected to a standard house service, 220v single phase. Only one company that I could find makes a single phase output drive. It isn't large enough for my system goal of 6-10kw. There are however, many vfd's that accept single phase power and produce 3 phase power.

    I am currently imagining that the above is possible if the right ratio of poles are selected between the rotor and stator.
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    why not use a tthree phase motor for an induction machine? the "breezy 5.0" uses one and its a simple system for generating grid tie power, and can be used to generate on its own.
     
  3. wvuengr04

    Thread Starter New Member

    May 28, 2015
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    Thanks for your reply. The Breezy family is what I was referring to in my original post. It is a remarkable design in many ways that I admire, but I don't like several things about it. It is my impression that the breezy turbines are limited by the speed range of the conventional induction motor. In addition, the gearbox adds weight and some losses, not that a lot of the megawatt class turbines don't do exactly the same thing, though most use an inverter or a DFIG in conjunction with inverter(s). I am interested in a direct drive option. I read an IEEE paper years ago about using a coreless stator on a permanent magnet wind generator. The radius at the air gap was a proportion of the blade radius. (something like 1/3). The magnets had to move past the coils at a higher rate to make the thing efficient. So I guess I am also wondering if coreless is an option here as well. Thought I might find some EEs that would be interested/able to help. There seem to be many folks willing to tackle building PM generator from fridge magnets and an old axle, imagine what they could do with something they could build and put on the grid without the grid tie inverter.
     
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
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    You could go DIY grid tie. Building your own grid tie unit is surprisingly easy once you know the little tricks behind how to synchronize things.

    Take your pick on designs. http://www.electro-tech-online.com/forums/alternative-energy/

    As for direct grid tying a larger three phase induction motor to a single phase system that again is not much a of a problem either.
    Start here and just use the formulas and circuits (just skip the starting circuit parts) in reverse to take three phase back to single phase.
    http://www.electro-tech-online.com/threads/3-phase-converter-schematic-miller-system.100563/

    Now if you want to really get wild you could use a three phase induction motor with a three phase rotor that is powered by a VFD unit and make yourself a generator system capable of sub synchronous to super synchronous generator speed operation while still having the stator direct tied to the line like the big boys do it. :cool:

    As someone who has played with wind power for over 20 years I have yet to ever follow the 'efficiency' mentality of home built wind power being if I want 1000 watts of output and my gearbox uses 15% at worst case then I just size my blades to put out at least 1150 watts instead of 1000.
     
  5. wvuengr04

    Thread Starter New Member

    May 28, 2015
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    Well, this is really what I am interested in learning about. If you sacrifice the sub synchronous quadrant, you can use a standard VFD and a contactor on the stator and make the local utility happy with your grid tie setup. (I have no personal experience with grid tie, but I assume that if you put a 50 foot tower in your back yard with a turbine on it, someone eventually shows up to ask why your electric bill is so small?)
    So if I take the standard high level explanation of a wound rotor generator, I need 133% of net rating for the stator, so if my goal is 10kw, I need an 18hp stator and 180uF of capacitance on each side if I want to use a 3phase motor to feed my single phase 220v service.
    The question is, which is better, a single phase stator with a 3phase rotor or the capacitance setup above on a wound rotor three phase generator?
    Would I end up with a power factor problem?
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    Yes, the speed range is limitted by the rpm of a conventional induction motor, thats so you dont have to have an expensive and complicated speed control system. how would you get the magnet type to synch with the power line? and a lot of portable "generatora' on the market are induction motor based. by basing the breezy on comercial off the shelf gearboxes and motors, you dont have to worry about magnets flying off and such at high speed. the blades dont turn as fast as some of the dc generators, but then they dont have to be ballanced as well either. the blade design is that the blades will put out maximum power untill the wind is too high, then go into braking and stop. the blades are sized so that the power generated by the blades equals the power the motor is rated for, that does quite a bit to limit overspeed. I dont make or sell them, but I helped my friend design some of it.
     
  7. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I am no expert on the induction motor based wind generator designs being my grid tie inverter units are fairly simple to build and allow for such a wide range of DC input which give them the ability to backfeed power pretty much any time the generator is spinning.

    I do know the that the using two phase balancing capacitors on a three phase motor to back feed it into the grid does work pretty well and does allow the motor to feed back it's full HP rating if the capacitors are sized right. I have a small diesel engine that I set up to burn used oil and that is connected to a normal three phase motor with a set of balancing capacitors and that has no problems with running at its full rated HP doing power back feeding for hours at a time.

    As far as your electric bill going down I have doubts that your utility will care all that much. Mine doesn't and in fact if I were to start back feeding enough power they will install a bidirectional digital meter and pay me for my excess.
    As far as I know there are no laws saying you can't produce your own power and use it for your own personal use rather than what comes from the utility. The only thing they care about is if you are backfeeding that you have a safe way to shut down your back feed systems if their power feed goes off for any reason.
     
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