Grid Tied Induction Gernerator Sizing

[RogerS]

    I have read often in this forum of the importance of balancing the size and design of the axial flux machine to blade size and profile.  With a grid-tied, constant speed induction generator, the balancing act of achieving stall regulation seems a little difficult to me, part science, part art.

    In an earlier post I asked about coupling two smaller induction generators together and just energizing one of them in lower winds, then switching on both as the winds increased.  My objective was to allow for good cut in performance, and also to make sure that there was enough total generator capacity available to hold the blades in stall as the wind increased, while not burning out the generator.

    After re-reviewing Nigel Smith's book Motors as Generators and some Dept. of Energy papers on induction motor sizing ( http:www1.eere.energy.gov/industry/bestpractices/pdfs/mc-2463.pdf ) it seems that watts of rated output is pretty much linearly proportional to % of slip down to about 25% of total rated power.  Go lower than 25% and larger motors tend to run at slightly higher efficiencies than smaller motors.

   My question then is, would there be much loss in kick in speed and overall efficiency if I just used one larger motor, say 15HP (llK) and  use a turbine diameter designed to give 6K watts total output?  I have been using Paul Gipe's  standard power rating numbers and using 30% overall efficiency value to help calculate power output at various wind speeds, which is pretty optimistic.  

   Basically, I would like to cut in at 400 to 600 watts in an 11 or 12 mph wind. I would be turning on the 15HP generator at about 8% of rated load, the 10HP @ 12% and the 5HP @ 19%.

   Let me add that while I understand that switching between a small 6 pole and a larger 4 pole generator would make more effective use ot the lower TSR of the 6 pole in low winds,  I feel that doing so would require using at least a PLC for control which is something I don't want to do.

    Thanks to all,

    RogerS

 

[oztules]

Sorry Roger,

This is not what you asked for, but I think the best thing you can do if you wish to ply this route, is to follow the works of others... rather than reinventing the wheel.

These things are pretty big, there is no little one worth building (due to gearbox losses, you need big just to get going)..... with this in mind, any design shortfalls will be expensive boo boos.... in material and time and probably money as well.

When I considered this style of mill, I bought the book from Breezy. They have made all the mistakes, have a shed full of failed blades to prove it, and solved enough of the problems to come up with a practical useful unit.

They are apparently happy to help with your project, and the book, and their controller will be the cheapest part of your project. (the book is detailed enough to not require much help I should think)

They seem to have happy enough customers, and are the only people on the planet I have found that persue this style of machine, and are happy to share it.... I don't know if they have the best design out there, but they're unit works... and that's a good place to start.

As you are aware, I also have first hand experience with bigger (50kw and up) units, and can verify that they are simple. But so is the dual axial flux, and motor conversions...and it seems a common thing to mess these up... even with all the information available on Hughs site and this one.

It is matching things up that is important, and building it strong where it needs it to be, and as the Dans have done for us..... solve all the little things you don't think about till they fail.

So, I know it is not what you wanted to hear, but it is the best thing I can think to offer you.

Best of luck with the project, mine will be starting when I finally get hold of a motor gearbox that will fulfill requirements..... Their blades are the key I think, as they solve several problems with a "fixed speed" head to a variable wind.... all the rest is open for modification.

A few analogue inputs and a few op amps will solve the control problems, but their pic solution is more elegant... and tested, which when you have that sort of mass up in the air is comforting.... I guess.

Remember to design it to go up pretty high to make it worth while... these ain't toys.

.........oztules

[RogerS]

     Thanks, for the reply.  I like the looks of the breezy and have seen 2 up and running.  The gear box I came up with is 15:1 and rated at 40HP input.   This should give at least a 3 or 4 to 1 service factor. It does turn pretty easy for its size.  I can just turn the high speed shaft by hand.  Needless to say it is pretty heavy, which did up the cost of tower materials.

    I have an old article from windmission regarding the bonus wind turbine to which I can't find the link. It mentions the blades having a short "stall strip".  I think this must be similar to the flattened leading edge on the breezy's blade profile.  I have a number motors available and think I am leaning towards using the larger one and accepting some loss of output in lower winds to gain more hold back power.    

    Thanks again for the input,  one person working alone can come up with the wrong solutions sometimes.  This forum can be a great place to learn.

    RogerS

[trailb4u]

Hey RogerS, I think using two motors is an excellent idea, but only if

the extra motor was uncoupled.  This would of course require a clutch

to engage/disengage the extra motor.  Running a large turbine on a small motor

would greatly reduce the drag associated with the extra mass, bearings and

cooling fan,etc.  What type of gearbox is it you found?

Does anyone have any suggestions as to what existing clutches might be

suitable for this application?

Clint

[RogerS]

Hi,  I really don't think I want to try to use a clutch.  I'm afraid  most types would be prone to failing when you needed it most.  Also they would have to draw power to release.  Most electric types would draw off quite a bit.  (one thing to note is that the Nord Brake  that the Breezy 5.5 uses draws less power to stay released than a lot of other brands).

The gear reducer is a Sumitomo cycloidal drive.  I recently saw that Enertech is introducing a new 5KW turbine (the E6) that looks to use this same type of drive.

Thanks for the input,

RogerS