Thinking about a direct grid-tie induction generator wind turbine...

[The Crazy Noob]

Hi,



First of all: sorry about the long post :-)


I've been thinking about a direct grid tie wind tubine. I know that it's possible (that's the way the big comanies do it) and it was mentioned a few times here. There weren't a lot of people making them here however (none that I know of).

The reason why I am interested in that type of system is becouse, to me, it seems to have a higher efficienty/costs-ratio than a normal (axial) wind turbine with a battery bank and a grid-tie inverter. I also do not like batteries. They are expensive, have a limited life-span and add up to the losses.



The pros and cons as I can think of at the moment when looking at the losses:
Traditional: Wind to electricity - power line loss - battery storage (convertion to chemical energy) - inverter loss.

Induction gen: Wind to electricity (gear losses: low RPM windmil to higher RPM generator) - low powerline loss (HV) - no battery storage: direct to grid.



About the "Wind to electricity" step: I have been thinking about this one because, if you let the induction motor run using mains power, it already uses a certain amount of power. (I have a 200W single phase induction motor (with capacitor) laying here so I will continue with that example) Turning the rotor uses about 100W without load. This would mean that the wind turbine must put 100W in the motor to cancel out the useage from the grid (0W used form grid) Is this power wasted? Because if you have a windmill that is supplying 250W, than 100W will go to canceling out the usage form grid and the remaining 150W will back-feed.

If the above is true, then I might need to re-consider the high efficienty that I tought an induction generator system would have.

Also: will it work with the capacitor or do I need a three-phase motor?



I am also aware of the fact that you would need a switching circuit that monitors the winspeed (availible power in wind) and when the speed of the turbine is higher than that of the induction generator, it connects it to the grid. Another one of my worries here is that this will couse an sudden slowing down of the turbine (the RPM is limited to the frequenty (here 50Hz) divided by the number of poles (in my case 2 I think) wich gives 1500RPM + the slip from the turbine (maybe 5 percent?)wich gives 1575RPM maximum.) The speed will not go any faster. This may lead to stresses on the blades when being slowed down and speeded up frequently. This can however also work in our advantage as to protecting the mill from overspeeding.

A furling system would still be needed because if you get to the maximum rated output of your induction motor, you will need to furl away as to not burn up you motor.



There would also need to be some type of gearing to increase the speed of the shaft that is coming from the blades to get in the proximity of the RPM of the induction motor.



As I said before I have a 200W induction motor that I can play with. I would like to build a wind turbine using that motor for direct grid tie. I've made this thread to see if you guys can give any more advice or tell me if this is a downright bad idea. In case I go on with the project, I will definately post it here but it will be a slooow project :-).

Oh, I also have a 1.1kW (around 3/2HP) three phase induction motor but this seems a litle heavy to play with as a first turbine. I might use this one for a normal convertion using neo-magnets.




Thanks,



The Crazy Noob.

[dinges]

A small remark: the required RPMs for an induction motor to work as a generator. You use 1500RPM (1575RPM). This is correct for a 4-pole motor (at 50Hz; if you're in the USA, your RPM would be about 1700RPM). If you could find a 6 pole, it would be about 1000RPM (1050RPM with slip). This already sounds much more reasonable. An 8-pole? 750-800 RPM.

It's just that all the induction motors I've seen (I haven't seen that many, BTW) are 4-pole. If you want to go the induction route, I think you'd better try to find an induction motor (or even wind one yourself? The rotor cage wouldn't have to be changed) with about 8-10 poles, perhaps even more, so no or just a little gearing up would be required.

Just a thought that came to mind.

Peter.

[The Crazy Noob]

Yes, I've thougt of that too, but it appears as though they are unfindable here in Belgium for a reasenable price.

About the rewinding: are you shure that you don't need to change the cage??

By the way: sorry for the long subject, if you want to reply: just shorten it...

[The Crazy Noob]

Hmm, could I rewind my 1.1kW three phase four pole induction motor into a one phase 12 pole motor? The speed would be 250RPM which seems nice.

But how about the voltage, the thickness of wire and the amount of turns? Also I have no idea as to how i'm going to get a nice coil in between the laminats.

[Stonebrain]

One thing you didn't mention:

Your generator will have to run at nearly constant

rpm.The "slip" will allow only a minor variation of

the rpm.

This is a major problem for the prop,it will be effective

for only a small range of windspeed.It will stall in higher winds.

To make this design have a better poweryield you need something

like variable pitch.

cheers,stonebrain

[drdongle]

I suspect that your local power utility will have a cow when you try to hook it up. Make sure that you have their approval and use equipment that is accepted for grid tie.

Personally I think it a waste of time as they usually buy your power at a rate that is

good for them and bad for you. Bite the bullet, get batteries cut the strings and get off the grid.

[wind4Reg]

Here is a link to a site that has done exactly what you are looking to do:

Breezy 5.5KW Home Built Grid Tie Wind Generator

 

[Nando]

There are several ways to connect an induction (generator) motor to the GRID.

The motor can be a single phase or 3 phases .

There is one site selling the idea of using a 12 or so KVA induction motor with a multiplier gear and large 4 blades Mill.

The smaller and simpler systems would be a small induction motor, single phase, with the phase capacitor removed (nay not) and driven by a power source that has variable power capabilities.

The system "loop" = the wind mill + 3 phase generator + DC motor + Induction motor + GRID

There is a controller for the DC motor and a ballast for the generator.

Efficiency

induction motor = 0.75 to 0.8

DC motor = 0.8 to 0.9

Generator = 0.7 to 0.8

Wind mill ?

Overall efficiency = about 40 % ; so por 1 KW harvested around 0.4 KW into the GRID.

The DC motor is used to maintain the induction motor GRID locked for much longer times during highly variable wind times .

Controller may need to detect GRID voltage and phase, also under certain conditions it may be used to do Induction Motor start kicks to easy up the whole running procedure for less initial power starting energy expenses.

###############

A second one is a = Wind mill + 3 phase Induction-Generator + single phase GRID + (in parallel to the 3 phase) a ballast load and a power supply converter DC/ AC to add to the induction motor from the 2 non-GRID tied phases.

A controller to set the running set up and to vary the DC/AC converter as required by the GRID and the wind conditions.

Over all efficiency around 60 to 70 %

###########################

The http://www.prairieturbines.com/ BREEZY 5.5 , though I would make quite different to attain higher GRID operating times

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

The IDEAL ONE WOULD BE THE SECOND system( either single or 3 phase) USING A Pitch controlled wind mill with convoluted blades for closer RPM setting maintaining the GRID locking time longer and with current feed back to the pitch control arrangement.

Single phase would be much cheaper.

Enough for now

Nando

[The Crazy Noob]

How big would the problem be of the blades being slowed down a lot in high winds (blades can't go any faster)?


I've also got a 50W induction motor, single phase with a capacitor but it has a 24-slot stator, I can make that 12-pole single phase so it's 250RPM. It won't run as a motor because there's no phase shift, but if you turn it and then apply power it should keep turnig and if you try to turn it faster than the 250RPM you are backfeeding into grid. Could annyone give some more info on rewireing induction motors?


The thing I don't like about the above post is the fact that you still need batteries for that more simple system (I've already made a thread about that here.)


Bottom line: would the stalling of the blades (withouth variable pitch) be such a big problem to sink the idea to make a windmill like that? Would a simple passive pitch system work? (I guess not as that only works with the force that pulls outhworths when spinning and the blades can't spin any faster so the force stays the same (I think the force is called gyroscopic force in English).)

[Warrior]

It can be done, read here:

http://groups.yahoo.com/group/awea-wind-home/message/4146

http://groups.yahoo.com/group/awea-wind-home/message/4149

Regards,

Warrior

[Flux]

The big problem is that to connect to the grid you will need approval that may cause lots of problems.

Yes it can be done and done effectively but it is not even worth trying at the toy level.

Single phase will present you with considerable problems and to try to do it with a motor rated at a few hundred watts is pointless. The efficiency will be very poor with low power, with single phase it will be far worse. With a stall limited control it is virtually useless to try it with an induction generator than can't hold you into stall at fairly high wind speed. 200W would not hold back even a 6 ft prop in a modest wind.

The speed issue is a nuisance, I wouldn't go much beyond 6 poles or 8 at the very maximum. Efficiency will likely be best with 4 or 6 pole and it will need a speed increase.

I don't see the worry about the blades being held down to constant speed, why should that worry them. The blade efficiency will be poor as you drag them down into stall but that will make their life easier.

Control over the working range is simple, but you will need some form of furling or other control to deal with higher winds, you will never hold it in a gale reliably without some form of power limiting.

Perfectly easy and practical if you can overcome the issue of being allowed to do it.

Flux

[dinges]

Agree with you on most points. It's nice to think about the possibilities, but there's too many practical problems to be solved. Apart from legal things.

But what do you mean with the remark about efficiency of 4-6 pole motors being the best? Is the efficiency of a motor, or an induction generator, related to the number of poles?

Peter.

[Flux]

Perhaps I should have stressed the bit about not going beyond 8 poles. In theory you could rewind for 12 poles but it would be virtually useless as a generator.

Don't start messing about changing the poles of a motor unless you really know what you are doing.

Choose a 4 or 6 pole, use it as it is and accept that you need speed increase.

Stall of the blades does restrict your efficiency, but if you choose a tsr that is rather high at cut in, it will be fairly well matched in the common winds and the stall will help you in high winds so you can leave furling to higher winds than would be the case without the help of stall.

There is not a major difference in energy capture between a stall regulated machine and one let run at constant tsr. I have seen figures of 12% for a typical wind site.Your prop matching will be poor but you are not compromising the alternator efficiency.

Flux

[jmk]

  The utility co is only intrested in the fact of your controls. If you use controls that are being used already they are familiar with the enginering. Basicly you need to have a comercial inverter ready for grid tie. Then you have to please the township with a permit. Right now theses are all pencil whiped. they don't know enough to inspect it! The dump load is what needs to be of concern. Many of the larger mills don't or do not discusss how they safly discharge the power! If you have all this power there has to be a way to neutrolize it! It's all about fire prevention. This is what I'm figureing out right now. I am actualy thinking about getting a job as a re inspector?      

[Murlin]

I would think, most power companies would frown on the aspect of wind turbine grid I would think.   Unless they know they won't be ripped off.

People with...lets say..... of less than honorable character, could rip off the power company very easy( with a couple 3-phase motors and some.....) well....they would be powerless to really stop it...

The reason they like solar is that it can be controlled.

Murlin

[Flux]

Induction motors work best with low pole numbers. Even at 8 pole the things are poor motors with poor power factor and efficiency. It is possible to make motors with large numbers of poles, but they are large and low performance. They work even worse as induction generators.

Fortunately there are geared motor units available for low speed output and these are possibly the best option to use for wind power.

Flux

[The Crazy Noob]

Just a short thing about the legal-issues: in Belgium, it is allowed to connect an asyncronous generator (induction motor) to the grid and to start backfeeding.



For the people in Belgium: go to www.vreg.be and send them an email, they will send you a .pdf with the regulations and on page 231 and after it's about induction generators (you can go up to 15MW as I understand it).

[In Dutch:]
Voor de Belgen: je kan gaan naar de site van vreg (www.vreg.be) en een email sturen met je plan. Ze sturen je dan een .pdf-bestand met de wetten in. Vanaf pagina 231 staat het van windmolens en inductiemotoren. Je kan tot 15MW gaan dacht ik.

[Flux]

If you don't have a legal problem then your biggest problem is solved.

If they allow you to reverse the meter then you are on a winner. If they insist on different meter rates for load and generating you may not find much gain.

Flux

[thefinis]

Very nicely done but wish I knew more about what his turbine was. I had considered using a centrifugal switch to engage line power but was worried about control around cut-in speeds due to it wanting to oscillate.

Finis

[thefinis]

Dang so far down it is hard to know what I was talking about. Here is the links Warrior posted that I was refering to

http://groups.yahoo.com/group/awea-wind-home/message/4146

http://groups.yahoo.com/group/awea-wind-home/message/4149

[dinges]

I Like the Breezy link. Interesting to read. Apparently it can be done.

But what would you build/design differently in that Breezy design?

Definitely would like to read/learn more about DIY induction motor generators (no conversion). If anyone else has a link or information, I would be interested.

One thing on my mind: if the machine is generating at 1575 RPM, whether RPMs will increase as wind increases, or whether the low impedance of the grid means that as RPM of prop increases, so does slip and thus more power produced; the extra power production loads the prop more, so the RPM stays constant at 1575 RPM. Is this correct or not?

I.e., is there no problem with RPMs after a certain lower threshold (minimum wind, needed to start operating at 1575 RPM) has been met?

Peter.

[The Crazy Noob]

Ok, so I'm going to use the 1.1kW three phase induction motor (all three phases). About the blades: What would be a good size and profile for blades that have to work at a constant speed? They should start spinning to X rpm and then there will be a load on them. If the wind increases they should be able te keep producing power at the same soeed and if the wind is too hard, they should stall (when there is 1.1kW of availible power in the wind).

The blades of the Breezy 5.5 turbine look like they don't have much airfoil/twist/taper, is this becaus they need to stall at high power winds?

[thefinis]

I knew I had this bookmarked somewhere. This is a little article and program that was posted sometime back. It is a pretty fun read and has a program to design a constant rpm blade. I don't know if it's results are right but you might give it a try and see what happens. I ran it and it takes a long time to draw a blade and has lots of options you want to play with but each change you make it has to run through the design phase again. Caution it says program doesn't work on XP/2000

http://supercoolprops.com/articles/wind_turbines.php

The biggest problem I see with a grid connected induction generator is the small range of wind velocity that will match a fixed blade, fixed rpm, direct drive machine. Too little wind and not enough rpms too much wind and stall sets in due to loss of tsr for blade design. Most of the ways to get around this cause a loss of efficiency but as many here have said just build it a little bigger and basicly get the same output.

Finis

[vawtman]

Well put Finis

I agree and often thought about the Double fed induction gen.But with homebrew fixed blades It seems like when things got up to speed without some sort of pitch change and clean air things would stall and the process would need to be restarted.I think these are for the big guys with government help.Those machines are huge.

[Flux]

You will most likely have to experiment with blade size and cut in speed.

I suspect there is not much experience of induction generators here, I haven't done such a thing, but I may be able to suggest a few things to think about.

For 1.1kW I think you would be looking at about a 12 ft rotor. Don't attempt cut in below 10 mph and you may do better to keep it nearer 12 mph.

Remember that you are not going to entirely stall control it under all conditions.

Depending on the motor characteristic, it will pull out at 2 or 3 times full load torque and when it does pull out you virtually loose all load and all control. There is no equivalent to the brake switch, short it out and nothing will happen.

A furling system will prevent you pulling the motor out, but still think how you are going to stop it if you have a supply failure.

For blades I would use something similar to DanB's blades with a tsr of about 6.

For 12 ft aim to cut in a bit over 200 rpm, perhaps up to 250. Don't try slower.

That means a gear ratio to give you 200 rpm for the synchronus speed of the motor.

There has been a lot of blade research done for this type of application but mostly on defined stall rather than performance. With furling the defined stall is not to your advantage.

The blade experts will no doubt tell you to go for a big twist and lots of chord. I think you will do better with something with little twist or taper hence the suggestion that you copy DanB.

You will need a speed sensitive switch to connect it to line at synchronus speed and a reverse power relay to trip it off line when the wind drops. The control should be very simple.

Don't at this stage attempt any form of power factor correction, it will land you in real trouble unless you really know what you are doing.

Flux

[hvirtane]

I think that your idea is in principle a very good one.

In the longer run it might help also many others trying to do something similar.

There is some very good information available about induction generators at 'Dr. Gary Johnson's Renewable Energy and Tesla Coil Page':

http://www.eece.ksu.edu/~gjohnson/

Please see especially:

wind7.pdf Asynchronous Loads

I think that it would be a good idea to use

a bit oversize blades with such an airfoil

that the stall control system would work

well with the blades...

I could think about a couple of quite contradictory ideas to get this to be done.

The very well-known Danish expert of small wind turbines Claus Nybroe

is using many thin bladesand a slowly rotating wind rotor.

http://www.windmission.dk/

His machines are normally not stall controlled, but I could imagine them to work well with such systems.

Wind rotors with many blades will have less variation of rotation speeds than fast rotating wind rotors with only a few blades? Then even if the rotor would go past the stalling speed it wouldn't easily break with too fast speeds?  

There is an Australian company 'Flowtrack', which is using a fast rotating two blade rotor. They are telling that the stall control system is working well with their fast rotating rotor.

http://www.flowtrack.com.au/

I think that you'll need some kind of furling method as well. If a strong wind will push your machine past the speeds, where the induction generator will work at all the wind rotor would brake easily without any furling system available.

- Hannu

[powerbuoy]

Flux:

I was wondering if you could elaborate on your statement that an induction motor with more then 8 poles is not suited for this application. What factors do create a problem here?

Powerbuoy

[hvirtane]

an induction motor with more then 8 poles

is not suited for this application.

What factors do create a problem here?

I'm not pretending to be Flux, but

the thing is that it is really difficult

to get induction machines with more

than 8 poles to work well as generators.

- Hannu

[powerbuoy]

Hannu:

I was wondering if there is a technical reason behind Fluxes opinion ...,  or was he more concerned about the fact that there are not many 12 pole machines available in the market? If his reasoning was not technical, maybe its worth trying to rewind a stator?

Powerbuoy

[hvirtane]

If his reasoning was not technical,

maybe its worth trying to rewind a stator?

His reasoning is technical.

As said, 12 pole machines work badly

as generators.

Maybe somebody could solve the

problem, but it is certainly

not easy. People have made

induction engines already

many tens of years. There is

really a lot of experience

behind them, but still

12 pole machines work badly

as generators.

- Hannu

[powerbuoy]

Nando:

You wrote:

A second one is a = Wind mill + 3 phase Induction-Generator + single phase GRID + (in parallel to the 3 phase) a ballast load and a power supply converter DC/ AC to add to the induction motor from the 2 non-GRID tied phases.

A controller to set the running set up and to vary the DC/AC converter as required by the GRID and the wind conditions.

Over all efficiency around 60 to 70 %

My comments: In theory it is possible to drive a three phase motor on a single phase grid by using capacitors. Would overspeeding (as done with a windmill) push current back into the grid with this arrangement? I thought it would.

The setup you mention above seems to be different ... could you elaborate on the setup you describe and how that would work?

Powerbuoy

[The Crazy Noob]

I was thinking about the multi-bladed prop too. The big advantage of this is that the blades will virtually never fly off if they are attached good. A three bladed prop reaches such high speeds that flying blades are an accident waiting to happen.

I know that a lot of people would not agree on the multi-bladed prop idea but scince I will have to gear it up to reach the high speed, I might as well gear it up to work with very slow speed. Another thing is that props with a lot of blades are actualy MORE efficient than a three-bladed one (see Hugh Piggott's words @ http://users.aber.ac.uk/iri/WIND/TECH/WPcourse/page9.html - the 12-blade design).


This higher amount of gearing will however bring higher losses. Within two weeks I might get my hands on a 6 pole motor or a motor with more power. The 6-pole motor would decrease the gearing problem with 33 percent :-)