AC motor as a generator

[DannySmith]

Hello, I have been thinking about building a wind turbine for a while but have never got around to it. Anyway recently I have been thinking about giving it a go. I have an idea that might work ok, but I would be interested to hear if anyone has tried it.

My plan is to build a turbine around the 12ft span and fit it with a 10:1 (ish) gearbox, electronic clutch and 2-3 BHP AC motor. My idea is when there is enough wind to turn the turbine over 150RPM, I will start the electric motor and lock the clutch. The turbine will then overdrive the motor creating electricity(50Hz). Because it will be connected to the grind, the turbine will then be stuck at this RPM regardless of the wind speed. Greater wind speed will simply generate more electricity, the motor will not speed up as it is locked on the grid. The main information I am missing is just how good an AC induction motor is as a generator. I am also aware that when releasing the clutch the tubine speed will be slowed down quite rapidly to the generators speed, this maybe an issue.

I intend to measure the turbine RPM and control the motor connection to the mains using a PIC setup, If in the even the turbine begins to exceed the speed it should run at, I will release the connection to the grid, as this must mean the grid connection is broken and I dont want to toast any workers trying to fix the cable and blow everything up in my house with the increasing frequency. In the invent I start to generator more power than which I am using I dont know what will happen to my electric meter, whether it will stop, blow up or start going the wrong way. Im abit unsure on that one.

Any comments would be welcome.

Cheers

Danny

[Flux]

The basic idea is sound but success or failure depends on the details.

I am not sure where you intend to put the clutch, to be any use it needs to be between the prop and gearbox input but it will need to be a monster. If the prop can start against the gearbox there is no reason why a couple of extra bearing drags are going to stop it.

You can sense speed to decide when to connect it to the line, you don't need clutches for that.

You can hold it in normal winds, can you hold it in a gale? What do you do if the grid fails, it will come off load and unless you have a back up means of limiting speed or stopping it it will go crazy.

What happens to the meter depends on your meter, some run backwards some don't. What the supplier will let you do is a different matter. If you generate less than you are using then it acts as an energy saver, if you generate excess then you have to argue that with the utility and they will want a million things that will make it not attractive.

Simple in theory, can be done effectively but a runaway 12ft turbine with no means of stopping is fine as long as I am a mile away.

Flux

[Flux]

I didn't answer your question about how good an induction motor is as a generator.

A 4 pole motor as you are proposing if 3 phase will be quite efficient as an alternator but dragging your prop down to a fixed speed will result in a falling overall efficiency with load. If you are lucky this may prevent your generator pulling out and over speeding but I wouldn't guarantee that a 3hp motor will hold a 12 ft prop in a gale.

If used on single phase the efficiency may not be so good and the thing will be much more inclined to pull out. In either case the power factor will not be good but as you don't pay for that it may not worry you. Power factor correction is best avoided as it will result in self excitation if the line fails.

Flux

[DannySmith]

I am thinking about a clutch because when I decide to connect the generator to the grid if for example the AC motor is doing 1700 RPM, the moment it is connected to the grid it is going to want to do 1540RPM in phase with the grid. I was thinking this maybe an issue as the turbine will slow down very rapidly. I thought it would be safer to start the motor so that its in phase with the grid and then connect the prop. A 1KW turbine generates 78N/m of torque at full ouput. At the point the generator is connected to the prop, the prop will have a much lower torque as its done before it gets to windy if you get what I mean. Also the clutch could go between the gearbox and generator where the torque is much lower due to the higher RPM. The other way would be to stop the turbine and then connect it to the grid, it wouldnt need any form of clutch then and would be the safest I guess.

I think i will have to use a single phase motor as I only have a single phase power supply. I was thinking about making the blades change pitch under torque, although this wont solve the off grid in high wind issue. I suppose 1 thing that mite work for that one is connecting it to a very large resistor such as an electric radiator maybe or going the extra mile and including pitch control in the control system. I read somewhere that a single phase motor as a generator was good for 500watts per 1hp rating. I dunno if this is correct.

Danny

[Flux]

An induction alternator does not run in phase with the grid, it is an asynchronous generator. You need a speed sensor to connect it at about 1500 rpm, at which point it will be neither motoring or generating and will come on line at minimum current. It does not need synchronising.

If you come on early it will take a small current as a motor. If you are a bit late it will come on generating a bit of current.

If you let it come on violently above nominal slip as you propose, if you are lucky it will decelerate violently to slip speed. More likely it will shoot over the peak of the torque curve and run away, not holding speed and producing little output. Don't use the clutch.

You need a reverse power sensor to disconnect it from motoring when there is not enough power to generate.

Probably right about 50% at single phase, high slip, low pull out torque, lower efficiency and about 50% thermal rating.

Pitch control should work if you can incorporate some method of maintaining coarse pitch with loss of load.

Without capacitors it will not load into resistors. With capacitors of suitable value it will but you may need more than a single value resistor, it's not a very stable state. Adding capacitors means adding control circuitry to detect loss of load as it will feed back live into the line. Once you loose line the power will be wild.

Flux

[DannySmith]

Ok, thanks for the information. I agree that the best time to connect to the grid is when the motor is at 1500RPM, but what I was thinking is how can you decide if this is a gust of wind or not. What I dont want is to connect the generator and then the wind to drop or it not be windy enough to generate any power if you know what I mean. If I included pitch control I could wait until the turbine is doing say 1700RPM for a a decent amount of time, at this point its worth while connecting the generator to the grid. I could then change the pitch of the blades until the prop slows to 1500RPM at which point I can connect it to the grid. I could then slowly re-adjust the pitch to build up load on the generator. This of course requires computer control of the pitch of the blades, but I have an idea of how to do this hydraulicaly. This would also solve the high wind off grid issue.

If the information about a 1hp (749watts) motor generating 500watts is correct, that would be 67% efficient. So that along with the gearbox is going to give about 63% which isnt great, but at least the power it makes is very usable with little effort.

I have a 1.5KVA generator, but I have no idea what type of generator it is. Is it possible to syncronise a normal generator with the grid, or so they require some modification? Would be to easy if you could connect one of these to the grid and it would spin up its self in phase I guess, (thats if it is syncronous). Just thinking it would give a much higher efficiency.

cheers

Danny  

[Flux]

You probably need an anemometer or something to decide if there is enough wind to go on line. I still think you need to connect at a speed below the hump of the slip curve otherwise you will probably pull out especially with single phase. If there is not enough wind it will soon come out on reverse power. There will be days when it is motoring and generating and you may decide that it is more economic to not run it.

Synchronising synchronous alternators to highly fluctuating loads such as windmills is nearly impossible even with 3 phase unless the alternator is drastically over powered. I would go as far as to say it is impossible with single phase without an intermediate medium such as a hydraulic transmission to level the torque fluctuation.

Electrically your efficiency will not be too bad compared with battery charging alternators but dragged to constant speed your overall efficiency including the prop will be far below a mppt dc link grid tie. Cheap and simple is the main virtue, hardly worthwhile in small sizes and single phase makes it a pain.

Flux

[tljones]

I was down last week to visit the guys at Prairieturbines.com and was impressed with the plans that they have developed to do this. The day I was there the wind was blowing hard enough that to get down the gravel to the place I had to drag a tree off the road, and the machines were just churning quietly along and PRODUCING... Alan was home building parts so I got to see exactly how to do it and look at the controls. I was impressed enough that I bought the book. I am starting to gather materials, but that is the way I am going. FYI they are using 9 foot blades approx on a 7.5 hp geared motor and if they are not designed right they WILL run away and fry things. When I asked Alans wife what she thought, she sounded like my own.. He was crazy when he started, but she pays the electrical bills and there aint no arguing with that.

Tom

[DannySmith]

Hello, thanks for the link, it sounds like someone is already doing what I want to. Doesnt matter how much you search on the net, somehow you never seem to find the best sites.

Danny

[DannySmith]

Just had a look through, as they are selling the plans they are not giving to much away. I think its interesting how they use a three phase motor on a single phase supply. Can anyone explain how this can be done? I take it they manage it because it is operating as an asyncronous generator.

Danny

[hvirtane]

What I don't want is to connect the generator and then the wind to drop or it not be windy enough to generate any power if you know what I mean.

I think that it would be best to use an electric sensor to connect the induction machine electrically on the line, when it is turning fast enough, not any clutch. You might put a magnet on the rotor and a sensor is reading the frequency... and a relay cuts it in at the right speed? You might even use a small generator and when the voltage at that generator is high enough it closes a switch magnetically?

You might use some kind of furling system to prevent the machine to overcome the generator.

- Hannu

[Flux]

A 3 phase motor just runs single phase when connected to a single phase supply. One phase is sufficient to provide the magnetising current.The phase connected to the line will generate single phase into that line. If you were so inclined you could extract power from the other phases for other jobs. This trick is the basis of the Ferraris Arno phase converter that is often used to run 3 phase motors from single phase.

Larger motors are normally only available as 3 phase and they don't have the added complications of start windings and splitting switches that are redundant when you use a single phase motor as a generator. This still leaves the low efficiency and duty cycle and poor pull out characteristics of induction generators on single phase. There may be an improvement if you add a capacitor from one line to the unused terminal as long as you connect it for the correct phase rotation. This is also a trick used to run 3 phase motors on single phase when a true phase converter is not justified.

Done correctly this may increase output to about 75% of the 3 phase rating.

Flux

[Aaragorn]

this friend of mine is doing what you propose. He runs a 18' 3 blade upwind rotor. his brochure on his website has full details see windwardengineering.com

I am working on building a 12' prop version of a similar system. I am currently looking at motors and wondering if it's best to use a single phase 2 hp motor (he uses a single phase 5 hp for his) which plugs into my home easily or use 2/3's of a 3 phase 3 hp motor and get a converter for the phase shift. What I don't know is what the converter does when I start generating power and pushing it back down the line does the converter  UN-phase the power back to single phase?

[Aaragorn]

I neglected to mention that I believe I have 220 power in my house but I think it's single phase only.

I also know that my friend Dean at http://windwardengineering.com is getting some pretty good efficiencies from his system but he's doing some special tweaking that I don't understand fully. I still have a lot to learn eh?