Generator Design (without PM)

[mckelvi...@gmail.com]

Hi all,

We are a group of electrical engineering students at Strathclyde University.

As part of our final year we are undertaking a group project looking at the generator design for the Hugh Piggott turbine.

Our aim is to design a generator without the permanent magnets (or even just using less magnets). We have started to look at alternative designs such as the 'self-excited' machine.

Does anyone know of any generator designs we could consider? Is this a subject that has been well researched? 

Many thanks in advance,

Ruth, Julie, Trina, Ross and Mark

[Hugh Piggott]

hi,

For many years I worked with self excited generators in the form of surplus vehicle dynamos for wind power.  There are a number of disadvantages.  The main one is that the field coils require power to energise the field.  This uses up all of the available power in low winds, which is the time that energy is most precious.  Using permanent magnets was a huge advance and I would never go back to using self excited machines.  

Other disadvantages include the brushes and slipring or commutators that are expensive to build and maintain.

You can use car alternator for wind turbines but the efficiency is dreadful.  You could attempt to build your own self-excited machines but this would be very laborious in comparison to using permanent magnets.  I have to admit that I am not very positive about your plan.  I can't encourage you at all but if you want to try it then go ahead and try it.  I wish you the best of luck!

Thanks for sharing your plans and to be honest I hope you think up a better one so you can be successful :-)

Hugh

[Luiz Fernando Lavado Villa]

          Hello everyone,

        I have been thinking about this type of approach as well. I agree with hugh that self excited machines are a pain and much complicated to build. However, something you could look into is to make Asychronous discoidal machines using Hugh's recipe book. 

        Basically, you would make rotors with winding and short circuit them. That makes your electric machine to behave as an asynchronous generator/motor, depending on your slip. The fundamental problem here is that asynchronous machines require more complicated power electronics drives, which are also a pain to design.

         If you feel like taking this project forward, I would immensly recommend seeing the people from the PEDEC research group at Strathclyde. We could have a chat about it as well, but they are the specialists. 

           Cheers

[Hugh Piggott]

hi

Yeah induction motors with capacitors are quite a cool type of generator and I highly recommend their use in Hydro power where the rpm is much higher.  Nigel Smith has written a very good practical guide to that.  The motors are widely available around the world.  I don’t know how practical they would be at 100 rpm or whatever but maybe they can be designed to work like that or you could use gearing.

Geared induction motor wind turbines were quite popular in the early 1980s and a few stand-alone ones were built in Denmark.

I have never had much success using induction/asynchronous motors with transformer/rectifiers to charge batteries though.

cheers

Hugh

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[mckelvi...@gmail.com]

Hi all,

Thank you for your help Luis and Hugh!

Currently, we are just in the research stage of our project. We are looking at alll the different types of generator and hope to narrow it down to 2/3 concepts. Our plan is to do some modelling and analysis to find an improved solution to build.

We understand that our design might be no where near as efficient as the PM generator. However, we are interested in this area and are going to have a go!

We are going to look at the Asynchronous discoidal machine as an option.

I was wondering if anyone knew about electromagnets? I understand that they are normally used in large scale generation, such as power stations. Has there been any research into using electromagnets for small scale wind?

Many thanks in advance,

Ruth

[Luiz Fernando Lavado Villa]

Hello everyone,

Electro magnets, in my opinion, are a bad idea for small machines. Bringing the power inside of them is too complicated.

For induction generators the tough part is the start. You might have to magnetize the iron disk if the rotor to have some remanent field. I would recommend looking up how they start squire cage machines to inspire yourselves.

Keep us posted

Cheers

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Luiz Fernando Lavado Villa
Associate Professor at the University of Toulouse

Researcher on new architectures of renewable energy systems at the LAAS laboratory

Toulouse, France

[Luiz Fernando Lavado Villa]

         Hello everyone,

        I found a PhD thesis on self-excited induction generators from Newcastle which has a good explanation of the general principle of how it works. The mathematical models are also interesting, but quite crazy. Take a look at the experimental part as well, that is quite interesting. 

         https://theses.ncl.ac.uk/dspace/bitstream/10443/1478/1/Ma,%20D%2012.pdf

       Cheers

[mckelvi...@gmail.com]

Hi all,

Thanks for all your ideas.

We have decided to look at the following ideas. We hope to narrow them down to 3 concepts for simulation and analysis

1. DFIG

2. Asynchronous Discoidal Induction Generator

3. 'Self Excited' Synchronous Generator - excited from a DC supply

4. 'Separately Excited' Synchronous Generator - excited from a smaller PM machine

Thank you for the thesis Luis, we are going to look into that.

Have also found a paper on a 'Voltage-adjustable Brushless Self-excited-type Single-phase Generator'. Our research continues!

Any thoughts/previous experiences?

Many thanks,

[Luiz Fernando Lavado Villa]

            Hi everyone,

     Here are my returns:

Above all, don't hesitate to go and talk to the PEDEC group in Strathclyde Royal College building, these guys rock on electrical machines! 

As for your solutions:

1. DFIG

I think that this is really complicated. Specially on what concerns the power electronics and the control system. You'll need vectorial control and a complex converters. Remember that, in the end, if you are on isolated systems you might have only a DC bus connected directly onto a battery, which kind of defeats the purpose of controlling the speed of your turbine. Always remember: you are competing with a very elegant and simple solution - permanent magnets and a diode bridge. 

2. Asynchronous Discoidal Induction Generator

Sure thing! But how? My recommendation is to start by using Piggott's design but replacing the magnets by short-circuited coils. Remember to make them with very thin wire and lots of turns. That will give you a very strong field and a reasonable winding impedance. I have no idea of how many turns you need, so check out the manual and look up some electric machinery books. 

3. 'Self Excited' Synchronous Generator - excited from a DC supply

Bad idea. You'll need brushes or an embedded source of power. Brushes suck in comparison with permanent magnets, specially if the magnets are ferrite. An embedded source of power requires some recharging and eventually maintenance. All in all, complex and inneficient. 

4. 'Separately Excited' Synchronous Generator - excited from a smaller PM machine

I don't see the point. You still have the magnets around, which means do a Piggott machine and be done with it. You'd still need to get the energy onto the rotor of the synchronous generator to excite it. So it does not really make any sense. 

My bet is to use capacitors in the stator of the discoidal induction generator in order to let it self-start with reminiscent magnetic field. You still have plenty of problems using an inducton machine as a generator (as you can read in the thesis below). Since they are self-excited, their voltage drops as you load them up, eventually leading to total field collapse. Their frequency also varies, but since you use a rectifier that's ok. The only and sole advantage is that you get rid of the magnets.  

https://theses.ncl.ac.uk/dspace/bitstream/10443/1478/1/Ma,%20D%2012.pdf

Remember, you are competing with the Piggott machine and a diode bridge. Cheap, easy and simple. The only competing option I see is Self-Excited Asynchronous Discoidal Machines. Which means a battle between magnets and capacitors. Which one is worse to find in the countrysides of the world? Even more, which one is harder to replace? Which one lasts longer? 

Pick your champion. 

On Mon, Nov 9, 2015 at 4:55 PM, <mckelvi...@gmail.com> wrote:

Hi all,

Thanks for all your ideas.

We have decided to look at the following ideas. We hope to narrow them down to 3 concepts for simulation and analysis

Thank you for the thesis Luis, we are going to look into that.

Have also found a paper on a 'Voltage-adjustable Brushless Self-excited-type Single-phase Generator'. Our research continues!

Any thoughts/previous experiences?

Many thanks,

Ruth, Julie, Trina, Ross and Mark

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[mckelvi...@gmail.com]

Hi Luiz,

Thanks for all your ideas!

We have been trying to contact PEDEC for a few week now. However, we have been unable to find anyone who knows about this machine. Do you remember when this research was carried out and who by? There is not very much information available online.

We have decided to narrow down our designs to the following

1. Self-Excited Synchronous Machine (excited from a DC battery)

2. Self-Excited Induction Machine (excited from a capacitor bank)

3. Asynchrnous Discoidal Machine (if we can find out more about it)

We now aim to start some simulations for the designs above. Hopefully, from this we can pick our final design to build! We realise that no machine will be as efficient as the PM Generator. However we are very excited to see how this project progresses and what we are able to achieve.

Any other designs are welcome!

Many thanks,

Ruth, Julie, Trina, Ross and Mark

[Luiz Fernando Lavado Villa]

              Hey guys,

     No one in PEDEC has worked with Piggott machine so far. I just suggest you guys to talk to them about the general principles of electric machines to get some help in the simulations and the design of your machine. 

      When building machines, I would strongly suggest getting in touch with Aran Eales <aran....@btinternet.com>, since he knows how to build discoidal machines. Building machines is quite trcky, and making non-discoidal machines is quite a nightmare. Get in touch with Kostas Latoufis <lato...@power.ece.ntua.gr> in Greece to get some feedback on machine modelling and bulding issues. He has pretty much tried everything else before using Piggott turbines. 

      Keep in touch as you progress. 

           Cheers

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[mckelvi...@gmail.com]

Hi Luiz,

Thanks for your quick reply!

Yes, we have been speaking to PEDEC thanks for the contact! Its more in relation to the Asynchronous Discoidal machine that you keep mentioning. No one from PEDEC can give us any information on it, or has even heard of it. We also haven't been able to find out much from the internet. Do you have any more information on this design?

We are in contact with Aran already, he is who suggested we post here! Will ask him about the discoidal machines and also give Kostas Latoufis an email.

Again, thanks for all your help,

[Luiz Fernando Lavado Villa]

             Hi guys,

           That is really not astonishing. Discoidal machines are hard to come by and I've never heard of anyone who has tried to do an asyncronous one. I would suggest asking for help on the sizing of coils for discoidal rotors. If you look at this blog post below you'll find some photos on the permanent magnet machine. 

http://pureselfmade.com/2015/07/wind-turbine-construction-course-glucksburg-2015-review/

          What you are looking for is a way to replace the magnets in the metal disk by coils. However, I have no idea of how many turns you may need. You could suppose that you'll use the same number of coils as there is of magnets, that at least helps you to define the size of your coils. Then you have to find a way to define the number of turns and, by consequence, the thickness of your wire. 

          

         Imagine that those windins will be sitting on this disk, that that you'll cast resin on them all together.

         Cheers

   

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[mckelvi...@gmail.com]

Hi Luiz,

Sorry, I thought you suggested thas Asynchronous Discoidal machine and told us that PEDEC, Strathclyde have research this previously? Is this not correct?

Many thanks,

Ruth

[Luiz Fernando Lavado Villa]

                 Hello Ruth,

        Yes, I suggest Asynchronous Discoidal machines. As for PEDEC, I don't know if they've done any research on it, but I though that they would be able to help you considering that there are machine experts in the team. And in help I mean suggesting some reading or reference of past endeavours with discoidal asynchronous machines. 

           Cheers

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[mckelvi...@gmail.com]

Hi everyone,

Thanks or all your help. We have decided to build the synchronous generator but use electromagnets instead. We have run simulations and have a final design that we are about to build. 

Our simulations gave us an output that is 80% of the PM generator. Not sure if we'll get this but it has been a really interesting project to work on. 

Will let you know how we get on.

Ruth

[Luiz Fernando Lavado Villa]

            Hello Ruth, 

       Great news. Could you share your design with the WE community? If you cannot, can you at least tell us how are you going to feed the energy into the electromagnets?

         Cheers

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[mckelvi...@gmail.com]

Hi everyone,

We will be more than happy to share our design with everyone. It is not quite finalised yet but will let you know when it is. 

We are looking at trying to make the stator iron cored. Any ideas on how to do this? Can we encapsulate a steel disc and the stator coils in resin, would this work? Not sure if the resin is strong enough or how steel reacts with resin?

Many thanks,

Ruth

[Hugh Piggott]

hi ruth,

I do not recommend an iron core.  If you do make an iron core then you may wish to consider making it laminated.  If you are using a steel backing disk for your stator then maybe check this design by Ariaan Kragten who is looking for a student to cooperate on testing it.

http://scoraigwind.co.uk/2016/01/possible-student-research-project/

Personally I have some problems with this approach, which is why I do not use it.

Have fun

Hugh

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[Luiz Fernando Lavado Villa]

                   Hello Ruth,

       Hugh is right, if you are going for a steel core you'll have to think about Foucault currents. By consequence to laminate it. If you take the thinking further, why are you using a steel core? It will channel your flux, making your machine more capable of yielding current. But it will also mean your machine will stop the blades faster, possibly stalling them. You may also have a problem with weight, since a steel core will mean a heavier machine. How will that influence your furling system? All of these questions are tough to answer but should at least be considered. 

          Cheers

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