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Forum Index : Windmills : turbines output decreased
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jason shaw Newbie  Joined: 31/01/2012 Location: New ZealandPosts: 8 |
Hi all, i have owned a 1kw 48v chinese built upwind turbine four years but recently i have noticed current generated in high winds has dropped by approx %50. I lowered and replaced the brushes on the slip rings but this didnt change anything. The current output and winding resistance is balanced on each phase. To stop the turbine i short out the outputs through a three phase switch and even in high winds it use to stop straight away, now it doesnt. Micheal Lawley from ecoinnovation says i may of cooked the windings. They do look like theyve had some heat, the string tied around the windings had melted through (broken), but the windings look ok. When i first installed the turbine i made the mistake of having faith in the control box supplied with the kit and it went open circuit on a high wind day, this was before i installed the electrical brake so it was running with no load for a few hours, Maybe some damage was done then? Im hoping someone has experienced these problems and can help. I will post some photos later. |
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| VK4AYQ Guru  Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Jason From what I understand from your description of events and the initial failing of the original controller, this happened in the early days and the power output was still there after the event,is this correct?, if so there is a strong possibility that this did no electrical damage to the generator as a run away like this is more likely to do structural damage due to high speed. If it survived the event then it would be still OK. I believe that you have damaged the windings by shorting out the windings to stop the machine, when you do this, an extreme amount of energy is dissipated in a very short time due to the rotational inertia of the blades. I have a set of these blades on a mill of mine and they are very heavy, so they would store a lot of energy to be dissipated to stop the machine. The internal damage you describe is much like I have experienced when examining failed alternators. If you must use a switch to stop the mill do it with a resistance connected to dissipate the energy rather than a dead short, The resistance should dissipate a maximum of,150% of the rated output of the generator. This can be as simple as a container of salty water with three strips of stainless steel sized to suite. A new set of windings may be in order or live with the reduced output. All the best Bob PS: In my humble opinion shorting switches should only be used on a stopped machine or at least very slow rotation without resistance in circuit. I have seen more alternators destroyed by shorting switches than any other fault. Foolin Around |
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| jason shaw Newbie Joined: 31/01/2012 Location: New ZealandPosts: 8 |
Thankyou for the quick responce, i will probably bite the bullet and rewind the stator and stop the shorting out to stop and try the resistance method you mention. Im a bit outa practice on calculating ohms law but i came up with 4ohms approx resistance to dissapate the energy? |
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shawn Senior Member  Joined: 30/03/2010 Location: New ZealandPosts: 210 |
Just a little to add that may be of use or not?? A frend up the road a little has a very good mill its a hugh pigot clone around a year ago he had trouble with a high wind and the blades hit the tower he rebuilt it but the mags were only half the strenth they used to be, he replaced them and put cause of the half dead mags down to heat, the stattor was fine and is still in use. so worth a looking at before a rewire. |
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| jason shaw Newbie Joined: 31/01/2012 Location: New ZealandPosts: 8 |
Ok shawn i will take a look at that. The rotor is a solid large magnet. Did you know that if you take the turbine apart but fail to maintain the magnetism of your mags by wrapping tin or a chain around them they will lose magnetism? I was told this by a motor rewinder the other day. I remember the old horse shoe mags had a bar across the poles to maintain them, and wasnt dropping them or hitting them with a hammer bad? You would think the chasis of these turbines would work as a heat sink and the wind being stronger dissapating more heat. Cheers. |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
I think thats an old wives tail. Magnetism is a force, like gravity. It doesn't leak or get used up. It's just there, depending on how the atoms in the magnet material are lined up. The only way to reduce a magnets strength is to do something that changes the alignment withing the material. Too much heat or mechanical impact, like a hammer, will weaken a magnet. If the stator has been too hot, there'possibilitylity the magnets have been affected, is sounds like that may be the problem here. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| jason shaw Newbie Joined: 31/01/2012 Location: New ZealandPosts: 8 |
Hi Glenn, are you sugesting that shorting out the outputs has over heated the mags? When you do this it doesnt even make one complete rev. |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Its possible, as Bob explains. Shorting out the turbine at speed will generate a lot of heat, even within a short time. Then it depends on how that heat is dissipated. Magnets have something called a curie termperature, http://en.wikipedia.org/wiki/Curie_temperature, its the termperature that the magnet lines can be aligned, either to make the magnet, or destroy it. If the magnets reach the curie temperature, even if for a short time, it will dramatically reduce their magnetism. Rare earth magnets can have a very low curie termperature, some types less than 100C. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| jason shaw Newbie Joined: 31/01/2012 Location: New ZealandPosts: 8 |
Ok thanks for the info probably cheaper to get a new mag than rewind, will look into it. Other option i have is to turn it into a smartdrive turbine. |
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Don B Senior Member Joined: 27/09/2008 Location: AustraliaPosts: 190 |
Hi Jason, Apart from heating, it is possible to demagnetise or reduce the strength of even rare earth magnets, although they ar a lot less susceptible to de-magnetising than some of the earlier magnetic materials like alnico. De-magnetising is usually done by exposing the magnets to a strong AC field, which is then brought down to zero. It is possible that the circumstances of your shorting switch replicated this situation. The effect of opening the magnetic circuit of a permanant magnet so that there is a large air gap does apply a demagnetising effect, although rare earth magnets can usually survive this. Other types of magnets really need a "keeper", to minimise the air gap when they are dismantled. This is what the motor repairer was talking about. When "pancake" motors used alnico magnets, they would often have a fixed re-magnetising winding that enabled the poles to be re-magnetised once the motor was assembled, and the air gaps were minimised. Magnetising can be done with a very high pulse of DC current through a coil around the magnet. There will often be a fuse in the circuit that blows to interrupt the current, but I don't know how you would go about doing this with your magnet. It is also possible that your magnet is OK, but that you have a shorted turn in one or more of the windings. You might be able to check this by very careful comparison of the resistance of each of the three windings, or by energising each in turn with low voltage AC and looking for localised heating. Regards Don B |
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| jason shaw Newbie Joined: 31/01/2012 Location: New ZealandPosts: 8 |
Thanks for explaining that Don, my investigations continue. Im sure that motor rewinder i was talking about can test the magnetism and as far as the windings go i think theyre probably suspect seeing the string used to bind the windings was melted and broken and discoloured insulation on them but i just want to make sure before i fork out a few hundy to repair. |
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