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Forum Index : Electronics : (Question) Dynamic Brakeing F&P
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JimBo911 Senior Member  Joined: 26/03/2009 Location: United StatesPosts: 262 |
Back from vacation and ready to get back in to the grind of every day living and trying to make some money in a bad economy. While on vacation I had some time to think about how to go about designing a better braking system for my F&P mill. I refuse to place it back in service until I have a sure fire way of stopping it. My thought is this. Instead of just shorting out the stator to stop the mill which we all know doesn't get the job done (with F&P setups)why not apply a voltage to the stator to help enhance or increase it's ability to fight strong winds. I haven't done any testing at all. Just wondering if applying voltage to the mill is practical or even possible. Some of you guys are much more informed about this sort of thing than I. I was hoping that this (electronic)idea of braking would be a simpler idea then designing a mechanical brake. Thanks Jim |
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Downwind Guru  Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Hmmm?? I dont think the idea has much merit. You say about shorting the windings to brake the mill, this is not the recommended method of controlling a F&P and if this is what you have been doing no wonder you have problems. With a F&P you must use a dump load to place load on the mill or the core will saturate and the thing will spin past the control point. The other requirement is get the furling right so when the mill is under maximum load it tends to furl out of the wind. It sounds to me you have made the same mistake as many with trying to be to greedy with the maximum power you try to get out at the top end. Loose a little off the top end and furl a bit earlier, play safe. The top end is only bragging rights to the total amount of power generated on the average day. If you logged your mill you would see that most of the power comes from lower in the rpm range with only peaks in the upper top end. The peaks dont add up to be much in the total power over the day, so to furl earlier you will not loose much across the average. A mill that lives for another day is far better than a broken one that gave you bragging rights while it flew. Just remember there is 100's of F&P mills operating without fancy braking systems and they all dont self destruct, so what are they doing different than you. Just my 20c worth of where you need to address the situation Pete. Sometimes it just works |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5028 |
Yeah the F&P can be difficult to control in high winds. Back in the days of 2 meter diameter turbines, you could short out a F&P before the wind picks up, and it would never break free. But with bigger diameters and better blades, the F&P can be driven out of electrical braking. The good news is the F&P will survive a run away condition, even when shorted. Once it goes into saturation its efficiency goes out the window and the design can dissipate the heat generated with no ill effects. Its the rest of the windmill that tends to fly apart 
So you either need to build it stronger, furl earlier, or add a mechanical way to control the windmill. Bruce did play around with disc brakes on one of his mills from memory, operated by a pull cable down the center of the tower. I was talking to Pete a little while ago about connecting a cable to the tail to pull it into furl manually. You could use a push/motor bike brake cable and sheath, one end connected to the tail at an angle that would pull it up when you pull down on the cable from ground level. The cable would be fed down the center of the tower. I haven't tried it yet so dont have and pictures to show you. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| JimBo911 Senior Member Joined: 26/03/2009 Location: United StatesPosts: 262 |
Pete Thanks for your in site and ideas. I am well aware of the 80/20 rule, as far as being greedy with max power the mill was placed in full brake mode BEFORE the storm hit. I do believe the tail did not function properly and has to be lengthen accordingly. No dump load with my mill is going to stop it from spinning in gale force winds at least not with the lift force of theses blades. I am confident that she would have survived the run away condition if the blades did not contact the tower. If I had provided enough blade to tower clearance she would still be producing electrons. I have to agree with Glen the newer more efficient blades that create greater lift played a large part. The neo mags in the rotor place a much higher drag then the ceramic magnets but again not enough to get the job done. Glen After hitting the ground from 14 meters up my mill is for the most part undamaged. (got lucky with the tail) The blades and blade rods are the only components that need to be replaced, with this in in mind I feel reliability confident that I am OK in the strength department. The Bergy Excel-S and the Excel-R mills use crank out tails for shut down mode. I have given this some thought and will continue to do so. My neighbor has a Sky Stream 3.7 that only uses Dynamic braking. I have done a little testing with what I would like to call Enhanced Dynamic Braking. I placed the alternator in my lathe and shorted the out put wires and by just using my hands to turn the rotor I was able to get it to spin but only very slowly which is of course what I had originally, difficult to spin but it dose spin slowly. Then I applied 12 volts to the out put wires (in delta) using the battery from my lawn tractor and the alternator locked up tight I was unable to spin the rotor what so ever. Thinking that increasing the magnetic flux in the iron cores of the stator will help to hold the rotor solid. As soon as time allows I will apply power from my lathe spindle in an attempt spin the rotor while power is being applied to the stator to see just how much holding power can be achieved? It would be nice not having to design and build a mechanical brake. In the mean time I will be increasing the blades to tower clearance and ordering some new blades. Thanks guys for your comments and ideas. Jim |
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GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi Jim, What you are describing is used in stepper motors, to increase the holding power at the step point. Power should only need to be applied to 1 phase, and this will create a strong magnetic pole that will move the rotor and lock it to the created pole in the stator. The current needed should only need to be enough to prevent the blades from coming out of stall. This would work well to hold a stopped mill, but I think that this will not work to slow a run away mill. Better to still get the furling right, and use a diversion load for a F&P style iron cored windmill. The Skystream 3.7 is a slotless design, so is similar in braking to an AxFx. The electrical shorting relays are directly across the windings, in the mill nacelle, across the windmill wild AC. These are an all in 1, GTI system. Gordon. become more energy aware |
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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
I have to agree with Gordon, The ampere turns is what kills your braking. When parked, you can energize to stop the blades from moving.... this will work, but if you are trying to stop them in flight, then it will only increase the ampere turns causing the runaway in the first instance I suspect... and may indeed exacerbate the problem. Keeping it loaded is still the only hope if your furling is way out. This way you drain power from the system, but shorting it out sets up big current at virtually no volts, so the amp turns are there to mess things up, but your not draining the power out of the blades anywhere near as much as before you shorted it. ... ie it will in all likely hood increase RPM when overspeeding... not decrease as expected. ..........oztules Village idiot...or... just another hack out of his depth |
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shawn Senior Member Joined: 30/03/2010 Location: New ZealandPosts: 210 |
HI I beleve your mill would still be ok if tower clerance was greater. High speeds do not hurt these motors seems if tail does not hit it or tower they just keep humming .My last mill also hit the tower (very high winds) but died only because tower contact motors still fine. ps my new mill furls better just to be sure... |
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| JimBo911 Senior Member Joined: 26/03/2009 Location: United StatesPosts: 262 |
Sorry guys that it took me so long to respond lots of things going on this time of year. My intent was solely to hold the mill in a parked position (complete still) not for governing. Thinking I would be able to energize the windings after shorting the windings or at the time when the mill is near stall or spinning very slowly. QZ If I am understanding you correctly by keeping the mill loaded, not shorted will place more load or drain more power than if the mill is shorted while spinning? In other words while spinning at HIGH rpm a shorted stator is basically useless for braking? This would con-cure with the comment that Pete quoted earlier. Shawn I will have me a boat load of clearance before she goes up again! Thanks Jim |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Yes, you stand the best chance of stopping it if you replace the batteries with a low resistance, not a short. Amp turns makes the magnetic field which effectively repels the permanent magnets field, and so allows the stator to run without generating more power. (current limited due to armature reactance). If you short the coils out, the terminal voltage drops over the coil, and the current increases. The increased current gives you higher back MMF (magnet force), for the same rpm. This repels the magnetic field even more, and so you don't bleed off power at the same rate as if you actually made less amps at higher voltage. If you have a lowish R dump load big enough, switching that in before the diodes should give the best result, and probably bring the thing to a halt sooner or later. Nothing worse than shorting the windings, only to see the mill take off instead... contra to what you thought would happen with iron cored machines. It works ok at lower speeds..... but you probably don't want to stop it then... ............oztules Village idiot...or... just another hack out of his depth |
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| JimBo911 Senior Member Joined: 26/03/2009 Location: United StatesPosts: 262 |
Thanks Oztules !!You are definitly the man!! 
And we continue to learn. Jim |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5028 |
Just thinking out loud, but what if we used large caps instead of a short? The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| JimBo911 Senior Member Joined: 26/03/2009 Location: United StatesPosts: 262 |
Glen !Now that's an idea that just might work! I do have three sets of caps one of which I do not use because they placed to much load on the mill. In the past when I used the larger caps the mill would never reach an rpm that would produce any real power just a trickle charge if you will. Together along with the neo magnets, correct tail furling and over sized caps might be what I need to keep things under control. I don't recall any other forum members that fly an F&P with neo's have tried this approach? !!!Best part is I still have the larger caps!!! Glen thinking out loud works for me.  Jim |
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| GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
I think that you will find that when I first stated testing with caps, that I tried caps between phases on my F&P mill. The caps loaded heavily up to a point, before the mill output dropped to zero output amps, and it ran away. The issue was with blades that can overpower, and inadequate furling. I was pushing for MAX output. Caps will form a parallel resonant cct across the windings, that will provide low load at low frequency, and increasing load at a particular higher frequency. Good luck with your testing. I found it easier to rebuild my F&P than all the extra effort required to get it to work with an overpowered rotor. Gordon. become more energy aware |
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| JimBo911 Senior Member Joined: 26/03/2009 Location: United StatesPosts: 262 |
Gordon Again, I wish to thank you and the other members for responding to my questions and ideas. Looks as tho you guys really have the F&P covered in all areas and this is a good thing. Any testing that I wish to do I am sure will fall short from yours and the testing that other forum members have performed in the past. I for one do not like to put time and or money on concepts that others have already worked through. In reality my mill is a mesh of ideas and designs (yours being two) that other people have came up with. I don't see any reason I shouldn't continue to go with the flow after all it's got me this far. The answers and ideas I have received tells me is that any attempt I may try to gain total control of my mill other than mechanical will not work. My original question has been answered. Gotta love the Internet. Jim Jim |
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