Minor failure - Major winds

[Dave B]

I decided to leave my 12' single phase mill up as the cold front with 60 MPH winds moved in. I heat water with this and when I set it up I decided a little less furl offset would yield a little more power as this would always be under load, no problem. Wrong, here's what happened: As some may already know if you have too little offset and strong constant winds there seems to be a point where the mill "locks in" on the wind and the tail only seems to help keep it there directly into the wind. The only relief is when the wind changes direction quickly enough to force the tail up to simulate furling and it spills wind only long enough for the tail to fall back down and direct the mill back into the strong wind. Prior to these winds I had measured 420 RPM max. under load (TSR  carved to approx. 7) Things happen at 800 RPM ! I am happy to say the only damage was a loosened coil rubbing on the magnets and a slightly bent brake disk. Lessons learned after lowering and evaluting the situation : Stick to early furling first making adjustments later, use quality epoxy like I did (Wests System) the magnets remained rock solid even with the beating and are not completely cast but just well glued with fillets. The coils were only glued flat on to the laminants also and fillets and only one of the several that were scrubbed by the magnets broke loose. Mount and support your blades with large diameter plates to help distribute the large gyroscopic forces generated at high RPM and yaw. This is what flexed my rotor just enough to scrub the magents into the select coils. My blades are also tipped back some to clear the tower which when spinning at 800 RPM would have created a tremendous force on the hub with the blades wanting to align vertically. There was also a lot of moisture inside the hub at the center of the blades. It rained very hard with the high winds so it looks like sealing the mounting bolts / holes when installing the blades is a good idea  especially if these are exposed with no spinner or nose cone. I hope some of this is helpful, comments and suggestions are always welcome. Dave B.  

[Flux]

Glad you survived without major damage.

At least you know about this strange property of the prop seeking the wind. Never reduce offset to increase output, add weight to the tail or change the tail pivot angle.

Things were probably made worse if your slotted iron cored machine shows reactance limiting at high speed. Most do and the power tends to level off instead of keep rising so you may have been stall regulating rather than furling and when you got away from stall it took off.

Flux

[SmoggyTurnip]

Wow - 800 rpm gives a tip speed of 343 mph. That must have been scary!  Glad your blades held up.  What are you going to do to make sure this doesn't happen again.

.

[Titantornado]

Yea!  Good a testament for well made blades.

[Dave B]

Flux,

  You hit it right on the head, exactly what happens with this single phase with laminants. It's the old design and the magnets are too close together along with the coils too thick and the gap too wide there is a lot of cancellation and basically after approx. 1KW it takes off. Lots of work to make this better and safer but a great learning experience for my current 18' dual rotor project. The blades are laminated white pine glued with Franklin water proof glue and the hub is Gorilla Glued and screwed sandwiched between 3/4" marine ply with a 1/4" steel plate on the front. 3 1/2" drop at the root tapered and twist to approx. TSR 7. Measuring the frequency on my Fluke the actual RPM maxed out at slightly over 800 RPM, recorded gusts here were 60+ MPH. and as you can imagine it sounded like a jet. I really didn't think I'd see everything together like the photo after this. If you think you've overbuilt think again, you'll find out soon enough. Thanks everyone for the suggestions and comments ..... furl early.  Dave B.

[force9BOAT]

Dave,

How did you measure frequency?

Rob

[Dave B]

My alternator outputs wild AC and I just measure the  frequency of the AC with my Fluke meter in cycles per second. There are 9 full AC cycles per revolution with my alternator and I measured 121 cps. or 13.4 revolutions per second. Multiplying by 60 is approx. 806 RPM. or over 340 MPH tip speed. It was an amazing show and very loud. Less than half of that tip speed will be my very max. for my next mill of larger diameter but quiet and slow.  Dave B.

[force9BOAT]

Thank you Dave,

I interested in measuring frequency (RPM) of my own single-phase wind turbine.  I looked at the Fluke web site.  They've got lots of different kinds of measuring and counting devices.  May I ask what device exactly you have?

Rob

[Dave B]

Rob,

  My mistake, I have 2 meters and the one I use to measure frequency is the Radio Shack 22-812. The Fluke I have is an old Model 77 that has to be 20+ years old and does not have frequency measurement but has been an incredibly reliable and accurate meter through a busy field tech. life. The Radio Shack 22-812 has been a tremendous value (about $60.00 I think I paid a couple years ago) which also  comes with logging software and RS232 connectivity. I use this for data logging of either the AC output or frequency of my alternator. I believe they still offer it and in my opinion it is a steal with those capabilities. I'm sure Fluke has similar but maybe not at the same cost as the Radio Shack, seems they have always been upper end equipment but at least from my meter 77 experience well worth the price. Dave B.  

[Flux]

If you use a multimeter to measure frequency of an alternator feeding a rectifier and battery you may need a filter to clean up the waveform. Ok with resistive load.

Flux

[force9BOAT]

Thank you Dave!

Rob

[force9BOAT]

Dave,

Can you tell us more about the tower in your photograph?  That's a great looking tower.  How tall is it?  Is it a tilt up or was it erected in-place?  Did you have to apply for a permit where you live to put up that tower?

Rob