Need help from a electronics freak

[Cor van Houtum]

Hello guys,
I have a idea about balancing turbine blades but need help
In what way is it possible to let a generator run free ?
Would it be possible to power the 3 phase of a generator with a frequens that equals the speed when turning the generator by hand
This could mean that the magnetic resistance is over won by the power added to the generator.

When this works it should be possible to static balance the blades while connected to the turbine

Help

[Dale Sheler]

Pretty much the standard way to static balance is to mount the blades on the alternator and spin it by hand in a windless place, if one blade consistently ends up at the bottom you add weight directly opposite the heavy blade close to the hub, it's pretty simple.

[Chris Olson]

In what way is it possible to let a generator run free ?
Would it be possible to power the 3 phase of a generator with a frequens that equals the speed when turning the generator by hand
This could mean that the magnetic resistance is over won by the power added to the generator.


Cor, you're talking dynamic balance, not static. Yes, it is possible to power a three-phase generator with current of the proper frequency to make it run at a specific speed, turning it into a motor. I've done it many times driving the gen as a motor with another three-phase gen. However, it takes a reasonably high freq (about 50 Hz) to get your generator to start as a motor, and to run smooth, otherwise it will "cog" because the magnetic field is not rotating fast enough to pull on the poles to make it go.

The best way to static balance a rotor is to remove it from the turbine and hang it from dead center in the hub. Then add weight as necessary to get it hang level.

Static balance does not, however, mean the rotor won't "wobble" due to an imbalance (dynamic) in the rotor disc as it turns. I dynamically balance rotors by running them in my shop with a set of high speed fans with a torque arm hooked to the turbine head to sense the dynamic imbalance problem and identify which blade or blades is causing it. Dynamic imbalance problems can usually be corrected by adjusting tip tracking. Sometimes it is caused by the blade mass not being equally distributed along the stations on the blade, and requires weight at multiple points on the rotor disc to "fix" it.
--
Chris

Edit: Cor, I made this video once when I was test running a 3.8 meter rotor after dynamic balancing it. It does not show the procedure used to dynamically balance it. But for a rotor this size about 40 rpm is a good speed for detecting dynamic balance problems. If you run it faster sometimes a slight dynamic imbalance is not detectable, but it will still shorten the life of the turbine. After balancing it I like to watch them run up close, especially watching the tip tracking on the blades, as a dynamic imbalance will be visible there as all the blades won't track right due to the "wobble" in the rotor disc.

I use a torque arm on the head and a piece of reflective tape on the back of the rotor. By sensing the pulses on the torque arm and visibly looking at the position of the rotor (using the piece of tape as a reference to identify it) vs which way the pulse is on the torque arm, I can determine which part of the rotor disc has the problem, and then correct it.
http://www.youtube.com/user/DairylandWindpower#p/u/22/CWaIinCzG88

[Cor van Houtum]

Hello Crist,
Thanks for your response
I suspect blades from not having the same weight
To spread them to a other position can help but the problem is the magnetic hold of the turbine
The plan is to make a tool that comes in between the hub and the generator to let the hub and blades run free
This makes it nessesary that the hub is removed from the generator and after balancing put back on.
The plan is just t have the blades run free and see what blade is falling down on it's own weight .
Adjust the spread between blades so the blades stand in every position and non I falling down.
This works but it is a he'll of a job to get the hub off a 10kw turbine with blades of 8 meter in diameter
Because the tower is hydraulic and also the yaw is hydraulic driven we can lower the tower to a level of 4 meters above the ground and let I hang there
By turning the generator to the site it will hang on the site of the tower with one blade down almost on the ground
Now we can turn the blades easy by hand
But the generator magnetic field is interfering and keeps a torque on the blades
What I trying to do is to power the generator in a way that the magnetic torque is total zero.
In this way the blades run free only on the bearing and the weight of the rotor
This should make it possible to see what blades fall down.
Without dismantling the hub and blades and put a special made "run free tool" in between

So putting a power source with a 3 phase variable frequentie on the generator that makes it run free could save a lot of work when building a turbine or doing service

And when the power regulator is powerful enough and the frequency is regulated it must also be possible to drive the generator and see if it is in balance

Cor

[Chris Olson]

Cor,

I don't know what sort of tool you could put between the rotor and stator to cancel magnetic force of the rotor attraction to the stator core.

Is it a fixed pitch rotor, or does it have a variable pitch governor? My 7 meter 10 kW machine has no provisions for adjusting tip spacing because it is built into the governor. Even if you can adjust tip spacing I would not do that to adjust a balance problem caused by a heavy blade because you could introduce dynamic balance problems by not having the tip spacing the same on all blades.

On a larger machine like that I've found that the best way is still to do it on ground, not on the turbine. Weigh each blade to determine which is the heavy and light, and if the heavy blade was not finished properly (wrong external dimensions) you can possibly work it down to the correct dimensions to remove some weight. If the blade is fiberglass with a hollow or foam core this is most times not possible, and then the best option is to replace the defective blade rather than add weight to the other two. The turbines I work with that size have quality blades that are balanced and sold as sets to eliminate those sort of problems. But I still put them together and hang the rotor from the center point of the hub to double check the balance before putting them on.

The larger the rotor, the harder it is to balance them by adding weights without causing a dynamic imbalance problem.
--
Chris

[Cor van Houtum]

The turbines we sell have a dome where the blades are mounted on
With a centric hole that fits the turbine
I want to make a double sited axel with a bearing in the middle
So the ends are not connected (only true the bearing )
The one end fits over the generator axel and the other end fits. In site the blades dome
Now I could use the already installed tower and generator to balance the blades

Remember we can hydraulically lower the the tower and lay it almost flat

This was the idea to make a tool that let's the blades run free from the generator

The blades are screwed on with 6 bolts flat connection
There is always some play on the setting
So loosen up 5 out of 6 bolts results in a tip play of several centimetres


Cor

[Chris Olson]

Hi Cor,

What brand of turbine? It would be helpful to see a photo of it perhaps. It sounds like a fixed pitch rotor possibly, which would be very unusual for a 8 meter machine. Unless the blades bolt to the governor shafts on plates or something.

I guess I'd have to see it to gain a better understanding. And I am also assuming it has a three-phase radial generator, which would explain the "cogging" or magnetic attraction you are talking about (rotor to core) which prevents the turbine from turning "free" to see if you have a heavy spot in the rotor.

It's also surprising that a 8 meter turbine would have a balance problem that severe. Most manufacturers of machines in that size category take the time to make sure things are right before they leave the factory. You are in the Netherlands, right? I assume it can't be a Proven turbine because Proven's don't have that problem of not spinning free with no load with their toroid generator. What sort of turbine is it?
--
Chris

[Cor van Houtum]

Hello christ,
I am talking about the swg turbine
See this video to look at the blade connection
http://m.youtube.com/#/watch?v=JdmlkLmnnwk

Cor

[Chris Olson]

I couldn't get that YouTube link to work. But I googled it and found the manufacturer of it and see what it is. I see now that they use electronic controls to yaw it out of the wind for power control and it has a fixed pitch rotor.

Interesting problem you have with that one, trying to balance it.
--
Chris