Author Topic: What size blades for this generator? (Read 4755 times)

mmurray70 · « **on:** January 26, 2019, 12:53:14 AM »

Hi Guys, I bought an AC servo motor to build a wind turbine and need some help with the blades. This will be used for resistance water heating in a grid connected home to try and get a slight savings on our monthly bill.

Anyway the servo is a Fanuc BiS 22/2000HV and has a KV of 210v/1000rpm, continuous current is 5.6a and resistance is 3.6 ohms line to line in star configuration (1.8 ohms per coil). Did some testing tonight in the lathe and the numbers seem to match up.

470 rpm
------------
No load
VAC 99.6
VDC 133.8

10.2 ohm load
VAC 65.6
VDC 83.0
Amps (DC) 8.08
Power 670w

625 rpm
------------
No load
VAC 129.7
VDC 174.6

10.2 ohm load
VAC 79.7
VDC 101.5
Amps (DC) 10.0
Power 1015w

So what do you guys think? What size blades would be a good match for this?

SparWeb · « **Reply #1 on:** January 26, 2019, 07:14:29 PM »

Ok, for the benefit of other readers, this is a follow-up to this thread: https://www.fieldlines.com/index.php/topic,149713.0.html

It looks like what you've chucked the shaft in the lathe to drive it, and secured the body to the tool-post rest. I did exactly the same thing to my motor conversions to test 2 of them. That keeps it secure, and it gets you the electrical power output, but you're missing some information. The answer to your question is "what is the mechanical power needed to drive the generator at any given speed?" Well, power in divided by power out is efficiency, and efficiency varies a LOT with speed, meaning that the input power required is hard to predict but it you can just measure it directly.

The blades driving the shaft are providing the mechanical power needed to drive the generator, what you need to measure is mechanical power input. It's not a straight-forward calculation to work it our mathematically, and I wouldn't suggest you try. The lathe isn't going to tell you that, either, but the torque required to secure the motor body is easy to measure. It's not hard to build, if you know what you're looking for. Have you thought of making a prony brake?

I put video on Youtube to show how I did it years ago... seems to be gone now...

My solution was a 2x4, some C-clamps, and a bathroom scale. When I posted my videos doing this before, some folks didn't get what I was doing. I think they didn't believe that it could be so easy. This gave me a direct measurement of torque. Multiplying that by RPM gave me input mechanical power. You're in a position to do exactly the same thing, and it will take all the guesswork out of making blades match up to your generator.

I expect using lower resistance would get your more power out, but only slightly more... maybe 8 or 9 ohms. I'm just going by the ratio of your no-load voltage to the loaded voltage. If the loaded voltage is half the open voltage, you've matched the resistance (sorry, "impedance") of the load with the generator, which in theory would be the peak power point.

SparWeb · « **Reply #2 on:** January 26, 2019, 07:16:43 PM »

At least I can still find a photo of the set-up. Sorry, not a big one...

mmurray70 · « **Reply #3 on:** January 26, 2019, 09:15:03 PM »

Quote from: SparWeb on January 26, 2019, 07:14:29 PM

The answer to your question is "what is the mechanical power needed to drive the generator at any given speed?"

....I'm just going by the ratio of your no-load voltage to the loaded voltage. If the loaded voltage is half the open voltage, you've matched the resistance (sorry, "impedance") of the load with the generator, which in theory would be the peak power point.

Thats quite the test setup you have there Sparweb. Nice lathe too, looks to be a Colchester triumph 2000 just like mine. I can probably rig up the same kinda thing if needed. But I wonder if its necessary? Just thinking about what you said above, if the loaded voltage were half of open voltage the resistance (and power) would be equal. So if we look at the 625 rpm test, 61.5% of the AC voltage is being used by the load, generating 1015w. Couldn't we cross multiply and figure out that the servo is wasting 635w for a total load on shaft of 1650w? I guess it doesnt add in the load from seals and bearings, but thats probably minor. Are there any other electrical losses that im not thinking of?

DanB · « **Reply #4 on:** January 27, 2019, 11:55:20 AM »

It's fun and useful to do exactly as Sparweb has suggested with a home built pony brake, it makes it easy to chase the best power curve. For your purpose, water heating... I'm afraid you wont get much from this. That'll be very expensive water heating at the end of the day. Fun project though!

SparWeb · « **Reply #5 on:** January 27, 2019, 01:30:02 PM »

So you aren't wrong, both of you: tests to measure torque is not ABSOLUTELY necessary. But when you have 99% of the set-up needed to get the mechanical power measured, and you don't bother, then IMO you have thrown away the opportunity to spend 30 minutes to get the blade match right, and decided to spend hours and hours guessing instead.

Not all of the mechanical power gets converted into electricity. A lot is lost in heating the wire inside the motor. It's pretty easy to miss the fact that generators and motors can be anywhere from 90% to 30% efficient, and that this varies depending on the load on the generator. I really don't know if this is one or the other, so guessing about the rotor blade size based on the electrical output means we could be way off.

More details of what I did: http://www.sparweb.ca/3_Gen_MoCo/Baldy.html
Scroll to the bottom to find the test on the lathe.
I hope you can make out that the 2x4 is clamped to the motor's mounting feet, that the free end of the 2x4 has another c-clamp on it, and the handle of that c-clamp pushes down on a scale. The length from the motor shaft to the clamp at the scale is measured exactly - that's the arm. The scale measures the force to restrain the motor.
Force X arm = torque
Torque X RPM = Power (if you use US units you have to fuss with "550" or "5252" or some other unit that depends on units of measure)

I'm sure it goes without saying.... be very very careful not to turn the lathe backwards!

I suggest you set (as close as you can) about 4 speed increments to make a good clear curve. Say, 200, 400, 600, 800 or thereabouts on the speed selections your lathe has. 800 RPM will really max out the generator, so don't make anything too hot. If that's what you see on the lathe test, then this should tell you something about max speed for making the WT tail furl in strong winds.

Once we understand the servo-motor's efficiency when driven as a generator, then we can be confident with the power you want the rotor blades to give it. With the information so far, I can't narrow it down closer than 10 to 8 feet diameter, but that's a 40% difference in swept area.

Battery-charging with wind is a bit more forgiving than resistance heating. If the rotor is too small, the battery cut-in unloads everything in light wind so the system can still start. With resistance heating, you have to rely on a disconnect system to get it right, and a small rotor just makes the fine-tuning harder. OTOH, having a rotor too big lets it spin up fast in strong wind. You'll need a furling tail or some sort of shut-down system anyway, no matter what, but again, battery charge is a bit more forgiving than heating.

SparWeb · « **Reply #6 on:** January 27, 2019, 02:29:28 PM »

Here's a better photo of the beam:

mmurray70 · « **Reply #7 on:** January 27, 2019, 02:47:17 PM »

Quote from: DanB on January 27, 2019, 11:55:20 AM

It's fun and useful to do exactly as Sparweb has suggested with a home built pony brake, it makes it easy to chase the best power curve. For your purpose, water heating... I'm afraid you wont get much from this. That'll be very expensive water heating at the end of the day. Fun project though!

Im sort of afraid of the same thing. I was hoping to find a servo maybe double the size of this, but this came up for a great price, and the numbers didnt seem terribly bad so I thought id give it a shot. Maybe try building a smaller turbine first. I don't absolutely have to use this servo, can keep looking for something larger.

Curious to see what everyone recommends for blades and realistic expected output and ill make a decision then to proceed with it or look for something slightly larger. I know everybody here tends to use axial flux generators but im in a marine environment with freezing temps in winter and really dont want to go this way due to corrosion problems and possibility of ice jamming the rotor. I want something enclosed. Id really like to use a BLDC servo if possible or I could also do a TEFC induction motor conversion fairly easily, have a full machine shop here so can easily machine rotor.

Is it worth considering gearing up? Or will the extra power losses eat up the extra power of larger blades? Timing belt pulleys and belts and fairly efficient. This servo should have tons of power if I could spin it faster and setup a nice higher voltage system.

mmurray70 · « **Reply #8 on:** January 27, 2019, 02:50:14 PM »

Quote from: SparWeb on January 27, 2019, 01:30:02 PM

So you aren't wrong, both of you: tests to measure torque is not ABSOLUTELY necessary. But when you have 99% of the set-up needed to get the mechanical power measured, and you don't bother, then IMO you have thrown away the opportunity to spend 30 minutes to get the blade match right, and decided to spend hours and hours guessing instead.

Not all of the mechanical power gets converted into electricity. A lot is lost in heating the wire inside the motor. It's pretty easy to miss the fact that generators and motors can be anywhere from 90% to 30% efficient, and that this varies depending on the load on the generator. I really don't know if this is one or the other, so guessing about the rotor blade size based on the electrical output means we could be way off.

More details of what I did:
Scroll to the bottom to find the test on the lathe.
I hope you can make out that the 2x4 is clamped to the motor's mounting feet, that the free end of the 2x4 has another c-clamp on it, and the handle of that c-clamp pushes down on a scale. The length from the motor shaft to the clamp at the scale is measured exactly - that's the arm. The scale measures the force to restrain the motor.
Force X arm = torque
Torque X RPM = Power (if you use US units you have to fuss with "550" or "5252" or some other unit that depends on units of measure)

I'm sure it goes without saying.... be very very careful not to turn the lathe backwards!

I suggest you set (as close as you can) about 4 speed increments to make a good clear curve. Say, 200, 400, 600, 800 or thereabouts on the speed selections your lathe has. 800 RPM will really max out the generator, so don't make anything too hot. If that's what you see on the lathe test, then this should tell you something about max speed for making the WT tail furl in strong winds.

Once we understand the servo-motor's efficiency when driven as a generator, then we can be confident with the power you want the rotor blades to give it. With the information so far, I can't narrow it down closer than 10 to 8 feet diameter, but that's a 40% difference in swept area.

Battery-charging with wind is a bit more forgiving than resistance heating. If the rotor is too small, the battery cut-in unloads everything in light wind so the system can still start. With resistance heating, you have to rely on a disconnect system to get it right, and a small rotor just makes the fine-tuning harder. OTOH, having a rotor too big lets it spin up fast in strong wind. You'll need a furling tail or some sort of shut-down system anyway, no matter what, but again, battery charge is a bit more forgiving than heating.

Sparweb, I have an old dial type bathroom scale here in the shop so I will do that test. Ill post back results as soon as I can, might be able to do it tonight. Thanks again.

SparWeb · « **Reply #9 on:** January 27, 2019, 05:54:21 PM »

Quote

Is it worth considering gearing up? Or will the extra power losses eat up the extra power of larger blades? Timing belt pulleys and belts and fairly efficient. This servo should have tons of power if I could spin it faster and setup a nice higher voltage system.

No
The wind power is the same either way.
And the speeds match already.

richhagen · « **Reply #10 on:** January 28, 2019, 12:07:50 AM »

I don't know about others, but I am just a bit jealous of Sparweb's engine lathe. . . .

Bruce S · « **Reply #11 on:** January 28, 2019, 01:40:49 PM »

Rich;
I'd give up my best laying hen just for that amount of space

DamonHD · « **Reply #12 on:** January 28, 2019, 04:00:11 PM »

Indeed, that's about 25% of my home's floor area! B^>

Rgds

Damon

SparWeb · « **Reply #13 on:** January 29, 2019, 01:10:44 AM »

Woah woah. I still dream of such facilities, myself.
I had access to (the run of) that shop (playground) where I used to work up until 2012.
That business moved to another city, and I couldn't.

That lathe BTW was a Colchester Master 2500, with a 5HP motor geared between 35 to 2500 RPM (IIRC).
The shop also had a Bridgeport mill - and not the kind of Bridgeport with a chinese name on the headstock, mind you - a real Bridgeport, with DRO and powerfeed.
I still remember the day I put a hole in the bed of the BP mill by plunging too deep a 1/2" end mill.

I went upstairs to tell the boss right away, he had me dress out the burrs and give it a once-over cleaning, and later forgave me.

I may never have it so good again.

SparWeb · « **Reply #14 on:** January 29, 2019, 01:26:05 AM »

Here's a blast from the past:
Before I converted the Baldor 3HP motor, I converted this Toshiba 7.5 motor. I also ran it through the a similar lathe test, though with a different set-up:

I'm not sure why I ran the beam over the back of the lathe. Seems complicated to remember to run "backwards" but that's what I did and it was a long time ago. There may have been reasons why I first set it up that way, but I know I changed it later. The weights on the boards on the floor was too heavy to lift, and there's a spring scale attached so that must be how I got the "force" part of the torque measurement. I seem to remember it jiggling around a lot...

Also note the bicycle speedometer. I set it up to give me a speed cross-reference, which was handy because when the lathe was heavily loaded the speed dropped by about 10% below the nameplate speed on the gearbox. Induction motors have slip, yeah.

Here's a photo later that day, and I switched the beam around to the front.

No I haven't got the Toshiba flying, yet. It would need blades 14-16 feet diameter. It also needs a substantial tower which I haven't had money for. Until last year's new garage project, I also had no place to set up a new turbine. Now that I do have a garage ready to house a new battery system and be the "gateway" for grid-tie, new possibilities open up...

ruddycrazy · « **Reply #15 on:** January 29, 2019, 05:22:13 AM »

Yea oneday I'll have to get 4kw motor conversion back in the lathe with prony brake. The only test I did was put my old 4' lathe in backgear, clamped the output shaft in the 3 jaw and used the tool post to stop it turning.

got the lathe upto 250 rpm then saw a flash of 89 amps from the clamp meter then the VFD when into safe mode saying overcurrent.

Only got 88 16x13mmN50 neo magnets on the rotor too 22 in each pole on a 10 degree skew

mmurray70 · « **Reply #16 on:** January 29, 2019, 09:57:11 AM »

Hi guys, still havent ran this test yet. Im really considering the option of going with a timing belt setup for approx 2:1 speed increase to allow running a bigger rotor. I know theres more drag, but the electrical efficiency goes up with higher voltage and lower current. Looks like there would be much more potential for power overall, and still have a fairly lightweight setup versus a huge, direct drive unit.

Im going to see if I can find a 3750-4000w water heater element (around 15 ohms) to better match a 2:1 setup. Then when I rig up my test setup I can test both options and see which way I want to go. So just be sure im not missing anything, im going to be measuring DC voltage across load, DC current, RPM (ill try and double check with tachometer for slip), and force at a given length for each test right? Anything else im missing?

SparWeb · « **Reply #17 on:** January 29, 2019, 10:44:11 PM »

Length of the beam.
Open-circuit volts.
If you want to do your accounting very carefully, include the rectifier's voltage drop, too. Not very important tho.
Even more picky: Open-circuit and loaded AC voltage both measured at motor terminals.

You can usually make safe assumptions about rectifier voltage drop, and about AC-DC conversion in 3-phase, but it's not always true, and your multimeter can lie to you. Extra measurements help sort it out when the measurements don't make sense.

mmurray70 · « **Reply #18 on:** January 29, 2019, 11:30:28 PM »

Sorry Sparweb, never got your msg in time so I didnt get unloaded voltage. We can probably calculate that fairly close if we need it.

So I did a bunch of tests with three different loads tonight. I used the same 10.3 ohm load I used before, and also tried a slighly heavier 7.6 ohm load without much improvement in terms of power. And finally tried a 14.9 ohm element at higher rpms in case I decide to gear up. I feel fairly confident in the measurements. Amp readings are from a cheap multimeter, but I tested it in series with my fluke at one point and they both read the same. DC voltage was measured from my fluke meter. Force was measured with a digital scale at a radius of 26.5". I did check actual rpm with a tachometer too. Stayed within a couple rpm for all the tests but it did vary slightly from the dial on the lathe.

What do you guys think? What are my options in terms of rotor size? Should I consider gearing up? Efficiency did improve considerably at higher rpm and higher resistance, probably enough to offset any mechanical losses of a timing belt drive.

SparWeb · « **Reply #19 on:** January 30, 2019, 02:41:20 PM »

No time to crunch numbers now, but great work!
TTYL

SparWeb · « **Reply #20 on:** January 31, 2019, 01:01:05 AM »

Thanks again, this is very interesting. I've been number crunching and there are some surprises.

The input power curve is more of a straight line than I expected.
To get a good cut-in at 10-15 kph wind, you need a 10 foot rotor.
An 8-foot rotor will just stall below 15kph and have trouble starting.
But, because the generator's power curve is straight, the curve of the rotor power rises much faster than the generator's load rises.
By 40kph, the rotor is up over 800 RPM because the generator isn't loading it nearly enough.
That relies on the furling to be active and "regulating" the power more than usual, and much more than the "on/off" style of furling that suits me.

The effects of power curves not matching between rotor and generator is nothing new. But this is a little more than I expected.

There are some hints that rectifying to DC is wasting a bunch of power, but I don't think it's enough to account for what I'm seeing.
I'm not sure what to do about this. Lemme sleep on it.

mmurray70 · « **Reply #21 on:** January 31, 2019, 09:48:31 AM »

Yes it is interesting. Im glad I did took the time to do this test, and also that I found the 14.9 ohm element to try too. Im no expert, but im am surprised to see how close the output power, and input torque are when basically doubling the load. At 607 rpm and 14.9 ohms im getting 944w out, and with double the load at 7.6 ohms im only getting 1056w out. Actually got more power out with the 10.3 ohm load.

So seems obvious that this servo doesn't like large loads. Im thinking it makes the most sense to do a timing belt drive now, probably around 2:1 ratio. And aim for around 15 ohms max load. Should be much more predictable and extremely efficient with smaller loads switched in before going to 15ohms max. Seems like current is limited with heavier loads on this servo.

Do you think I could get by with a 10ft rotor in a 2:1 ratio? Could probably use 2000w 240v water heater elements (28.8 ohms) which are available cheap. Start with one at cut in, and add a second to get 14.4 ohms for maximum.

SparWeb · « **Reply #22 on:** January 31, 2019, 09:49:44 PM »

Funny, we're going in different directions.

The efficiency of your motor is a bit misleading. Take it further for the sake of argument - say you connected a 100 ohm load. The efficiency would be will over 90% but the current would be so low... 1 Amp, maybe... dropping the power off to nearly nothing. So you aren't chasing efficiency. Don't make any choices based on that.

And you only got about 1% more power out with the 10.3 ohm load than the 7.6. You've found the flat spot on the power curve. Anything in that range will give you similar results.

mmurray70 · « **Reply #23 on:** February 01, 2019, 12:50:17 AM »

Im just thinking about efficiency to help decide if I go direct drive or gear up. Just saying the increased electrical efficiency will probably offset the mechanical losses of the extra set of bearings and a timing belt drive.

SparWeb · « **Reply #24 on:** February 01, 2019, 01:42:05 AM »

Let's see if I can guess what happens with gearing up. Gotta write it out step by step to be sure myself. Following my gut feeling but putting numbers to it carefully as I go.

I'll say the GB steps the speed up 1:2 from rotor shaft to generator shaft. Ignore GB losses (probably -10%).
Let me see what happens at 300 RPM for example.
A 10-foot rotor turning at TSR of 7 would do this at 25kph. Nice breeze.
If the rotor is 40% efficient, it's putting 600W into the shaft.
For 600W of power at 300 RPM the torque is 19 Newton-meters.
The gearbox turns that into 9.5 Newton-meters at 600 RPM.
That's no match for your servo. It needs 1.5kW to be turned at 600 RPM.
The rotor slows down. Probably gets to 200 RPM, because 600W won't get the servo over 400RPM.

Without the gearbox, the servo is looking for ~600W to be turned at 300RPM. Maybe a little faster depending on the load resistor.
So it's already close to being matched to the rotor giving it 600W.

How about a 1:1.25 gearbox...?
Same rotor & wind speed... same 600W into the rotor shaft turning at 300RPM and steps up to 400 RPM. The servo needing ~7-800W to stay at that speed (for either 7 or 10 ohms load) is going to slow the system down. It's not perfectly matched, buts close enough that the WT will run slow, probably at a TSR about 6 instead of 7. No big deal. But you didn't get much from that gearbox, because the rotor is still only giving you 600W to work with.

Well, that's what happens at one speed. The picture changes for stronger wind, and it goes the other way for lighter wind. I don't think it gets much better. Direct drive is already matched up at a common average wind speed.

Electronics are much more versatile. This is why most WT's do better with switching regulators that track the maximum power point, or at the very least buck at low wind speeds. Are you inclined to look at that?

mmurray70 · « **Reply #25 on:** February 01, 2019, 05:39:12 PM »

Yes I will be using some kind of controller to lighten the load at lower rpms. So Im thinking it would be better to have the generator overpower the blades and then reduce load to match with the controller right?

Its interesting that you think I could possibly get by with a 10ft rotor direct drive, I didnt think I could go that big. At what point do you think I would need to be furling with this setup?

SparWeb · « **Reply #26 on:** February 02, 2019, 01:17:11 AM »

Quote

Yes I will be using some kind of controller to lighten the load at lower rpms. So Im thinking it would be better to have the generator overpower the blades and then reduce load to match with the controller right?
Its interesting that you think I could possibly get by with a 10ft rotor direct drive, I didnt think I could go that big. At what point do you think I would need to be furling with this setup?

You've got the idea. So if the controller packs it in, the generator doesn't let the blades fly away. If you follow that principle, you have to accept making a smaller rotor though. Eight or nine feet instead of ten. I do recommend doing that. Your electronics have to manage the low power anyway, so reducing the blades doesn't hurt at the low end. And at the top end you have to set the furling to a safe wind speed. Using a smaller rotor makes that safe wind speed a higher threshold, giving you more latitude to get it right.

I hope some members who have used the Classic controllers, or made their own, can provide you with more insight into doing the electronics.
I haven't gone MPPT or tried wiring up my own buck converter before. I'm doing direct battery-charge here.

It takes some effort to work out, but well worth it for the insight it gives you, to work out a wind power curve for 10 foot blades, and compare to your generator input power curve. That's all I was doing yesterday. You can repeat my calc's for higher wind speeds and when it tells you the blades will literally run away, then it has to furl well before that wind speed. It's probably 40-50 kph wind. The blades still make some power, depending on how you set the furling, it doesn't stop the blades completely.

mmurray70 · « **Reply #27 on:** February 02, 2019, 11:53:49 AM »

Ok so when you say eighth or nine feet your saying direct drive right? Thats kinda the range I was thinking too for DD. Im thinking i should easily be able to handle 10-10.5 foot blades with a gear reduction setup but I will double check your calculations at other speeds. Going with something in the 10-11 foot range seems to make the most sense to me. Any bigger then that and things start to get really big, and any smaller seems like a lot of work for limited output.

Biggest question I have now is how can we possibly know what efficiency the blades will have for the calculations? I will most likely be drawing the blades in CAD and cutting on a CNC mill. Should I look at other airfoils or just stick with the simple profile in huge piggots designs?

SparWeb · « **Reply #28 on:** February 02, 2019, 12:43:48 PM »

If you're using CAD and a CNC then you can go nuts, have fun and it won't affect much, not like it would if you were cutting to measurements on a workbench.
Making curvy blades is an improvement, but the tried-and-true work well enough, just by basic dimensions of diameter, chord, and pitch.
This part has some "art" to it, so do what suits your taste and interest.

This may be hard to accept, but you don't want the rotor blades to be so big that they are in charge of the speed all the time. The generator really has to be in control of the system speed, because once the genny overspeeds or overheats, then all control can be lost. I've run my machine with blades that are a bit too small and been satisfied the whole time, because nothing could get out of control that way. Now that the 10-year old blades are worn out, I'll risk a little more diameter on the replacements, but not too much.

I expect you've plotted your power in & out numbers on a spreadsheet by now. I did so the other day, and that's when I saw that your power IN curve is so straight, not curving up very much. That curve is fixed and only slightly affected by your choice of resistors. The wind power curves up quickly, so big blades just make the RPM's go way up. Don't worry, you'll get plenty of power with smaller blades, and less often feel the need to shut down in strong wind storms.

JW · « **Reply #29 on:** February 11, 2019, 07:19:27 PM »

Try this link...

https://www.fieldlines.com/index.php?topic=143874.0

Like this post from Sparweb in the link-

Quote

It's too easy to assume that what's easy for me is as easy for you.

I'd say that carving is 1/2 muscle work 1/2 brain work, so if anything puts you at a disadvantage on the first count, there's always the second part that makes up for it.

The chainsaw business is rather hard to manipulate, but maybe you can consider using a router, mounted on guides to keep it from cutting too deep. It's been tried before, but I don't know if a lot of pictures can be found to help you picture what should be done. I actually tried it myself, but I wasn't having fun and it was actually taking longer than just going at it by hand. You need rails on each side of the blade, clamped down so they don't move, with a tilted shape. The router needs to slide back and forth, following the rails in such a way that the curvature and twist are cut by the router bit.

News:

Author Topic: What size blades for this generator? (Read 4755 times)