Brian’s Morning Newsletter for May 27th 2009

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Good Morning
I’m feeling a little better this morning, I had to work yesterday, thank goodness.  It was tough, obviously because I was exhausted and to make matters worse, I forgot half of my tools.  Somehow I made it through and you know what they say, "What don’t kill ya, makes ya stronger." So with this in mind, I’m feeling good. I don’t know if I mentioned this, but it was my intention to make every effort this Tusas to abstain from alcohol as much as possible, I mean I had no intention of being crazy about it, just pay attention to how much I drank. My report is one of success, Friday night I managed to stay awake and keep the fires in the horno and main fire ring going until 2:00 AM, thanks in part to  David’s camp coffee, but also because I didn’t drink beer all day long. What a concept, huh?

Washing away all the aftereffects of alcohol poisoning was my usual program after Tusas, but this year was indeed different. No hangover, but serious exhaustion just the same. Every part of my body aches. I need a vacation from Tusas. Sunny beach, somewhere, right? Well, that ain’t going to happen, but I can tell you what will happen: After we go back in the Central Meadow, and haul the trash that Danny, Bear, Jackson, Austin, Amelia, Vidal and Kevin picked up yesterday, and sorry if I missed anyone, of course I wasn’t there, then we’ll unhook the batteries, and load up the stereo, and inverter.

Potential benefits, bldg higher voltage machine, step-down transformer?

By Seyiwmz, Section Wind
Posted on Mon May 25th, 2009 at 01:41:25 PM MST
?

Just finished the Homebrew book.  Great info, and I'm ready to apply it to my situation.  My experience with wind power is based upon using an induction motor for generating.  I built one of those 20 foot diameter Breezy 5.5 machines last year. Fun project, but want to stand up an axial flux and expirement further.  My only fear is the high amps, when building the 48v machines.  There seems to be alot of concern in the book about heat in the stator.  My question is, can or has anybody wound for higher voltages  (96 or higher) and then used a step down transformer to bring it back down to the 48 volts that a grid-tied inverter could accept.  I don't have batteries in my current set-up and would rely on an inverter tied to the grid.  I'd like to go with a bigger swept area than my last one, maybe 24 or 26 ft.  I'm know there's some loss of power using the transformer, but would it be worth it if the stator would stay cooler?

Seyiwmz
Potential benefits, bldg higher voltage machine, step-down transformer? | 11 comments (11 topical, 0 editorial)
Re: Potential benefits, bldg higher voltage (3.00 / 0) (#1)
by tecker on Mon May 25th, 2009 at 10:14:55 AM MST
(User Info)

 might work out ok if the weight is right . The primary is very low resistance
and will have to run out of a max load condition which is not the case with rectified circuits ,I guess I would have to say the stator heat would be an even greater problem.
Best to charge at 48 and control stator heat by bring out the both ends of all three phases and set to wye delta switching.Or Thermal insert to the stator and pwm .



Re: Potential-down transformer? (3.00 / 0) (#2)
by Flux on Mon May 25th, 2009 at 01:02:07 PM MST
(User Info)

Don't forget that these machines were intended for battery charging. If you start messing with grid tie then many things change and you will have a very steep learning curve.

It makes little difference winding for 48v or some high voltage and transforming down. in fact the transformed case will almost certainly be worse because you introduce transformer losses. For charging a 48v battery it may make sense to do this if you have an impossibly long cable run to the battery and the cost of cable is prohibitive at 48v. There will be no difference in stator heating for the same power out.

I suspect the 48v grid tie inverter is intended to work with batteries, the direct connected ones usually are meant for a higher voltage. If it is a battery based inverter it will not work direct. You can use the higher voltage ones direct with a suitable winding but you will need some form of voltage limit to protect the inverter.

Also a direct grid tie scheme will loose load if it comes off grid and a machine designed for battery charging is never expected to loose its load and won't be able to cope with it. It will overspeed and the furling intended for charging on load will offer no protection. You will have to do an awful lot of rethinking especially with a large machine like you are proposing.

The control scheme that you use to prevent excess inverter volts will have to be able to dump full rated output of the machine in a high wind at least until you stop it by some means. If it will reliably stop with a short circuit brake then you will need to rely on that for protection. Otherwise you need an alternative means of restricting prop output when not on grid.

Most of the worry about stator heating is as a result of loading to a battery with no proper load matching. If you load to a tracking grid tie inverter you will have little to worry about from stator heating but you open many other cans of worms.

Flux



    Re: Potential-down transformer? (3.00 / 0) (#4)
    by bobfandango on Tue May 26th, 2009 at 12:22:47 PM MST
    (User Info)

    "There will be no difference in stator heating for the same power out."

    Not quite sure I follow you here....  There is certainly something I'm missing since I know you know all the basics.  If voltage doubles, current halves for a given power which means heating loses are 1/4 assuming the same resistance.  That means stator resistance would have to increase by 4x to get the same heating loss.  Twice the turns fit into the same volume should require wire half as large which would only double the resistance.  When all is said and done, it seems to me, doubling the voltage should result in half as much heating loss in the stator for a given output power.  What am I missing?

    Thanks...

    [ Parent ]


        Re: Potential-down transformer? (3.00 / 0) (#5)
        by Flux on Tue May 26th, 2009 at 02:54:10 PM MST
        (User Info)

        If you take a specific case of say a 24v machine and lets say for argument that it has 20 turns per coil of 2 in hand wire of some size.

        If you wind it for 48v then it will need 40 turns and you have exactly the same winding space so you can use 40 turns of the same size wire.

        In the first case if the resistance of one strand is R, you have R/2 ohms.

        In the second case you have 2R ohms, ie 4 times the resistance.

        The current for the same power will be half the 24v case for 48v.

        When you look at I^2R the loss comes out exactly the same I^2R for 48v or (2I)^2.R/4 for 24v.

        I really can't see where this misconception comes from. If you take a 1hp motor designed for 200v to a rewind shop he will rewind it for 400v and it will still give one hp. You can't squeeze 2hp out of it because you have doubled its voltage.

        Flux

        [ Parent ]


            Re: Potential-down transformer? (3.00 / 0) (#6)
            by TomW on Tue May 26th, 2009 at 03:13:59 PM MST
            (User Info)

            Flux;

            Nice explanation! I also often wondered at that same myth / misconception. I think the bit about "all else equal" is not immediately obvious.

            That should clear up "why" for some.

            Thanks.

            Tom

            RoboEd V 1.001 Rev 2


            [ Parent ]


Re: Potential benefits, bldg higher voltage (3.00 / 0) (#3)
by electronbaby (roy<at>windsine.org) on Mon May 25th, 2009 at 01:16:32 PM MST
(User Info) http://www.windsine.org

Stator heating is in direct relation to I^2R losses. Although you can effectively change the resistance of the stator by switching between Wye and Delta windings, most people opt not to switch because it induces one more point of failure in the design. If you wanted to use the machine in a grid-tie scenario, you would effectively wind the stator to produce the voltage required for the inverter to work at cut in speed. This is usually in the 150v -250v range, but it varies with each inverter design. Since the stator resistance would stay the same (Wye), you are effectively lowering the losses (to an extent) in the stator by winding for such a high voltage because you are allowing the current to be lower for any given power output. Power point tracking then helps a lot by widening your operational RPM window, but you would need to have the power curve data for inverter programming, and this data will change with each respective build. As Flux said, you are going to bump into a whole other set of problems to overcome with grid tie turbines. If you know what you are doing, these can be figured out, but if not, the learning curve could indeed be "steep", specifically with a large machine like you intend to build. Main areas to watch out for are furling and programming, and of course a way to safely deal with inverter limiting, and loss or rotor loading due to grid failure.
Have Fun!! RoyR KB2UHF


    Re: Potential benefits, bldg higher voltage (3.00 / 0) (#7)
    by Seyiwmz (guytrudell@msn.com) on Tue May 26th, 2009 at 05:10:18 PM MST
    (User Info)

    First of all, thanks for all the input.  If I understand correctly,  there might not be any benefits for winding for higher voltage in my situation.  I'd like to stay away from batteries.  I was relying on the fact that short circuiting the machine when I lost connection to the grid would stop it, but do I understand correctly that may not be the case.  Here is a link to a video of my machine that I'm used to building.  http://www.youtube.com/watch?v=ci9bS7Ep9OA       What about imbedding a couple of temp sensors into the stator.  I could have them short circuit the machine when it nears a critical temp.  My machine does not furl and was hoping to get away with doing the same for this axial flux machine because I have no experience with bldg that design.   I could use a controller and hall effect sensor to short circuit it if a preset rpm is hit.  Still pondering in Michigan.

    Seyiwmz  

    [ Parent ]


        Re: Potential benefits, bldg higher voltage (3.00 / 0) (#8)
        by wooferhound (tim((NoSpamAt))wooferhound.com) on Tue May 26th, 2009 at 09:08:44 PM MST
        (User Info) http://wooferhound.com

        How would you prevent the spinning magnets from producing voltage in your Heat sensor wires ?

        W o o f -={(

        [ Parent ]


        Re: Potential benefits, bldg higher voltage (3.00 / 0) (#9)
        by Flux on Wed May 27th, 2009 at 12:25:11 AM MST
        (User Info)

        To build a non furling design will be difficult if you use a conventional grid tie inverter that reasonably tracks the load.

        Your induction machine is effectively constant speed and the prop is stalling above cut in. By the time you get to reasonable winds it will be stalling hard and the prop will be running very inefficiently.

        With the axial and a grid tie inverter the prop will stay near peak power right up to full load. It is much more difficult to brake a prop from maximum power point than from hard stall. Even if your alternator is powerful enough to hold the thing braked in any wind it will need to be even more powerful to be able to brake it from full power in a high wind.

        If you can meet this requirement your ideas will work as long as nothing fails. There should always be a back up so that you can control it if anything happens to the alternator or its connections ( still applies to a lesser extent with furling). Ideally you need tip flaps or some adequate mechanical brake as a back up.

        Wooferhound raises an interesting point about voltages induced in sensors buried in a stator. This can be dealt with but it is not easy, choose the correct sensors and twist all leads with the smallest possible loops for volts to be induced into and you may need a lot of filtering even then. Even thermocouples in a large commercial alternator can have several volts ac induced in them on top of the several mV of the required dc signal.

        Good luck, keep us informed of progress.
        Flux

        [ Parent ]


        Re: Potential benefits, bldg higher voltage (3.00 / 0) (#10)
        by oztules (oztules__at__bigpond.com) on Wed May 27th, 2009 at 03:47:10 AM MST
        (User Info) http://www.anotherpower.com/gallery/Oztules-toys

        I work on three types of Mill over here. One is transformer coupled, one is direct to battery axial, and two are bigger induction grid tie machines.

        Whilst the transformer coupled machine works much better than the direct coupled version from the same manufacturer, it has more to do with the iron cored alternator than anything else. It adds a complexity that in an axial flux unit, would give no benefit. Or said differently....in the AWP alternator the inductive reactance seems to be better controlled at higher voltages, than it's lower voltage brother, and consequently it's output is about 20% higher than it's LV version.

        In an axial, these problems don't exist (materially), so a transformer is of no benefit anyway. (unless you had a prohibitively long cable run)

        The induction motor turbines are bigger machines (30kw and upwards), and operate completely differently than the parameters you are intending to do with an axial and a grid tie inverter.... and you are familiar with these things too.

        Flux has pretty well spelled out the problems you will hit. It would seem to me that a Jacobs style variable pitch blade mechanism would probably allow you to do that which you have outlined.... ie speed limited (dynamically) to stop over powering the grid tie, self governed rpm wise (like the breezy, but by different method... active pitch change, not blade stalling).

        It would add another cost, but if done properly, would be the very thing to make this scenario work... in my opinion.

        ............oztules



        [ Parent ]


Re: Potential benefits, bldg higher voltage (3.00 / 0) (#11)
by tecker on Wed May 27th, 2009 at 06:56:08 AM MST
(User Info)

I can tell you the there will be a hard start for a stator to  three phase primary and a stall somewhat with every radical wind direction change . The stator sees a what amounts to a direct short.  









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