AC versus DC

[jbear]

Gentleman, The burning question I have is as follows:

Why build the wind generator that supply's AC power, convert it back to DC with a rectifier, then run it to the batteries for storage if your using an inverter in the system?

Seems to me it would be simpler to just run the AC to the inverter with converts to DC for storage in the batteries or am I missing something? To much incoming power that can not be controlled that may in turn burn out the inverter maybe?  Jim

[Bruce S]

Jim;

   Way back when windmills were first on the scene, they were generally old generators from the ole chevy and ford era. These produced DC power.

However, most nowadays produce AC. The main reason here is due to power loss going long distances to were the power is being used. AC allows for longer distances with less loss. Converting the wild varied voltages to DC smoothes out the levels, this also allows it to be stored for later usage.

There are of course losses, but it remains the safest with respect to controllers and inverters.

This is a short answer to a long history , but hope it helps.

Bruce S

[finnsawyer]

The ac put out by the mill can be quite "wild" varying widely in frequency or waveform.  The appliances that you want to run the ac into may be quite fussy as to what they will accept and continue working.  So basically it becomes easier to rectify to dc and then use an inverter that can control the frequency and waveform if necessary to give the appliances the type of ac they were designed for.  Also, I suppose one answer would be, "Because we can (economically) thanks to modern diodes".  

[asheets]

A wind generator, unless exceedingly well designed, does not supply either constant voltage or frequency.  You could do as you suggest (although you would use a transformer and a regulator instead of an inverter), but the fluctuations in frequency would prevent you from using many typical household items.

[jbear]

Genltmen, thanks for the response to my question's. However, I must continye with the discussion.

The AC I understand is at a fluctuating output, the issue that seems to be missed is that I would still run it thru the inverter which coverts the AC to DC for storage in the batt's. later to be re converted to AC as called for, would not be running anything direct off the incoming AC from the Wind genny so I do not see where the fluctuating would hurt the equipment we run on the system.  Face it guys, ya got a newbie here with a lot to learn and little time to do it! I have got to get something up and running with-in the next 3 months. I figured to build the plans by the book for the 8ft. machine and use 10ft. blades. I am concerned about the output not having enough to keep up with the batt's. I have 16 225ahr units

@ 12volt as that is all the inverter will handle. no cash to buy a bigger unit for some time. As it stands we are charging everyother day for 3-4 hrs to bring them up to required specs. before the temps droped we would get about 3 days before needing to charge. at $2.60 per gallon I'm going in the whole so fast there wont even be a sound when I hit bottom!  

Am I about to embark on construction of a unit to small? Remember, I have built nothing of this nature in the past and can not have an $800 plus windmill setting in my back yard to make noise and not power that I need.

the power input from my 'conventional' genny is erratic in itself, anywhere from 11-20 amps AC into the inverter. dont know why it fluctuates as it does when the engine does not rev. beyond it's setting. Maybe it's getting ready to puke?

Thanks again, Jim

[Seaspray0]

In a house setting, your power deals with three values:  voltage, frequency, current.  For your appliances to work properly, they expect the voltage and frequency to be 120V and 60HZ.  The amperage is what your electrical devices will draw and that can vary.  Basically, you supply the voltage at a fixed frequency and the devices determine how much amperage is used.  Note:  voltage x amperage = watts (a 1200 watt hairdryer will draw 10 amps at 120 volts, 60HZ).  Vary the voltage or frequency by over 10% and you run a good risk of damaging your appliances.  The power company usually maintains them within 1%.

Now lets look at the output of your typical permanent magnet wind generator.  The voltage and frequency vary well beyond 10%.  There is no device out there that will transform a variable voltage and frequency directly to 120V and 60HZ.  You CAN convert that wild generator output to DC and regulate the voltage.  From there, you can use an inverter to produce a regulated 120V 60HZ.  Batteries are often used in the DC to assist in the regulation and store the energy.  This type of system does not rely on grid power to produce usable energy but can be connected to sell excess energy back to the grid.

An alternative is to use an induction type generator (rather than permanent magnet generator) tied to the grid.  This route has several requirements:  1.  It must be grid tied.  The shear scale of the grid is what regulates the output of your generator.  The control circuit must disconnect it during a power failure or you will have wild output.  2.  The control circuit must cut the output of the generator to a loadbank when the wind is insufficient to generate power (or it will draw power from the grid to keep the blades spinning like a fan).  No batteries are used in this type of settup.

[Seaspray0]

11. -20 amps from the AC?  volts x amps = watts
    
120. x 11 = 1320 or 1.3KW
     
120. x 20 = 2400 or 2.4KW
     
     

You're using between 1.3KW and 2.4KW at any given moment.  Figure in 30% losses for the system and my guess is you'll need a wind generator in the 3KW range just to maintain.

As for running it directly?  Even something like a light bulb can blow out on the unregulated power from a wind generator.  Heating elements (i.e. water heater) have been used with success (mostly as the dump load).

[PaulJ]

   Jim, sounds like you're using some sort of combination inverter-charger. This won't take the "wild" AC from a windmill as an input, you will probably blow it up if you try.

   Also, 10' blades on a windmill designed for 8' will probably disappoint you, you'll make less power in low winds and risk burning it up in high winds unless you make other changes.

   Paul.

[Bruce S]

jbear;

  Okay, I took a longer read on your first post and on your second.

The point which I think you may be getting side tracked is: the inverter.

If I read your posts correctly, you are wanting to run the windmills AC directly to the inverter's AC side for charging the batteries?? Is this correct?

If I read this right, then the problem is that the AC coming in from the windmill more than likely will nothelp. The main reason is; most inverters are built for taking in DC voltage and stepping it up to the 120Vac your stuff is looking for. Inverters are not normally built for going the other way around.

I have as a dump load, a rather small inverter that when my PDU starts seeing 15Vdc it kicks in a rely and turns on the battery trickle charger that is charging or toping off winter stored motorcycle batteries. If this has a very noticeable hum to it.

The aforementioned inverters are just not built to do what you want them to do.

NOW if the inverter you are talking about is one that's currently built into a fuel powered genny, then that is a whole different thing.

Mention of an existing unit perhaps, if you give us some deeper details of it we can help you get it's power output better controlled.

There are available but, NOT cheap,power conditioners that will take in a  range of AC and smooth it out but even those have their limits. The ones I've seen are made by a company named Oneac, they hum and are not efficent for what you're asking, but they do work.

And perhaps save you that $$$ that can be used elsewhere, like more batteries or better charge controller:-)if though you seem to have one heck of a large amount for 12Vdc:--)

Hope some of this helps

Bruce S

[jbear]

Thanks, mow it makes some sense to me.

[jbear]

Paul,  what would be the better size to build considering we run wind speeds at rather inconviennet times (middle of the night) to reach 40-45mph gust and @ times sustained during winter.

Usually through the summer we run up to 20 mph for high's.

Could I build in an auotmatic relay that under extreme output or rpm shut down the 3rd. leg to brake the system and keep it from burning itself up, or worse yet going into the dreaded high speed 'blow it all over the country mode'?  Jim

[jbear]

Paul

To get specific, we are using a heart interface 2500, 12 volt inverter. bought it at a give-away price from the only off systen dealer up here. the unit came off a rather large boat, had about 59 hrs use on it. the owner pulled the boat for winter, had some re-work done on it and moved it to another location. took the interface to the dealer and had it completely re-worked before re-instalation. the guy stroked out so the dealer was left with the inverter. bought it for the cost of maintenance and the chech-up.

I suppose it isnt large enough to do what we recuire. with only 2 of us here and little power draw except for the pump which I think I installed a 1/2 hsp., PC and some lights.  with 16 new 225ah batteries we get 3-4 days in summer and every other day in winter before re-charge. Which genny will be the best for us to build? bear in mind the occasional highe winds of which I think I  can accomodate with an auotmatic shut down.   Any and all help is greatly appreciated.  

I am sure I am probably driving you guys nuts, Sorry! I just allways figured it's best to deal with people that have a better understanding of things to get their experiences before I have to learn the hard way and end up building a lot of exspensive junk, or worse yet, commit great bodily harm to myself which I seem rather good at of late!  Jim

[PaulJ]

   Jim, I'm not familiar with your particular inverter, but if it's like the inverter-chargers I've seen it will only run off the local grid voltage and frequency  in charger mode. It won't work off the output of a windmill, it will probably blow up, and you'll be left with an unloaded, out-of-control windmill.

   Most of the windmills described on this board need their outputs rectified and then connected to a battery bank; some sort of charge controller capable of diverting excess power into a shunt load is then needed to prevent the batteries overcharging when it's windy (google search this board for "shunt controller" or "dumpload controller").

   As to the size of windmill you should build, well, how long is a piece of string? There are a lot of things to consider, just for starters:

   -your power usage

   -how much wind does your site have

   -how much can you afford, bigger costs more and needs a more expensive tower

   -your construction abilities (personally, I think 8' is big enough for a first attempt, but I have no idea of your capabilities).

   My point about blade size earlier was more to do with not messing with a proven design. Are the 8' plans you mentioned earlier from Hugh Piggotts book? If so, don't change them, follow them exactly. If you want a 10' machine, the hosts of this site have a good design, I think they're also selling kits now.

   Paul.