A wierd question on AC/DC

[Devo]

If you have a 120 volt DC battery bank to keep your voltage from a wind mill constant or close to it can you extract power from the AC side of the rectifiers & just use the batteries to hold the voltage near 120 volts or will it still be all over the place before the rectifiers?

Devin

[nick1234]

  120v battery bank  

a rectifiers are a one way devices when the electricity goes in its a one way street you have to take the electricity off the battery if you need ac you have to convert it to ac with a inverter. As dc you may use light bulbs, universal motors.eg hand drill they do not care if is ac or dc.  Your  wind mill produces a variable voltage and frequency  due to the wind conditions to use it you convert to dc with a rectifier and send it to the battery to act as shock absolver  and storage device

nick

[RP]

It'll still be all over the place.  I undertsand what you're thinking but it won't work.  The rectifiers act like a turned-off switch until the AC voltage is just little more than the battery voltage at which time it switches on.

The bottom line is when the AC is less than the battery, it's like there is no battery and the voltage is proportional to the rpm of the mill.  When the AC is higher than the battery voltage, it will be clamped (limited) to the battery voltage.

[Opera House]

Define all over the place.  The battery will act as a shunt regulator and act to limit the maximum voltage within the limits of the battery.  So, there will be an upper peak voltage limit.  Ac waveform will become a modified sine wave approaching a square wave.

[Devo]

I knew you couldn't pull from the batteries I just wondered if it could be used as a sort of dump load so you could run easy AC devices like lights,heaters etc only when the mill was turning & when the voltage got to high dump it into the batteries through some rectifiers.

I kind of figured it wouldn't work but wasn't sure why & even if it would I am still at a loss on how you could use the batteries when full unless you had 2 banks, changed them when full & rewired the bank for a normal voltage.

I was thinking for example of powering an electric heater with no loss charging batteries as it's tied to the mill but at a certain voltage over 120 when it's to much for the heater some kind of charge controller could let extra power charge the battery.

If a battery is at 120 volts & the mill is running but producing 110 can you take that 110 & use it (before the rectifiers)if there is no charge controller & the bank is always "on" or will you get no power until the mill gets higher than the battery?

Devo

[veewee77]

One other thing to consider with that arrangement is this. When the windmill is turning, it will produce AC voltage from the moment it starts turning. However, it won't start to put power into the batteries until it reaches the "cut-in" voltage, or the voltage at or slightly above the voltage of the batteries.

Now, if you put a large resistive load on the AC side of the rectifiers, that load will begin to load the windmill as soon as it starts to turn, not when it reaches cut-in. For this reason, it will make it harder for the windmill to start, and will likely raise the cut-in RPM of the windmill. A small load that just loads a small percantage of the load rating of the windmill won't affect it much. But if it is a light bulb, it will vary wildly in brilliance until the windmill reaches cut-in. Then it will stabilize. So in low winds, you'll have a hard time using the light output for anything useful. Heaters won't care but if you put a heater on it, that isn't a small percentage load. . . that is a major load.

Light bulbs are inefficient, and have been nclassified as heaters with a byproduct of light. Use the power from the batteries and run CFLs for more efficiency.

Doug

[snowcrow]

  Devo, DaveB's postings may give you a few ideas.

http://www.fieldlines.com/story/2007/8/22/23522/0302

  I would go with heating hot water off the AC side, and it can be done!! A good rule of thumb for dump loads is to use a resistive load, power resistors, air or water heating elements, etc.  NEVER use motors, light bulbs, pumps, they are unreliable and may fail, leaving the mill unloaded.

Blessings, Snow Crow

[robl]

Devo

I had a 2KW 120VDC wind generator, 200VDC(VOC)/500W Pv and a 240VAC/500W hydro plant all running into a 120VDC battery plant for about 14 years. I used a simple on-off relay circuit (with a hefty arc-supressed P & B DC-rated relay) to divert DC from the batteries to a standard hot 1500W/120VAC water heater. Lots of clicking and lots of hot water. Later on I added a TRIAC based diverter on the AC side of the hydro plant to the same hot water heater (switchable between the two sources) and also to a 240V baseboard heater. All these variations worked as desired. It is just a matter of deciding what is easier to install and maintain.

Currently I am only using PV and hydro and running on 48V batteries. By using a 2000W/120V hot water element I am still able to divert about 500W at 60VDC direct from the batteries.

Hope this helps

Rob

[Flux]

Yes you can take raw ac power before the rectifier and you have some form of voltage regulation as long as you take less than is going into the battery.

The waveform will change drastically and the frequency will change with speed. You will also have a problem of balancing the 3 phases.

Within these limitations it will be fine, you may find a load somewhere it will suit, but the honest truth is that for most real loads it is going to be useless.

Flux

[robl]

Flux

Wth all respect, unless I msunderstood this thread, if he just rectifies the three phases and dumps into a hot water heater or baseboard then it will not be useless power. Ideally, it will be intelligently controlled either on the ac or dc side, but even a relay off a Morningstar or Xantrex controller's diversion circuit would suffice. I have put together several small load dumps that take wild three phase AC from little turgo-type units running magnet-excited three-phase motors as generators. As for myself, I get 40 gallons of hot water every night. See my earlier posting in this thread.

Regards

Rob