Author Topic: Why rectify close to the turbine?  (Read 4206 times)

0 Members and 1 Guest are viewing this topic.

Mohrenberg

  • Newbie
  • *
  • Posts: 7
  • Country: us
Why rectify close to the turbine?
« on: March 31, 2014, 02:25:35 PM »
From what i understand, you can carry AC farther than DC without as much loss. So why do all these builds rectify the AC ot DC power right at the turbine?
Why not run the lines to your house and then rectify it?

Flux

  • Super Hero Member Plus
  • *******
  • Posts: 6275
Re: Why rectify close to the turbine?
« Reply #1 on: March 31, 2014, 02:50:01 PM »
Not so simple really. For general power transmission ac can be transformed up to very high voltage to reduce line loss. If you can find a way to get dc to very high voltage it has the lower loss and is used for power transmission between England and France.

Anyway none of this is relevant to what you are discussing. Even for normal low voltage transmission 3 phase ac is considered to be the most cost effective way to deal with ac and dc is not normally used.

You are looking at a very special case of a rectified 3 phase alternator directly feeding a battery, this is a special case not normally met in any other field and it doesn't behave as you would expect.

At any instant the rectifier will conduct with two phase active and the other phase inactive, this is not standard rectifier operation for conventional rectifier loads with resistive or inductive loads where there is overlap in the diode conduction.

The result of this is that the loss in 2 leads of a 3 phase supply is the same as in two similar leads on ther dc side. The 3 phase case needs 3 wires of the same size as is needed for two on the dc side. It therefore works out cheaper to run 2 cables than 3 if any distance is involved.

There is also another case where running dc makes a lot of sense and that is with a 12v machine which is near impossible to wind as a star connected simple winding as the turns are small and the wire thicker than advisable for eddy loss. Parallel windings are prone to circulating currents that hinder start up if connected before the rectifier. If these parallel windings are rectified first and the output of the rectifiers paralleled then the losses are blocked below cut in. If you have several rectifiers it needs a multitude of ac connections and it makes more sense to rectify on the machine and just run 2 dc cables.

If you are not at 12v with multiple rectifiers and the cable run is modest I find it simpler to run 3 leads and have the rectifier near the battery but on a very long run you can save considerably on cable by using 2 wires instead of 3 by running the dc.

Flux


JW

  • Development Manager
  • Super Hero Member Plus
  • *****
  • Posts: 4021
  • Country: us
    • Flashsteam.com
Re: Why rectify close to the turbine?
« Reply #2 on: March 31, 2014, 06:35:27 PM »
I think the Dan's can backup what Flux is saying here. There isn't much of a loss issue using DC from the turbine..

ChrisOlson

  • SuperHero Member
  • ******
  • Posts: 3642
  • Country: us
Re: Why rectify close to the turbine?
« Reply #3 on: March 31, 2014, 11:30:21 PM »
Due to each leg of a three-phase AC source being rectified to DC handing the full current load 2/3's of the time, it is more efficient to run DC as far as possible instead of AC.  However, when dealing with MPPT turbines where you may need to run control wiring out to the tower, or at least to where the rectifier is located, it is usually better to run the AC from the tower to keep the control wiring shorter.

tomloods

  • Newbie
  • *
  • Posts: 7
  • Country: be
Re: Why rectify close to the turbine?
« Reply #4 on: April 01, 2014, 05:53:15 AM »
Hello,

From a last project i got some rectifiers left over . those are combined rectifiers ( 3 AC in 2DC out). In last project they heat up quit a lot... As i have some rectifier spare, is it possible to put 2 rectifiers in parallel? To reduce the generated heat in rectifier and to serve as back up in case one of them blows....

grts tom

ChrisOlson

  • SuperHero Member
  • ******
  • Posts: 3642
  • Country: us
Re: Why rectify close to the turbine?
« Reply #5 on: April 01, 2014, 09:29:52 AM »
If your rectifier is being consistently pushed to over 50-60% of its rated capacity I think I would consider paralleling two of them.  It will not change how much heat is dissipated as that is based on the forward voltage drop in the diodes.  But it will spread it out over a greater area.

Mohrenberg

  • Newbie
  • *
  • Posts: 7
  • Country: us
Re: Why rectify close to the turbine?
« Reply #6 on: April 01, 2014, 02:54:45 PM »
Thanks guys. I just read that you lose voltage over distance with DC but had no idea on how much. The best location for a turbine is probably about 200 feet away from my home, so I wasn't sure if i'd experience really bad losses from that.

I plan to do a similiar build as the 7 or 10ft build on OtherPower.

I'm trying to get ahold of his book. I wish there were an E book for sale as I hate having to wait to get it if ordered.

Flux

  • Super Hero Member Plus
  • *******
  • Posts: 6275
Re: Why rectify close to the turbine?
« Reply #7 on: April 01, 2014, 04:00:59 PM »
Tomloods

" From a last project i got some rectifiers left over . those are combined rectifiers ( 3 AC in 2DC out). In last project they heat up quit a lot... As i have some rectifier spare, is it possible to put 2 rectifiers in parallel? To reduce the generated heat in rectifier and to serve as back up in case one of them blows...."

Yes Chris is right, try paralleling two rectifiers but mount them on separate heat sinks if possible. You will get some improvement with both on the same heat sink but nothing like as good as spreading the cooling between 2 sinks.

Sadly manufactures are less than honest about the rating of these potted bridges, your use is equivalent to a capacitive load and although never mentioned there is a very significant reduction in current handling. Still worse is that the rated current is at a base mounting temperature of 20 deg C and you can't achieve it with any real heat sink as the internal heat transfer is not good enough to do it.

Mohrenberg

You don't mention the system voltage, there is some line loss that is desirable to keep you out of stall so modest losses are not a big problem.

200ft at 48v will be easy, at 24v it will be more costly in cable but not a big problem, if it is at 12v then you will need very heavy cable for that distance and running the dc would be wise to reduce the cost.

Flux

boB

  • Sr. Member
  • ****
  • Posts: 389
  • Country: us
    • boB
Re: Why rectify close to the turbine?
« Reply #8 on: April 02, 2014, 05:49:08 PM »
Either DC or AC will have wire losses.   Higher turbine voltage and either step down or MPPT is a good alternative to reduce losses.

2 wires for DC means only 2 slip rings instead of 3 for 3-phase AC which is a plus.

Running 3 wires for AC means you have access to AC zero-crossings if that is important for RPM sensing, dump-load clipping, etc.    (I wonder if AC is easier than DC on slip rings ?)

Also, since the rectifier is a semiconductor, it may be susceptible to lightning up on the tower where it would be easier to replace it down near the rest of the electronics.

boB

ChrisOlson

  • SuperHero Member
  • ******
  • Posts: 3642
  • Country: us
Re: Why rectify close to the turbine?
« Reply #9 on: April 02, 2014, 07:03:02 PM »
Running 3 wires for AC means you have access to AC zero-crossings if that is important for RPM sensing, dump-load clipping, etc.    (I wonder if AC is easier than DC on slip rings ?)

I have never seen where it is better with AC.  Either way those slip rings get dirty and high resistance with age and cause problems.

SolarShadow

  • Newbie
  • *
  • Posts: 1
  • Country: us
Re: Why rectify close to the turbine?
« Reply #10 on: April 26, 2014, 03:43:22 AM »
BoB it would seem to me that AC would be a bit easier on it as DC would tend to hold any arcing longer. I tend to think that slip rings would tend to have a tad of bounce to them, but if any dirt or high resistance points occur then I would think a DC arc would sustain and pit it worse increasing the arc until it fails.

Flux

  • Super Hero Member Plus
  • *******
  • Posts: 6275
Re: Why rectify close to the turbine?
« Reply #11 on: April 26, 2014, 04:27:49 AM »
Slip rings can be trouble free or they can be a real problem. The slip rings on a yaw head are likely to be more troublesome than those on rotating machines.

With this non rotating application the ring must never open circuit, the rotation is slow and bounce shouldn't occur, mechanical accuracy needs to be good and brush contact pressure can be higher than on rotating machines.

The caliper type brushgear works well, things with brushes sliding in boxes don't fo so well in general.

If the atmospheric conditions are reasonable and the worst of the weather is kept out things behave better. Marine environments and other conditions where various chemical vapours are persent can cause serious problems.

For these slow rotating rings a film of lubricant can sometimes help to reduce corrosion of the rings, but in other ways it can hasten disaster, if the slightest break of circuit takes place the lubricant will make a very effective spark eroder and destruction will be rapid.

I have noticed little difference with ac or dc, once you get to the state where things are bad enough to sustain an arc then the game is over.

Well designed, construcetd and maintained slip rings can work fine for many years, badly designed or amateur codged rings are best avoided unless you live in a very difficult wind area where thisting cables is a serious problem.

Flux

Rob Beckers

  • Full Member
  • ***
  • Posts: 146
  • Country: ca
Re: Why rectify close to the turbine?
« Reply #12 on: April 26, 2014, 08:09:20 AM »
Hello,

From a last project i got some rectifiers left over . those are combined rectifiers ( 3 AC in 2DC out). In last project they heat up quit a lot... As i have some rectifier spare, is it possible to put 2 rectifiers in parallel? To reduce the generated heat in rectifier and to serve as back up in case one of them blows....

grts tom

Tom, rectifiers in parallel is risky: They may not share the load equally, because silicon diodes have a negative temperature coefficient (their resistance goes down as they heat up) they tend to thermally runaway when put in parallel. Meaning, that of each rectifier leg some diode ends up carrying most of the current while the other carries very little (leading to failure).

-RoB-

tecker

  • SuperHero Member
  • ******
  • Posts: 2183
Re: Why rectify close to the turbine?
« Reply #13 on: April 26, 2014, 08:42:42 AM »
From the AC at  stator a wave extends out along  the conductors  . To a rectifier in most cases .  The total impedance of the wire the stator and the rectifier accounts for charge attraction through the load held on the positive pole at the rectifier . The longer the wire at the Rectifier the smaller the attraction .
Batteries have ions in solution to be attracted . Resistive loads react to the stator . So are best connected straight the the AC . Inductive loads have Phase changes and timing of the phase changes to account for .  Pick your load then match the wire and stator placement