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Forum Index : Electronics : Intelligent rectifier
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| adonis Regular Member  Joined: 13/02/2010 Location: SwedenPosts: 42 |
I was thinking that there must be some way to rectify the output of the generator without the voltage drop and powerlosses of a diode rectifier. maby all of you already is using something intelligent, or there is a good reason not to? i have not really thought this through, but maby it is best to ask you if this is anything worth bothering with first, there is no doubt some experienced people on this forum. maby one could use a voltage comparator to controll some power mosfet? (well, you would need 6 comparators and 6 mosfets, but it might be wort it). expecially when you are running a 12v-system, the power losses in a diode rectifier must be significant. im guessing around 1/5 of the total energy is lossed? is this something that may be worth spending time on? |
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| davef Guru  Joined: 14/05/2006 Location: New ZealandPosts: 499 |
If it was a 20% power loss it would possibly be worth doing something. The Vdrop for Schottky diodes is around 0.3 Volts, so even two times that is only about 4% in a 12V system.. The Vdrop for MOSFETs are say less than 0.1 volts. However, the power to run all the control circuitry is likely to eat into the savings you MIGHT make. |
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| adonis Regular Member Joined: 13/02/2010 Location: SwedenPosts: 42 |
ok, thanks. then i will not bother. what kinds of schottky diodes do you use? (i guess that depends on the size of the system) maby you could give me a hint what to look for. the one im putting together will not give more than 180W, 12v system. i figure the current will not exeed 15A after the rectifier, but how many amps should the schottkys be able to take? also, i saw some datasheets for different MOSFETs, and they seem to have a resistance (around 0,4 - 1 ohm) instead of a voltage drop? this is not good either and just might cause bigger losses than diodes. |
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Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
The V-drop for an average power diode is around 0.6 volt and i doubt it is worth messing with the extra 0.3 volt gained with schottky diodes. Look on ebay for 3 phase rectifiers as they are dirt cheap. Add a Gordon style voltage doubler and get the so called lost power back through the doubler. The more basic you can keep the rectifier the less problems you will have. Pete. Sometimes it just works |
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oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Beware: Not all shottky are the same. In your computer power supplies, you will find D83-004 or SBL304 or C85 etc etc as the shottky rectifiers for the 5v output. They are common cathode devices, so can get messy to mount the negative poles.... but their forward voltage is still .5v or so. Your normal diode will do not much worse, but may be easier to mount. At least there would be an endless supply of the computer PSU ones I guess. You never know, the voltage loss may give you a better TSR due to getting out of stall .7v later  , and you may not need to add line loss (if cut in is too early)
For your target audience it is more important to keep it simple perhaps. ............oztules Village idiot...or... just another hack out of his depth |
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| adonis Regular Member Joined: 13/02/2010 Location: SwedenPosts: 42 |
ok. i have alredy built a rectifier, i used 6 "power aplication" diodes, im not sure what they are called... a metal lump with a wire on, quite big and heavy. anyway, they can handle about 30-40A and 400-800V. now im considering they might have a big voltage drop and i dont need that powerful diodes? but maby i should just stick with them for now, at least they work fine. the stall thing is not really an issue since im building a VAWT. voltage doubler? never heard of it. will check up on that. by the way, oztules, i think i have got the hang of the PMW-controller. i found a picture in a datasheet that, together with your inputs, really explains all correlations. i have not built it yet, waiting for a "lab-plate" so i dont need do solder and redo it 15 times. but it just might work and i think it will be very simple! |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Regadless of the diode size the voltage drop across the diode will remain the same regardless if its a 1 amp or 30 amp power diode. The only thing that changes is the current each diode can handle. My thoughts is use what you have at hand and if needed change it later, as once something is up and running it is much easier to test and work out where the pit falls lie and correct them if needed. The old saying ....suck it and see rings true. You can get to hung up on finer details that will give less than 5% difference rather than getting it to fly and refine the system where needed. Pete. Sometimes it just works |
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GWatPE Senior Member  Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Main Capacitor Multiplier thread Hi Adonis, This thread is where most of the discussion on caps was at. the thread is currently on page 3 of the forum. The 3phase Capacitor Multipliers used to load match a windmill, to my knowledge was my idea. I have only seen references in literature to series type cap multipliers, that are unsuitable for load matching of a windmill. I did ask the question on an other forum, as well as this one, and have not had any references to others who have also done it yet. This thread is a major read. I cannot point you to any particular page, so I guess you will find what you need. There is an article here, with a general layout for a capacitor voltage doubler. cap multiplier article Gordon. become more energy aware |
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KarlJ Guru Joined: 19/05/2008 Location: AustraliaPosts: 1178 |
Ah Gordon, regardless of any previous interpretation of the voltage doubler you brought it to the forum and as far as we are concerned you are the man. Without it my F&P mill would be lucky to crack out anything on those low wind days and with it I'm good for 500W/hrs that would otherwise not exist. Add that up over a year and its the difference between something that pays for itself and something that wont. I'm a believer! Adonis, cheapest place to find caps and bridge recs (you'll need an additional 4 of those and 12 caps) some things that werent mentioned until after I built mine were to keep the wire runs as short as possible to keep electrical noise down and SOLDER all the connections to the recs as resistance is larger than you might think using spade terminals. Mount the main current handling recs on some kind of heatsink and the doubler ones at least on a piece of steel sheet (side of an old microwave i used -big enough to glue your caps on too) There isnt much talk about correct size for 12V but the consensus from what I read was bigger caps than 24 or 48V I used 680uf 450V which are physically massive and when wired back to back make for 340uF which should be in the ball park for 12V too albeit at the low end of the scale suggested Luck favours the well prepared |
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| adonis Regular Member Joined: 13/02/2010 Location: SwedenPosts: 42 |
thanks for all the help. the voltage doubler thread really is a major read, 37 quite hevy pages, and im not through it yet... but extremely interresting and impressive. there really is some brillient minds at this forum! for now i think i will go with Petes suggestion and dont get too complicated, my turbine is not even finished yet and the diodes i use seems to be quite good, only about .53v forvard drop. but i will certainly try to understand how the voltage doubler works. it may allow me to build a bigger turbine and still have a early cut-in. again: im very impressed Gordon! |
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| KarlJ Guru Joined: 19/05/2008 Location: AustraliaPosts: 1178 |
Tip, no one really understands how it works (other than Gordon) but the fact is it does! kept me awake at night for more than a couple of weeks, in the end just built it and it works Karl Luck favours the well prepared |
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| adonis Regular Member Joined: 13/02/2010 Location: SwedenPosts: 42 |
haha. ok. but at least i will try to understand what you all were talking about when it was discussed. i understand there is no failproof formula to calculate the best capacitance, but at least i will try to get a general idea about the cap size that would be good for my generator. |
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| GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Hi adonis, Do you have any idea of the operating frequency of the alternator at expected maximum rpm? or Can you give the number of rotor magnet poles? I may be able to work the simulator that "sparweb" found, and determine cap sizing for a doubler. Gordon. become more energy aware |
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| adonis Regular Member Joined: 13/02/2010 Location: SwedenPosts: 42 |
thats very nice, but i dont think i will be able to give you enough information just yet. im trying out some different geometries on my balcony for the moment, this summer i hope to have the fist working turbine running at my country house. i am building a savonius/benesh turbine, i think H=2m and D=0,60m to get some small power produced even in small winds. the location is far from ideal, we rarely (if ever) see wind speeds over 10m/s, and usually it is not more than 2-3 m/s. (i realize it is not really worth the trouble putting something up there, but im looking at it as part of my education. im studying mechanical engineering - sustainable energy technology. and its good fun). i have no clear idea about what TSR i can expect, and i hope to find a complete LG-washing machine with shaft and bearings intact. right now im fiddleing with just a stator and rotor from an LG, with a home-made hub. so i dont even know if the generator i in the end will use will look like the one i have now. the stator of the one i currently posess have 36 poles, 3 Phases in delta. im not sure about the wire size but it seems to be around d=.8mm. the resistance over two phases is 7,8 ohm. the rotor has 12 magnet lumps with NSNS, giving it in total 48 poles. i have mounted the motor so it is driven by a bicycle pedal to be able to make some measurements. i can charge a 12v battery through my rectifier bridge (Vdrop=2*0,53v), and at 6,43 rad/s = 61,5 RPM the pole voltage is around 12,3v and the motor is charging the battery. i guess the max rpm will be (61,5/12,3)*14,8=74 RPM. does this make the max frequency 74/60*(48/2)=29,6Hz? i assume i will never produce more than 100W, so Imax after the rectifier will probably be around 8A, im not sure what the max. current AC will be. but as i said, if this is time consuming for you, dont bother. im not sure i will use this motor in the end. thanks anyway. |
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| adonis Regular Member Joined: 13/02/2010 Location: SwedenPosts: 42 |
sorry, i think i made a mistake... the phases is probably connected in what you call star, my swedish terminology differs somewhat from yours. all the three copper wires are connected at the end, this must be star? |
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