Author Topic: 3 phase versus single  (Read 5461 times)

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Offline artv

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3 phase versus single
« on: January 08, 2012, 06:48:39 PM »
Hi All,
I've been testing a modified car alt I received from a very generous member from this and the "other" board.
It was wired star ,with a built -in rectifier, it had one lead out +,..I assume the shell is -,
Took it apart, found the star point ,now it's wired IRP
Anyone of the single phase's (by itself) put out ,..21.4Voc, ..connect 4 lights,..7.3Vdc,...9.3amps=68w
When I series' the 3 outputs ,...62Voc,...lights,..8.3Vdc,...10.43amps=86.6w
Why don't the phases add up ???
I've tried everything under the sun,  but they just don't add up??
Shouldn't it be,...3x68=204,...minus doide and resistive losses??
Each phase puts out 68watts (with 4 lights)....

Does this have to do with "matching the load"???
Be careful what you ask for ;)...........Thanks.......artv

Offline rossw

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Re: 3 phase versus single
« Reply #1 on: January 08, 2012, 07:01:32 PM »
I replied to your (almost identical) post on the other forum.

Offline Wolvenar

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Re: 3 phase versus single
« Reply #2 on: January 09, 2012, 12:23:25 AM »
I don't know if you got any responses on the other board except Ross pointed out here that he responded, I haven't read it there yet. But if I know him at all, I am sure Ross pointed out that since it's 3 phase the phases wont just add up. When one phase is fully charged in one polarity the others are not going to be at that same relative point. You could play with capacitors, but they wont work with anything big without making it more costly than its worth.
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Offline artv

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Re: 3 phase versus single
« Reply #3 on: January 09, 2012, 04:20:32 AM »
The #'s above have nothing to do with caps.
IRP, each phase has it's own bridge, then you wire the bridges in series, for the overall output.
Is that not how it's done?
68 watts out of any (one) of those outputs, series them up and I only get 86.6 watts??
Seems the space would be better utilized just making single phase??
I know I'm wrong but why?.....artv

Offline rossw

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Re: 3 phase versus single
« Reply #4 on: January 09, 2012, 04:22:48 AM »
The #'s above have nothing to do with caps.
IRP, each phase has it's own bridge, then you wire the bridges in series, for the overall output.
Is that not how it's done?

Short answer?  No, that's NOT how it's done.

You NEED to understand AC theory here as well as the simple arithmetic sum of voltages in simple DC circuits.
If you draw the 3 phases IN THEIR CORRECT RELATIVE PHASE, then draw what they look like once they've gone through a bridge rectifier, again IN THEIR CORRECT RELATIVE PHASE, and *THEN* sum them, you might see the answer.

"I can explain it to you, but I can't understand it for you"

Offline Wolvenar

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Re: 3 phase versus single
« Reply #5 on: January 09, 2012, 05:52:11 AM »
Here is a good website with a primer on single and poly phase motors
It may *help* you understand enough that a bit of googling could fill in the gaps

http://www.coilgun.info/northrup/theory.htm

I can use it to give you a basic idea, though not a complete explanation
If I am wrong, anyone, please step in and correct me

Here is representation of a single phase AC

This has a single rise and fall of potential, As illustrated a cycle is measured from one peak to the next.
If you had another alternator which is locked to match the rise and fall of this phase, you could increase your voltage through placing them in series.




Here is a 3 phase AC representation

This shows how each phase separated by its position relative to its rise/fall of between max+ 0 and max-

It should be easy to see by this image, that while phase(a) at its max+ none of the others are.
In fact phase(c) is on its way the opposite direction, but not at the same degree as phase(a).
They are 240 degrees separated.

I am not an expert at this, so keep that in mind.
I hope this helps give you an idea why it does not simply "add up"


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Offline rossw

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Re: 3 phase versus single
« Reply #6 on: January 09, 2012, 04:46:23 PM »
OK, I don't usually do this... but to make the point, I just quickly mocked it up for you.

Here's a graph showing ideal 3-phase. Voltage (vertical scale) against time (horizontal). Each phase is 120 degrees apart.
ila_rendered

Here's what each phase looks like once you full-wave rectify them.
ila_rendered

And if you were to simply add them in series you get this
ila_rendered

The volts scale is relative, so if you have a 30v input, "1" = 30V.  "2" = 60V etc.

Does that make it any clearer?

Offline artv

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Re: 3 phase versus single
« Reply #7 on: January 10, 2012, 06:19:50 PM »
Wolv & Ross,
Thank-you for taking the time,...but I'm pretty sure I have a handle on 3 phase ..
I'm using IRP ,when you put the 3 individual rectifiers in series, the output voltages add..
But being 3 phase ,only one of those phases is at max output,..one is climbing towards full output, while the other is declining from full output...
Wouldn't it be better to have all the coils at max out all the time (single phase) as opposed to one third of the time....I know this would result in sort of a pulsed situation (bad vibration)...
When the 3 phases are putting out 68watts each, you should be able make use of it , instead of only 86watts........doesn't make sense to me.........artv

Offline rossw

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Re: 3 phase versus single
« Reply #8 on: January 10, 2012, 06:31:16 PM »
Wolv & Ross,
Thank-you for taking the time,...but I'm pretty sure I have a handle on 3 phase ..
I'm using IRP ,when you put the 3 individual rectifiers in series, the output voltages add..
But being 3 phase ,only one of those phases is at max output,..one is climbing towards full output, while the other is declining from full output...

That's a pretty 2-dimensional view of it, but in the context, yes.

Quote
Wouldn't it be better to have all the coils at max out all the time (single phase) as opposed to one third of the time....I know this would result in sort of a pulsed situation (bad vibration)...

You know the problems people complain about with turbines not "getting away" at low wind speeds? If all phases were in-phase, it'd be an unimaginably difficult thing to handle. 3phase applies virtually constant torque (load). Single phase is very "lumpy"

Quote
When the 3 phases are putting out 68watts each, you should be able make use of it , instead of only 86watts........doesn't make sense to me.........artv

You certainly CAN make use of it. It's just that you're trying to use it the wrong way.

Sure, a shifting spanner CAN be used as a hammer. It rattles some, and it doesn't hit square, and it's hard to hold. Should they re-design it so it works better as a hammer? Or should you use a hammer (the right tool for the job)??

Offline Wolvenar

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Re: 3 phase versus single
« Reply #9 on: January 10, 2012, 07:03:54 PM »
I think this is a matter of not fully understanding your needs artv
as 3 phase is superior if matched to your load and needs. it DOES have the capacity to deliver exactly the same power, if matched the same as a single phase.
The more phases the smother the genny will load the blades, and the lesser the ripple to the rest of your RE system as the phases will cut the time that in between the total output drop/reversal
on one hand a single phase unit is technically "stackable" but such a setup has many other drawbacks, and essentially self defeating in nature to rewire a polyphase to a single.

However, what we are pointing out is you just cannot *stack* 3 phases to make up for that mismatch.

You can see if you can change your wiring config inside  from  Delta to Star to see if you can obtain a better match.
I do not know where you are with this, or the details of your alternator. If you can supply more information , where yo are, what you have tried, and maybe some images I think I, or anyone else with experience could better help you.

We are currently having to make assumptions how your setup, this leads to misunderstandings, and maybe not the best answers. It can be frustrating to everyone.

Quoting a friend, this is all part science, part black art.
I'll do my best to help where I can
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Offline artv

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Re: 3 phase versus single
« Reply #10 on: January 11, 2012, 07:06:50 PM »
Hi Guys,.. I really appreciate the replies, I should state that, I am just trying to get the most out of the generator .....I figure the more I can get out the more I can use..I'm thinking of building a big tower, with big blades,something that won't even see one of these generators as a load...
have an engage an disengage to the genny's when needed........
The genny's will be on the ground in a shop , the only thing in the air will be the blades and the differential gears .........It might be a wild dream , but the tower is going up this spring.....
I'll definately need help with the blades...........
Just want to build the best generator first ,before I go to all the work involved with tower construction......I can build just about anything under the sun .....but all this electrical stuff has me confused........ok you can laugh now .....but time will tell......Art V

Offline tomw

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Re: 3 phase versus single
« Reply #11 on: January 11, 2012, 08:25:05 PM »
but all this electrical stuff has me confused........ok you can laugh now .....but time will tell......Art V

There is nobody on this planet that was born knowing this stuff.

Period.

Do NOT mistake me for any kind of "expert".

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Offline Wolvenar

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Re: 3 phase versus single
« Reply #12 on: January 11, 2012, 08:28:52 PM »
Sounds like you have a lot of fun work ahead.
 
We will help as much as we can but the more information on what you got,
within reason of course we don't want to know if you farted building it or anything.
But general the better we can help you then
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Just to abuse what I make. (and run this site)

Offline ChrisOlson

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Re: 3 phase versus single
« Reply #13 on: January 12, 2012, 08:16:38 PM »
I'm using IRP ,when you put the 3 individual rectifiers in series, the output voltages add..

I think is one of the problems - you don't put the bridges in series with IRP.  The DC side is hooked in parallel.  IRP delivers the voltage of one phase, with three times the current of one phase.  It's kind of like a delta connection without the delta connection.

On the single phase issue I have found, despite the loading characteristics and relative "inefficiency" of single phase vs three, that in the real world it does work OK on a wind turbine.  The reason it works is because most three-phase gens are too "stiff" and won't let the blades spin at optimum Tip Speed Ratio once a load gets applied to the gen.  Single phase is less "stiff" because it only loads the input shaft when the voltage of the sine wave is high enough for the diode to conduct.

When it comes to getting power out of a wind turbine, having the rotor running at optimum TSR delivers the most power it can make to the input shaft.  That is usually way more important (with higher power yield) than how many phases the generator has.

With the single phase experiments I have done with geared generators, they worked fine and ran fairly smooth with no noticeable vibration.  At slower speeds and frequencies, however, single phase can be a bear because it will make your turbine and tower rumble pretty good due to the cyclic loading of the input shaft.
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Offline DBCollen

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Re: 3 phase versus single
« Reply #14 on: January 13, 2012, 10:34:59 PM »
but all this electrical stuff has me confused........ok you can laugh now .....but time will tell......Art V

There is nobody on this planet that was born knowing this stuff.

Period.



 I was born knowing EVERYTHING, but I forgot almost all of it before I learned to talk.  :o
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