Selection of three phase configuration

[Titantornado]

Hi all.  I've been trawlin' the board for a while now, and starting to get things together to build my first mill.  I've extracted a ton of info from Otherpower and this forum, (which is a bit overwhelming, perhaps intimidating) but I've managed to get a lot of things figured out.  I'm waiting on Hugh's book to arrive, which will no doubt probably answer most my remaining questions, but I'd like to field a few here as well.  In this case, I'm trying to figure out which configuration to wire the stator.

From what I've read, using the same coils, wiring in wye will give you higher voltage with lower amperage, and a lower cut-in speed.  Delta gives lower voltage with higher amperage, and a higher cut-in speed.  But running at equal RPM's, the delta will provide higher wattage overall (mathematically anyhow) due to the lower resistance.

Seems to me wye would be better for low wind performance, delta for higher wind situations. (that is using the same coils) So my question is, (ya knew that there had to be one in here somewhere) what would be the result if the delta configuration had some extra turns in the coils to make up some of the voltage difference comparable to the wye configuration, to get the cut-in speed down a bit?  

Just in my mind's thinking, I'd expect a voltage rise, with a slight resistance rise, providing a higher amperage, and overall wattage increase, now at a cut-in speed comparable to the wye configuration. I know Ohm's Law, but many of the formulas I've seen discussed here are a bit beyond my (current) understanding to accurately conclude the results. I realize nothing comes free in physics, but I'm curious.  So, what would happen?  Benefits? Consequences? Opinions?

Thanks for listening to my rambling mind.

Rod

[johnlm]

>But running at equal RPM's, the delta will provide higher wattage overall (mathematically anyhow) due to the lower resistance.<

That statement is not entirely true.  At lower RPM the WYE configuration will supply more power.  At some higher RPM (well above the cutin RPM for either the WYE or Delta configuration) the power output of the Delta configuration will catch up to the WYE and surpass it.  You cannot say that at any given RPM the power output of a Delta is greater than a Wye.

Johnlm

[Titantornado]

Yes, that's correct.  I should have included that detail.  (just thought I was gettin' a bit long winded already)   ;-)

[Flux]

There is only a difference between star and delta because the effective turns are different.

If you raise the number of turns on a delta winding to have the same cut in speed then it will be the same as a star winding of the same cut in speed. It will have the same effective resistance and will have the same current carrying capacity.

There is a difference in performance for the same number of turns, but it has different operating speeds.

The only way to get some of both worlds is to change from star to delta in high winds,you can't do it on one winding. In a good wind area you can improve overall energy capture by winding for a higher cut in speed, you just have to decide how much you value a couple of ampere hours a day in very light winds.

Unless you want to go down the star delta route, I can see absolutely no reason to use delta, it is always less efficient than star when driving rectifiers.

Flux

[JW]

Hi Flux,

 I agree with your post (as usual), and just wanted to say thanks for your well thought out responses here on the board. One quick question thou.

 '(delta) is always less efficient than star when driving rectifiers'

 Is this because the voltages generally obtained with star are higher? And that most, if not all, semi-conductors operate with better efficiency at voltages over 12v.

 Just for clarification- the difference between wye and star is that star omits the center tap. A wye connection has 4 leads and star uses 3.

 I installed this big air compressor that uses a star/delta switch, it switches to delta for more torque when making air. Then it switches to star when idle. But this is a 'driving' application, different from the 'driven' example disscussed here. And its a 3ph ac drive motor so rectifiers are not used.

JW  

[Flux]

JW

I always assumed that wye was a North American name for what we call star, in the same way that some call delta mesh. If you are right about 4 wire star being called wye then I have learned something.

It was common for large motors to start in star with reduced line current and run in delta to develop full torque. Nowadays we just expect even quite large motors to start direct on line in this country. If on the end of a long line we may have to resort to a soft starter, few motors used here are designed for star/ delta any more. They are now wound for 230/415v working with 415 being star.Anyway I digress.

The issue with delta on any form of supply or load other than a pure sine wave is that odd order harmonics (predominantly 3rd harmonic) circulate within the closed delta winding and cause drag and loss. A star winding with no neutral can not circulate any harmonics, it does do strange things with the neutral point voltage but that doesn't bother us.

The effect is not so great as to make delta unworkable but there is typically about 10 to 15% extra loss in delta and it shows most at low loads. That is why I say use it for star/delta connection but otherwise avoid it.

Flux

[kitno455]

how does separate per-phase rectification compair? you can series/parallel those as well.

allan

[Flux]

As far as I can tell, separate rectification of phases with the outputs paralled seems to be exactly the same as delta except that any winding voltage unbalance does not show as a circulating current below cut in speed. As soon as the rectifiers conduct the thing behaves as delta and the magic losses reappear.

I am totally suspicious of series connecting 3 single phase rectifiers as there are phase differences. I have never looked seriously at this separate rectification thing, but if ever it shows to advantage then something wasn't optimum in the first place. To rectify 3 phase, the 3 phase bridge has been the industry standard and no one ever tried to do it by other means, so I shall not be going that way.

Flux

[JW]

Thinking about that, I would suppose that the geometric arrangement/relationship (trigometry) of the coil's too magnet arrangement/ratio is related to the multiphase 360* of waveform. I often wonder about this, with some of the coil to magnet ratios that are not prime. Or an even number of coils to magnets. I guess in theory, its best to shoot for a 3 coil to 4 magnet ratio or the like, at least for conceptual purposes. But in the real world(i think) its ok to waste a little space. Neat disscussion. Its really amazing to me, how much of this stuff has been worked out thru current designs, but it becomes even more facinating when someone explores why this is.

JW

[Titantornado]

"If you raise the number of turns on a delta winding to have the same cut in speed then it will be the same as a star winding of the same cut in speed. It will have the same effective resistance and will have the same current carrying capacity."

Well, that's pretty much what I expected to hear in the end.  "No free lunch" theory continues to hold true.  So it looks like I'll be wiring wye.....I mean STAR, (I learned something too) to take advantage of the much more common light winds.  Getting into a star/delta switching is not something I think I should be getting into for a first mill. (though I how to)