Generator Efficiency - Again!

[WStayton]

Hi Guys!

  I've been wrestling with the generator efficiency thing as a function of the number of waveforms being generated.  I.e. single phase, vs, three phase.

As Lloyd has pointed out to me in another post, efficiency of a generator head is influenced by a factor of one over the square root of the number of phases. So, a three phase head has an efficiency inherantly better than a two-phase/split-phase head (isn't this what a 120/240 head with the capability to make the 240 out of two 120 phases is?) by the ratio of one over the square root of three vs one over the square root of three.

But, I THINK (always dangerous!) that there is something wrong with the equations that Lloyd gave me, because when I do the math with a one HP generator running at 120 volts and 6.21667 amps (for a 1 hp output) with a power factor of 1 - which I guess would have to be a purely resistive load - I get the following efficiencies:

     Number of Phases       Efficiency
                1                     1.000
                2                     0.707
                3                     0.577
                4                     0.500
                5                     0.447
                6                     0.408

  I think that what I have calculated here is the current through the return wire for each case, so for a single phase, 100% of the total current goes back through the return wire, and 70.7% of the current is returned through the neutral phase for a 2 (split) phase sytem and 57.7% of the current is returned through the neutral for a three phase system, etc., etc. no?

  Anyhow, that is just a slight side track to my REAL question, which is:  Does a generator get more efficient as I add more and more "phases" to it?

  If, in fact, I get increased effiency, ad infinitum, as I add more and more phases to the generator - how come I can't google and find a four or five phase generator to buy?  Obviously, there is a limit, since there actually has to be a coil of wire wound and stuffed in the generator for each phase that I add, and, eventually, there just isn't going to be room for them, but I would think that if I can fit three coils of wire into a generator, I could, by just being a little more ingenious and careful and attentive to details, add another coil or two to the pile.  And, if in fact, this will make a more efficient machine, why doesn't anybody do it!

   I suppose that the obvious answer is that its not a simple as just making the generator, I would have to build five phase motors and controllers and . . . ad infinitum, too, but it seems strange, to me, that if there are gains to be made this way, that nobody does it!

  Anyhow, that's my question for the day!

  Responses are hereby solicited!

Regardz,

Wayne Stayton

[mobile_bob]

when all the math is done, all the testing completed i think you will find that most single phase heads
in the sub 50kva class will come in at between 78-80% efficient
and most of the three phase heads will come in at between 82-85% for the sub 50kva class units which covers
everything most of us are going to be using.

the low side going to the smaller units and the upper range of efficiency going to the upper side.

that will get you as close as any calculations

the problem with trying to calculate is lack of data for the various factors involved
not knowing iron losses, copper losses, windage and frictional drag, airgap, and other more detailed
factors leave too many variables to come to an accurate calculation in the real world.

bob g

[WStayton]

mobile_bob:

   But, how about if I had a four pole or five pole head, would they be more efficient yet than a three pole head? 

  Of course, that assumes that I could find something to drive with a four or five phase AC!  <grin>

  This all has absolutely NO practical value, since I can't find a single four or five or more pole generator head for sale any place, and if I could find one, what do think my chances would be of finding a corresponding number of poles motor?  I'd say probably something like 1 in 10 to the 20 power!  <grin>

  I am just curious if  more poles, than three, would increase your efficiency, yet more than a three pole head is relative to a one pole head!

  Enquireing minds, and dumb ones like mine, want to know!

Regardz,

Wayne Stayton

[Lloyd]

Wayne,

You came close to answering your own Q here.  

http://www.microcogen.info/index.php?topic=2048.msg24156#msg24156

bschwartz:

 With "single" phase you have just TWO sine waves of voltage going back and forth between +120 and -120, for a net of 240 when you pick up both sides that are exactly (180 degrees) out of phase.

 With three phase, you have THREE each, single phases going back and forth between +120 and -120 just the same as in 120/240, but they are 120 degrees out of phase, not 180 like with the two phase, so when you go across two of them you don't get 240 volts net, but something like 210 volts net.  Most things designed to work on 240 volts will work on 210 volts, but that isn't universally true and there may be some loss of power and/or efficiency.  

 Again, I hope that my clarity or lack thereof has not further confused the issue!

Regardz,

Wayne Stayton

But single phase falls to 0 each cycle, and three phase never falls to 0, during a cycle, it's always conducting, that one of the reasons for the better eff. sq3, next your generating higher power on smaller wire.

And finally typically 3-phase you get 208 volt instead of 210v, which is used in 3-phase industry all the time.

Lloyd

[mobile_bob]

on very well made alternators from back in the day or the golden age of design
it was determined that the point of diminished returns happen very quickly after you do three phases
for instance if you have an 85% efficient three phase you might gain another 1% going to 4 phases, less than another 1/2/% going to 5 phases
and less for more phases.
they found out that a good balance was reached at 3 phase and quit there, finding that increasing efficiency was easier by simply going to larger diameter rotor/stator assemblies which of course led to higher outputs, they also found that efficiencies really increase with much higher voltages and much larger machines, both of which are orders of magnitudes larger than what we will use.

if you are interested in alternator/generator design there is a book on the subject which title escapes me just now
but is in the white paper section of our forum, you can find a copy and buy it or download the online version as  pdf file
and learn everything you ever wanted to know about design.  bear in mind 99% of all that is known about alternator/generator design
was well understood back between 1880-1890, and certainly by the early 1900's when 95-98% efficiencies were attained with regularity
albeit with very large, high output machines relative to the puny little units we have to work with.

bob g

[WStayton]

mobile-bob

  Yet another demonstration that if some is good, more is not necessarily better!  <grin>

  I DO see what you are talking about though - there just ins't much unused real-estate (phase wise) to pick-up by adding more phases to the picture!!!

  Okay, another dumb idea, re-retired to its resting place!

  Thanx for humoring me, guys!  <grin>

Regardz,

Wayne Stayton