Gen head efficiency - big vs small?

[Lloyd]

Hi All,

I'm going to propose that in a 3-phase, each phase is only energized 1/3 of the time so you have less heating losses, and less hysteresis losses.

While the rotor accounts for much of the above losses, the stator still has a portion of those losses.

Lloyd

[BioHazard]

Just thought this was interesting, here are replacement fans for the ST heads:
http://stores.ebay.com/Central-Georgia-Generator/Metal-Fan-/_i.html?_fsub=1076849012&_sid=54603752&_trksid=p4634.c0.m322
Looks like the same fan is used on 5-12kw, 15-24kw, and 30-50kw gen heads.

[bear]

eny fool should know that a gen has to spin at a set speed .to get corect volts out.next the size of driving moter will give you usible amps.bigger the drive force in the more amps out.up to the size of the getset

[WStayton]

Lloyd:

  I don't think it has anything to do with the phases only being energized for 1/3 of the cycle, since each phase is a, more or less, perfect sine wave that goes from zero to + 120 to zero to -120 to zero, on each revolution of the rotor, or on each half rotation for four pole,  1800 rpm units - so if you just look at one phase, it SHOULD look exactly like the single phase in an ST head.  For three phases, they just "stack: three of these "simple" sine waves at 120 degrees to each other.

  The above was the reson that it seemed strange to me that a 3-phase should be more efficient than a single phase.

BioHazard:

  If you look at all of the specs for a range of ST-STC generator, you will discover that they have more in common than just the size of the fan - like, for some lines, the 15 kW, 20 kW and 24 kW all have the same shaft size, like wise, lots of different size units have the same size brushes, etc., etc.

  I THINK that this is just to make life less comjplicated for the guys assembling the units . . . so for the 15 kW / 20 kW / 24 kW example, they just used a shaft that was "adequate" for a 24 kW unit and way over "designed"  (word used advisedly!) for the 15 kW unit - the cost of making the shafts "too big" for the smaller units must not be as great as the cost of stocking/having/inventorying exactly the "right" size shaft.

  All of which lends credence to the theory that they don't really engineer these things to an exacting spec, but rather just use lots and hope it is enough - clearly demonstrated by the fact that a commercially made US generator is about half the weight of an ST/STC unit.  If you were "real" concerned about the cost of materials, you could easily take 1/3 of the weight out of the unit - albeit at the sacrafice of some of the reliability of having the unit exceptionally solid/firm/unyielding.

  That's my take, YMMV!  <grin>

Regardz,

Wayne Stayton

[Lloyd]

Lloyd:

 I don't think it has anything to do with the phases only being energized for 1/3 of the cycle, since each phase is a, more or less, perfect sine wave that goes from zero to + 120 to zero to -120 to zero, on each revolution of the rotor, or on each half rotation for four pole,  1800 rpm units - so if you just look at one phase, it SHOULD look exactly like the single phase in an ST head.  For three phases, they just "stack: three of these "simple" sine waves at 120 degrees to each other.

 The above was the reson that it seemed strange to me that a 3-phase should be more efficient than a single phase.

unyielding.

 That's my take, YMMV!  <grin>

Regardz,

Wayne Stayton

Wayne,

Stop and think for a minute.  2 gens of equal output 1) single phase, and  2) 3 phase.

The 3 phase you can make the same power on smaller size conductor windings, with less heat losses, and associated magnetic losses.

Why you ask, bc you are carrying the same volt/amps over three different circuits, each phase is only energized 1/3 of the time, to make the same power.

Lloyd

[Lloyd]

3-Phase Efficiency =                 746 x Horsepower              
                                    Volts x Amperes x Power Factor x 1.732

Single-Phase Efficiency =           746 x Horsepower       
                                        Volts x Amperes x Power Factor

[WStayton]

Lloyd:

  Join me here for a minute in a little "thought experiment". . .

  Suppose., on one hand I have a 15 kW STC 3 phase generator and on the other hadn, I take three eash 5 kW ST generators and "weld" their shaft together so that they are 120 degrees out of phase with each other.

  If I combine the output of the 3 ea 5 kW heads into a three wire bundle, do I not have 15 kW of three phase power???

  There will be the same current in each phase of both set-up, presuming I l;osd them with the same load. 


   Why would the three generators welded together be less efficient than the single 3-phase generator, neglecting the extra bearing loads - presumably they would have the same size wire in their windings, etc., etc.

  Doesn't any generator have the wire windings sized for the current that they are going to carry, so then, the losses SHOULD be the same, no???  If I run 30 amps through a 1,000 ft of # 10 wire or 20 amps through a 1,000 ft # 12 wire aren't the losses the same - more or less??

Next question: Where did the "1.732" cpme from in your efficiency eqution for a 3 phase generator???  If that is true, then a three phase generator should be 43% more efficient than a 3 phase unit - is that right??  Remember, I'm a MECHANICAL engineer and only just barely belive in electrons which I havn't ever seen (But have FELT!) <grin>

Explain please, I WANT to get less dumb!!  <smile>

Regardz,

Wayne Stayton

[sailawayrb]

Lloyd is correct.  A 3-phase generator (i.e., a 3 wire polyphase system with square root of 3 power advantage...1.73) is more efficient than a 1-phase generator simply because a 3-phase uses significantly less conductor material than a 1-phase generator.  Using less conductor material means less conductor resistive losses (i.e., less I**2 x R losses) to generate the same power...therefore, more efficient.  Or another way of looking at and quantifying it, 3-phase transmits 73% more power but only uses 50% more wire...therefore, 146% more efficient.  3-phase was also invented by Nikola Tesla, so there is little doubt that it is a better approach for power generation and why it is so widely used worldwide.  I am a mechanical engineer and a electrical engineer working for an aerospace company my entire career.

Bob B.

[mike90045]

.....   Why would the three generators welded together be less efficient than the single 3-phase generator, neglecting the extra bearing loads - presumably they would have the same size wire in their windings, etc., etc.  .....

You now have 3 field windings, which is 2 more than 1.  That's a good chunk of power.

[deeiche]

rm /

[Thob]

Here's one way to look at the issue of efficiency of a three phase generator vs single phase.  A certain amount of power is lost in the field winding, this power is heat generator by current flowing thru the resistance of the wire.  If this is DC (see the thread on ST waveforms for what happens when it's not) then the power is constantly being wasted.  In a single phase generator, you have a part of the time when the waveform goes thru zero that the generator is not generating any power, but power is still being wasted in the field.  In the three phase setup, due to staggering the power windings, there is always some winding generating power.  So slightly less waste.

In the case of using 3 single phase generators to make 3 phase, you have 3 field windings, each one wasting power, so you wouldn't get this advantage.

As for rectifying the output and using an inverter, yes that is done on small generators.  Search for inverter generators.  I think they usually employ permanent magnet 3 phase generators, rectify that, and then use an inverter to create the AC.  The other nice thing about this is that the frequency is now controlled by the inverter, not by the shaft speed.  So the engine RPM can be varied to match the load, and the inverter pulse duty cycle varied to get the proper voltage.  But you have to include the inverter (in)efficiency in the equation.  People also use a 3 phase head to charge batteries (thru a rectifier) and then run an inverter from that.

[Ronmar]

I'm pretty ignorant when it comes to generators so this most likely is a stupid question.
Given the greater efficiency of a 3 phase generator can you rectify it's output, then combine the 3 DC sources in to an inverter?

You just described an automotive alternator.  They are a 3 phase AC generator, with the outputs rectified and combined.  The beauty of combining and rectifying 3 phase is that because you have 3 times as many half cycles, once rectified, they overlap, so the valleys are much smaller, and require much less filtration to get a clean DC output.  Here is what single phase looks like rectified:


Here is what overlapping 3 rectified phases does to the waveform:


But since it is pure DC below the bottoms of where the valleys meet, you are left with this waveform which is nearly pure DC with only a little ripple to clean up...



I havn't tried it, but with the supposed efficiency of induction motors used as generators, a 3 phase induction motor with the output rectified should be a pretty efficient way to make DC.  Inductions as generators do not have any way to easilly regulate the output, but with fixed caps to provide an acceptable output that could be rectified and fed to an inverter, that might be the ticket.   With the variable input voltage of solar systems, the tech is already available to deal with this, you just need to select capacitors to get the output of the induction motor into a range that is acceptable to the inverter.      

[LowGear]

I sure wish someone had an off the shelf system that even I could hook up to my 8 HP Witte and produce about 350 VDC to feed the Sunny Boy.  Its getting close to crunch time as the back hoe is coming in next week to start the generator building hole.  If I get right on it I'll be producing electricity after dark in about two years.   

Casey

[WStayton]

LowGear:

  It sorta looks like you operate with the same "sense of urgency" that I do!  <grin>

  Glad to see that I'm not the only one who like to sit back, take aseond, and a third, and . . . look to make sure I'm not screwing up!   And I STILL manage to screw-up occasionally!  <smile>



  About the increased efficiency of three-phase heads over single-phase heads:  I THINK I am finally beginning to understand why a thrre phase is more efficient - now, don't anybody say anything and confuse me AGAIN!!


  Thanx for all the input to my confused musings!  See, if you point enough and shout enough and pat on the back (low enough!) even a mechanisal engineer gets it . . . eventually!

Regardz,

Wayne Stayton

[sailawayrb]

http://www.allaboutcircuits.com/vol_2/chpt_10/2.html

http://www.allaboutcircuits.com/vol_2/chpt_10/5.html