Report KD 712 about a 5-phase and a 9-phase PM-generator available

[Adriaan Kragten]

Report KD 712 can be copied for free from my website: www.kdwindturbines.nl at the menu KD reports. The title of this report is: "Ideas about a 5-phase and a 9-phase PM-generator". Especially the 9-phase generator has a very low fluctuation of the rectified DC voltage. The way of rectification with three separate 3-phase windings as described in chapter 6.2 is most promising as this way gives the highest power. A low fluctuation of the DC voltage might be favorable if the generator is grid connected with an inverter with a DC input voltage.

[Adriaan Kragten]

Chapter 6 of KD 712 has been extended. A 9-phase winding also produces more power than a 3-phase winding if the 4-pole armature, the stator stamping with 36 slots, the total amount of copper and the rotational speed are the same. This is because for the 9-phase winding, all coils have the optimum angle of 90° in between the left leg and the right leg of a coil but for a 3-phase winding there are six coil bundles with three coils around each other with angles of 70°, 90° and 110°.

[Adriaan Kragten]

A new chapter 7 has been added to report KD 712 about 5-phase and 9-phase rectification. The title of this new chapter is: "Ideas about a 32-pole generator with a 9-phase winding". This winding is much simpler than the 9-phase winding of a 4-pole motor as described in chapter 5. A side view of the armature and stator is given in figure 10 which is added as an attachment.

[SparWeb]

Hi Adriaan,

Have you ever tried publishing your papers in technical journals?

[Adriaan Kragten]

Hi Adriaan,

Have you ever tried publishing your papers in technical journals?

Yes, I have informed several Dutch magazines and several news papers about the KD-reports written in Dutch and about recently written Dutch notes about the energy transistion but never got a reaction. I think these reports and notes are too specific for the general public. But report KD 35 is used as basic report about wind energy at the Technical College of Rijswijk. I have posted the availability of my KD-reports on several forums and if someone is interested in a certain described subject, the report will be found through Google. May be some ideas will be picked up only after many years. I don't mind as I no longer have any commercial interest.

About this report KD 712 about 5-phase and 9-phase rectification, I have informed the Laboratoy of Electromechanics of the University of Technology Eindhoven and asked their comment. But even they don't react. I think that especially 9-phase rectification is of interest for big grid connected wind turbines as these turbines always have a DC step in between the generator and the inverter. The fluctuation of the DC voltage for a 9-phase generator is very much smaller than for a 3-phase generator and this eliminates the need for capacitors. Another advantage of a 9-phase generator above a 3-phase generator is that all coils can have the ideal pitch in between the left and the right leg of a coil and therefore the maximum power which you can get out of a certain generator size at a certain rpm, is higher.

If you look at the 3-phase winding of a normal 4-pole asynchronous motor with a stator with 36 slots, you will see that the coils are laid in six bundles of each three coils. The three coils of a coil bundle are lying around each other. A coil bundle consists of a coil with a pitch of 70°, a coil with a pitch of 90° and a coil with a pitch of 110°. Only the middle coil has the optimum pitch as 90° is the pitch in between two armature poles. So the 70° coil and the 110° coil supply a lower voltage than the 90° coil if all coils have the same number of turns. A lower voltage also means a lower power. For a 9-phase winding of a 4-pole generator, all coils can have the ideal pitch of 90°.

[DamonHD]

To SparWeb's point, not just the general press: actual technical/scientific/engineering journals, or the energy trade press?

I am just embarking on a PhD and I have been used to writing things up on my own site, but I am about to have to make the transition to formal papers in journals, at least alongside, and I assume that that will be uncomfortable.

Rgds

Damon

[joestue]

Your 32 pole, 36 slot machine has a winding factor of .945 when configured for 3 phase, when connected for 9 phases it will probably have the same. the theoretical lap winding of 3 slots per magnet pole has a winding factor of 1, in practice this is never achieved. but if your winding has a winding factor of say .86 (for a 24 pole 36 slot machine) it will only produce 86% of the voltage that a different coil configuration could theoretically produce with the same turns, and same volume of magnet. i have generally found this site to be pretty accurate https://www.emetor.com/windings/

the concentrated pole machines have significant third and higher harmonics and i suspect this will hold true for higher order phase connections as well... as the fundamental problem is that in order to get a sine wave machine you have lots of coils distributed around the magnet, or you have magnets tapered in ways to get a sine wave... but if you make a sine wave from distributed coils.. it adds copper losses. if you taper the magnet to produce a sinewave from a single coil, then you have removed magnetic flux from the machine that could have otherwise been flowing in the machine.

for example:
i rewound a very common 2 pole, 24 slot, 5KW "generator head" with a 3 phase lap winding with a pitch of 10. i took out 3 kilograms of copper and put 5 kilograms back in the machine. i was able to measure about 16 missing volts when comparing the line to neutral vs line to line voltage, representing third harmonic that was canceled out in the star configuration. you seem to be well aware of this with your measurements of shorting the neutral point to the line, for more resistance to turning forces. this is also why i realized a few years ago that the best test of a high torque low rpm generator is to short the windings and apply a torque and see how fast the shaft turns. if re-configuring the magnets for 9 phases produces slightly better results, then its better. technically, this has to be done after the rectifier.

so as for a 9 phase 32 pole machine, vs the same coils configured in 3 phase... which one is more efficient when driving a "dumb" 9 phase rectifier...  that may depend on the specifics of just how much third harmonic is generated. it is true that the dc coming out of a 9 phase machine will be on the order of 3 times smoother than a three phase machine, but as far as the electrical efficiency of the generator.. it may not be improved at all.

much larger machines are designed to produce clean sine waves for a second reason: less eddy and hysteresis losses in the machine.

a small machine intended for simple 3 phase recification (or 9 phase) would likely do better to be designed for a trapezoidal voltage waveform.

most small BLDC motors for electric airplanes, scooters, ebikes.. are designed this way as far as i know. or it may be that its just simpler to drive them with a "dumb" 3 phase driver that simply pwm's the coil for a set number of amps, then turns the coil off and turns the next one on. making no attempt at all to push a sineusoidal waveform into the motor.

"dumb" rectifiers draw basically square wave amps from the machine, and a 3 phase trapezoidal emf generator will produce nearly constant dc current out of the rectifier, especially if an inductor is added after the rectifier.
a 9 phase machine required a third as much inductor (by energy, which means the inductor need be but half the size)

[MagnetJuice]

I am not sure if this has any relevance to this subject, but it looks interesting.

Power Transmission Engineer Lionel Barthold explains how 3 phase, 6 phase, and 12 phase power works, advantages, disadvantages.

https://www.youtube.com/watch?v=HqZtptHnC2I

[Adriaan Kragten]

There is a difference in winding factor in between the 9-phase winding of a 4-pole generator as described in chapter 5 and the 9-phase winding of a 32-pole generator as described in chapter 7. For the 4-pole generator, the angle in between the left and right leg of a coil is 90° for all coils which is the same as the angle in between a north and a south pole. So the winding factor is 1. However, the coil heads of these coils are rather long and so the coil resistance is larger than for the much shorter coil heads of the 32-pole generator. For the 32-pole generator, the angle in between a north and a south pole is 11.25°. The angle in between two armature poles is 10°. This result in a winding factor of 10 / 11.25 = 0.889 (if I use the correct meaning of winding factor). So this is worse than for the 4-pole generator. But the winding itself is very simple as there are no crossing coil heads. Another big difference is that the frequency of the 32-pole generator is a factor 8 higher than that of the 4-pole generator. This might be an advantage for slow running direct drive generators of big wind turbines for which one may take even more than 32 poles like 64 poles for a stator with 72 slots.

A normal 3-phase winding has six coil bundles with each three coils. Figure 1 of report KD 718 shows such a stator winding. The inner coil of a coil bundle has a winding factor of 70° / 90° = 0.7778. The middel coil has a winding factor of 90° / 90° = 1. The outer coil also has a winding factor of 0.7778 because for the part of the coil in between 90° and 110°, the magnetic field is flowing in the wrong direction. So the average coil factor of all three coils together is (2 * 0.7778 + 1) / 3 = 0.8519. This is worse than for the 32-pole generator. 

[joestue]

Winding factor is a little more complex than just the angle of the magnet compared to the stator, but it's close.

It also has to do with the position of all the coils in the machine and how they add or cancel out.

For example, some coil magnet combinations, winding every stator tooth might be .96 winding factor, but winding every other tooth gets you .933, or vis versa.

Winding every stator tooth separately reduces the end turn length ever so slightly, and it also increases thermal conduction to the stator.


As for the performance of this type of machine, I have a 30 pole 36 slot machine made from 30x .25 by .125 by 2inch magnets. It has no problem generating 500 watts of 120vac at 400hz with half as many losses as the 1/3rd hp induction motor it once was.

[SparWeb]

I am just embarking on a PhD [ ...]
Damon

I hope that goes well.  GOOD LUCK!

[DamonHD]

Thanks SparWeb!

Damon