Magnet Shape

[LazyYogi]

Hi, I am trying to find out which magnet shape fits better with a double layer concetrated winding (trapezoidal coils) and why...

I am trying to choose between trapezoidal and rectangular magnets. The easy way out would be the trapezoidal, becouse of the same shape but I would like to know more... In addition, trapzoidal magnets cost more...

Does the shape of the magnet (trapezoidal, rectnagle etc) have any impact on the quality of the induced EMF? Are there going to be harmonics if the shape is not correct and how much should this concern me when building a 3 kW coreless axial flux generator?

I have seen home made wind turbines with rectangular magnets and both rectangular and trapezoidal cois but none with both magnets and coils trapezoidal (though I have met them many times in papers...)
In addition, it is said in literature that choosing a pole arc/pole pitch ratio ai=2/pi means sinusoidal emf... But in the generators I have seen ai is different. Doesn't this mean that the induced voltage will not be sinusoidal? Should I care about it?

Is there any way to assure that the voltage will be sinusoidal? And generaly, does anybody know what is the optimal magnet shape for my trapezoidal coils and why?

Thank you very much for your help...!

[ghurd]

Not much help from me.

Wondering why you care what the wave form looks like, or how sinusoidal it is?
Nearly all outputs get converted to rippling DC anyway.

The general concensus is trapezoidal coils gain a hair more surface area, but have considerably more resistance.
G-

[Flux]

For axial machines trapezoidal magnets make sense but as you say they cost more, standard blocks produced in large quantities are cheaper than specials.

Waveform depends on far more things than magnet shape and for most purposes if not very relevent. When you rectify the output and clamp it to a battery the waveform is messed up beyond recognition so you may as well choose the magnet shape on other considerations. Many very satisfactory machines have been built with rectangular or circular magnets.

Output mainly depends on total flux and you can just as well choose the cheapest shape to obtain the flux you want as long as the shape is reasonable.

Flux

[LazyYogi]

Isn't there going to be any change in the efficiency of the generator?
 Perhaps trapezoidal magnets with trapezoidal coils will be more efficient or not?
And an emf that is not sinusoidal doesn't imply that there will be harmonics that are useless and will only produce losses?

[Hilltopgrange]

Quote “I am trying to choose between trapezoidal and rectangular magnets. The easy way out would be the trapezoidal, because of the same shape but I would like to know more... In addition, trapezoidal magnets cost more...”


There is another option the pole can be be made up from multiple magnets of the same polarity. I used this approach on a 16ft machine, I used the common/cheap 2x1x.5 magnets to form poles in the shape of a capital T. It was quite easy to do and a lot cheaper than buying shaped magnets, It has worked well for me!

Details of the build are here,

http://www.fieldlines.com/board/index.php/topic,138389.html

In my case I got a good deal on the magnets and had them at hand so that's what I used.
I toyed with mounting the magnets side by side to make a 2x2x.5 block but when I drew it out I found the T shape allowed more room for copper with a wedge shaped coil.
It is all about compromise and cost, there are many ways to undress a feline!

Russell

[Flux]

Isn't there going to be any change in the efficiency of the generator?
 Perhaps trapezoidal magnets with trapezoidal coils will be more efficient or not?
And an emf that is not sinusoidal doesn't imply that there will be harmonics that are useless and will only produce losses?

What do you expect with efficiency. If you are generating sine waves for a conventional load then yes harmonics will lower efficiency.

Once you decide to rectify with simple diode rectifiers then the harmonic content must go up and the power factor will be less than unity.this is true of rectifiers feeding resistors. Once you clamp the voltage to a battery then harmonics are inevitable and you will suffer losses, why worry about a few % harmonic distortion in the open circuit waveform when the loaded waveform is chopped beyond recognition.

Another factor is that if you could achieve 100% alternator efficiency you would have a constant speed machine that would produce a dreadful match to the prop power curve and you would be an order of magnitude worse off overall than accepting an alternator efficiency below 60%.

If you want absolute perfection then go for mppt and use a low distortion active rectifier such as a Vienna rectifier. You will produce an excellent machine but it will be impossibly non cost effective. Fine if it is your pet project done for love, but not a commercially viable scheme.

Just to add a little more to your problems, using trapezoidal magnets and coils will not guarantee any approximation to a sine wave, the inherent flux linkage is trapezoidal ( near rectangular so to get a sine wave from phase volts you will need to proportion the winding distribution so that the odd harmonics are mainly suppressed.

With normal proportions the phase volts of most windings is near triangular with in phase 3 rd harmonic. The line voltage of a star winding will be very close to sine wave but this will only apply into resistive or at least linear loads. All gores out the window with any form of non linear rectifier load.

Flux

[LazyYogi]

I will use the generator in an ac connection... This means generator-rectifier-inverter.
I am aiming for an 90% efficiency.

Question: You say I will need to proportion the winding distribution so that the odd harmonics are mainly suppressed. Could you give me an idea? The connection is a star connection and the neutral is not connected.

You also say that the line voltage of a star winding will be very close to sine wave but this will only apply into resistive or at least linear loads. So if I have star connection I don't have to care about the harmonics?

Thank you very much for your help.

[electrondady1]

you will get good efficiency if you let the volts float.
 if price is no object then go with trapezoid mags or shape the pole with multiple mags russel has suggested . (greetings to county down)
there is something to be said for mag surface area and the total number of lines of flux cut.

[Flux]

If you are rectifying and going via a dc link to an inverter then the load is a rectifier, not a normal ac load. Unless you use a special low distortion rectifier you will chop the waveform up, not as badly as when clamped to a battery,but you will create harmonics all the same.

Don't spend any time worrying about creating harmonic free open circuit voltages, when you connect the rectifier it becomes pointless.

Star is usually ok for rectifiers and will avoid much of the harmonic circulation of delta. With the inverter you should be able to let the voltage rise with load ( wind speed) so you are not constrained to the low efficiency of direct battery charging and the inverter should keep the prop on its power curve.

I seriously doubt that you will reach 90% alternator efficiency in any small machine and it is worse with a rectifier load but you should manage to get into the low to mid 80%.

To achieve 90 % you will need far too much material to be cost effective and you will need highly specialised rectifiers( in my experience they don't gain you much). If you can get over 80% at full load then you are probably at a decent cost effective point with sensible use of magnet and copper and you will have no real worries about stator temperature. Those problems start to be an issue when you get below 60%.

Unless you can get trapezoidal magnets at as good a rate as rectangular ones for the same flux then they just aren't worth the expense. If you can equal the price then they may let you make a better job.

Flux

[wooferhound]

I will use the generator in an ac connection... This means generator-rectifier-inverter.

The correct configuration would be Generator-Rectifier-Battery-Inverter
If it's not connected like that your inverter will never function properly.

[ChrisOlson]

I am trying to choose between trapezoidal and rectangular magnets. The easy way out would be the trapezoidal, becouse of the same shape but I would like to know more... In addition, trapzoidal magnets cost more...

I've used both wedge and bar magnets on many generators, comparing the two on the same stator.  The wedges will deliver about 3% better efficiency than the bars simply because they have a more ideal shape for an axial generator, and have slightly more surface area than a comparable bar magnet.  The extra surface area allows you to use less turns of wire to get the same voltage at the same rpm.

I've found the wedges to only be cost effective if you're pushing the outer limits on output vs efficiency, and stator heating becomes a problem.  If a given generator is on the edge of running too hot at high outputs with bar magnets, switching to wedges allows you to reduce the number of turns in the coils, thereby reducing stator resistance and heat, and use the smaller wire with heating problems.

Otherwise, assuming you have the space to do it,  it's typically more cost effective to use one size bigger wire to reduce the resistance, and use the bars.

You said you've never seen a generator with wedge mags and coils?  Here's one I built not too long ago.  One warning on this though - you can't just invent coil dimensions, rotor dimensions, and coil spacing in random fashion and expect the generator to work properly when you use wedge magnets.  It's best, unless you have experience with generator design and building, to use a design that has been proven and is known to work.




--
Chris