Do Large commercial turbines mostly use air cores or solid cores?

[hatch789]

Hi Guys,

I'm having a hard time finding good information on what the large commercial turbines have inside of them. Is there any good resources that one can reference to find answers to technical questions like solid-core vs air-core in these large machines?

I find a lot of papers on theoretical work but nothing about what is in practice really.

[joestue]

Direct drive turbines are using on the order of 20mm thick magnets, 6 meter diameter for a 4MW direct drive. Probably at least a meter deep for that size.

I cant find the photo again, but my guess is the direct drive 7 and 8 Mw generators are larger than 100MW hydro turbine generators. On the order of 10m diameter.

Of course there is a core.

[hatch789]

so the cores I'm referring to are in the coils themselves. I know there's a rotating core so I didn't want you to think I meant that.

In smaller residential turbines they like to use air-coils to overcome startup cogging and lockup.

[Mary B]

Pretty sure they use an iron core design... at least they did 10 years ago on the one I saw being repaired at the base of a tower. They had craned the head down and had it torn apart when I went with a friend to see it.

[hatch789]

So how are these large turbines with solid cores starting up then ...especially if they have 90:1 gear ratios. If they're using electromagnets to excite the coils, then I can understand startup with no cogging since there is no active attraction until it gets spinning.

But I thought quite a few of them were in fact using NiB magnets in their generator designs. I am just trying to understand.

Perhaps they have some sort of motor assisted startup when wind is above a certain m/s?

[joestue]

If they are using a concentrated pole design, then the lcm will be so high that there wont be any cogging torque.

Also they are all variable pitch so startup torque is high. Resonances due to torque ripple are probably a much larger issue.

[hatch789]

Not sure what LCM stands for in this case? Lens something? Least Common Multiple? I'm just trying to make sense of what you were saying. Thank you for the reply and for trying to help!

[Ungrounded Lightning Rod]

So how are these large turbines with solid cores starting up then ...especially if they have 90:1 gear ratios. If they're using electromagnets to excite the coils, then I can understand startup with no cogging since there is no active attraction until it gets spinning.

But I thought quite a few of them were in fact using NiB magnets in their generator designs. I am just trying to understand.

Perhaps they have some sort of motor assisted startup when wind is above a certain m/s?

If you shape the pole pieces just right, on a polyphase design, you don't GET cogging.  At any angle some gaps are pulling forward, some backward, and the total cancels out.

If you're designing a tens-of-megawatts purpose-built generator system for a commercial mill, you already have a bunch of engineers and CAD tools deployed to tune up all sorts of aspects of the design.  Getting the cogging flattened is just a small piece of the job.  (You want it gone because, in addition to the startup issue, it turns some of the power into vibration - which is both wasteful and can be a hazard to the rest of the mill.)

If you're making a single-digit-kilowatt one-off design, it's easier to sidestep or approximate it.  You want a design that's forgiving, that can be thrown together sloppily (i.e. without a lot of engineering effort and precision fabrication) and still work well, not one full of critical tolerances that keep it from working if you don't get everything just right.  So for an axial-flux machine the easy path is to skip the pole pieces through the coils and use stronger magnets to compensate for the increased gap.  For a motor conversion driving a rectifier-and-battery system (with or without an MPPT controller) you just spiral or offset the poles a bit (or use one of several related approaches).  You only have to get the cogging down to where the mill will start at a speed below the cutin, while the variable-frequency vibrations will be small compared to those from the pulsing current loads due to the rectifiers peak-clipping the generated waveform.

[Ontheronix]

Does anyone know what factor the magnets have to increase in size to account for not using solid cores? Just out of curiosity

[joestue]

Does anyone know what factor the magnets have to increase in size to account for not using solid cores? Just out of curiosity

that depends on how large the generator is and what your efficiency target is.

the largest 60hz transformers on the order of a gigawatt in capacity have leakage inductance that makes the impedance of the transformer drop 22% voltage under load. they are 99.8% efficient. my estimate is a transformer of 4 times the physical dimension (so 64 times the volume, which would make for an electrical capacity of 256 gigawatts assuming it could be cooled off), would operate more efficiently without a core.

the reason for this isn't really intuitive until its explained to you but basically the capacity of a motor or transformer scales faster than the volume if its possible to cool it off. a 50 megawatt transformer might weigh 50 tons, for a capacity of 1kilowatt per kilogram. and it would be better than 97% efficient.

a 97% efficient 1kw transformer will weigh about 10 kilograms, 5 kilograms if its a toroid (power density increases with flux squared, 1.3T for E core and 1.9T for good quality tape wound toroid)

the reason why is simply that the cross section area of the core sets the voltage per turn, and the cross sectional area of the core sets the amps. both increase with the square, and volts times amps makes for the 4th power, but the volume only increases to the cube.

so a 1 cubic inch 60hz transformer might be 50% efficient at reasonable power densities such as what's found in a "wall wart" and you might get 1 watt per cubic inch power capacity at more reasonable efficiencies, which is a power density of 7.8 watts per kilogram.




the magnetic core in a small motor let you wrap the magnetic flux around an electrical conductor of 100 times the cross section area that the flux lines would flow through air on their own. so the torque is set by the magnet (for the most part) but the increased magnetic path that the flux flows through, drops the copper losses perhaps 100 fold from what they would need to be to generate the same torque in the motor. i think it was tim williams (7 transistor labs) actually worked some real numbers out to illustrate a similar problem and he basically said if the resistance of metals such as copper were 1/200th what they are now, it would be practical to build air core motors.