Homebrewed Electricity > Wind

Please discuss stalling and adding resistance to the line.

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Rainwulf:
Thank you for your details response and thanks for updating me.

However there is a correction, i mentioned shunt regulation, which means actually shorting out the line, not "disconnecting" it.

I know that under no circumstance is there ever to be open circuit on a wind generator as it would overspeed signficantly.

My wording of using "PFC" was not to suggest that power factor correction is needed, but more the technology involved in pulling power from the parts of a sine wave that are under the designed voltage, while at the same time making sure that the peak of the sine wave, where the current is highest and i2r losses are significant (due to the current being squared) to be left alone, which would actually allow the rotor to spin up, as the places where it has the most load, at the max magnetic flux position, the load is actually reduced, kind of like an "electronic" anti-cogging system.

The alternator will only ever stall at the positions where its generating the most electricity, so if those points are left alone, while the pfc circuit (i know its not pfc, but the same tech) is used to pull the power out of the sine wave where its not loading down the alternator so much would allow it to spin up and get out of stall.

At that point standard MPPT would take over. I think this entire system could be integrated, basically a high frequency buck/boost module with integrated shunt controller as well.

SparWeb:
Hmm, now the image in my mind is of "ripple".

Yes, these alternators produce a messy waveform, definitely not a sine, but ripple is not the cause of power loss or inefficiency.  For any specific given wind speed, I would expect the power wasted due to the nasty waveform to be 1/10 to 1/100 of the power wasted by mismatching of blades to alternator power curves.  Perhaps a rough estimate will help:

When I think of the total power in, say, 20 mph wind in my turbine blades, and that I'm getting only about 25% of it into the shaft of the alternator, I know I'm leaving 670 Watts behind that would otherwise be available to make electrical power if my blades ran up to the Betz limit.  With my alternator typically operating at 60% efficiency, then I could have converted that 670W of mechanical power to 400 Watts electricity.  If I decided to install a MPPT controller, I could boost aerodynamic efficiency by 15%, gaining me 177 more Watts in the wires.  If I try to fix ripple, I might get a few VA back.  I'd rather be greedy.  :) 


--- Quote ---where its not loading down the alternator so much would allow it to spin up and get out of stall.
--- End quote ---

Yes, actually, that's how the alternator is sized in an effective MPPT scheme.  The alt will be stiff enough to stall most of the time, and the MPPT scheme will switch current off sufficiently to reduce load and allow the blades to spin.  Disable the MPPT and the system goes back to stalled - that's the fail-safe I mentioned before. 

joestue:

--- Quote from: Rainwulf on February 12, 2018, 04:35:12 AM ---My wording of using "PFC" was not to suggest that power factor correction is needed, but more the technology involved in pulling power from the parts of a sine wave that are under the designed voltage, while at the same time making sure that the peak of the sine wave, where the current is highest and i2r losses are significant (due to the current being squared) to be left alone, which would actually allow the rotor to spin up, as the places where it has the most load, at the max magnetic flux position, the load is actually reduced, kind of like an "electronic" anti-cogging system.

--- End quote ---


The most efficient way to get power out of an alternator/generator, is to connect a resistor to each phase. this is simply because the resistor will draw a current proportional to the voltage of the generator, regardless of the harmonic content. but using a pfc rectifier to specifically not draw current proportional to the terminal volts is a step backwards. --this is done when you want to drive a motor and produce zero torque ripple at the motor shaft, but it comes at a cost of additional copper losses.

due to the third harmonic content that my 3 phase 5kw generator head produces, each phase produces 145 volts but only 230volts phase to phase. if i connect it in wye, for the same copper losses i could pull 230 watts out of it, or i could pull 250 watts from it with a resistor connected to each phase directly (requiring a 4 wire connection to the wye connected windings). (because the third harmonics are subtracted by the wye connection, the voltage goes away)

due to the significant voltage drop under load, the wye/delta switch is a good idea for wind turbines. wye for startup, delta once the wind gets going. yes, delta has recirculating third and 9th harmonic currents. but because the battery holds the phase voltages down (and draws bad power factor when there is no inductor present after the rectifier) once the generator is loaded there is no third harmonic current recirculating because the voltage drop through the resistance of the generator itself exceeds the third harmonic voltage being produced. --any third harmonic content is being sent into the rectifier directly.

Rainwulf:
Thank you. Not having a wind generator i have no ability to play with one :)

Donations? hahaha

I think a specially crafted MPPT algorithm would be the best solution, as wasting power in resistors is.. well wasted power. Unless of course all your batteries are charged.

An MPPT system that has an RPM input and secondary wind speed input would be able to be programmed with a "map" to both keep the generator out of stall, and allow it to pull the power out at the most efficient voltage would to me seem to be be the best.

Ungrounded Lightning Rod:
Was just going over this and had a realization:

While a fixed resistance does well at matching the blade's power curve to the wind, having a resistance that's low when the wind is low and goes up as things get blustery should match even better, losing less at those middle ranges.

Fortunately, that's exactly what you get if you use things like heating element wire for your resistors and size it so they heat up substantially at higher winds.

(There used to be a similar thing in some early tube radios.  It was called a "ballast tube" - a thin resistance wire in a vacuum bottle that heated up and increased resistance when the input voltage was high, to provide some regulation on the current through the heaters of the rest of the tubes.  If the line voltage was high most of the extra energy was dissipated in the ballast tube, which could handle it, rather than the pricey ones that did the actual work and weren't able to dump much excessive heat rather than burning out.)

======

One nice thing about using a powerful, efficient, "stiff" genny and resistive tuning when you're first setting up is that it's exactly the sort of genny you want if you later decide to switch to an electronic controller and salvage the power the resistors were burning.  Changes in the powerhouse, not up the tower.

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