Author Topic: Please discuss stalling and adding resistance to the line.  (Read 39228 times)

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WoodWaterWheel

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Re: Please discuss stalling and adding resistance to the line.
« Reply #30 on: June 04, 2011, 09:14:59 PM »
Chris,     Your method of adding variable restiance using the springs is great.       kudos




 
« Last Edit: February 13, 2018, 04:16:32 PM by JW »
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Rainwulf

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Re: Please discuss stalling and adding resistance to the line.
« Reply #31 on: February 10, 2018, 08:47:32 AM »
This looks like something that a reprogrammed MPPT controller could deal with.

Using PWM, and off cycle voltage measurements to measure the actual voltage (like they do with brushless motors, as this is basically a brushless motor in reverse), you could do whats called "active PFC".

The problem with AC alternators and supplies is that when rectified, you are only using the very peak of the ac waveform, as that's the only time when the diodes are forward biased with respect to the battery bank or dc rail or capacitor bank.

During the other times, there is zero load, zero current flow, so the rotor sees less "loading" and will spin up faster. It also means that instead of a nice regular current flow, the current flow ends up as a series of short pulses of very high current, with the rest of the time no current at all is flowing. Due to the fairly large current pulses, you need large cables to help reduce I2R losses.

What active PFC does is engage a boost regulator during the up and down sides of the waveform. The regulator tracks the sine wave, and engages higher boost when the waveform is closer to zero volts.  What this does is allow you to pull power out of 70-80 percent of the waveform, instead of only the top 5 percent of the peak.

Once thats done, you end up with a fairly stable actively rectified DC rail. You then use standard MPPT controller schemes in conjunction with the aforementioned RPM and voltage detection to intelligently vary the load the wind generator sees. Basically, instead of a standard ohmic load like a battery bank or series of resistors, you load the generator down with an "active" load.  This would be ideal during low wind and a flat battery bank, as you could reduce the boost voltage to reduce the load on the generator, getting it to spool out of stall speeds.  Basically, MPPT.

As your battery bank charges and you end up with excess electricity you could simply shunt regulate the alternator using high frequency PWM.


You could approximate the first half of this system with a high power DC-DC buck regulator that runs of the rectified but not capacitively filtered output.  The second half, the shunt regulator, would have to be switched in at full charge, but then act like a solar panel PWM shunt regulator.

Just my 2 cents. I could be very wrong as i dont have a wind generator.

SparWeb

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Re: Please discuss stalling and adding resistance to the line.
« Reply #32 on: February 10, 2018, 10:49:47 AM »
Hi Rainwulf,
There are actually two subjects to deal with here.  You've put your finger on one of them, but is not always the chief factor.  You can explore some discussions on this matter that I've had recently with another member "Dave P68" where we thrashed out some details, and then he ran a bunch of tests to figure it out.

The other factor, put simply, is when the blades just don't have the torque to drive the alternator.  It's a balancing act:  blades too big and the thing runs away in high wind, blades too small and it never starts except in high winds!  What the resistors do, when put in series on the power lines, is to reduce the current in the alternator, which also reduces the torque it imposes on the shaft to the blades.
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

Rainwulf

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Re: Please discuss stalling and adding resistance to the line.
« Reply #33 on: February 10, 2018, 05:10:20 PM »
Yep i understand. What you could do is use a second input, that being windspeed using either a hot wire element or a small cup anemometer.

if your controller detects that there should be more output with respect to windspeed, digitally remove some of the electronic loading by reducing the boost voltage target. That would unload the alternator allowing it to spin up.


SparWeb

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Re: Please discuss stalling and adding resistance to the line.
« Reply #34 on: February 11, 2018, 10:57:36 PM »
Hi Rainwulf,
Maybe you'd find this (rather old) discussion interesting:
https://www.fieldlines.com/index.php/topic,127288.0.html

Flux built the discussion and details slowly, but very thoroughly examined buck-boost options for making the match.

When I read your post from Saturday I got a little sidetracked by the term "active PFC" but now I see you really wanted to talk about DC buck/boost. 
I have not personally delved into the MPPT world, either as a user of off-the-shelf equipment or as a DIY builder.

Regular hard-wired connections through rectifiers don't suffer from too much inefficiency, and their chief advantage is being dead simple and reliable.  In this scheme, the EMF generated by the alternator grows linearly with speed.  In high winds this EMF can be many times the voltage at the battery.  When rectified with a 3-phase rectifier, the whole electrical system is clamped to the battery voltage.  Roughly speaking:

I = (EMF - Vbatt - Vf) / Z

In words, that says the current equals the difference between the generator electromotive force and the battery voltage minus the rectifier forward voltage drop, divided by the total system impedance.  In practice, the equation needs adjustment to account for the AC EMF and the DC battery voltage - it's just to give you an idea.

There is a time near "cut-in" where the EMF is only barely higher than the battery voltage, and the "short pulses" do happen as you say, but the current is very low.  Once the EMF rises, then there's plenty of delta-V over the battery voltage for a majority of the sine for current to flow most of the time, and this situation gets set up at RPM not much higher than cut-in.  From there, the AC waveform stays pretty "square" as RPM increases.  Different mills respond differently, and mine is a horrible mess, but in the Axial-flux machines they are neat and tidy.  One major difference between wind turbines and solar panels is that solar energy always tops out at about 1000 W/m^2 and many solar panels start to produce a trickle at 100 W/m^2 or so - a factor of 10x.  The power in the wind rises as the cube of the wind speed, so for a turbine that cuts-in at 10mph wind and furls at 40mph, the range of power it has to accept is nearly 100x.

The big advantage you get from MPPT or a DIY buck or boost controller is that you don't have to get the blades perfectly matched to the alternator, and in fact you can do other tricks that allow you to improve even a well-matched set of blades and alternator.  That comes from the difference in power curve between passive electrical machines and wind power.  They never do match; you can only get them close.  With MPPT, you can make them much much closer, keeping the turbine running at peak blade efficiency at a wider range of wind speeds.  Without it, you can only pick one wind speed to optimize, and accept less than optimal matching at other wind speeds.  This one feature is what offers a 50% increase in energy yield from a MPPT-controlled turbine.

The electronic scheme that the MPPT controller uses is not, I don't think, like power factor correction (PFC).  I believe units like the Outback use high-frequency switching - much higher than the turbine's generator frequency - to manage the load.  The generator may be giving 60-100Hz but the controller is switching at many kHz, and sampling the turbine output frequency/voltage frequently to sense the turbine conditions to be regulated.  I believe it's closer to your last suggestion "...you could simply shunt regulate the alternator using high frequency PWM." but I don't believe it's simple.  In a wind turbine, there is no condition where a disconnect is permissible.  Any scheme used for regulation has to fail-safe.  This can be done with a shunt regulator, and with the controller shut down or failed the load remains connected to the turbine.
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

Rainwulf

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Re: Please discuss stalling and adding resistance to the line.
« Reply #35 on: February 12, 2018, 04:35:12 AM »
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

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Re: Please discuss stalling and adding resistance to the line.
« Reply #36 on: February 13, 2018, 12:15:35 AM »
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.

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. 

No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

joestue

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Re: Please discuss stalling and adding resistance to the line.
« Reply #37 on: February 13, 2018, 03:38:52 PM »
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 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

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Re: Please discuss stalling and adding resistance to the line.
« Reply #38 on: February 15, 2018, 07:11:02 AM »
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.