Three-phase bridge SCR control

[SparWeb]

I think Oztules is talking about "R", while Joe and ULR are talking about "Z".

Just stepping in to mediate - I don't know enough to contribute.   I can nit-pick the non-electrical comments that have gone by, but that's not what's at issue. 

If I was to speak from my own experience, there is usually only 1 point where the genny's power curve matches the rotor's on a simple direct-drive wind turbine.  Everwhere else it's "close enough", unless you do something drastic to force the blades's power curve to move (pitch control) or the genny's power curve to change (by monkeying with the load, either by MPPT, bucking, switching, whatever).  Isn't that the scope of what's being discussed?

[Ungrounded Lightning Rod]

(quoting ULR):

"If you only allow current for part of the cycle you're only pulling energy out of the flywheel for part of the cycle - while the wind is pumping energy into it for the full cycle."

I agree with it up to here..... but now we differ. Because we have a fixed R, we can only see EMF-BattV/R for our current in the stator and so into the load.

Unless we mystically decrease R we CANT increase current (torque) unless we increase RPM or  EMF or TSR (same things for constant wind input). I fail to see how we can increase the torque without first raising the TSR, as R will hold it to the max value we started with with full conduction at the original TSR or RPM or EMF.

If I'm right, we can't even increase instantaneous torque/current without first increasing TSR.

So far so good - except you're not distinguishing design TSR and operating TSR.  The former is constant - the TSR for maximum conversion of wind to mechanical energy.  The latter varies depending on operating conditions.

In this case if rpm does not increase (and it WILL with less than 100% duty), current can't increase. Until (and while) the TSR catches up with the lesser load current, the new current figure cant increase magically for a shorter period in order to have the same output. Only when TSR increases to the new "level" reflecting the lower duty cycle can we close the gap and have the same output as we started with, and still use a smaller duty cycle

Yes.  (But think "RPM" to avoid the design/operating TSR confusion.)

so it will have hysterisis too.

Not "hysterisis".  That requires a difference EMF at a given RPM when the RPM is rising vs. when it is falling.  The RPM vs. EMF curve would be a loop, not the single (and essentially straight) line it actually is.  Red herring.

"Current and torque are proportional so average current remains the same regardless of duty cycle."...... only if the TSR rises so the EMF is high enough to compensate for the shorter duty cycle.

True.  But now the design/operating TSR confusion raises its head.  It's clearer if you say "only if the RPM rises so the EMF is high enough to compensate for the shorter duty cycle."

Which is precisely what happens.  Note that the free-wheeling TSR and RPM-for-a-given-wind-speed are about twice that under optimum load - and the raio is more than 2:1 between freewheeling and stalling.

So:

We can clip, ..... so we relieve the load on the mill  (decrease continuous torque ) until it climbs to a higher RPM..... then with a higher EMF, we can get higher continuous current  through the stator.... but not when we started the clipping.

Now can we benefit from this?

In fact as has been said, we lose efficiency in the stator, ....but.... we may now get into a better position for the blades to do their work with far more efficiency  than the losses we sustain in the stator from the higher current spurts with the higher EMF. (similar to  the old resistor in line trick)

Except that when using a switcher you can avoid the resistive losses.

Thats the only reason I can see for doing this in the first place..... to fine tune the mill, to move it's operating specs up or down the wind scale, and how it behaves in those winds.

And what you have just described is a max-power-point controller.  Adjust the average load current so the RPM rises to pull more power from the wind, maximizing the AVERAGE current into the battery load.

But you can do that BETTER with a filter cap and switcher AFTER the rectifier.  This does the same job WITHOUT raising the RMS current seen by the genny coils due to short current pulsing in the genny itself.  The short pulses are only between the post-rectifier filter cap and the load.

[Ungrounded Lightning Rod]

whether or not the wind turbine stores any energy... it certainly does...
BUT ELECTRICALLY IT DOES NOT.. it is a voltage source and a resistance... there is no capacitance, inductance is trivial at 10hz line.

But it doesn't matter whether the storage is mechanical or electrical.  What is seen at the generator output is the same.

The wind, turbine, and generator form an equivalent cricuit like this:

 - Voltage source proportional to square of the wind speed.
 - Nonlinear variable resistor - approximate it as linear for RPMs between freewheeling and stall (i.e. max power RPM is about half freewheeling RPM) - with a value in that region inversely proportional to wind speed (i.e. max current at optimum load goes up with square of wind speed)
 - Capacitor (HONKING BIG one), with value a function of the turbine's moment of inertia and volts/RPM, with genny's EMF equal to its charged voltage (iproportional to RPM),
 - Resistance of genny's windings (and we're actually electrical from here on out).

Electrically it doesn't matter at all if the voltage source, wind, and capacitor of the model are electrical components or wind, aerodynamics of the blades, and spinning mass.  The result is the same.

What i am concerned about, and indeed, just about saying its impossible, is to bring your turbine out of stall (tsr 2-3) and get it up to 8 where it belongs.
you can't do this with phase control, but your alternator could certaily handle it with a resistive load.

But you CAN do it, trivially, with a switcher AFTER the rectifier.

[joestue]

whether or not the wind turbine stores any energy... it certainly does...
BUT ELECTRICALLY IT DOES NOT.. it is a voltage source and a resistance... there is no capacitance, inductance is trivial at 10hz line.

But it doesn't matter whether the storage is mechanical or electrical.  What is seen at the generator output is the same.

Within the context of my arguments outlined in the last 4 posts you are telling us that the rms to average current ratio doesn't matter.

The wind, turbine, and generator form an equivalent circuit like this:
 - Voltage source proportional to square of the wind speed.

nooooooo.....

- Nonlinear variable resistor - approximate it as linear for RPMs between freewheeling and stall (i.e. max power RPM is about half freewheeling RPM) - with a value in that region inversely proportional to wind speed (i.e. max current at optimum load goes up with square of wind speed)
 - Capacitor (HONKING BIG one), with value a function of the turbine's moment of inertia and volts/RPM, with genny's EMF equal to its charged voltage (iproportional to RPM),

i really could care less about the inertia of the turbine, it could be infinite and it wouldn't make any difference electrically. The torque ripple of a 3 phase system at 10-40hz loaded at 80-30 degrees is guaranteed to jack something up, seeing as people already have issues with the vibration from direct connections. and i'm not seeing any non linear resistors unless you attempt to model the blades as a non linear resistor, which unnecessarily complicates things. just build an rpm vs torque vs wind speed chart for your blades (yep, 3-d, you'll need at least 200 data points.)

- Resistance of genny's windings (and we're actually electrical from here on out).

Electrically it doesn't matter at all if the voltage source, wind, and capacitor of the model are electrical components or wind, aerodynamics of the blades, and spinning mass.  The result is the same.

i'm not sure what your trying to say. the results are *not* the same in the context that i could care less how much energy the system stores (ignoring for one second the issue of torque ripple and various resonances that could cause)  because the only issue we're talking about is whether or not phase regulation is doable. my position is that it is too expensive, difficult as outlined in my first and second post in this thread.
perhaps if you already had a 12 pulse rectifier set up and wanted to try it. but you would have to derate the secondaries to the point that it would probably be less copper to just wind some more turns and build a tap changer.

[Ungrounded Lightning Rod]

- Voltage source proportional to square of the wind speed.

Typo:  Voltage source proportional to the wind speed - i.e. to the unloaded RPM.

(The equivalent resistance modeling the aerodynamics of the blades being inversely proportional to wind speed handles the other power of wind speed in the square law for torque.)

[oztules]

Thanks ULR, and yes actual TSR not design TSR  was what I was using . When you hack your 4 meter  blades out of a tree with a chainsaw, the concept of design TSR is ..... well... not quite as clinical as it could be. (... wild guess at best for me).

If Jon can get the triggering solved, I think it may help the  load matching.

With just diodes we are phase switching (me when the wave approaches 48v), so zero crossing and small overlap is not a design difficulty I would be all that excited about. The hash and harmonics and square wave we get from just doing it with normal diodes  will be hard to make worse I think.

 I see the losses as small (current peak to rms), as the duty cycle will be not far from 100%... maybe as low as 85% at low power.

It needs real world testing.



...........oztules

[kevbo]

better to just use the top half diodes,and replace the bottom half with mosfets
then control the mosfets.

The body diodes of your MOSFETs will prevent you from turning them off.   You'd just be switching between diode drop and Id * Rds on drop.

no need to control them with some phase angle scheme typical of scr control of three phase because we don't need three phase as a finished project, just control them as a group with some sort of pwm scheme

When you drive all three gates at once, you have shorted out your alternator, No?

 (MOSFETS are happy to conduct in either direction, and are normally used "backwards" for sychronous rectifiers)

[Ungrounded Lightning Rod]

whether or not the wind turbine stores any energy... it certainly does...
BUT ELECTRICALLY IT DOES NOT.. it is a voltage source and a resistance... there is no capacitance, inductance is trivial at 10hz line.

But it doesn't matter whether the storage is mechanical or electrical.  What is seen at the generator output is the same.

Within the context of my arguments outlined in the last 4 posts you are telling us that the rms to average current ratio doesn't matter.

No, I'm saying that whether the physical mechanism of energy storage is spinning mass coupled to the rest of the circuit by coils and magnets or stored charge coupled with wires doesn't matter.

RMS vs. average matters a BUNCH.

The wind, turbine, and generator form an equivalent circuit like this:
 - Voltage source proportional to square of the wind speed.

nooooooo.....

Correct.  Was a typo.


Wind is equivalent to a voltage source, and the voltage is the genny's EMF at the freewheeling RPM for that wind speed.

The other half of torque-is-square-of-windspeed comes from the resistor that models the blade aerodynamics having conductivity proportional to wind speed (corresponding to higher mass flow past the blades at higher wind speeds).

- Nonlinear variable resistor - approximate it as linear for RPMs between freewheeling and stall (i.e. max power RPM is about half freewheeling RPM) - with a value in that region inversely proportional to wind speed (i.e. max current at optimum load goes up with square of wind speed)
 - Capacitor (HONKING BIG one), with value a function of the turbine's moment of inertia and volts/RPM, with genny's EMF equal to its charged voltage (iproportional to RPM),

i really could care less about the inertia of the turbine, it could be infinite and it wouldn't make any difference electrically. The torque ripple of a 3 phase system at 10-40hz loaded at 80-30 degrees is guaranteed to jack something up, seeing as people already have issues with the vibration from direct connections. and i'm not seeing any non linear resistors unless you attempt to model the blades as a non linear resistor, which unnecessarily complicates things. just build an rpm vs torque vs wind speed chart for your blades (yep, 3-d, you'll need at least 200 data points.)

For a single cycle you'd be right.  But we were talking about the current rising as the duty cycle falls.  That's a multi-cycle phenomenon:
 - Duty cycle falls.
 - Average current falls.
 - Load torque resisting the rotation falls.
 - RPM rises.
 - Voltage rises.
 - Average current rises.
 - Everything stabilizes at a new equilibrium.

- Resistance of genny's windings (and we're actually electrical from here on out).

Electrically it doesn't matter at all if the voltage source, wind, and capacitor of the model are electrical components or wind, aerodynamics of the blades, and spinning mass.  The result is the same.

i'm not sure what your trying to say. the results are *not* the same in the context that i could care less how much energy the system stores (ignoring for one second the issue of torque ripple and various resonances that could cause)  because the only issue we're talking about is whether or not phase regulation is doable. my position is that it is too expensive, difficult as outlined in my first and second post in this thread.
perhaps if you already had a 12 pulse rectifier set up and wanted to try it. but you would have to derate the secondaries to the point that it would probably be less copper to just wind some more turns and build a tap changer.

I think we have a conversational disconnect going here.

I agree that direct phase regulation of the genny is not a good idea.  You object because implementing it isn't very practical.  I object because, even if it were doable, it would reduce the efficiency and dump the lost energy in the genny, while the desired behavior can be obtained without these downsides by a post-rectifier MPPT controller.

As we discussed this we've had several digressions.

[joestue]

No, I'm saying that whether the physical mechanism of energy storage is spinning mass coupled to the rest of the circuit by coils and magnets or stored charge coupled with wires doesn't matter.

Without backing up said statement with a theoretical model built with passive components, then it means nothing to me and is likely to be taken out of context.
for example, you failed to specify that the capacitor in post #30 has to be connected to the generator and load through a T network of resistors.

As for the rest of the points you are attempting to clarify, i already covered, most if it in reply #1 and 4

Furthermore, a proper mathematical analysis of the impact of a phase controled rectifier on the turbine does Not require multi cycle analysis.*
all you have to do is look on the torque-rpm graph. the length of time it takes to slide up and down that curve is only of interest when building the algorithem to set the firing angle, and keep it from oscillating at some low frequency related to the time constant of the turbine. a time constant which is not constant, but varies with the windspeed according to some non linear relationship.. all of which can be simplifed by analizing the system from a 3-d torque-rpm-windpeed graph, rather than a bunch of non linear components.
*unless you're attempting to get better gas milage by adding an ac component to the throttle, and want to know exactly how much extra gas that costs you.