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Gizmo
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Posted: 05 June 2017 at 10:10pm | IP Logged Quote Gizmo

Been a bit of interest on adding diodes to the center point of a multi phase star alternator.

About 10 years ago I was talking with a auto electrician mate who said he was starting to see car alternators with the diodes on the star. He said on the test bench they were definitely making more power.

I've been trying to find some more info on google, but its knowing the right question to ask thats the hard bit. On one thread I did read a 10% gain, at a fixed rpm into a battery load. Of course this also means more power is needed to drive the alternator at the testing rpm.

I suspect the power output for a fixed rpm would be somewhere between star and delta.

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Posted: 05 June 2017 at 11:37pm | IP Logged Quote Warpspeed

Just to mention something I did a while back that may or may not be useful or relevant to all of this.

I have a large three phase transformer that originally came out of a 24v industrial battery charger, must be about 5Kw, not really sure. But it does have three independent secondary windings, rather like an F&P.

As we know, star or delta connection gives a choice of two different dc output voltages, but there is a third way.

What I did was connect each winding to its own conventional four diode bridge rectifier. The three bridge rectifier dc outputs could then be connected either in parallel or in series.

The parallel connection behaved just like a normal six diode delta connection giving a nice smooth low ripple 30 volts dc output as expected.
Connecting the three bridge rectifiers in series exactly doubled the output voltage to 60v, also giving a nice smooth low ripple output which is higher than you can get with star connection.

I have no idea if this has any practical application for wind turbines, but just thought I would throw this into the discussion.

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Posted: 06 June 2017 at 3:17am | IP Logged Quote oztules

This is a copy of a response I did on another thread, but more inline with this thread. It was in response to Dave's test, that did not yield any response from extra diodes in his test rig.... and why this may be so.

"Dave,

The extra diodes won't work on a properly loaded machine.... only when it is poorly matched. Then the star voltage will sag, while the dleta catches up to it, and then it can augment it.... before that ( or if matched, and the emf allowed to rise proportionally) then the star will always be above the delta, byt about 1.6:1, so the delta emf will be lost in the background, and cannot contribute until the star has sagged at least below the delta figure.

Because you know what your doing, you are likely to be matching the load fairly well, and so the delta "switch " will not happen, you will have to loose at least half the EMF in the coils in star before the delta should kick in.

We should transfer this part over to Gizmo's other thread, in fact I will copy this answer over to there, and that may help others understand what i/we are trying to explore.


........oztules"

Warp..... that would require cap filtering to work?

Once you use transformers to match impedances, you can dial up whatever you want really I guess.


.........oztules

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Posted: 06 June 2017 at 3:41am | IP Logged Quote oztules

For those interested in experimenting, this is my take on whats happening, and why I would try to do it if I had a F&P mill.

If we wire in star, we get an early start up, which is a good thing... except that our output will suffer terrible reactance problems fairly early on at say 10 amps...(or less?) and then thats it really... the end of the power increasing as the wind does.

In star we are using two phase coils in series, so we have twice the turns for the resistance value than would a delta or single phase coil configuration. Our voltage is about 1.6 times as high as would be delta, but or resistance is double.... but just as importantly the number of turns has doubled for the same current.. so our amp turns is a lot higher.

These amp turns generate an opposing magnetic field that will react against the ferrite magnets doing the magnetising in the first place, and a stand off occurs when the repelling field from the amp turns in the stator interfere enough with the magnetic field from the ferrites, as to cause a top limit to current.... no matter the rpm after that, we reach stalemate.

If we add two more diodes to the star point, we create another path for the electrons to get out of the stator... this time, the amp turns is for the single phase coil not the two in star.... likewise the resistance, so at this point it should start acting as a delta stator. It should return the same results as a delta configuration now... as that is what it really is.

As the rpms drop off, the delta will lower it's emf output to the point where the star starts to take over..... thats a simplified version, but I think gets the idea across.

The reactance should be such that it is pretty messy at cross over, and I would expect it to be relatively smooth.

It can only work with iron cored alternators who's synchronous impedance is such that they can run safely in reactance current limit state..... A air core axial flux would just burn up if it tried too emulate this.

So properly matched units will not be able to do this, as the star will not sag below the delta point ( about 60% of star ).... nor would it need it, as the emf would rise with rpm.... so there would be no current limiting by this mechanism.... eg mppt grid tie unit.

A traditional hook up would benefit from this I suspect, where you could start lower rpm, but still squeeze a few more amps in when the wind picks up... maybe another 20% or more.

Bench testing a car alt would require a big battery bank that would keep the lid on 70 amps or more, without rising too fast. That may be why the 10% figure was mentioned Gizmo.... but there is no reason I can see for it to be that low only... it really is a delta connection, and a car alt should be fairly feral in delta I would think.... maybe test out the belts if the batts were low for a few seconds.


.......oztules

Edited by oztules on 06 June 2017 at 3:42am


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Posted: 06 June 2017 at 4:25am | IP Logged Quote Boppa

I dont know about the fp stators, but it was quite common back in the eighties to change star wound car alternators to delta for the extra amps they provided- they didnt start charging until about 2500 rpm, but once they did a bosche 55a would easily pump out 90a, some broke 100a

very popular with anyone who wanted to run more than 4 130w spotties before the modern 80/100a alternators became available

(ps hate typing on tablets/phones- gimme a real keyboard anyday)
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Posted: 06 June 2017 at 6:18am | IP Logged Quote Warpspeed

oztules wrote:


Warp..... that would require cap filtering to work?


The waveform coming out of the three bridge rectifiers series connection is almost pure dc with only a very few percent of ripple. No capacitors required anywhere.

It appears that all three phases when added together in series (through individual rectifier bridges) add up to twice the peak voltage of one phase. And due to the magic of sine waves this addition produces an almost constant dc output.

I was curious about this as I have never seen it described in any engineering texts. But it definitely works giving exactly twice the dc output voltage that you would expect from a delta connection plus single six diode bridge.

If I had a camera I could show you the beast, the measured voltages and the waveforms.
I bet it could beat a capacitor multiplier, especially at very low frequency.

How it could be incorporated into a wind turbine I have no idea.
But I leave it for others to try out and experiment with.
Its easy enough to try.

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Posted: 06 June 2017 at 6:46am | IP Logged Quote Warpspeed

Boppa wrote:
I dont know about the fp stators, but it was quite common back in the eighties to change star wound car alternators to delta for the extra amps they provided- they didnt start charging until about 2500 rpm, but once they did a bosche 55a would easily pump out 90a, some broke 100a

I actually bought one of those upgraded alternators, remember it well.
Never really knew what they did to them, which makes your comment very interesting.

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Posted: 06 June 2017 at 7:20am | IP Logged Quote oztules

Not quite on topic, but Warp, your comments are very interesting.

Using a ac waveform to charge batteries is very wasteful in single phase, in that the power factor from crest damping is very severe, and a petrol genny struggles to do well... ie the power factor is miserable due to the crests used only.

If your setup is delivering almost ripple free DC..... gee I suspect we could get better kwh into the batteries than normal rectification.
In theory, we use no extra power in petrol with poor power factor, but in practice we still need to generate much higher current in the alternator coils to deliver a vector product into the battery. I notice this loads the genny down significantly more than the power it is actually seems to deliver..... current equates to torque I think.

I have rewired and built 48v alternators from normal car alternators for battery bank charging, and even though they are three phase, they still load the motor up more than I would anticipate.... maybe if I used your three rectifier system, I could reduce the motor load for the same power output.....( less wild harmonics??) I guess I will have to try this... ie bypass the normal diode block, and use three single phase blocks in series.... always something else to try. ( and change the pulley ratio too )

..... thoughts before I do this?

Boppa, that sounds encouraging too.


.......oztules

Edited by oztules on 06 June 2017 at 7:24am


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Posted: 06 June 2017 at 8:26am | IP Logged Quote Warpspeed

I have been sitting here thinking about this myself.

A standard three phase rectifier, either star or delta can put out an almost pure dc voltage with only a very slight voltage ripple, and the load current will be continuous. This continuous load current commutates abruptly between whatever diodes are conducting.

The current through the windings must always be a square wave that chops on and off.

With the series connection, this is also true, but the current through all windings must be continuous, but rapidly reversing in each winding during each zero crossing.
Really fascinating stuff.



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Posted: 06 June 2017 at 9:10am | IP Logged Quote Warpspeed

This is the general idea:



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Posted: 06 June 2017 at 9:47am | IP Logged Quote Warpspeed

The voltage doubler could be switched in and out with a (horrible) four pole relay, at least to try the idea initially.

I would expect the parallel system to work the best once it has reached cut in speed, and gets going as it will produce a higher current.

But the voltage doubler might provide some useful output below that point.

If crossover could be arranged where both systems are providing similar output power, switching over could be pretty seamless.

Not terribly thrilled with the idea of using a relay to do this !

But if the concept works, something a bit more sophisticated could be used to do the switching electronically. Need to think on that a bit more. But the quick and dirty way is with a relay.



Edited by Warpspeed on 06 June 2017 at 10:25am


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Posted: 06 June 2017 at 11:11am | IP Logged Quote Boppa

Warpspeed wrote:

I actually bought one of those upgraded alternators, remember it well.
Never really knew what they did to them, which makes your comment very interesting.


they were actually a really easy job on the bosche one, not so much so on the lucas ones

inside you had 3 wires twisted together, soldered and sleeved in a ceramic sleeve- you just unsoldered the three together and the 3 off the diodes plate, joined together in a delta (you had to make doubly sure to get the starts marked with a blog of liquid paper), joined start to finish of next winding and resolder the joined wires to the diodes (usually had to extend the old y join ones in the bosche) and bingo, done

some people told you you shouldnt do it because you would `run out of voltage', but most of the ones I did were fine, the usual 55a bosche would get up to 19vac or so in y connection, even in delta they would still get up around 16vac which was enough to get your 13.8dc at the battery
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