Use of an asynchronous motor as generator for a wind turbine (Part II)

[SparWeb]

The first part is here:

https://www.fieldlines.com/index.php/topic,150484.0.html

Following up on this topic, since I've been doing tests and measurements of this for the past few months.  As time allows, I change a few things, do a test run, measure a bunch of things, then see what the numbers can tell me.  After a while patterns emerge.

Here again is the equipment setup.  There is 315 uF of capacitors connected in Delta across the motor outputs.  The motor itself is wired in Series-Star.




This is a schematic of the wiring to clarify how it's all put together.




I have only been varying the capacitor values so far, and keeping the load relatively modest (30 ohms per phase).  They have a direct effect on the cut-in speed.  I have tried testing with the motor windings in parallel but the cut-in speed "can't get there from here".  In the chart the parallel windings would need about 2000 uF to reach a cut-in of 200 RPM - already quite high for a 10-ft rotor.  So I have focused on series-connections.



With open-circuit tests, I have noticed that the capacitors apply a substantial load on the generator, even when there isn't an external (resistance) load on the output.  Here you can see again that using 370 uF of capacitors allows cut-in to be below 200 RPM, but notice how much power is taken just running the capacitors alone.  At 270RPM, the generator needs over 1kW to keep turning!  This is without producing any resistance heat or battery charging at all.

[SparWeb]

Next, I need to try applying more load to the outputs to see if a valuable amount of power can be extracted.
Currently, I've kept the output loads light.  Just enough to measure something, but not enough that I break my lathe.  Which is a bit of a snag at the moment because this is a 2HP lathe and I'm putting more load on it than that.  The belts are slipping (making noise) by 50% !

The baseline resistance of the motor windings is about 2.5 Ohms, so a properly matched load would also have 2.5 Ohm resistance.  By using a 30 Ohm load I'm not heavily loading the generator or the lathe.  In theory this means that my 100-200 Watt outputs could be raised to 1000 to 2000 Watts.  That prediction makes it reasonable to expect that testing this generator with a 2.5 Ohm load would require more than 5 horsepower.  So I have to come up with a new way to drive this beast.

Any suggestions?

[Adriaan Kragten]

Nice experiments. Measuring of a big generator requires a driving motor with a very high maximum torque level if the transmission is direct drive. In my report KD 78, I describe measurements performed on a PM-generator frame size 90. A test rig of the University of Technology Eindhoven was used for these measurements. The driving motor was a big DC motor which was supported in air film bearings and so the reaction torque could be measured very acureately. The outside diameter of this motor was about 40 cm and the length was about 50 cm. But the maximum torque level of the PM-generator was that large that the torque could be supplied by the motor only after short-circuiting of the motor fuses and performing the measurements very fast to prevent that the motor was burned. So for direct drive, you need a very big motor.

In my report KD 595, I describe a test rig which I have built at home. I used a PM-DC-motor and reduced the rotational speed and so increased the torque level by a chain transmission but I could measure only PM-generators with a low maximum torque level on this test rig. I have thought about replacement of the chain transmission by a poly-V string transmission for which a much higher gear ratio is possible in one step. A gear ratio of 1 : 10 is possible if you use a big wheel on the generator shaft. The advantage of a poly-V string is that only the small wheel on the motor shaft must have grooves for the belt. The big wheel on the generator shaft can be flat because the spanned bow at this wheel is 240° if the spanned bow at the small wheel on the motor shaft is chosen 120°. This kind of transmission is also used in certain washing machines.

The biggest problem of building your own test rig is measuring of the torque. For the test rig of the University, the driving motor had an arm which was connected to a balance and the procuct of the measured force times the lenght of the arm gives the torque. For my private test rig, the generator was connected to the driving shaft by a fixed coupling and the arm was connected to the generator. But this is only possible for light generators as the weight of the generator gives a bending moment in the driving shaft. If the generator is heavy, you need a supporting bearing at the back side of the generator but the friction in this bearing influences the measured torque. But a small bearing can be enough and the friction of this bearing can be neglected.

[MattM]

Sparweb,

Is there any way to use smaller capacitors in series to power larger capacitors run in delta, but have the smaller capacitors cut out as wind velocity kicks up?  Perhaps trickle charging at low speeds would cut the drag down and allow you to begin collection at wind velocities that are typically inconsequential.

You are truly tackling an interesting topic here.

[kitestrings]

Is that board we see attached to mount of the motor, with a scale?  Perhaps it's not this one, but is that how you plan to measure torque?

[Astro]

Sparweb,

Is there any way to use smaller capacitors in series to power larger capacitors run in delta, but have the smaller capacitors cut out as wind velocity kicks up?  Perhaps trickle charging at low speeds would cut the drag down and allow you to begin collection at wind velocities that are typically inconsequential.

You are truly tackling an interesting topic here.

 That is what I was talking about for a breaking system. I would imagine with the same kind of circuit and caps, you would have a low wind start up system.

[SparWeb]

Hi
Thanks for the interest.
Those of you who asked about measuring torque, and about the board, you are right.  Sorry, I haven't shown the whole setup in the photos above.  I need to go and take another photo, so you can see the rest of it.  At the end of the board is a kitchen scale, with a few pads of rubber to soften the vibrations.  Load on the scale X length of the board is equal to torque.  Multiply by the RPM of the shaft and a conversion factor and you get the mechanical power driving the shaft by the lathe. 

I still can't get over how much power it takes to drive this thing even when it's not producing anything on the outputs.  It seems there will always be a "tax" on the system, efficiency will suffer, and it will run hotter than a permanent magnet conversion would.  Not only the extra current in the windings heating them, but the capacitors too will stay hot.  In my modest tests so far, the caps are getting warm.  That's not to say this is discouraging.  If more output power can be extracted from the generator, or the overall system can be made more robust or less expensive than a PM conversion, then I don't want to give up.

Another strange thing I've noticed is that when I switch on the load (small as it is), the shaft power seems to go down.  That's strange, and the only explanation I can give is that the Delta capacitors are wasting a lot of energy with circulating currents.  I'll have more to say about this once I get higher current loads on the output.  Maybe adding the output load improves the power factor, even in small amounts.  The PF is probably awful when it's just capacitors alone.

I'm still in the early steps, and feeling my way carefully through this. 

I've received a few "try it and see" suggestions like yours Matt, and I'm interested in these.  I have no idea if anything good will come of them, but yeah, why not, eh?

Driving the shaft is the next obstacle.  Thank you for the suggestions Adriaan to get me started looking in the right places.  I figured you had done something like this before.

[Adriaan Kragten]

The advantage of a PM-armature is that no energy is needed to create a fluctuating magnetic flux in the stator. If you use a short-circuit armature, there will only be a strong magnetic flux if strong currents are flowing through the aluminium short-circuit bars. These bars will have a certain resistance and so these currents will generate heat in the armature and require a certain mechanical power. Especially at low power, these losses will have a negative influence on the efficiency. This is the reason why a PM-generator made from an asynchronous motor can have a higher peak efficiency than the original asynchronous motor from which it is made.

If the winding is connected in delta, there will be certain circulating currents in the stator winding. In KD 78, I have measured the generator for star and for delta rectification and you can see that the maximum efficiency for star rectification is higher than for delta rectification. So the fact that there are no circulating currents for star rectification has a positive effect on the efficiency. But the efficiency also increases at increasing voltage and this effect is stronger. 

[bigrockcandymountain]

I always thought a 540 rpm pto on a tractor would make a nice test rig. At idle you can usually run about 200rpm or lower.  You would need a cradle to measure torque, but otherwise it would have plenty of power. 

This is an interesting experiment.  I'm not sure i can add much, but I'm following along for sure. 

[Astro]

I always thought a 540 rpm pto on a tractor would make a nice test rig. At idle you can usually run about 200rpm or lower.  You would need a cradle to measure torque, but otherwise it would have plenty of power. 

This is an interesting experiment.  I'm not sure i can add much, but I'm following along for sure.

 That's what I said. Maybe spin it off the garden tractor, idk. First I just wanted to build a solid generator, then I will figure out how to spin it. But I think I have a plan now and I am going all in on the wind turbine thing.

[joestue]

The power is eaten up in the motor saturating, not the circulating current.

Add the load, the voltage and current and flux density go down.

[Warpspeed]

This really only has one useful application, charging a battery from a CONSTANT RPM source such as a piston engine of some kind, that has a very effective speed governor.

There are two characteristics that make this unsuitable for more general use.

Self excitation of the alternator relies on tuning the windings to resonance with associated capacitors. There will then be an electrical resonance, that more or less sets the rotational speed to a very narrow range to get the most efficient buildup of resonant energy.

The energy keeps building up in amplitude until one of two things happens, either the steel in the motor magnetically saturates, or enough energy is bled off into an external load to keep the circulating currents going.

Its all very tricky.  If there is no electrical load, the ac voltage can rise to very value, many hundreds of volts perhaps.
Connecting a load will dramatically reduce the voltage.  The more load, the lower the voltage will be pulled, until the damping of the resonance is so great, the voltage collapses to zero at some point.

The only way to start it going again once this has happened is to completely remove all load, and let the resonance build back up.  Its no good just decreasing the load slightly hoping the voltage will come back, it will not.

These are just some figures I invented to illustrate the problem:
No load 350 volts ac
One 100 watt light bulb 300 volts (very briefly, then *plink* the bulb blows)
Two 100 watt light bulbs 220v
Three 100 watt light bulbs 90v
Four 100 watt light bulbs 0v (total darkness).
Switching out some load progressively......
Three 100 watt light bulbs  0v
Two 100 watt light bulbs 0v
one 100 watt light bulb 0v

All load disconnected it springs back to 300 volts.

It may not be exactly like that, but its the sort of thing to expect. You see U tube some grinning guy lighting a couple of light bulbs from a 15Hp motor driven by the PTO on his farm tractor.  What the U tuber will not tell you, or show you is what happens if the load is varied, or tractor rpm is varied.

So one application for this is charging a battery of suitable voltage from a fixed rpm power source through a bridge rectifier.  Resonance build up, and there is no current into the battery until the battery voltage is reached. So that allows the system to start up naturally without any electrical load.

As battery voltage is reached, the current into the battery suddenly increases, and that holds back what would otherwise be the massive peak voltages produced by unchecked resonance.  The battery effectively regulates the voltage !

The more torque you feed into the generator the higher the charging current, being careful not to exceed the rating plate figure for full load amps on the motor.

Its not going to be efficient at charging a low voltage battery, but it is possible with a higher voltage battery, maybe 100v or 200v to get a pretty good system going, but it needs fairly constant rpm which you will never get from a wind machine.
It may be possible with hydro, but definitely the best will be a piston engine gasoline/diesel/steam  with an effective speed governor.

There is nothing really new about any of this, and go ahead experimenting. 

But I think you will eventually realize that its just not a practical way to generate useful power, especially with a highly variable load.  Load MUST be constant, and driving rpm almost so.  Any attempt to reduce loading will likely destroy whatever else is connected through over voltage.

Any attempt at increasing load will drastically reduce voltage and maybe cause a blackout.
Voltage regulation will be worse than horrible with anything other than a large battery as a load.

[joestue]

so i disagree that the self excited induction motor can't be used as a wind turbine...

but i think that in order to make it work you need to have resistors in series with the capacitors. and the voltage across that resistance can be rectified to DC and dumped into a dc-dc converter to make it into a useable voltage.

this might work...
https://imgur.com/a/Yy4trg9


you can increase the resistance across the bridge rectifier which will reduce the effectiveness of the capacitors.. to prevent the run away voltage increase that wastes energy in the rotor.

the problem being is that you still have a cut in voltage at which you need a minimum volts per hz at a minimum rpm in order to generate any power.

as the rpm rises, if the voltage at the load is clamped to a constant value than the rotor flux density decreases... not good. we need it to increase in order to push even more volts into the load.


so basically what i think has to happen is that the capacitors need to be rectified across a resistor, and a dc-dc converter be set up so that any volts present across the resistor.. be boosted and shoved into the load.

some passive controls can likely be good enough to solve the complex issues of how does it start up and what is its maximum power generated before the motor burns out.

this machine also needs to have a mechanical brake.

[Warpspeed]

There is no mystery about how it starts up, or anything else.  how these work is very well understood, and so are the associated problems. Its all pretty basic electrical engineering.

There will always be some pitifully weak remnant magnetic flux stored in the motor steel.  As the rotor turns, that might say for example generate one millivolt. That starts the resonant energy cycling, all one millivolt.
The resonant current causes the flux in the steel to start swinging, and that might generate ten millivolts for the second cycle.
More circulating flux and current, next cycle 100 millivolts, and so it goes.
It keeps increasing until the steel saturates and there is a huge overvoltage.

If there is ANY load at all at startup, your one millivolt wimps out and there is no buildup of energy to tap into.
This very violent resonant buildup feeds on itself, buildup only takes a very few cycles and it appears instantaneous to a human observer.

Its not magic or voodo or mystery.  If you load it down with resistors, either in series or parallel, or place any load on it at all, its just not going to be able to even start up.  You can disagree with me all you like, but show me proof if you can make it work.

Anything can be used with a wind turbine, it may not work very well, or at all.
But you can certainly try.

Not trying to put anyone down, just tell you what I know from my own past practical experiments with self excited alternators, and what I have learned from a great many years as a power electronics design engineer. 

[JW]

what do you think about this quote

"Laminations about the transformer's core give small gaps in between, which enhances the coil's resistance. This resistance will decrease the total current and thus holds the eddy current losses. Lamination is made to decrease the eddy current loss by enhancing the resistance of the core."

[Warpspeed]

Haha, as clear as mud, unfortunate choice of words, and frightful English.
But basically correct.

[Warpspeed]

Getting back on topic.

One experiment Sparweb might like to try.
Set it all up in the lathe and get it running, push the load button and measure the power.

Stop the lathe.
With the load button pressed, start up the lathe and see if there is any output.  There might be if the load is light enough, but probably it will not produce any output at all if you start the generator turning with load applied.

That problem alone might present a small problem for a wind turbine application.

[SparWeb]

Hi Warpspeed!
I'm glad to see you stumbled on my little experiment.
Yes, I don't make any pretense that this is novel or that I'm about to invent something.  I'm just diving into the arcane methods of AC motor control, because what I read in books and (you said it) see on Youtube, just doesn't tell me the whole story.

Thank you for the clear picture about core saturation and especially the play-by-play of the field collapse.  I have literally seen that happen many times, now, and gotten used to it.  If you scroll up to my schematic diagram, you'll see a 3-pole switch on the star-point of my resistance load.  When the machine is running and I close the switch, the load comes on.  Open the switch, and the load drops.  Noticeably, it's a fraction of a second after I throw the switch.  If I happen to stop the lathe with the load still connected, and I restart it, the machine will run unloaded, just like you say.  The implications of that on a wind turbine are not pleasant to think about!

I once had a rectifier fail, shorted to ground.  When it happened (years ago) to a PM generator, everything slowed down.  If this happens to a capacitor-excited field, the opposite would happen.

Your tip about the battery load is fascinating and thank you for explaining it so concisely.  The load from a battery bank is indeed disconnected by the rectifiers - exactly what this type of system needs.  High battery bank voltage, you say...

Scrolling back to the schematic that Adriaan posted on the Part I thread, now I realize that any time the generator's field fails, the voltage driving the control circuit will remove the loads from the output.  Same thing, it needs to build the field before loading it.

In various test runs, when I add the load, the line voltage drops, sometimes substantially.  If you work it out, it's pretty easy to see that the more you load it, or the slower it's going for a given load, the more the voltage droops.  I've seen that happen, so it comes as no surprise when you say that more load can cause the field to break down completely.  Voltage to zero, then.

Variable load (hence, current) and variable speed (frequency) makes for quite the moving target.

[SparWeb]

A brake would likely be necessary as Joe pointed out before.

I have a lot of confidence in the tachometer that I have developed in my datalogger.  It seems to be noise-proof, now. 
As long as it's running there may be some "software" solutions available to control and manage loads...

Silly, I have the tachometer on the lathe and I have a tachometer for the WT and I just realized that this is an opportunity to see of they read the same value!
The essence of "calibration".  Two instruments give the same result when measuring the same thing.

[Warpspeed]

There are fascinating things to play with, and can do some pretty peculiar things if you don't know what to expect.

The bit about the high voltage battery only means that if the rating plate on your motor says something like 10 amps combined (from all three phases) maximum, that is all you can safely expect out of it as an alternator.
If your battery is only twelve volts, that's going to be a piddling 120 watts.
If your battery is 200 volts, its a far more respectable 2Kw.

So trying to charge a battery is fine, but the results will be far better with an impractically high voltage battery.

[Warpspeed]

Oh yes, and you can do braking by applying a dc voltage across any two phases.
Once again do not exceed the rating plate amps for one phase.
That will require a low dc voltage and several amps, depending on the ohmic resistane of the windings.

How you can do that safely during a storm with high ac voltage already across the windings, without blowing thins up,  I have no idea.
But an induction motor makes a quite a reasonable eddy current brake, but it will tend to get VERY hot if run continuously.

[SparWeb]

Hi again,
I've done some investigation into that one and I agree about getting HOT.  I've found a few ways to work out with math how much wind power can be braked by a generator with its outputs shorted together.  The match is borne out by my own observations of having an 8-ft rotor on a PM conversion generator, and then swapping it for a 10-ft diameter rotor.  The 8-ft rotor could be braked with shorted leads, the 10-ft can't.  I now cannot rely on stator braking and the only protection of my current WT is by the tail furling system (and plain ol' robustness of the fat blades and heavy bearings etc...).

The battery - yes that is what I thought you meant.  For hobby purposes, 48VDC is about as high as anyone should go, if you ask me.  For a professional system, with locks on doors and trained personnel, I guess the sky's the limit.

[Warpspeed]

A more fruitful approach might be a home brew eddy current retarder. That should not be too difficult to build.

Some of you guys may be familiar with commercial eddy current brakes (retarders) used on large trucks/busses and also dynamometers.
These can hold hundreds of horsepower, but a small one should work pretty well on a wind turbine.
Its just a cast iron disc, rather like a vehicle brake disc, and some dc iron cored electromagnets arranged north, south sequentially around the disc.

Its very similar to an axial flux alternator in construction,but uses steel cored coils energised with dc.

[Warpspeed]

A random thought just occurred to me...

What if you coupled a small hydraulic pump, such as a junked power steering pump up to your turbine ?
That should turn pretty easily with unrestricted free flow of transmission fluid.  If the flow was restricted that should offer considerable resistance, right up to full solid hydraulic lock.

A motorized needle valve, or similar means of fluid control, might be able to slowly bring on some serious braking.
Any hydraulic Gurus here ?

[Adriaan Kragten]

A big advantage of a PM-generator is also that it can be used as a brake by making short-circuit in the winding. Short-circuit in delta gives the highest peak torque at a rather low rpm (see KD 78). Using of an asynchronous motor as a brake by sending a DC current through one or two phases works for a short time but a lot of heat is generated in the armature and you need an energy source for the DC current.

After reading all posts, I now get the feeling that all disadvantages of using an asynchronous motor completely neutralizes the advantage of the fact that a standard or slightly modified armature can be used.

[Mary B]

A more fruitful approach might be a home brew eddy current retarder. That should not be too difficult to build.
(Attachment Link)
Some of you guys may be familiar with commercial eddy current brakes (retarders) used on large trucks/busses and also dynamometers.
These can hold hundreds of horsepower, but a small one should work pretty well on a wind turbine.
Its just a cast iron disc, rather like a vehicle brake disc, and some dc iron cored electromagnets arranged north, south sequentially around the disc.

Its very similar to an axial flux alternator in construction,but uses steel cored coils energised with dc.

Using precious battery power...

[Warpspeed]

Using precious battery power...

Yes indeed.

[joestue]

what it take to make a wind turbine from one of these?
https://www.ebay.com/itm/153460794838

[SparWeb]

Just going down the rabbit hole a bit further - on the subject of brakes.  They've been tried with some success:

Fabricator: 
https://www.fieldlines.com/index.php/topic,138504.0.html

Dan Lenox:
https://www.fieldlines.com/index.php/topic,129198.0.html
https://www.fieldlines.com/index.php/topic,129665.0.html
https://www.fieldlines.com/index.php/topic,140333.0.html

[mab]

what it take to make a wind turbine from one of these?
https://www.ebay.com/itm/153460794838

Possibly quite good if it's permanent magnet - sort of like a beefed up direct drive washing machine motor - but I'd want to know volts / rpm and how 'coggy' it feels before forming an opinion. If not pmg then you still have the IMAG issues.

[kitestrings]

Some of the Enertech induction turbines used a hydraulic brake.  It basically had an electric solenoid valve, normally held open.  We power was removed either manually or by the controller in high winds, the thing pumped against a closed valve, and so braked.  It worked, but... it was really a mess on the tower, especially if one leaked or failed.  I still remember that smell.  And, it presented some unwanted resistance.  I think there are better ways.

Some of their later, larger turbines (40-60 kW) used a dynamic brake.  A bunch of capacitors in a BIG junction box, and on the back of it there were several water heater elements.  This worked quite well, but you had to give the resistors time to cool if you were doing any sort of repetitive stops.  It stopped in similar fashion to your cut-off saw (some of them).

[SparWeb]

Here are some detailed results from the tests. I’m collecting current at various points in the circuit, and in each phase to make sure they’re balanced.

For example, with 420 uF of capacitors in Delta, across the leads of the motor in series-Y, I see this:

Starting with the generator running Open circuit:
251 RPM
10% slip (belts and lathe drive motor)
12.6 Hz
142V line voltage
11.3 V / Hz
9.5A line current (2337 VARs)
5.9A capacitor current (1451 VARs)
18.8 Newton-meter torque
497 Watts mechanical power

Same setup, now loaded by just 60 ohms:
221 RPM
16% slip
11.1 Hz frequency
90V line voltage
8.1 V / Hz
4.5A line current (857 VARs)
2.9A capacitor current (447 VARs)
5.5A Load current (281 VARs)
242 Watts on wattmeter
86% load factor
22.3 Newton-meter torque
517 Watts mechanical power
47% mechanical efficiency

And that’s just one of the test runs. I’ve repeated this for various speeds for each capacitor value, and varied the capacitors many times. It’s a big spreadsheet…

One general observation: the V/Hz looks good when lightly loaded. The basic motor is dual voltage, and having it in series-Y makes it 460V at 60Hz. If you consider the rated speed actually has some slip, and the base frequency actually extrapolates to 480 V, then the standard V/Hz for this motor is either 7.7 or 8.0 depending on how it should be defined. Either way, the tests where I have the induction motor loaded, it is maintaining a loaded voltage that corresponds to this ratio rather well. Except in some cases where the speed isn’t high enough to maintain the field and the voltage is about to drop out anyway. It is as important to me to use different speeds on the lathe to investigate the BAD speed ranges as well as the good ones, to know what’s going on in both scenarios.

I go back to the books and I can sort-of see how some authors (Gary Johnson for example) have definitely done work like this and understood this better than me, but they didn’t offer enough detail for me to appreciate what really happens. Not as much as just doing it myself. Now that I have, I am going back to the books and realizing “oh, that’s what they meant when they wrote that”.

[SparWeb]

Here's what the test setup looks like:

https://www.youtube.com/watch?v=LbX3Ia0jx3A