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Forum Index : Windmills : 7.5HP Three Phase Motor Conversion

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vawtman

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Joined: 14/09/2006
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Posted: 10:27pm 20 Jan 2009
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linkI tink
 
johnj
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Posted: 06:31am 21 Jan 2009
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thanks guys
cheers, J
 
SparWeb

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Joined: 17/04/2008
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Posted: 04:51pm 21 Jan 2009
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Welcome to the Backshed John J.

I checked out "Vawts.net" once. I didn't like much of what I saw - too many lawn ornaments. Those who want to produce lots of power come here!

If 6000/RPM gives 6 poles @ 1000 RPM (equation for 50Hz equipment), then 920 RPM under load is 8% slip. That sounds like a lot! Any motor I've converted had about 3 or 4% slip, judging by the data plate.

A question for the larger audience:

Would the high slip in this motor affect its success as a conversion?


Steven T. Fahey
 
oztules

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Posted: 07:27pm 21 Jan 2009
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Sparweb,

The frequency gives us the rpm. At synchronous speed, we have no torque. To get torque, we need to get a differential between the rotor and the rotating field... asynchronous speed.

The torque for a 2 pole for 1000w is different to the torque for a 4pole 6 pole etc. The lower the rpm, the higher the torque, and if slip gives us the torque... then it would be easy to think that that is all the answer... however

The slip is also a function of rotor resistance. The higher the rotor bars resistance, the greater the slip ratio becomes. For low slip motors, low resistance in the rotor bars is necessary.... however to get starting currents down to a reasonable level, some resistance is desirable.

Also bigger motors tend to have lower rotor resistance and so lower slip (and also use interesting start circuits to get around the huge start up currents associated with it)....

So once you have taken the rotor out and bastardized it to your requirements, all the slip components are removed from your conversion.... and so should not I would think come into your calculations.


...........oztules
Village idiot...or... just another hack out of his depth
 
Dinges
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Joined: 04/01/2008
Location: Albania
Posts: 510
Posted: 07:48pm 21 Jan 2009
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John,

You seem to be misunderstanding some aspects of induction motors and motor conversions. It happened to all of us, I know it feels overwhelming in the beginning. So, before you go any further taking apart motors or removing turns, I'd strongly suggest to read (and re-read, and re-re-read) these excellent files by the master himself, Zubbly:

http://www.otherpower.com/images/scimages/4/z_conversion_all .pdf
http://www.otherpower.com/images/scimages/4/zubbly2.pdf

Then, browse through all his pictures in his IRC gallery:

http://www.anotherpower.com/gallery/zubbly

You may also want to read a few other stories on FL regarding motorconversions. Google 'motor conversion' on the Fieldlines forum. Some users who have built conversions and did a proper write-up are 'Zubbly' (of course), 'Sparweb', 'Behoof', 'JacquesM', and myself, 'Dinges'. (there are much more, btw; these are just the names that pop into my head right now). You may also want to check out my photo gallery:

http://picasaweb.google.com/motorconversion

When it comes the time for designing a new rotor and determine how to decog, you may want to read this too:

http://www.otherpower.com/images/scimages/3538/decogging_tut orial_V1.pdf

There's so much information in the links above that it'll confuse you the first few times you read it. In the end it'll start to sink in though.

I agree with Steven on the vawt forum; it's nice for vawt afficiniados to have their own forum, but it's not my cup of coffee either. To each his own. Keep in mind that motorconversions generally have lower efficiency (due to iron losses) than axial flux alternators. Especially in a VAWT a high efficiency generator may be particularly important, as there's already so little mechanical power available; so losing 20-30% of it in the generator (motor conversion) may be even more of an issue than with HAWTs.

Peter.
 
Tinker

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Joined: 07/11/2007
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Posted: 01:20pm 22 Jan 2009
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Thanks Peter.
That essay on decogging was particularly helpful, I wish I had seen it before I encapsulated the magnets of my little project in epoxy
Klaus
Klaus
 
SparWeb

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Joined: 17/04/2008
Location: Canada
Posts: 196
Posted: 06:50am 31 Jan 2009
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Hi,

Back to add a new chapter to this story. At last I've been able to test the conversion's performance. Since the assembly I haven't been able to get back to the project and finish it off. The complete motor weighs 125 pounds, so even moving it around is difficult.





The cogging is noticeable, but not excessive. It seems bad when turning it by hand, but with a suitably sized propeller, there will not be enough resistance to prevent early start-up. I get a laugh out of my GE motor-conversion, whenever I see it tick-tick-tick to a stop, so I expect to see the same from this one, if I ever get it up a tower.


Early bench tests showed me that cut-in will be very slow in this one. Wired in series-star, the 24V cut in is just 40 RPM! It just begs to be wired in parallel, or operated with a 48V battery bank. Later tests show that very high voltages are present at high RPM's, making direct electrical water heat a serious consideration. I had to take unusual precautions while running it up to avoid electric shock. More on this later.







To give Toshi a complete run-up test, I used a lathe at work. This lathe has a 5 HP motor, which sure seemed like enough before I started, but I was wrong! I chucked the main shaft into the jaws and the tail (fan) shaft into the tailstock. Then I bolted a board over the mounting lugs to serve as a torque beam. Sorry if the messy pile of stuff beside the lathe makes it hard to tell how the torque was measured. I just dropped a board on the floor, put an eyebolt through it, and weighed it down with bags of lead shot. It was just a convenient way of anchoring something to the floor under the torque beam. A spring scale connects the torque beam to the floor. Load on the scale indicated torque (force X arm). In all tests the arm was 36.0" (91 cm). If you can see it, sticking out the back was another arm to which I taped a counterweight to get the scale to read somewhere close to zero when the machine was stopped. I still had to deduct a "tare" from the scale reading.


I brought a pile of batteries with me for this test; enough for 24V and 48V trials. Quite a mish-mash here. The smallest battery, from my tractor, was far too small. It suffered a lot of gassing and spilling during the test! This actually affected the test results, so I had to make allowance for the over-voltage when calculating the output that it would actually show if a charge controller was regulating the voltage.


There were a lot of combinations available to test. Unfortunately, I did not have time, nor the extra rectifier, to test "Jerry" connections. I really regret this because it was the perfect opportunity to make comparisons for myself. Not since Flux wrote his "Matching the Load" thread on Otherpower have I seen such a clear comparison of Star, Delta, and Jerry connections, and he wasn't using a motor conversion to do then.


The choices I did have were 24 & 48 volts, series & parallel, star(Y) & delta(D). In the end, I had time for parallel-Y in 24+48V, series-D in 24+48V, and series-Y in 48V. I did not test in parallel-D because the vibration in series-Delta was frightening! The lathe was putting out more noise than I've ever heard it make. After realizing that I should have checked the data plate on the lathe's motor, I discovered that I was on the verge of locking it up! In the end, fear, not power, was the limit to my run-up tests. Hehehe.


It makes for quite a mess to plot all of the curves on one graph. Here they are anyway. Splitting them into separate graphs for Inputs and Outputs will make more sense.






Some results aren't surprising, like the output curves at 24V in parallel-Y and 48V in series-Y, which match up very well, because the increase in voltage is proportional to the decrease in current. What WAS a surprise was more output power in series-delta at 24V than parallel-Y. I did not expect that. Nor would I have guessed that the Parallel-Y curve at 48V would line up with the Parallel-Y curve at 24V like it does at 300 RPM!


Another surprise is how closely the input power required for 24V series-Y and 48V parallel-Y are. You would think that the higher current necessary to make the same power at lower voltage would incur a resistance loss, which would show up in greater input power demand. And yet, the two curves are nearly the same. If anything, it took more power to turn at higher voltage. I'm still scratching my head about that.


None of the output power curves are straight lines. They bend down at higher RPM. This is probably a characteristic of motor conversions, where the copper is wound around iron teeth. The windings have a certain amount of reactance to current, due to the iron laminations, which increases when the AC frequency increases. Delta was least affected by this. It is possible that the output would plateau at some speed. I couldn't explore such high speeds due to the immense power required to do so.


Delta-connections seem to be the winner looking at the curves, but something I haven't mentioned yet is the vibration! Sure there was noise during all of the tests, but nothing compared to the heavy vibrations running in Delta! The torque measurements are, in fact, averages of the scale readings, which were often 20 pounds +/- 4 or 5. Windmills have enough trouble with vibrations coming from blade imbalances, tip tracking differentials, cogging, and wind turbulence, I don't think I really need to add any more... Too bad. I can only imagine what parallel-Delta would have yielded, but alas, I didn't dare! Perhaps if I decide to try some Jerry connections, and haul the genny and batteries back for more tests, I will risk a few parallel-delta tests to see what happens.





Here's the last graph, where the rubber meets the road, so to speak. After wondering if Parallel-Y would be worth anything at all, here it rises to the top, though only under certain conditions. The P-Y connection offers the highest efficiency in both 48V and 24 Volts. There may be other combinations that give the same or more output power, but more power at the prop is needed to get there.

Which leaves me with the next question. What size and TSR would make a good prop for each of these combinations? Higher TSR is needed for the faster connections, bigger diameter for the less efficient ones. Ah the joys of matching the cubed power wind with the square power generator load!

I'm going to leave it here for now because I could just keep going on about the skew, the open-circuit voltages (I got up to 250 volts before fearing for the rectifiers), and more.

Steven T. Fahey
 
GWatPE

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Joined: 01/09/2006
Location: Australia
Posts: 2127
Posted: 08:27am 31 Jan 2009
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Hi sparweb,

This is a fairly large motor conversion. It seems that the efficiency with permanent magnets is still quite low as an alternator at low speeds. The performance of the parallel delta suggests that winding resistance is a major culprit in the low efficiencies. The shuddering of the machine in delta suggests recirculating currents between windings. It is likely that the output waveform for an individual phase is not sinisoidal.

I hope other readers appreciate how much effort goes into producing comparison data. This is a great effort. Unfortunately the tests confirm my direction to build another AxFx machine.

Gordon.
become more energy aware
 
SparWeb

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Joined: 17/04/2008
Location: Canada
Posts: 196
Posted: 06:05pm 31 Jan 2009
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[quote=GWatPE]This is a fairly large motor conversion.[/quote]

Hi Gordon,

This is about as big is I could go. I can barely lift it!

When wired in series-star, the winding resistance is indeed high, like 6 or 7 Ohms. The resistance loss is huge. Heating was a concern, so I only ran the lathe for 30 seconds at a time. (Turns out that was as good for the lathe as it was for the Toshi).

You could make a case for re-winding the stator with greater wire gauge. If a prop can be matched to the input power curve as it is, however, there might not be much benefit to re-winding when all is said and done.


Steven T. Fahey
 
Dinges
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Joined: 04/01/2008
Location: Albania
Posts: 510
Posted: 07:22pm 31 Jan 2009
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Steven,

Thanks a lot for making all those measurements and publishing it here. Very valuable information. First time I've seen this kind of information for a larger motorconversion. And yes, indivdual rectification of all phases would have been very interesting information too...

I had one conversion that worked perfectly in delta; on attaching a scope, it turned out to put out near-perfect sinewaves, i.e. very little harmonic distortion. I figure the problems we see in practice with delta is not because of delta causing circulating currents in itself, but because of non-sinusoïd waveshapes with 3rd order distortion, which, in delta, can cause circulating currents.

Your efficencies are quite a bit lower than what I expected and hoped for, but considering that you're using the original windings (high voltage, thus high resistance) it's not very surprizing. You say:

[quote=Steven] You could make a case for re-winding the stator with greater wire gauge. If a prop can be matched to the input power curve as it is, however, there might not be much benefit to re-winding when all is said and done.[/quote]
Matching to the input power curve needs to be done anyway... but if rewinding gives you effiencies in the range of 80-50% (as for my small 500W conversion) I think it'd be worth it. You'd still need to match to the blade, but you get more electrical output power and less wasted heat in the conversion. But that's just me. You know, there *is* a reason that people go through the trouble of rewinding... it's not just a way to keep busy... Rewinding, though not hard (if you can get the materials), is quite a bit more work. With the current winding it may indeed be a better way to utilize it for water heating, not battery charging.

Here are the measured curves for a much smaller conversion, but one that was rewound. The efficiencies are quite a bit higher... so I think rewinding is worth the effort:



[quote=Steven]This is about as big is I could go. I can barely lift it![/quote]
Yes, know what you are talking about. That's a big issue with my 10hp conversion too. It takes some logistics to move it from one place to the next.

Off-topic: Have you considered swapping bearings (front to back, back to front) on the rotor, and swapping endbells too? That way the larger bearing would be at the rear, where it could take the full axial thrust load of the blades.

Again, excellent write-up. Thanks.

Peter.
(but please, tell me the red hose is not compressed air, used to 'clean' the lathe...)Edited by Dinges 2009-02-02
 
oztules

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Location: Australia
Posts: 1686
Posted: 10:25pm 31 Jan 2009
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Sparweb,

Great to see such well gathered and presented information. I would be inclined to run it at as higher voltage as I could... to minimise the R loss. Maybe 48v jerry will be as good as it gets.

.........oztules

edit: Dinges, I'm sure he only uses the air hose to cool his coffee. He wouldn't use it to drive fine swarf dust into the slides and chuck spirals..... Edited by oztules 2009-02-02
Village idiot...or... just another hack out of his depth
 
SparWeb

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Joined: 17/04/2008
Location: Canada
Posts: 196
Posted: 01:56am 01 Feb 2009
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Hi Peter,

Thank you for your comments. I hope you especially will benefit with your concurrent conversion project. The efficiency curves you've posted there are interesting to look at beside mine. At every connection or voltage there is clearly a peak soon after cut-in, and a rapid decline. The slow decline in efficiency of your 500W conversion starting at 80% testifies to the benefit of the re-wind.

I don't worry much about the thrust load... Have you ever worked out the gyroscopic loads? I want the big bearing on the front for that!

As for the air hose, that wasn't my idea. If it was my Colchester, I'd be cleaning it with a toothbrush. Actually, years ago, it was my job to clean it from time to time. Time, new shop personnel, and changing responsibilities at the company carry me away from those "good old days".


Oztules,

I know what happened when you tried resistance loads instead of batteries...
Steven T. Fahey
 
divemaster1963

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Joined: 28/01/2009
Location: United States
Posts: 46
Posted: 02:09am 01 Feb 2009
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hey guys I am a newbie trying a experiment with a cieling fan for my shop. I have a question about the magnets? i have 12 small 1x1x1/2 mags. that will fit perfectly around the casing. I also have 6 1x3x1/2 mags. that will work also> my question is which ones will preform better. 12 small or the 6 long ones. any info would be welcomed.

thanks for your time.
 
herbnz

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Joined: 18/02/2007
Location: New Zealand
Posts: 258
Posted: 04:17am 01 Feb 2009
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  SparWeb said   Hi,

Ah the joys of matching the cubed power wind with the square power generator load!




Hi Steven a ratical thought has just struck me.
With permanent magnets we cannot get more than a squared charcs, if we were separatly excited the field could be increased as the load current increases by series winding.
Now we know a squirrel cage can be excited by addition of capacitors to provide the excitation normally in parallel but as proven in the FP motor also series works.
So as well as adding magnets to the rotor form rotor bars between the magnets shorted at both ends as in a squirrel cage. This is commonly done in large sychronous machines to stop hunting. This will not have any effect normally except if there is hunting. However if a leading current is created ie load current with series capacitance, the rotor bars will provide increased excitation as in a self excited induction motor. This will give an increased output potential as the load increases breaking the square relationship and heading to a cubed relationship .
Herb
 
SparWeb

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Posted: 06:52am 01 Feb 2009
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Hello diver,

If you're choosing between different sizes and numbers of magnets, you might want to check the size of the coils that will receive the flux. Part of the motor's principle, when you are using it as a permanent-magnet generator, is that the magnetic field crosses through each winding of wire, inducing a voltage. If you were to arrange your magnets into, say 6 poles (N-S-N-S-N-S), then you would have either 2 square inches of magnet face per pole, or 3 using those big ones. If the coils accepting the flux are only 2" wide, then putting 3" wide magnets around the rotor will not help much.
Have you determined for sure that you need 6 magnet poles? If you want to get into more details, try starting a new thread, and more people will see your questions.



Hi Herb,

You really made me get my thinking cap on!

That's an interesting idea. How do the rotor bars increase excitation? Do you mean that they would be copper? If connected at the ends, then a current could flow in response to the demagnetizing current in the main coils... Do I follow your reasoning? Current flowing between the bars would be a feed-back response that would delay the drop off of output current. Not so radical because it was bars like that which I was cutting through, turning down the rotor diameter for the magnets in the first place!


Steven T. Fahey
 
GWatPE

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Posted: 10:10am 01 Feb 2009
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I see a fair bit of work to be done yet with matching the alternator to the wind energy. The example alternator above coupled with a typical blade set would yield approx only 1/9 of the wind energy getting to a load. I would not be prepared to add the complexity of slip rings to the rotor in an effort to change the rotor magnetic field strength, in this conversion. Maybe above 30kW would be worth the effort.

Gordon.

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herbnz

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Posted: 06:13pm 01 Feb 2009
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  GWatPE said   a load. I would not be prepared to add the complexity of slip rings to the rotor in an effort to change the rotor magnetic field strength, in this conversion. Maybe above 30kW would be worth the effort.

Gordon.

Where do comments refering to sliprings come from ???
If it is in relation to my comments re re establishing the rotor cage alongside the magnets to allow the use of capacitors to increase excitation as the load increases as you have been doing in the FP's I am amazed that you wish to confuse the promotion of this to other applications.
Again the simple addition of a low resistance bars shorted at both ends is relatively simple now though if a leading load current is provided current will be induced here that will boast the permanent magnets.
compensating for the fall off in flux due to the de magnetising ampere turns of the stator.

Herb
 
herbnz

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Posted: 06:15pm 01 Feb 2009
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  SparWeb said   Hello diver,


Hi Herb,

You really made me get my thinking cap on!

That's an interesting idea. How do the rotor bars increase excitation? Do you mean that they would be copper? If connected at the ends, then a current could flow in response to the demagnetizing current in the main coils... Do I follow your reasoning? Current flowing between the bars would be a feed-back response that would delay the drop off of output current. Not so radical because it was bars like that which I was cutting through, turning down the rotor diameter for the magnets in the first place!


Hi steve
Current would only flow in these bars due to the demagnetising current if it was made to lead induced emf by use of Capacitors in the output. As Gordon has shown this works with series caps on FP's.
This will increase the MMF compensating for normal fall off and in fact can add flux.
The exact workings that go on with the demagnetising ampere turns is very complex I have been trying to put it together for presentation here but so far no joy. I have researched text books googled etc only one book so far analysis the action most books will explain the action that is similar in transformers and DC motors but delightfully avoid AC generators. The one book uses complex phasor diagrams that would go down like a lead ballon here.
Yes the cage i propose is the same as the one you destroyed lol, as you would now have less volume heavy copper would be the go bars just alongside the magnets.
I notice might be difficult on your present rotor as gap not clear an straight,can you machine neo magnets ie grind a channel clear .

HerbEdited by herbnz 2009-02-03
 
SparWeb

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Posted: 06:08am 02 Feb 2009
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Hi again Herb,

Okay so I was sort-of following you. If I was to set a rod in the space between the poles, I could fit a 3/16" diameter rod (5mm) equivalent to about 5 gauge. That would not affect the clearance to the stator. The rod could be welded/brazed in before the magnets go on (I have zero proficiency with welding aluminum) or soldered and cast in epoxy to keep it together. You know, there is some aluminum still down there. I didn't machine it all out. This rotor was a surprise to me because it seemed to have two layers of aluminum bars. Once I was through the first set I was sure that was it, but on the next pass of the lathe I discovered yet more bars to cut through.

But does simply restoring the bars do what you expect? You don't need any more trickery to make the secondary flux lead or lag?

[quote]Current would only flow in these bars due to the demagnetising current if it was made to lead induced emf by use of Capacitors in the output. [/quote]

Uh oh that's the kind of trickery I was afraid of.

I don't understand what Gordon suggested, either, but he is as likely to be wrong as to have jumped ahead of us and realized that some tweaking will be required - of a kind that requires slip-rings...? What do you say G?

Steven T. Fahey
 
herbnz

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Joined: 18/02/2007
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Posted: 06:44am 02 Feb 2009
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  SparWeb said   Hi again Herb,

This rotor was a surprise to me because it seemed to have two layers of aluminum bars. Once I was through the first set I was sure that was it, but on the next pass of the lathe I discovered yet more bars to cut through.

But does simply restoring the bars do what you expect? You don't need any more trickery to make the secondary flux lead or lag?



Ok what you have there is a motor designed for high starting torque the outer rotor winding has higher R and lower Xl so when starting and rotor frequency high it does the work when frequency drops to near DC the lower bars work. If its still intact it will maybe do the job.
However placing copper bars between magnets and shorting out both ends then epoxy them in would be good insurance.
If you set up your test bed as before but instead of connecting directly to the rectifiers place a capacitor in each phase line as Gordon did to get a handle on the effects try say 100mfds or what ever size you can scrounge keep the voltage rating up and better on test with batteries connected or if open circuited at low speed watching the cap voltage.
Well worth a go to see effects all a learning curve .

Herb
 
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