large motor conversion

[wind4free]

Hi guys,

Here are pictures of the rewind I have been working on. The motor was a 7.5 Hp 230 volt single phase, 1725 Leroy Somer motor originally direct coupled to a pressure washer. The original winding had about 90 turns of what looks like # 20 wire inside each slot.(I don't have a micrometer to test it).Here is a picture of one stator lamination

There are 36 slots in the stator. Each slot is about 1/2" deep by 1/4" wide.





There is just less than 1/8" clearance between the rotor and stator when the rotor is centered inside the stator. I found it difficult to figure out the dimensions for the rotor so I had the machinist make it a little smaller rather than too big. Machining is really expensive.













I had the machinist make a new shaft for the alternator. The front shaft is 1 1/2" in diameter and I have a 3/4" diameter shaft sticking out the back in case I want to couple something to it (like a break).

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Here is how I oriented the magnets on the rotor.

The frame of the motor is aluminum and and has a small lip against which the laminations sit. So to get 10 degrees of skew I put a protractor on this lip and made a mark at 10 degrees. I then drew a line at this angle on the motor housing. I then placed the laminations in one at at a time each time lining up a hole all the laminations have with this line. You can see it on the left side of the stator. This resulted in all the laminations being skewed to this line. Not terribly accurate, but all I could think of. Once they were all in I squished them together with c- clamps and put screws through the housing so that the portruding screws press against the last lamination holding the pack together.( The second line was an attempt to skew it more to see if this helped reduce cog even more which it didn't.)  So I guess it doesn't cog badly, but I can feel small steps as I turn the shaft by hand.

 



I have wound one phase with #16 wire so far. Each slot has 25 turns which was all I could fit. At about 250 rpm I am getting about 32 volts phase voltage.







This is my last turbine I had working till it burnt out- a 12 pole axial flux machine driven by a 15' 3 bladed prop. In a decnet wind I would get between 1000- 2000 watts and I've seen peaks of up to 3600 watts. (I live in windy Southern Alberta.) This is the second time it has burnt out which is one reason I'm trying a motor conversion. The first stator it had was wound with about 200 turns per coil of # 18 wire. It seemed to be okay up to maybe 1500 watts or so but higher wattages created large heat. When it burnt out I had a stator temp of over 100 degrees C. ( I had a temperature probe inbedded in the stator.) The second stator I wound with # 16 wire about the same amount of coils. It also burnt out. On this one I actually had flames shooting out of the stator as the polyester resin burned. I'm hoping to use this 15' prop to drive this motor conversion.

I use the power from this turbine to heat water. I take the ac from the turbine which I run between 90 and 180 volts and put it through a 3 phase full wave bridge.  I then put the dc through contactors ( with the contacts in sereies) to heating elements. To keep the turbine between 90 and 180 volts I turn contactors on and off using a small plc- an easy relay. I take the Ac of two phases and put it through the secondary of an old microwave transformer. I take the output from this transformer and put it through a rectifier. I put this dc through a resistor to drop the voltage just a bit so that I have an anolog 0 - 10 Volt dc signal which I can feed into the plc. I have the plc set so that if the signal goes over 8 volts it starts turning contactors on. As soon as it stops going over 8 volts, the contactors that were on stay on and the contactors that were off stay off. If the voltage goes below 4.5 it starts turning contactors off, until it rises again. I have timers in the programming to avoid load being added or subtracted during gusts etc. I find it works well and so far the contactors have managed to handle the dc. But with a burned out turbine my system is obviously not working which is why i'm attempting a motor conversion.




<BR

 This is the setup I use to raise my 40' guyed tower. I pull it up with a winch.

So I'm wondering, is the little bit of cog I have normal or should I be able to get rid of it completely? Also 32volts at 250 rpm which is what my prop runs well at is way to low. Only 55 volts line voltage. Maybe I can get 40- 50 coils of # 18 into those slots which might bring me up to 100 volts ( line voltage) or I could go even smaller # 20 - 90 turns like what was originally there. That would give me the voltage at the speed I want but I don't know how many amps it will handle.  I could try a 1:2 gear ratio but that's risky. I know from past experiences that a gear ratio wastes lots of power but then I haven't tried it with a 15' prop. I could run it at a lower voltage but that will increase the current which is a shame for water heating. I am hoping to get up to 3600 watts from it.  Any comments or suggestions would be appreciated.

[Flux]

That's better, we have something to work on now.

I assume the magnets are 2 x 1 x 1/2 neo but I don't think you mention it.

Your magnets are a bit wide for 12 pole and your air gap is also rather big. when taking into effect the extra gap at the centre due to using flat magnets in a curved stator, you have probably got to accept that your leakage flux may be nearly as great as the active flux.

Your 32v phase will give you about 54v line or about 76v dc. You will need to increase the number of turns to get to the sort of voltage you are aiming for.

You could certainly get more turns in those slots as it stands, but by the time you have to cross the other phases I don't think you will manage much more.

If you are going for heating then you will probably end up with a similar wire size and turns as the original. If they got 90 turns of #20 you should get about 45 turns of #17 so your 32 seems a bit on the low side for #16 but You might as well keep it an easy winding as struggle to get a few more turns for all the difference it will make.

I doubt if this will be any more efficient than the axial but it will get rid of the heat better so you should be reliable if you push it to the same level.

Just a few comments about the axial.

I suspect your temperature measurement is wrong, 100C rise should be ok for the polyester resin and very easy for the wire ( unless it was polyurethane $#|+). I think you burnt out at a temperature well above this.

Used for heating you had the option to keep the efficiency up but I suspect you have forced the load too low to keep the speed down rather than accept the fact that you have to use the furling to do this. I suspect that you have loaded the alternator down to 50% efficiency or probably lower in an attempt to hold the speed. If you had kept it at 70% or more you would not have burnt it out.

You were either trying to get too much for the magnets used, failed to make it furl as it should or you were misled by your temperature sensing. I would have been happy to let it run at 100C rise and I am sure it would have been ok if that was a true figure.

Flux

[tecker]

The problem will be the same Your excceding 10amps on the 18 wire .  You can't do steady 10 + ampacities on you coils without damage.

[vawtman]

 Has far has the skew angle goes,Zubbly told me to simply hang a weighted string from the center of an upper slot and the correct angle would be when its at the center of the next slot at the bottom.

 Motor rewind shops have coil formers that can make many different size coils and they just slide them in.Seen it done

 In order to match the copper of the original winding they often use 2 in hand or more windings.

[Flux]

I have just run through these figures and it seems as though your gap flux density is about 750mT. This is not much higher than those magnets would have given you with a dual rotor. You are therefore as I expected loosing half your available flux in leakage.

Even with 750mT in the gap, those teeth will be approaching saturation as they will have about half the total gap area.

To get any really large output out of that motor you are going to have to push the current density in the wire to the limit and rely on the good heat transfer to the core.

You may find some improvement if you fit steel shims under each magnet to reduce the air gap to .020" at the corners of the magnets. I am not sure how badly tooth saturation will defeat your efforts but it would be worth a try before you make the final decision.

Flux

[zubbly]

hi Wind4free,

as soon as you said "Leroy Somer" it fully explained the lack of coil space in the stator slots. of all the motors i have rewound before, Leroy Somer always seemed to have the least space of all.  the bright side of your conversion though, is that the frame is aluminum and will do an excellent job of getting rid of excess heat build up within the stator.

your stator bore from the picture looks like 5 inches, and the one in my 7.5hp Baldor (which was also originally a single phase unit) is 5.475 inch, which is quite a difference. that explains why i am able to get so much more wire in mine.

you have a few choices here. proceed as you are and accept a lower voltage output, use more turns as Flux says but at a cost of using a much smaller wire, or salvage the mags and look for a more suitable stator to acheive what you want. what may be worth doing, is to remove half of the lams, so you use just 12 of the mags, and accept less of an output and use a smaller prop running at a faster speed. the unit really will make an awsum unit for batt charging. order another 12 mags so you still have 24 for a conversion capable of giving you what you want and able to hold much more wire in the stator slot.

motors for larger conversions.  my favourite seems to be Baldor, as they do contain more stator slot room. premium efficient motors (3 phase) such as WEG, WESTINGHOUSE and others seem to contain enormous amounts of coil room.

another alternative and i think possibly the best is to look for a 3 phase motor in the 10-15 hp class. remove all but 4 inches of the stator, and skew those 4 inches of laminations. you will end up with very large wire slots, huge bearings and shaft. this will give you an excellent unit to mate with that 15 foot prop of yours.

since you are in Alberta, i think you should be able to locate a few motor repair shops there. contact them, ask for a burnt 10-15 hp motor (do not use 2 pole motor-3600 rpm), and preferably a rolled steel frame to help keep the weight down. you should be able to get it for scrapp price. ask them to be sure that the bearing journals on the shaft and bearing housings are good. if the unit has already had its winding stripped, ask if they have the rewind data so you will know what wire the slots in the stator originally contained.

hope this helps,

zubbly

[RP]

Would it help him to add radiused pole shoes on top of the magnets?

[SparWeb]

Howdy neighbour,

If you are interested in more big motors, you could try a place near where I work in Calgary.  They have a junkpile full of old motors.  Problem is they're 25 HP and up.  If you want to get one, bring a crane.  Actually, if you were to knock on the door and ask, they might have some more reasonable sizes that they could part with.  (I haven't tried myself because I have no use for a 25 HP motor, even to re-wind.  My re-wind project used a 3HP motor, and that was big enough, thank you!)

I wouldn't get worried about the cogging.  If the "step" feeling is small, when you turn it by hand, then a 15' blade will overcome it just fine.

Have you considered Star/Delta switching to squeeze more performance out of it?  If you want high voltage at low speed, then switch to Delta when the speed is high enough, you can deliver more current because, technically, the wires are in parallel.  Better yet, switching from Series-Star to Parallel-Star will allow double the current a given wire would normally allow.

[wind4free]

Thanks everyone for all the thoghts and answers. I tried what you said flux- I added a 1/16" thick piece of steel behind each magnet to bring them further out. I now get about 40 volts- somewhat better. So if I wind it with 4O turns of # 18 I might get about 130 volts line to line - not bad. But then I'm not sure if the # 18 will carry the current I need. It should be better than my axial because the resistance should be substantially less and we have a good heat sink.

I actually have a premium efficiency weg 3 phase 20 hp motor in my shop which I thought I might use in a bigger conversion later. The inside of the stator measures about 5.5" - much the same size as yours zubbly. I tried putting a test coil into these slots and found I could easily fit 45 turns of # 16 into them- I could probably fit 50 or more with a bit of effort. If I get a similar volts/turn ratio to the one I have now that would put me around 140 volts. I might get a better volts/ turn ratio if there is less flux cancelation. I was thinking I could use the 12 sided rotor, shim up the faces with flat bar steel and have the shaft of this motor turned to fit it. I have to use the rotor again as the rotor with tapped holes and custom  shaft cost close to $900 canadian! The only thing I don't like about this motor is it weighs about 180 pounds. But then it is really strong, probably would never burn out and could take all the 15' prop could throw at it. So it might be better.One other thing is that it's an open drip proof motor. But that could change if I built shrouds to go over the end. Any thougts/ comments on this idea would be appreciated.

Flux you are probably right about that temperature in the axial - I had the sensor attached close to the outside of the coils.- the internal temperature was probably way hotter- also when I checked the temperature it was still putting power out- it burn't out later so it probably got far hotter.  Also I may not have been furling quite enough I guess.

[wind4free]

Hi guys, I removed one # 16 coil and put a # 18 in it's place. I was able to fit 45 turns of # 18- it wasn't easy. On this one coil I get 12.2 volts which when multiplied by six I get about 73. This works out to about 126 line voltage.  I think I might go with this and run the prop a bit faster. Does any one know if it would work to make the stator slots longer- if so what would be as good method? - I think They would need to be punched out somehow.

[Flux]

I don't think it is practical to alter slots in punchings unless you have access to a notching press. In view of the extreme labour required to do it any other way I would advise against it.

Results in the long run may not justify the effort, deep slots increase the problems associated with leakage reactance, those slots look deep enough to me for a 12 pole stator. You would definitely obtain more volts for the same size wire but you may not benefit much in terms of power output. Once you pass the point where resistance is the limiting factor you don't gain much with thicker wire except for a lower temperature rise.

With the shims under the magnets I suspect that is about the best you can do with that core and if you are prepared to let the prop run a bit faster I think it should do quite well.

The larger motor core will undoubtedly do better as you have a better ratio of magnet width to gap, this will help you use the flux better. The larger slots will help in some ways but again reactance with deep slots may partly offset the gain from a lower resistance winding.

There is no denying that low speed alternators need a large diameter and using a part core of a very large motor will probably work out better than a full length core designed for a low pole number. High speed alternators are long and thin, low speed ones need to be short and fat. As true today as it was when this was deduced at the turn of the last century.

Flux

[dinges]

I wonder whether the original poster tried this out (shims to reduce airgap) and what his results were. I'd be very interested in hearing about it. I noticed in the follow-up story that the conversion runs pretty hot (http://www.fieldlines.com/story/2007/8/8/5113/51670)

Flux, your remark about reducing the air gap has me puzzled; if it means it'll drive the teeth into saturation, then it would be a bad thing, as it would just increase the losses in the stator (iron losses) without any increase in output ?

Regards,

Peter.