Motor Conversion

[Off grid in Tonopah]

If some of the motor conversion folks on the board could give me a little input to on a project I'm undertaking  it would be appreciated. I came across this motor in the trash heap it's 3 phase and looks to be 4 pole if my figuring is correct. I was thinking on using  the 2"x 1"x .5" magnets for the rotor. After reading some of the posts on the board on air gap on the conversions being a problem I thought if I used some iron arc shaped shims on the top of the magnets and used the magnets with the holes through them to secure the shim and magnet to the rotor it would help with these losses. I got the ides for the shims after someone on the board posted that iron was something like 1000 time better at transmitting the flux than air.

What I'm wondering from the folks that are a lot more versed on this than I is what kind of output could be realistically expected from this setup. That way I can start planning on the size of blades to carve. Any help is appreciated.

                                      Thanks Bob

[Off grid in Tonopah]

The data on the name plate photo was a little hard to make out. It's a 1hp 3phase 1729 RPM 220/440 Volts 4.4/2.2 amps 60hz  hope this helps. Again thanks

                                                          Bob

[Countryboy]

Interesting idea.

However, I will note that if you want to use 2X1X0.5 neo magnets, you will need to skew the laminations (a complete rewind), or else use the JaquesM method of skewing the magnets and deal with getting it balanced.

I imagien you will have a heck of a time securely fastening the curved iron piece to the magnet.  You will need to use a lathe to make the curved pieces perfect, and you will cut off the screw if you aren't careful.  

This is an excellent idea though.  If we can place a thin piece of iron over the top of the magnets, we could essentially create a different magnet shape.  For example, in conversions with many round magnets for a pole, placing a thin piece of metal over the entire pole might yield closer to a 'perfect' pole shape.

[Flux]

That was the conventional way to build PMGs in the days of Alcomax magnets. It has some advantages but also has disadvantages and you will likely make a lot of work for little or zero gain.

The reduction of air gap is small and it will cause surface iron losses in the pole shoes. To avoid this you need to laminate the shoes or work with a larger air gap. if you work with the larger gap then you are back to where you started. Laminated pole shoes that thin will be virtually impossible.

The only possible advantage I can see is that it gives you another way to reduce cog by skewing the pole shoes and machining the curve afterwards. If you don't want to rebuild the core with a skew then there might be some benefit as long as you can get the extra length in and have sufficient core at the centre.

This is no substitute for curved magnets, but obtaining curved skewed magnets is difficult. It does give you the option of the correct pole width with a smaller magnet but will not be very effective unless the shoes are fairly thick and that means more surface loss.

Flux

[Warrior]

Hi Bob,

I did this with a 3/4 HP single phase and with round magnets. I managed to get a very small airgap with this setup and it made a huge diference in output and cutin speed. It did cog but it wasn't that bad. It ran for a very long time like this, made about 20-25 amps peak at 12 volts. I used 4 round magnets per pole, 1.00" diam x 0.125" thick

Warrior

[Flux]

With your small quantity of small round magnets that are well below the correct width you would see a very significant improvement. If the rectangular ones are wide enough then there would be much less improvement especially with magnets 1/2" thick, your little round ones are quite thin.

With the bigger rectangular magnets and with added surface iron loss the starting may be affected, but every case is different.

Absolute efficiency seems of no consequence as long as it will start reasonably.

To be worth using the pole shoes it is probably better to think in terms of smaller magnets rather than ones that are probably adequate anyway. I am inclined to think that using thinner magnets, thicker skewed pole shoes and preferably laminated would make the best use of magnets.

To be of benefit the laminations could be fairly coarse and a few runs of weld across them would not remove all the advantage, just lots of work.

flux

[Off grid in Tonopah]

    Thanks Guys. I was planning on skewing the magnets on the rotor and than using some 2" x 1" x .5" iron blocks to simulate the magnets on the rotor than place the shims on top and cutting them on a lathe to size. That way I don't have the chips sticking all over the place. I'm not familiar with the term surface loss for the shoes? The shoes rotate with the magnets so is the loss from eddy currents from the counter EMF from the coils?

    Also thanks for the output figure Warrior I was hoping for some were in the 500 watt ballpark. What cut in speed did you see? I'm going to use this on a 24 volt system so I'm sure it will be different, but any information is good to have when stumbling around in the dark. Thanks Again.

                                         Bob

[Flux]

Remember that if you want to skew the magnets the shoes will need to be thicker than you have shown and the shoes must be skewed ( the magnets don't have to be).

Don't aim for a very small air gap, anything less than 1mm will cause lots of loss.

The loss comes from the high frequency ripple of the core slots disturbing the flux in the gap and inducing eddy currents in the solid pole shoes.

If your magnets are not wide enough you you can make the shoes wider. Aim for the pole shoe to fit inside the smallest stator coil, that normally works out at poles being twice as wide as the gap between them ( 2/3 pole , 1/3 gap).

That is a good idea to use blanks in place of magnets for machining.

Flux

[Warrior]

Cut in was at 230 rpm, but I had it geared up to a two blade, 2 mt prop because the output was low. It put out 15 amps at 1000 rpm, so it did need to turn quite fast.

Eventually I got tired of the chain drive noise and pole shutter so then I flew it with a smaller 1.4 mt prop direct drive. With this set up I had to reconfigure the coils to series/paralel because it would stall with a dead battery.

Cut in now went to a high 490 rpm, and it still managed to put out 15-20 amps.

It was a three blader this time, and surprisingly quite.

Listen to Flux's comments as he's one of the most knowledgeable guys out there.

With the block magnets you have it shouldn't be hard to meet your goal.

I used these small neos because it was all that was available in my country.

Also, do a search for Zubby, he's built ton of induction conversion with very good results.

Good Luck,

Warrior

[Off grid in Tonopah]

Thanks Warrior. I've read a lot of Zubby's posts and was hoping he might chime in with any thought he might have on this project. The only reason I was thinking of using the iron shoes is that skewing the large block magnets will give me a large air gap. I didn't show this in my drawing as paint brush isn't the best cad program. But Flux say's I don't need to skew the magnets only the shoes this could make things much easier. Flux... Would just using the centerline of the shoe as the line of skew be adequate or should I be using center of  iron mass of the shoe as the centerline as I think the mass line will be different from the physical line if I skew the shoe and not the magnet and than machine round. It's hard to see this in my head with out having the parts to play with, I think I'll have to go pinch my kid Play dough for a few hours.  

[Flux]

If you put the pole shoe blank rectangle on the magnet and rotate it to the skew angle you will do what you want. The centre of mass stays in the middle, each side of the shoe has a thick bit and thin bit, that is why you need enough thickness to do it. Ideally the thin edge shouldn't run out to nothing as it will saturate and the skew angle will not be what you think it will be.

Try the play dough, it will end up a queer shape but that doesn't matter.

Flux

[Ungrounded Lightning Rod]

The flux will be stronger in the portion of the shoe under a stator pole piece than it will be under a gap between them.  That means it wiggles around through the rotor pole piece's surface as the rotor moves from one stator pole to another.  Field dragged through a conductor generates eddy currents, and those are the "surface iron losses" Flux referred to.

Flux:  Won't you get the same effect in the magnets themselves, which are also conductive?  Isn't the "surface iron loss" a symptom of the reduced gap (making the "field wiggle" take place more within the conductor rather than within the gap)?

Of course the losses increase with increasing conductivity.  So if iron is more conductive than the neo alloy (I'm not sure if it is) the losses will be greater if the narrow gap is achieved with a curved magnet than whit pole shoes.

In any case: once your magnets are strong enough that you're saturating the stator you don't get any gain by reducing the gap further.  So doing that at the cost of increasing the surface iron losses is a net loss.

I'd use shoes (and damn the surfaces losses) with weak magnets.  But with big neos I'd assume I'm getting to stator saturation or near it, and say "Why bother?"  Or try it both ways and see if I get more power out at a given RPM with the shoes (and then need to make a call on whether the drag and/or its heating's limit on furling point is more of a loss than the power gained).

In any case:  If you use magnets with screw holes you'll want to use a non-magnetic stainless steel screw to hold the magnet down, to avoid "shorting out" part of the field.

[Ungrounded Lightning Rod]

Of course the losses increase with increasing conductivity.  So if iron is more conductive than the neo alloy (I'm not sure if it is) the losses will be greater if the narrow gap is achieved with a curved magnet than whit pole shoes.

Wrote that backwards.  If the iron is more conductive the losses will be greater with shoes than with curved magnets.

[SparWeb]

OG,

IMHO, you can skip the shoes altogether.  The decreased clearance will not figure into a marked increase in performance.

I picture it this way: lines of flux are fixed through the axis of the magnet.  The lines of flux will emanate out of the magnet's North Pole and seek the easiest path around to the South.  When the stator teeth of your motor are 1/16" away from the magnet face, then those lines WILL go through the stator.  There's nowhere else more attractive to go.  If the clearance is 1/16" or 1/32", there isn't much room for the flux lines to dissipate before entering the stator teeth, so I don't expect clearance matters much.

But maybe you want to try it to test the idea.  We would all learn if you post results like that.  Good luck with the project.

[Flux]

I don't know the conductivity of the bulk neo material, it is probably fairly high.

Even the nickel plating resistance is probably high enough to avoid much effect.

I have no figures for this, but I had extreme difficulty starting  a PMG with solid steel poles and Alocmax magnets although cog was negligible. None of the motor conversion people seem to be worried by this so I can only assume it is not a big issue with neo, though to be fair not many have used curved magnets with small air gaps. Alcomax had such a low energy product that large gaps were not viable. With neo there seems no point in having gaps less than 1mm and at this point the surface loss may not be a big issue.

I suspect to high a gap density and very short air gaps are counterproductive.

Flux