3 Phase motor conversions

[windy]

 I recently aquired a 7.5 hp three phase Baldor motor that was burnted out. I made a 3 hp 3 phase conversion that works good so I know how to do it. Don't have it on a tower yet, but already thinking of a bigger genny.

 The question I have is when remaking the rotor,could I use heavy wall steel pipe with both ends capped with a hole drilled through both caps for the shaft? Or does the rotor have to be solid for the flux to flow through. I'm just thinking I could shave some weight off the rotor. It would be easier than trying to drill through a 6 inch diameter shaft,7 inchs long. I'm not sure yet if I will offset the stator rings or offset the magnets on the rotor.

 A question for zubbly!

If I offset the stator teeth and put the magnets in line on the rotor the way you did on your last 7.5 conversion, could I cut the rotor down another 1/2 inch and increase the thickness of the magnet to increase the total cubic inches of magnet or is there a point at which there isn't enough iron on the rotor for flux flow. Instead of using 2" x 1" x .5" thick I would use 2" x 1" x 1" thick neos. This project is just in the thinking stage.

 Any comments would be greatly appriecated!

 windy

[SparWeb]

I'm already doing what you propose for my 3-HP conversion project.  The flux has to "flow" from one magnet to the next, and the thickness of the pipe will be enough for our purposes.

While you are thinking of plates on the ends of the pipe, I've opted for an aluminum plug between the pipe and the shaft - it's lighter and easier to bore out than a big block of steel, as you pointed out.  Using plates might be lighter still, but maybe harder to balance...?  Maybe not an issue, either way.

I milled flat surfaces on the pipe for the bar magnets (2x1x0.5).  I left enough room to rotate the magnets slightly to provide the skew.

Pictures coming soon, keep an eye out for my diary...

Others have done similar projects, (not just Mr. Z) but haven't elaborated on how they built their rotors: you may be able to pry some info out of them.

[Countryboy]

A heavy walled pipe?  May I recommend a pipe with an OD of 6 inches, and an ID .005 smaller than the shaft diameter?  Don't even worry about the end plates that way.

Drilling through 7 inches of steel isn't really that bad.  A little time consuming, but it's not that much work.  I imagine cutting a pipe, cutting plates, welding the plates, and then boring the plates will take more time.  Depending on your lathe/mill, it may be easier to drill 3.5 inches into each end, rather than trying to drill straight through in one shot.  Drill it slightly undersize, and then bore to size.  (If you don't have everything set-up perfect, a drill bit can wander off center a bit over 7 inches.  Drilling from both ends lessens your chances of scrapping the part because the bit walked too far.)

I would imagine a solid steel rotor is near flux saturation as it is.  Why take a chance of reducing flux to the coils by skimping out on the magnet's back iron?

[Flux]

For a 2 pole machine you will need virtually a solid core. For 4 poles the tube will need to be fairly thick ( 1" thick wall ?). For 6 pole probably about 1/2" and when you get to 12 pole about 3/8" thick will do.

This will make sure you use all the flux. You will almost certainly get away with a fair bit less as you will be forced to have large air gaps with rectangular magnets in a round bore.

There does come a time when you saturate the iron circuit, particularly the teeth and if you get things right I think you will be there with 1/2" thick magnets. Using 1" ones will probably only increase leakage flux. The improvement at best will be negligible.

Flux

[WXYZCIENCE]

Windy,


That is how I did the rotor on my 1hp unit. I used a 4" schedule 80 pipe and then machined end caps. I used the original shaft. There is only 1/8" of pipe wall after the machining. The conversion works very well. Also it was an accurate way to get the rotor to spin true. I also balanced the rotor by drilling the end plates. Joe

[WXYZCIENCE]

Windy,


My 1 hp is a 12 pole unit, so I do not know how a hollow rotor would work with 4 pole as Flux has mentioned. Joe

[hiker]

on my little motor conversion my first rotor was made from washers stacked on a 5/8s bolt..each whasher had to be hand cut with a hacksaw[4 sides]..

turned out all right but was a little off balance..so i had some vibration in it..

looked around the shop and found some 1.5" pipe laying around -cut a section of that off for a new rotor-using the lowes ceramic mags i could fit eight mags on the rotor-4 pole-2 mag per pole length wise..i then slide the rotor over a 5/8s bolt and centered it-for end caps i just used two 1.5" whashers-with lock whashers and nuts at each end.

plus super glue on the threads to lock it all togeather..

as far as power goes i may have lost a fraction--no room for the harddrive mags on top of ceramics--but my airgap is really close now-so that makes up for some of the loss of not having the h.d. mags on..and my power output is a lot more even..

at a count of one thousand two thousand-the time it takes to rotate my pedals one turn i can power a 13" tv[45 watts] or power up a couple of 40 watt florencent tube lights

[total power 80watts]-or my 1970s stereo...

with all 3 phases hooked in series-power output is about 100 watts a.c ..

with all three phases rectified i get close to 300 watts d.c. out of it..

just for fun i plugged a 110v 4 amp 12volt batt charger on to it and it lite up a 50 watt 12v headlamp almost to full brite[belt was slipping some what]..

well i better stop now........getting off subject here as usuale......

heres a old picture of it-its mounted on a diffrent exersize bike now.

[zubbly]

hi Windy,

i think you will find that if you try to cut the rotor to accept 1 inch thick mags, the rotor diam will be too small to accommodate 12 rows of mags ( i almost guarantee that ). careful calculations on paper will show you this.

you didn't mention how many pole you are going to make the unit, but 1 inch wide mags in the 7.5hp is just about perfect for 12 pole. if you are going to make it a 4 pole or 6 pole and try to put mags tight beside each other to form a wider magnet pole, you will find that you have a very large gap between the mags at the outer surface, whick may cause some problems. also, i did once make a rotor with the 2x1x1/2 mags with same polarity up and tight beside each other (it was an absolute nightmare to get them mounted)

pipe rotor would be fine, and i think Flux"s recommendations for pipe thickness are a pretty accurate estimate. its up to you how you wish to proceed and what you think is the best way to go given what equipment you have.

if you do go with a solid rotor as i did, i suggest no more than a .0005 inch interference fit (1/2 of 1 thou). over a length of 7 inches, that is an extreme amount of press fit. boring the rotor to the same OD of the shaft will still give a tight fit and just weld a few spots on the rotor to the shaft.

good luck and keep us informed of your progress,

zubbly

[windy]

 zubbly

 I forgot to mention that it would be a 12 pole conversion similar to your last 7.5 hp conversion. I'll take Flux's suggestion of using 3/8" tubing for the rotor. I will probably use 1/2" thick tubing if I decide to cut flats on the rotor for the magnets unless the rotor will get to thin. I'll have to draw it out on paper to see what it looks like.

 Again thanks everyone for all the suggestions!

 windy