Author Topic: 2hp induction motor conversion - how much neo magnet required?  (Read 10982 times)

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mab

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2hp induction motor conversion - how much neo magnet required?
« on: November 11, 2015, 04:34:02 PM »
I'm currently using a 2hp 4 pole induction motor with capacitor exitation as a hydro generator and was thinking of making a pmg replacement.  I've been looking at some of the motor conversions folks have posted on here (and elsewhere) including Zubbly's archives that have been referenced recently.

But I was wondering if I really need the thick (& pricey) neos that seem to be used (3/8" - 1/2" from what I've seen) as I plan to run this at ~800ish rpm rather than the 200ish you might be aiming for with a wind turbine?

I do want a fairly high output voltage for transmission purposes but does anyone know if there's a rule of thumb for relating output volts  as a pmg to output volts as a cap exitation system when using the standard windings? I guess it all depends on the magnetic flux linking into the stator but I don't really have a handle on how zubblys pmg conversions compared to the motor flux density in it's normal operating state.

also looking for a uk supplier of reasonably priced neos if there is such a thing.

thx

joestue

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #1 on: November 12, 2015, 02:59:32 PM »
you may be able to get away with 1/8th inch thick magnets, but they are more expensive than 1/4th inch thick magnets.

so what you can do is mill out the slots for only the north or south poles, not both.

use https://www.emetor.com  to simulate your motor.

also how many watts are you presently getting?

mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #2 on: November 12, 2015, 03:20:25 PM »
currently getting about 400w but could get more with more/bigger nozzles (maybe 500), but the bearings have developed a noise suggesting I'll need to replace them soon - so I'm a bit reluctant to push it harder ATM - and also why I'm thinking about modifying as it's going to have to come out soon anyway. If I go pmg I should get a bit more again? and more importantly more power when there's limited water?

I do have a 3hp motor sitting around which I could modify, but presumably that would require even more magnets. I did wonder about the merits of finding a smaller motor - but thought that the winding resistance would be higher. Also using another motor would allow me to modify whilst the old one keeps generating - assuming the bearings don't give out.

Doing norths or souths is a good idea. :)

I must admit the best magnet prices I've seen (per unit volume) are actually 46mm x 30mmx 10mm for hugh piggot AF - not the best size/shape for pmg conversion - so you may well be right about pricing.

mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #3 on: November 12, 2015, 03:46:19 PM »
have to confess I'm not sure where to start with that simulator.

hiker

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #4 on: November 12, 2015, 07:35:57 PM »
What about just using caps..only takes a small amount of currant to get things started..12v..
WILD in ALASKA

joestue

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #5 on: November 12, 2015, 08:25:13 PM »
have to confess I'm not sure where to start with that simulator.

i can't figure out how to login either at the moment.

so the three hp motor should be more efficient than a 2 hp rewind.. that is if you can effectively use the volts it will generate. it may not require more magnet volume, but rather the problem may be trying to source 3 inch long magnets. 2 and 4 inch long are easier to find.

you should be able to improve your efficiency from 50% to 75% with a permanent magnet conversion, so add the second nozzle and i would expect with a properly engineered system you can double your generated power.

so, i may be able to login on another computer.

what is the od and id of the stator, od of the rotor.

and what is the slot opening (the gap the wires have to go through, usually 2mm)
and the slot depth and width.
is the slot sides parallel, or is the iron tooth sides parallel (most likely the iron teeth have parallel sides.. if so, what is the width)
and what is the length or depth of the lamination stack..

emetor will let you save the configuration in a text file btw, so you don't have to type all this in every time.

and what is the rpm you're presently running at?

mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #6 on: November 13, 2015, 01:02:10 PM »
Thanks Joestue,

I don't have dimensions for either motor at the moment - I'll open up the 3hp tomorrow, but the 2hp is generating right now and I didn't note anything down properly before I put it into service - annoying as I can't order new bearings until I take it out of service. The 3hp I have sitting here is wound for high volts (415v 50hz delta)... that gives me quite a high output volts but as the plan A is for transmission at high voltage and to use a grid-tie inverter to connect to the house grid that may be OK. Alternatively, I may be able to split the coils and parallel to halve the volts if needed.

I'm not sure of exact revs but I estimate 800rpm ish - I think I calculated that from head and pelton runner diameter (runs best at 26-27hz at 400w; falling to 22hz at 50w, but I have no real idea how much slip there is).

What about just using caps..only takes a small amount of currant to get things started..12v..

that's what I'm using now: partly it's for those times when there's limited water and efficiency becomes an issue, but I get the feeling that cap exitation works better at higher frequency(?) i.e. you can use less capacitor, therefore less cap current.

mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #7 on: November 14, 2015, 09:37:20 AM »
well the 3hp motor is quite an old one ( I do wonder if I might be better off with a newer one) but it has 36 slots, the internal diameter of the stator is 5 1/2 ", and the length of the stator iron is 2 5/8 ", although the rotor seems slightly longer.

will try and attach some pics, but as there have been issues recently...

SparWeb

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #8 on: November 14, 2015, 09:27:16 PM »
Hey
That looks like my first motor conversion project.

I found some wonky magnets for that one.  They were 2x1x1/2, with two holes through them, but they have become much more expensive since 2006.  I understand your pain.  You can get an equivalent result from putting on the same volume of magnets, but you can also improve on my result with smaller magnets - because you can keep the gap smaller.  I got carried away on machining the flats on the rotor, as you can see.  I ran it at first with 2 magnets per pole, then took it apart and put 3 mags per pole - and measured a 50% increase in current as expected, but not much difference in cut-in speed.  In hindsight I understand why.
 
It's the flux that crosses the gap that determines the open-circuit voltage/speed ratio.  But beware, if you scrimp on magnets then the performance could flatten out at high speed.  That's where the total volume of the magnets comes in.  Perhaps run-away is not so big a risk on a hydro unit.  Just bear in mind that cut-in voltage isn't quite as important as the current that is actually being driven when running at speed.

Do you have wiring choices?  For example six wires for Wye/Delta choices? 
Avoiding converting the motor with higher winding resistance is a good choice.

Another trick I've used successfully is to alternate magnet sizes.  When the rotor length (like yours at 2.6") doesn't match a typical magnet length, putting two together (2" + 0.5") covers the rotor more completely.  It also gives you a different way to skew the mags, by shifting the small one a bit.  Doesn't eat up as much rotor/stator gap, and still spins smoothly.  Further, two small magnets are often cheaper than one big one of the same total volume.

GE 3 HP motor conversion



No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #9 on: November 15, 2015, 10:55:12 AM »
Hi Sparweb,

that does look like the same (old) type.

I've been looking at small magnet last night - there are some bargains but they seem to be 1/8 - 3/16 " (3-5mm) thick whereas I'm looking (I think ) for 3/8 - 1/2". I did see some 60x 10 x 5 mm which would work OK if I could double them up to get 10mm thickness. Using small magnets to achieve the skew is a good idea :) .

I've changed my mind about doing only two of the four poles with magnets as I'm not sure about how to turn down the rotor on 2 of 4 quarters rather than turning down the whole thing.  The other engineering challenge, i suspect, is keeping the magnets in place at high rpm - if the genny were unloaded it could go up to 1500+ rpm.

Ideally I'd like the volts/speed ratio to be low enough that I can use standard 50Hz/240v transformers in the short term - trouble is, this motor is wound for 415v delta rather than 415 wye/240 delta (it does have 6 wires), so I was planning on looking to turn it from a 6 wire to a 12 wire which would give me options for: 207/359/415/719 nominal volts - if I can - otherwise I may look for a 230/415 motor.

mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #10 on: November 15, 2015, 11:12:01 AM »
Quote
It's the flux that crosses the gap that determines the open-circuit voltage/speed ratio.  But beware, if you scrimp on magnets then the performance could flatten out at high speed.  That's where the total volume of the magnets comes in.  Perhaps run-away is not so big a risk on a hydro unit.  Just bear in mind that cut-in voltage isn't quite as important as the current that is actually being driven when running at speed.

hmm - that's what I'm trying to get my head around - max power point for the pelton wheel should, in theory, be at a fairly constant speed - which should correspond to a constant emf for a pmg genny(?). At the moment I'm going by what others have done (3/8-1/2" thick) rather than understanding why they've used that thickness. But if I'm going to the trouble & expense of doing a conversion, then I should do it properly the 1st time - I don't want to get it built and installed only to find I ought to have spent a bit more on magnets.

joestue

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #11 on: November 15, 2015, 02:54:37 PM »
the 60x 10 x 5  magnets are probably the best option.

one layer of those magnets, nnnnnn-sssssss-nnnnnnn-sssssss.. and skiewed should do it.

seems to me if you turn the rotor down to fit 5mm thick magnets in, if they are 10mm wide you can put 40 of them in, if you don't skew it. 40 magnets is an expensive way to saturate the core in my opinion when you don't need to get anywhere close to saturation to get 400-800 watts out of a 3 hp motor.

I think you can do with 28 magnets, 10mm wide. skiew them of course and space them out.
for holding the magnets in.. you can tightly wrap .010" diameter stainless steel wire around them and add a layer of epoxy but as far as how to hold them in in place while the epoxy cures, i have no idea. might have to clamp them individually and wait for the epoxy to setup.


given the very large diameter of your rotor, I think you could try a ferrite conversion using the ferrite magnets found in treadmill motors or other 90 and 180 volt dc motors. they will have to be cut with a tile saw but the outer diameter of those magnets is close to 5.5 inches.

hiker

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #12 on: November 15, 2015, 03:06:05 PM »
sence its a water powered gen..wounder if you might get a bit more power without the mags skewed ?  dount really have to worry about start up..with water power........
WILD in ALASKA

mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #13 on: November 15, 2015, 04:34:48 PM »
sence its a water powered gen..wounder if you might get a bit more power without the mags skewed ?  dount really have to worry about start up..with water power........

don't know is you get more power without skew, but I think I'll add a bit of skew anyway as I don't want it making any more noise than it has to.


Kind of drew a blank searching for treadmill motor ferrites so unless there's a magic search-term I missed (quite likely) I'll leave that for now.

I was slightly pleased to see you suggest 28 of the 60x10x5's as (following Zubbly's post) I'd estimated 7 per pole (plus a couple of smaller mags to add skew) - I must be doing something right. I'm even more pleased to hear that I don't actually need 10 or 12 mm thick as that was sort of what I was trying to pin down with my original question. :)

Next thing then is to verify the windings and see if I can split the windings into 12 wires before I start to buy...


curiously, I find that the best deals on neos seem to be very small ones or very big; I did find these which got me thinking of the merits of building an axial flux:-
http://www.ebay.co.uk/itm/1x-Super-Big-Strong-Magnet-50x25x10mm-Rare-Earth-Neodymium-Fridge-Magnet-N50-/272017098505?hash=item3f557b8709:g:YNgAAOSw5VFWH47z
or even:
http://www.ebay.co.uk/itm/1pc-N50-Neodymium-NdFeB-Fridge-Magnet-Rare-Earth-Magnets-Block-50x50x25mm-/291512148994?hash=item43df7a6402:g:q5cAAOSwMmBVn5TO
Though I do wonder that they're described as fridge magnets  :o

Trouble is, if I start thinking about too many generator solutions I'll end up not getting anything done.

SparWeb

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #14 on: November 15, 2015, 04:44:47 PM »
They are for holding the phone book on the fridge door.
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

SparWeb

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #15 on: November 15, 2015, 06:28:38 PM »
The original motor had been wired for 220V/440V (parallel/series-Y) and had 4 poles.  I did separate the wires to experiment with Delta, but I didn't stick with that for long.  I think I had an excessive gap between magnets to stator, as you can see from the crazy skew below.  You can probably do better on the fabrication work than I did. 
I tried various arrangements of capacitors and discovered that certain values helped but only slightly.

I used:   50mm X 25mm X 12 mm = 15 cubic centimeters per magnet.   15 cc's X 8 magnets = 120 cc's of neodymium.
With this I logged data for a while and got this:
Series-star:  24V Cut-in at 150 RPM.  Max 300 Watts, which flattened out at 700 RPM
Series-Delta:  24V Cut-in at 250 RPM.  Max 350 Watts, which flattened out at 500 RPM

Later, I added the missing magnets for a total of 12, (180 cc's of neo)  and got this:
Parallel-Star:  24V Cut-in at 250 RPM.  Max 650 Watts, which flattened out at 500 RPM

I suppose I should have made more comparisons, but each change meant lowering the tower to re-wire or disassemble the generator, which was time consuming.



Bringing this experience to your estimate:  60mm X 10mm X 5 mm = 3 cubic centimeters per magnet.   3 cc's X 28 magnets = 84 cc's of neodymium.  That is less than I used to start with. 

Your motor is wired for high voltage but you are also running at higher voltage:

1500/359*240=1000 RPM

I expect you will need to do some re-wiring to give yourself the low-voltage parallel-winding options you need if you expect to normally run at 800 RPM.

I bought mags from KJ Magnetics last time.  I don't know how their prices compare now, since I haven't done a motor conversion in several years.
This one looks like a bargain:  http://www.kjmagnetics.com/proddetail.asp?prod=SB8X04-IN&cat=11
The feature on the edge may be useful for holding them down.
N42 grade are OK and lower cost.  I've never been too picky about magnet grade (on advice from Hugh Piggott) and never been disappointed.

No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

System spec: 135w BP multicrystalline panels, regulated by Xantrex C40, DIY 8ft diameter wind turbine, regulated by Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024

joestue

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #16 on: November 15, 2015, 10:50:25 PM »
the 90 volt motors i have are 4.675 outside diameter of the magnet. this is a close fit to my 5 hp motor, but it may prove nearly impossible for me to remove the ferrite magnets from the motor, and i'm not exactly interested in scraping two perfectly good dc motors.

anyhow my experience with half a dozen induction motors, is nameplate volts per hz is about 20% beyond the iron's saturation point.

meaning, it is significantly more efficient to run an induction motor at 80-86% of nameplate volts and derate the nameplate hp by 30%.

so anyhow if you start with a 70% efficient 2 hp induction motor, you should be able to end up with a 70% efficient PM dc motor that dumps out nameplate volts per hz and nameplate shaft hp watts out of the wires. however, it is significantly more efficient to derate the "generator" by about 50% when you cut the flux density by 20%. (keep in mind this is at nameplate rpm)

when you cut the rpm in half, you cut the nameplate ratings in half, but you keep the same losses. so efficiency drops like a rock.

anyhow if you load it full of neodymium magnets i suspect you could easily burn up 200 watts of iron losses
« Last Edit: November 15, 2015, 10:55:01 PM by joestue »

mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #17 on: November 16, 2015, 03:38:20 PM »
OK, well I definitely don't ant to drive it into saturation, so I'll stick with the 28 60x10x5's

Just to avoid confusion it's the 3hp I'm converting - though I did refer to the 2hp in the original title.

Don't know how long this'll take as my engineering skills are not really up to this - I should be able to turn the rotor down but beyond that we'll have to see. At least with the proposed magnets and using small extra magnets to create the skew I should be able to get a reasonably small gap; but with thin magnets glued to a curved rotor I don't know how well the flux will link across.

thanks everyone  :)

Mary B

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #18 on: November 17, 2015, 10:22:33 AM »
Instead of turning it machine flats on the rotor...

mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #19 on: November 17, 2015, 02:03:22 PM »
Instead of turning it machine flats on the rotor...

mmm.. maybe, but turning down on a lathe is easy (relatively), but how to machine flats? all to the same depth? I can sort of see how I might do it with a suitable jib on the lathe but this would be pushing the limits of my engineering skills.

hiker

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #20 on: November 17, 2015, 06:59:41 PM »
heres a 3hp i did a few years back--stock windings..six wires out...made a steel rotor from some thick pipe...allthread..then glued down 98 small double stacked neos to it....ran it on a little 2hp gas engine for a test--came close to blowing out a 28v  1000 w aircraft landing light--had to do a quick disconnect...
cogs somewhat but can still turn over by hand---a fast turn by hand with vicegrips mounted to shaft-you can light up a 50w 12v auto headlite--tried it out on my pedgen as well---works okay..voltage kinda high for that...or just a bigger pulley............should work great as a hydro gen--to dam heavy for me as a windgen..cant really see that hanging off the back of my motorhome..............................
WILD in ALASKA

hiker

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #21 on: November 17, 2015, 07:02:22 PM »
gen..
WILD in ALASKA

hiker

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #22 on: November 17, 2015, 07:04:40 PM »
hmm...didnt mean to double post..secound pic. didnt show when first viewed..
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mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #23 on: November 18, 2015, 02:11:41 PM »
Aha so you used double stacked neos - I'd wondered if anyone had tried that; Do you happen to know what rpm you took it up to?

I seem to be heading for a single layer but I am curious as to how well glued-on neo's stay put when you go to high rpm's.

Another new rotor from scratch too - not sure I have anything handy to make one from scratch.


hiker

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #24 on: November 18, 2015, 06:07:39 PM »
i used some hard as rock--epoxy stick---its like a putty in your hands--forget all the specs on the stuff--its 3000 psi strength ?  good up to 300 degrees...
they have diffrent types..should have use the steel free kind...its really super strong ..ive dropped things ive used with it on a concrete floor and not even a dent
or  break away..its kinda pricy...but well worth it...       as far as rpm--not sure--i had a big pulley on the alt and small on the engine..didnt really need to rev it up..like i said i had to disconnect the  wires from the lite to keep it from burning out...forgot most of the specs--maybe ile drag it out of the shop and play around with it----getting quite the pile of alts stacked up -over the years ive done truck alt conversions--motor conversions--dual rotor alts--pedgens.............i took a break for quite a while untill-- the  all in one  exersize bike alt idea  hit me....nows its sparked even more ideas.....................seems to never end... good luck with yours..
WILD in ALASKA

hiker

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #25 on: November 18, 2015, 06:16:05 PM »
just another thought from awhile back on using large mags on a motor conversion---seems they oversaturate the lams---and cause some powerloss -- incressed cogging..so perhaps smaller mags are the way to go on motor conversions ?   
WILD in ALASKA

12AX7

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #26 on: November 18, 2015, 06:34:44 PM »
A number of years ago I picked up several servo motors at junk yard scrap weight price.   

They all develop impressive power (three phase) at low RPM"s 

Many servo motors are scraped/ junked due to problems with their built in tack/encoder/resolvers.    Bearings/wndings and PM rotor often are okay.

Find one in the scrap yard.. short  the two/three HEAVY pins on the connector and give the shaft a turn.   
Should spin,  open circuit  and stiffen up with any two of the legs shorted.   Short the three and it will be VERY difficult to turn.

I'd pass on the servos with built in brakes (need electric power to DE-energize).