Author Topic: 2hp induction motor conversion - how much neo magnet required?  (Read 10983 times)

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mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #27 on: November 20, 2015, 01:10:01 PM »
Thanks for the info: I'll try to reinforce mine with wire like Joestue suggested when I get that far.

I'll keep my eye out for a big ac servo motor - sound like an easy solution if I can find one for scrap value.

I haven't got any further with this project as work has got in the way - but I need the money.

Ungrounded Lightning Rod

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #28 on: November 20, 2015, 10:08:19 PM »
OK, well I definitely don't ant to drive it into saturation, so I'll stick with the 28 60x10x5's

NO. You DO want to drive it into saturation.  That's when you get the maximum possible flux through the coils. and thus both the maximum open circuit output voltage and under-load output current.  Unlike a transformer or motor, a PM alternator is not using the inductance of the windings and cores to resist current from an external source, so core saturation is not a disaster (unless you're paralleling two of them, spinning differently, directly at the wild AC terminals, or otherwise feeding power back into it, which would be silly).

That's part of why you use neos instead of ferrites in a conversion, if possible.  Neos are strong enough that you can drive the core to saturation without quite fully paving it, while you can pave it with ferrites and it won't be saturated, and thus won't produce as much as you could have gotten from a conversion of the same motor with neos.

Meanwhile, if for some reason decide to hang a transformer between it and the load (and accept the losses in return for shifting the voltage while avoiding a rewind), you can size the transformer slightly larger.  The saturation of the core of the alternator guarantees there's never quite enough power available, under any operating condition (except backfeed), to saturate the transformer.

mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #29 on: November 21, 2015, 11:19:19 AM »
Uhh, well I'm confused now. If I saturate  the iron ,won't I be wasting energy heating the iron?

joestue

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #30 on: November 21, 2015, 01:41:41 PM »
Uhh, well I'm confused now. If I saturate  the iron ,won't I be wasting energy heating the iron?

yes, a lot of wasted energy.

my 2 hp induction (diy synchronous) motor sucks up 200 watts when excited to nameplate volts per hz at nameplate rpm.

you're running at half nameplate rpm so you'd be looking at about half of 200-300 watts of iron loss if you run it at nameplate volts per hz.

joestue

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #31 on: November 21, 2015, 06:02:11 PM »
my 2 hp induction (diy synchronous) motor sucks up 200 watts when excited to nameplate volts per hz at nameplate rpm.

Just to clarify, this is how many watts it sucks out of the outlet, when running as a synchronous motor at unity power factor.

As an example, i just recently acquired a pair of 6 pole, 1/3rd hp motors. i managed to reduce their no load consumption down to 170 watts from like, 190 or something by changing the ~100uf start cap for a, i can't recall exactly, 7.5uf run cap or something. but once the line voltage is dropped 20 percent, that 170 watts is reduced to 114.

200 watts is 200$ a year of wasted energy at 11.4 cents per kilowatt hour.  that's a lot of incentive to build something better.

SparWeb

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #32 on: November 21, 2015, 06:36:58 PM »
In simplest terms, saturation is good, mostly, in this case.  This generator is not a transformer, though many principles are similar when you dive deep in the details.

What saturation does is "push back" against the induced flux that comes back from the stator when current flows.  Picture two arrows.  One arrow points out of the magnets, and the size of the arrow from the magnets is determined by the flux they produce across the gap to the stator.  The other arrow points the other way, coming from the stator, pointing back toward the magnets.  The size of this arrow depends on the current that flows in the wires.  When there's no current, this arrow is zero.  When the current in the stator is high, then this arrow is large.

If the magnets on the rotor do not saturate the stator laminations, then the arrow coming from the stator simply subtracts from the magnetic flux arrow, decreasing the net flux across the gap immediately.  If, on the other hand, the magnets do saturate the stator, then the flux generated by current in the stator has to overcome the amount of saturation already present before it can begin to reduce the gap flux.

Grossly oversimplified, but hopefully clears up the confusion.
If you do want to learn more of the details, you want to look at "electromotive force" "reactive power" and "back EMF" just to start.
There are a bunch of negative consequences to doing this, but IMHO they aren't as bad as having a generator with no "muscle" at the top of the power curve. 
The difference is EXACTLY what I was experimenting with, using 8 mags versus 12 magnets on the genny I showed you earlier.
No one believes the theory except the one who developed it.  Everyone believes the experiment except the one who ran it.

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hiker

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #33 on: November 21, 2015, 10:17:02 PM »
Few years back...their was talk of over saturation from using to big of a neo mag. On motor conversions...so how do you know how much mag is enough or to much ? From what I recall it kind of shorts out the magnetic field a it travels over the lams..maybe flux can jump in ? 
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mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #34 on: November 22, 2015, 12:52:18 PM »
...so how do you know how much mag is enough or to much ? 

That's exactly what I want to know too;  ;D maybe Zubbly & others have done calculations to determine how much mag to add or they just had a feel for it, but I don't truly understand how Zubbly and others have chosen the mag's they used.

Is it a case of calculating the Flux in the motor at full load amps and selecting mags (and gap) to match this? (my head's hurting already).

presumably if you do massively over mag the motor you will wast energy heating iron? But I need enough to get 800w+ from a 2200w motor, ideally at 700-900rpm - I guess I should look at Sparwebs numbers again, and I could do a crude extrapolation but I don't know if I'd get it anywhere near right.

Hope my old genny's bearings will hold on a bit longer.  :)

SparWeb

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #35 on: November 24, 2015, 02:17:50 PM »
It's OK.  Took me a long time to sort it out, too.
My humble opinion is:
Lots is good.  More is better.  Too much is just enough.  :]

What would be more helpful, though, is an idea how to maximize the money spent on magnets when they're so expensive.  Get the most bang for the buck.

I've never used magnets thicker than 1/2" for example.  Using thicker magnets would increase the flux, but my guess is only slightly, and the practical problem of fitting a rectangle on the outside of a cylinder makes thicker and thicker magnets a losing battle.

The other dimensions left, the length and circumference of the rotor, are pretty good for you, since the rotors on older motors was much bigger than they are now.  I just did a sketch of your 5.5 diameter rotor.  I can only fit 27 1/2" x 1/2" magnets around the circumference, not 28 like was said before.  Spacing out 24 magnets equally around the circumference fits better.  Too bad because 28 would cog much less than 24.

The rotor looks long enough to fit 3" of magnets, so I'd look at fitting 3 rings of them around, staggered a little to reduce cogging.
Back to KJ Magnetics...   1" long x 1/2" wide x 1/2" thick...  Uhhhkk   sorry   500 bucks.

So this is the point where we get motivated to make it more affordable!

What would happen if you used cheaper 1/4" thick magnets instead?  The funny ones I found earlier are much cheaper, and because they're thinner you can go back to fitting 28 around the circumference.  90 mags x 3.77$ is about 340 dollars.  Their website knows I'm in Canada, so it's probably gouging me - check for yourself if you're in the US and you might have a much better price.   (It's called the hoser discount, for a reason.)

There's half the magnet thickness, but thankfully there isn't 1/2 the flux.  It's lower than it would be with 1/2" thick magnets, but we've also gone up to 28 magnets around the ring.  We also haven't decreased the gap space.  Back when I was learning about conversions of my own, I found that these factors compensated quite a bit for less magnet.

http://www.sparweb.ca/3_Gen_MoCo/Baldy.html

If this is acceptable, then the total volume of magnet is...
28 x 3 x 1 x 0.5 x 0.25 = 10.5 cubic inches.
That's pretty good for 2HP

My GE motor conversion (3HP) had a magnet volume of 12 cubic inches.
On my Baldor motor (3HP), I managed to cram on 15 cubic inches.

I was very happy with the performance of the GE, and I'm still happy with the Baldor conversion.  I'm sure you'll be satisfied if you can get 10 cubic inches of magnet on your Brooks motor.
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 #36 on: November 24, 2015, 03:27:18 PM »
 N45 Neodymium Magnets 3 inch x 1/2 inch x 1/4 inch Thick NdFeB Bar Rare Earth Magnets
Magnetized through the 1/4" Thickness

4.20 each if you buy 30 of them. at magnet4less dot com
Rare Earth Magnets 4" x 0.5"x 0.25" Thick Neodymium NdFeB Block Magnet N42
Magnetized through the 0.25" Thickness

4.15 each...
the 1 inch wide magnets are cheaper by volume but the airgap volume will be increased as well.
« Last Edit: November 24, 2015, 03:33:07 PM by joestue »

mab

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Re: 2hp induction motor conversion - how much neo magnet required?
« Reply #37 on: November 29, 2015, 12:41:07 PM »
Aha! fieldlines is back  :)

OK, I went away & thought about what you've said - can't say I fully understand yet, but I'll be guided by your wisdom. So I'll have a rethink on which magnets to use. Just to be clear the rotor dimensions I gave are for the 3Hp, not the 2hp referred to in the title, so I guess I should go for 12c.i. or more if I can manage it.

I'm in the UK so I doubt those US suppliers will work out cheaper after adding shipping even if they ship over here.

although if it works out cheaper to buy 2x1x1/2 " mags for an axial flux I wonder if that'll give me a more efficient generator anyway (or at least easier for limited machining skills - maybe).

Thanks again :)
m