Author Topic: Question : How do I convert an Induction Motor into a Generator? (Read 8318 times)

KeithOlivier · « **on:** October 03, 2010, 09:19:41 PM »

ghurd · « **Reply #1 on:** October 04, 2010, 08:35:09 AM »

Many reasons.

There is a point where closer does not help much more.
Cutting neo magnets accurately is difficult, and a Dremmel tool diamond disk does not last long at all ($20 worth of HF Dremmel-type diamond disks to half 2 original #29's lengthwise).
The dust is toxic, flammable, and explosive.
If the magnet gets too hot, it looses strength.
Breaking the factory coating leaves it exposed, and galvanic corrosion happens fairly quickly.
Thicker neos, accurately placed, should get about the same results.

Not worth the amount of cost and effort?

Can get curved magnets, or have them made.
I have at least 4 sizes of curved neos here, and did not resort to having them custom made.
G-

Flux · « **Reply #2 on:** October 04, 2010, 09:33:08 AM »

I think Ghurd has pretty much summed it up.

Induction motors are special in that they must have a very small air gap to work correctly, in fact the smaller the gap the more they behave like the ideal but you never get there with any air gap that is practical. I don't think it is necessary to go into all the details of this.

Synchronous machines are very different and in many ways the larger the air gap the more stable their properties as long as you can maintain the flux density.

Permanent magnets really behave as air as far as this is concerned, the things are saturated and you are really looking at the relative gap of the true air gap compared with the total gap consisting of the mechanical air gap and the magnet thickness. If you have 12mm of magnet then a 1 mm clearance at the centre of the magnet compared with the gap at the edge is of no real consequence as far as flux is concerned. It does affect the way that you need to do things to avoid cogging but grinding the magnets to correct profile will not solve any cog issues, it marginally affects what you need to do.

Obviously very badly fitting magnets will need to be thicker for the same results. For a 2 pole machine you would hardly try to use two rectangular blocks, but when you get up to six poles or more things are not bad enough to want to do much better. Obviously curved magnets are ideal and manufacturers will go that way but it is never worth the effort for a single conversion to obtain special magnets and it is more trouble than it's worth to start grinding things.

For 4 pole machines you will probably need more than one rectangular block per pole but decent low speed alternators need lots of poles and large diameter. The rectangular blocks are not far from the ideal when you get to 8 poles or more.

If you do a motor conversion with a standard winding you will have to stick to the low pole number and suffer many of the disadvantages. Two pole is not likely to be a good starting point and 4 pole will not be ideal even with correct curved magnets but that is the one case where you would gain most with ideal curved magnets.

There are large differences between the operation of synchronous and asynchronous( induction ) machines.

Flux

SparWeb · « **Reply #3 on:** October 04, 2010, 04:20:25 PM »

I have tried to drill and cut NEO magnets and my experience is the same: watch out!

My last project involved using small blocks which kept down the need for "skew" and the gap small. Designing a motor-conversion to work with stock magnets is what works for me. I would also be worried about corrosion in a cut NEO magnet much more than one with just a few scratches on the plating. The cut surfaces can be very rough, meaning lots of microscopic cracks and edges that water can creep into and rust beneath the paint. The stuff is so hard it really wears out the tools, too.

KeithOlivier · « **Reply #4 on:** October 06, 2010, 06:34:13 PM »

Folks, considering that cylindrical grinding machines produce some of the finest finishes and best tolerances available short of lapping processes, I'm personally somewhat baffled by some of the comments. I'm not talking about hacking something with an angle grinder, I'm talking about a "real" grinding machine. I have included a picture to illustrate. I would say that the rotor is the part where one has the greater degree of control of the exact diameter, surface etc. The stator is just made from stacked laminations, after all...

ghurd · « **Reply #5 on:** October 06, 2010, 06:52:00 PM »

Once it is close enough, closer doesn't really help much.

"The stator is just made from stacked laminations, after all..."
Good point.
So why go through all the hassle to make a super accurate rotor?
And if you think those laminations are cut with much accuracy, you are mistaken.

I made more than a few rotors that should fit fine in the stator and can, according to the calipers.
Just to find out the can and bells are not really centered,
or the round hole in the laminations is not really round,
or the hole in the laminations is not parallel with the outside of the laminations.
G-

joestue · « **Reply #6 on:** October 06, 2010, 07:35:02 PM »

i'm not sure the increase in power or decrease in losses justify losing 20% of the magnets to grinding them down, when the fact that you have a grinder that could do so implies that you could pull this off with the rest of the equipment in said machine shop:http://www.cedrat.com/fileadmin/user_upload/cedrat_groupe/Publications/Publications/2007/10/Machine_with_rotor_structure_supported_by_magnet_ISEF2007.pdf

SparWeb · « **Reply #7 on:** October 07, 2010, 02:24:48 PM »

Quote from: KeithOlivier on October 06, 2010, 06:34:13 PM

... cylindrical grinding machines ...

What background do you have, which motivates you to bring this up? Whereas the Do-It-Yourself project scope usually only implies hand tools and simple machines to get the job done, you invoke a specialty machine that is not easily found, nor cheap. Sure, lots of us can sneak in an hour at a lathe, but what you're talking about is not nearly as common. How were we supposed to guess that you keep one in your basement just waiting for this task?

Feel free to apply any technology or process into the mix if you think it will help. Posting a general question about the subject will get replies based upon the assumption that you are using a hacksaw and an angle grinder, unless you mention that you do have a special machine like this. Should the "standard package" answers assume that you also have a sintering oven in your garage, too?

If that grinder in the picture does not belong to you, you had better buy the real owner a set of replacement bearings and bedways because the magnetic neo dust gets into the nooks and crannies and gums up the works.

KeithOlivier · « **Reply #8 on:** October 07, 2010, 03:37:02 PM »

I have accumulated over the past few years, certain equipment to secure some "independence", since like many others I have the intention of moving to a more remote location in the future.

The first was a Kent Vertical mill ($1200)
Then a 16x8 surface grinder ($950) which came out of a "government strategic reserve - I'm picturing a bunker in the side of a mountain)
Next was a Sheldon 15x36 lathe which I am rebuilding ($950)
I do in fact have my sights on a cylindrical grinder and there have been several for sale recently for $1200
Miller Dialarc welder (good for 200 amps at 100% duty cycle) $400

In addition, I recently acquired a 3 cylinder water cooled Deutz genset for conversion to CHP. I would like to exchange the single phase head for a equivalent 3 phase 6-8kW brushless head so that I have a ready source of 3 phase power for my machines. So far I have been using a 3hp Hitachi single phase in to 3 phase out variable speed drive.

I also purchased a Bobcat B200 Loader Backhoe a couple of years ago as a "labor argumentation" device and I am not sorry I did.

I have my sights set on some land in Custer County Colorado, where development has been kept in check for the last 30 years and residents have fought the "empire building" attempts of the local government with the consequence that property taxes are still modest. The county is home to less than 4k people and "year rounders" are less that 2000.

I still need a screw type compressor to support sandblasting and restoration, as well as a 4 post vehicle hoist for the never ending wrenching that one does on older vehicles....

Flux · « **Reply #9 on:** October 07, 2010, 04:09:48 PM »

You are quite right that a cylindrical grinder is perfectly capable of making your magnets the correct shape but once again I agree with joestue that in this case the precision is not necessary, and you are likely to loose by wasting magnet rather than gain by reducing mechanical gap.

If you were building an induction machine where the power factor was critically dependent on maintaining a close air gap then it would make sense. If you are building a synchronous machine with neo magnets then the effective air gap can never be less than the magnet thickness. As I said before if you have magnets 1/2" thick then small discrepancies in the physical air gap are swamped. If your rectangular magnet shape resulted in gaps comparable with the magnet thickness then it would be worth doing something about.

After going to great lengths to grind the magnets you are then left with a problem of protecting them after destroying the vital nickel or epoxy coating.

Flux

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Author Topic: Question : How do I convert an Induction Motor into a Generator? (Read 8318 times)

KeithOlivier

Question : How do I convert an Induction Motor into a Generator?

ghurd

Re: Question : How do I convert an Induction Motor into a Generator?

Flux

Re: Question : How do I convert an Induction Motor into a Generator?

SparWeb

Re: Question : How do I convert an Induction Motor into a Generator?

KeithOlivier

Re: Question : How do I convert an Induction Motor into a Generator?

ghurd

Re: Question : How do I convert an Induction Motor into a Generator?

joestue

Re: Question : How do I convert an Induction Motor into a Generator?

SparWeb

Re: Question : How do I convert an Induction Motor into a Generator?

KeithOlivier

Re: Question : How do I convert an Induction Motor into a Generator?

Flux

Re: Question : How do I convert an Induction Motor into a Generator?