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High speed induction motor update

Hi,

If anyone remembers I was messing about trying to modify a 2 pole 240vac induction motor to go at higher speed and lower voltage driven from a dspic based 3ph pwm setup.

well I got some of that magnetic paper wich goes black in the presence of a magnetic field, I put a circle of it in the center of the stator but was very little to see, it apears the paper only goes black if the megnetic field goes perpendicularly through the paper, wich it doesnt with it aranged this way.

but anyway I had a realy extra strong cup of coffee and traced how id conected the 4 windings to the 3phase (there were 16 slots wich made it awkward) and realised id got one phase backwards.

changing that it now runs at a whoping 20krpm. it realy starts to sing, and its not realy balanced well enough as it only made for 1500rpm.

it was only an experiment, I only put a few turns of wire loosly through the slots, I also need to bore the rotor out so it can sit in the middle of a 1M long

12mm dia shaft. I think I might need to trim the cooling vanes on the rotor too. however I realy need a motor 2"x2" this one is a bit to big to fit in easily at 70x70, its from an old 8" floppy drive to drive the main spindle. ive been looking around at all sorts of junk, but nothing just the right size yet.

my usual stockists only seem to have larger motors.

Colin =^.^=

Reply to
colin
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You must be driving the motor with a much higher frequency than 50/60 Hz. Probably more like 800 Hz. I'm surprised that an induction motor was used in a floppy drive. It is more likely a synchronous motor. Even so, I wonder about the 20krpm. You might not get very high torque at such high speeds, and there may be heating problems due to magnetic losses. You may need additional shaft support bearings if it extends so far from the motor bearings. If the shaft end vibrates and goes off-center, at 20krpm it may become a lethal weapon.

What is this gadget supposed to do?

Paul

Reply to
Paul E. Schoen

I suspect there's a fairly simple basket of wire one could shape that both suspends and spins an aluminum sphere, driven by 2 or 3-phase varoable-frequency AC. In a partial vacuum, such a gadget could rip any rotor apart.

John

Reply to
John Larkin

Modern chem lab centrifuges and turbomolecular vacuum pumps use magnetic suspension bearings and ac drive. They measure rotational speeds in kilohertz.

John

Reply to
John Larkin

Ive wound it with 2 poles the same as it was before, so 20krpm =666hz. I havnt checked it properly thats just going from the drive frequency. if the motor slows down much the slip increases and it rapidly slows down so its not far off 20k.

theres only 6 turns of wire on each segment to keep the inductance down so i can drive it from 24v. it struggled to get to 20k as my current limit kicked in before it reached ful voltage so i probably need a few more turns. but this was just an experiment.

I dont need torque, well no more than necessary to overcome bearing and wind losses, and im not realy sure how much that is.

although im puting less than 50w into the motor most of that is copper/iron loss.

the bearings dont seem to get warm exept when I had to much preload.

the floppy drive was an old one, very old, sa801 8".

it does scare me a bit when it gets going, there are bearings at each end however and the motor in the middle, I also have 2 more bearings inbetween.

Colin =^.^=

Reply to
colin

they have magnetically suspended high speed rotors in that gravity experiment satelite, but theyr driven by compressed gas.

im sure someone here could derive the failure speed of an aluminium rotor of a given size.

they dont use titanium in jet engines for nothing, well they got hot as well ...

Colin =^.^=

Reply to
colin

The electronics and software will go to over 100krpm, its hard to get the moving parts, bearings etc, to run smoothly enough even at 10krpm.

just getting the shaft straight is taking ages.

Reply to
colin

colin snipped-for-privacy@ntworld.NOSPAM.com posted to sci.electronics.design:

There are inexpensive motors and controllers that will do those things readily available on the market. And they are less than 2 inches (50 mm) diameter and can deliver up to 400 watts (over 1/2 horsepower).

Reply to
JosephKK

colin snipped-for-privacy@ntworld.NOSPAM.com posted to sci.electronics.design:

Hardly, dental drills and similar have done and do 100,000 rpm for decades routinely.

Reply to
JosephKK

Air powered, air bearing PCB drills run up there, too.

John

Reply to
John Larkin

yes but they dont have a 12mm dia 1M long shaft.

It probably wouldnt be so difficult if I had an engineering workshop instead of an electronice workshop lol.

Colin =^.^=

Reply to
colin

Yikes! That will wobble itself to death at that speed. The fan shaft on the JSF has similar issues; it transmits 32,000 hp to the forward lift fan.

John

Reply to
John Larkin

well with bearings at 400mm intervals if its 10um out of tru in the middle it does wobble a lot at over a few k rpm the catastrophic problem is at the point of resonance wich is ~ 5krpm, it absorbs a lot of power too.

if I can get it to have no visible bend on my dial guage wich is probably about 1um, then it runs fairly smoothly, however to straighten the last 10um is proving difficult, I can bend it so its straight but an hour later it just goes back.

another bearing inbetween so theres 200mm between bearings is better and il probably need it, but the extra friction is a problem and even with these it still needs to be straighter than it is.

Colin =^.^=

Reply to
colin

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