spherical ball roation under roatating magnetic field 1

[stevenz]

I am considering to let a permanent magnet or just a stainless steel spherical ball to rotate just like a rotor.This ball will be put in the center of two pairs of horse shoe electromagnet coils which are feeded by alternative current to generate a roational magnetic field.I hope this ball will follow the roational magnetic field.Is this idea possible?

 

[jraef]

No.
2 horseshoe shaped coils is not going to make it rotate. It will be attracted to them and stick. A motor works by creating another magnetic field in the rotating part which repulses against the field in the stattionary coils (over-simplified).
If your object were a permanent magnet it might make a difference, but I am not sure about the effects on a sperical magnet. Hmmmm.....

And by the way, stainless steel is non-magnetic.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Barry1961]

While austenitic stainless steel in not magnetic when annealed (normal state), martensitic and ferritic stainless steels are magnetic. While you can make austenitic stainless magnetic by cold working I don't think it would be worth the effort.

If the rotor must be stainless for some reason you need to pick a stainless steel with a number in the 400 series. Many of the 400 series steels can be hardened and are commonly used as knife blades, 420 and 440A/C being common. 440A/C is also common in ball bearings and balls can be purchased individually from mcmaster-carr. You can also get hollow balls....

Austenitic stainless steels are the most common in construction and are in the 300 series, such as 304 and 316 to name two. Series 300 steels usually have a wider range of chemical resistance and are non-hardenable.

As far as getting enough current flow in the sphere to produce a usefull magnetic field....? I will leave that to greater minds, of which they are many here.

Were you planning to laminate the sphere?

Barry1961

 

[tinfoil]

jreaf:

I agree with most of what you said, but I have one nitpick:

SOME stainless steel does respond to magnets, depending on the alloy. For example, some solenoids use SS in the plungers that react to magnetic coils.

 

[electricpete]

I certainly don't know the answer.

Just for clarity, the term "magnetic" has been used. But the important parameter is permeability, right?

You mention "Two horseshoe shaped electro magnets fed by alternative current."

Is that simple ac current/voltage? From the same source for both magnets? In that case you have the equivalent of a single-phase motor. You can generate a rotating magnetic field, but you need to get the rotor started moving by external means.

I know there are many different devices which use the principle of rotating magnetic field to spin that do not require rotor to have bars like squirrel cage motor. For instance "hysteresis syncronous" motor and some other oddball small motors that have a rotor which is a solid or hollow conductor without bars.

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[Skogsgurra]

Guys,

I think that you can forget the stainless steel/magnetic/non magnetic discussion as long as you haven't solved the real problem - and there are two. Jraef already discussed them, but I think that the points need to be said a little bit stronger.

First: It is impossible to make anything stay stationary in a magnetic field as long as it is not superconducting or part of a closed loop system where the position is fed back to a power stage that controls the field strength. If you do not do that, your object (ball or whatever) will smack against one of the poles (the closest one) and stay there.

Second: You will not have much rotation with a single phase field. You need at least two phases to have some rotation (induction motors have three). There is a slight possibility that a prelininary rotation of the ball can split the single phase magnetic field into one CW and one CCW and that the rotation makes one of them weaker so that you get some net torque. But that is one of the minor problems. Making your ball hover is the big one.

The hovering ball can sometimes be seen in science centers and museums. It is always guided by a photo cell or some other sensing device that controls the current in the magnet coil. Magnetic bearings also have active feed-back (except for the magnetal bearing which has a passive feed-back).

 

[buzzp]

I actually made a similar device in college. It used DC current feeding a coil wrapped around cold rolled steel. The current was varied depending on the position of the ball below the tip of the steel electromagnet. The position indication used an array of photo cells. A PID control loop was used. After playing with the constants of the PID loop we were able to get the ball to levitate in mid air. It was a really neat project. A plus of the whole thing is the steel ball rotated slowly and I suspect that they would all rotate slowly. We called this project a "magnetic ball levitator". Our device was confiscated by the school to use at science fairs. I have always wanted to make another one but just have not had time.
Unless I am picturing your scenario incorrectly, I think you will need more than is mentioned. Something will have to hold the ball in the vertical direction. As far as it working, I would have to see a pic or something to see what your proposing. However, I think more 'poles' would be needed.

 

[electricpete]

ok jraef and skogsgurra - I see the point you made which I missed.

The ball either needs to be restrained from radial movement or perfectly centered. I agree.

Just wondering though....

An interesting thing I have heard if you put steel balls in the stator of an energized induction motor stator with rotor removed (energized at reduced voltage to prevent overheating of course), the balls will travel around the stator. (The balls are small in comparison to the stator bore.) It is used by repair shops after rewind to check proper phase sequence/direction of rotation, is it not?

It has been discussed before here on the forum but I don't remember all the details. Not sure if it would work in this simpler simpler stator with different geometry etc but I'd be interested to hear discussion of that.

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[SteveKW]

An interesting thing I have heard if you put steel balls in the stator of an energized induction motor stator with rotor removed (energized at reduced voltage to prevent overheating of course), the balls will travel around the stator. (The balls are small in comparison to the stator bore.) It is used by repair shops after rewind to check proper phase sequence/direction of rotation, is it not?


Yes it is. I have done it on more than I can count!

 

[busbar]

Doesn't the Rosenburg repair text mention doing the same thing with a can of tomatos?

 

[Skogsgurra]

Never seen that, pete. But it sounds possible.

I had a similar thing going on in my pocket once. I had been asked by the large company to find out what the problem with one of their induction stirrers (used to stir smelted metals) was. They were running at 0.62 Hz and the current was supposed to be 540 A in the coils. But the cooling water indicated a much lower current. Funny way of measuring current, I think.

Anyway. When I got close to the beast, something moved in my pocket. It was my little pocket knife that rotated in the strong magnetic field.

I did not understand why my card did not work when I tried to fill up my car later. I did not understand why I couldn't get money out of the automatic teller. It was a lousy day, I thought. I entered the bank and asked them to fix the machine. It was only when the girl tried to read my card and looked pussled; "What have you done? Been close to a magnet?" that I understood what had happend.

 

[aolalde]

Our winders do a removed rotor reduced voltage test to every stator they re-wind. They apply 3- phase, 460 Volts, 60 Hz to 4160 Volts stator windings and a small ball bearing attached to a holding bar is introduced into the stator bore. The current is very close to full load and the bearing outer race must spin evenly around the bore.

 

[panelman]

do it with a tin of beans and you can have a hot lunch as well

 

[electricpete]

Thanks for all the info from Stevekw, aolalde, panelman. If I'm understanding Steve does it different than aolalde.. Steve puts just a ball in the bore and watches it bounce around and aolalde puts a bearing in there supported by inner race, and busbar and panelman use a steel can.

One question for my own benefit - in the case of the can and the ball, does it go around the bore in direction of rotation or direction opposite of rotation? (in case of bearing supported by inner race I'm sure it spins in direction of rotation but the other cases are more complex since there are two types of spinning: ball/can on it's axis and ball around the bore and they tend to make ball/can go in opposite directions).

Back to the original question, with benefit of comments from all the others, I think probably jraef and skogsgurra are right that the ball would just go radially to one of the magnets and stick there. The induction motor stator is different because it has the benefit of distributed winding which creates more of a smoothly varying mmf wave along the bore. What do you guys think?

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[SteveKW]

The ball runs around the stator sounding like a car following the rails and hitting every railroad tie. I never paid attention, but I would assume it would go in the direction of rotation. Just a lot slower because of the slip.
There are other methods I also have used... A large Start capacitor on a string and/or a very small rotor with a bearing supporting it on the end of a stick! Both of those do give correct rotations.

 

[YorkMech1]

SteveKW (Electrical) Jun 27, 2004
The ball runs around the stator sounding like a car following the rails and hitting every railroad tie. I never paid attention, but I would assume it would go in the direction of rotation. Just a lot slower because of the slip.

Nope...Sorry Steve...
The Ball Bearing "Will" go "Opposite" the direction of the motor shaft....
I've had handfuls at a time in stators just to see where they would end up after shutting off the power...
A Ball Bearing in a Stator "Always" whizzes around inside "Opposite" the true direction of the motor shaft...

 

[SteveKW]

Ya.. I knew better than to commit to a definite YES on this board about the ball! And now that you have said it, do recall being told that it does run in the opposite direction! I will have to do a test again when I get a chance

 

[YorkMech1]

Now I gotta' crayfish on this one...I just got through playing with the Ball Bearings again...
While it is true that the ball is whizzing around inside the stator "Opposite" the direction of the true rotor, The Ball "Itself" is rotating in the same direction that the rotor will.
Since the ball is now sorta' "Geared" to the Stator iron, It gives the visual impression of rotating reverse...
However....The rotation of the "Ball" itself...Is "Normal"...
It's all in the perspective I guess..I apologize for any confusion..

 

[jraef]

This sounds so fun that I am going to disassemble my test motor to try it out. I get a lot out of this forum, but entertainment was beyond my wildest dreams. I am doing a "show and tell" session for a local high school technology expo and I am going to try to rig a plexiglass cover on each end of the stator so they can safely watch the balls spinning. What a great way to demonstrate rotating magnetic fields to lay persons!

The tin can issue ammuses me to no end. If it works, the next time I host an open house I'm going to use that to serve hot snacks!



"Venditori de oleum-vipera non vigere excordis populi"

 

[skiier]

I remember seeing this back in college as well. We used a large tinfoil ball inside a large stator. I remember it well because the ball spun around pretty evenly and seemed stuck to the stator itself. Only problem was that I had forgotten my lessons on Eddy currents - the ball burned my hand when I picked it out of the stator!