Tesla's Egg of Columbus

Discussion in 'The Projects Forum' started by Munk, Jan 2, 2011.

  1. Munk

    Thread Starter Member

    Jan 2, 2011
    36
    0
    Here is a video to reference.

    Simply put, I want to make one of these. I don't really have any experience with inductive motors, but I have been doing some research. What is happening in the video is that the metal egg is being spun by a rotating magnetic field created by, what I assume, is a 3 phase stator.

    I would like to make something similar that could run off of a standard 120v AC outlet. From my research of tesla's egg, I should be aiming for 35 - 40hz. For the sake of simplicity and cost, I was thinking of using a single phase stator (or just connecting a 3 phase stator to a single phase power source) - granted I would physically have to start the object spinning before it would keep itself in motion...but that's ok if it avoids significant complications or expense.

    The problem, as always, is that the devil is in the details. I really don't know what type of single/3 phase stator to get (size, energy ratings, etc) or how I can safely connect that to 120v AC outlet. Any information regarding what type of stator I would most likely need, where I can purchase/re-purpose one, or any information on how to properly do the wiring would be much appreciated.

    Help this newb ;)

    * Video 2 (with other objects)
    * Video 3 (with electronic controller)
     
    Last edited: Jan 2, 2011
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    One major problem is the lack of 3 phase electricity in residential areas.

    However, the effect is due to a rotating magnetic field. You can obtain a strong bar magnet and use a DC electric motor to spin it. Have an aluminum or glass plate over the spinning magnet to demonstrate the effect with the iron egg, washers, etc.
     
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,761
    924
    The safest way to connect to the AC mains would be with BOTH an isolation transformer and a variac for control of the motor.


    Are you wanting to make your own stators and magnetic circuit? Tough to do

    can you aim a little smaller and use a three phase motor mounted beneath a tupperware bowl. Disguise the motor with a box over it like the device in the video. Put the variac dial on the outside of the box. You can make a capacitor dividing section for driving a small 3 phase motor from your 110 AC.

    See if you can get something with bad bearings at a repair place. Tell them you need the just the stator body. They might let you have a fairly big one for free. Your gonna junk the inner pieces anyway.


    Does any of that match up with what you are wanting to do so far?
     
  4. Munk

    Thread Starter Member

    Jan 2, 2011
    36
    0
    @beenthere
    Indeed, and all of the phase converters I have found online are either too large for my application, too expensive, or both. One important aspect of this project is that the object I intend to spin is not a magnet itself. It is, however, stainless steel (video 2 shows several different non-magnetic objects being spun...but that is with 3 phase).

    I read here that "A three phase motor may be run from a single phase power source -- however, it will not self-start. It may be hand started in either direction, coming up to speed in a few seconds."
    http://www.allaboutcircuits.com/vol_2/chpt_13/9.html

    Is it possible for a stainless steel object to be able to maintain spinning with a single phase power source (connected to either a single or 3 phase stator), or does it have to be a magnet?
     
  5. Munk

    Thread Starter Member

    Jan 2, 2011
    36
    0
    @Kermit2
    I did not want to make my own stator, but rather purchase/re-purpose one that would work for this application. I will need to do a little research on the isolation transformer and variac controller to see if that matches up with my intentions.

    I was planning on hiding the stator and whatnot beneath a box. Could you explain a bit more on what you mean by this (I'm not familiar with what you are referring to)
    As far as finding something with bad bearings, what types of appliances/machines (generally speaking) should I be looking into? The object I intend to spin is no bigger than an inch in any direction and is solid stainless steel. I don't have the weight of the object, but can find out if its important.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Consider a junked automotive alternator. The stators are generally wound as three phase in a wye configuration.
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Just for the record; home shop machinists have been using (for years), what that industry terms ' Static and Rotary 3 Phase Converters ' to run 3 phase machines on 240V-SPH in their home shops. The motors do not require any coaxing to get them going. To the best of my recollection the cheapest, and also least efficient is the 'Static' versions. These employ nothing more than >240V, AC rated, caps to 'semi fool' the 3 phase motor by creating phase shifts for the three legs. These converters have lost a lot of luster over the past decade because you can now buy one of these brand new little honeys. VFDs. They can do magic tricks for a lathe or a mill.
     
  8. Munk

    Thread Starter Member

    Jan 2, 2011
    36
    0
    @CDRIVE, Sgt.Wookie, and Kermit2

    Those VFD's will convert 1P/120v to 3P/230v, which seems like exactly what I need. Would the isolation transformer and variac controller still be necessary with a VDF, or are those capabilities built into the VFD already?

    Also, are automotive alternator stators rated for up to 230 volts?

    If the VFD doesn't have a voltage controller built-in, and the alternator stator cannot handle the 230v, I assume I could use a variac controller to bring the volts down to a safe range for the alternator stator. Would something like this work?
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    VFDs, as their name implies, controls speed of three phase motors by varying the frequency delivered to the motor, not the voltage. Lowering the voltage to induction motors is very undesirable and can damage them. My guess would be that VFDs would not like being fed from a variac and it would probably damage them too.

    Unless things have changed, Automotive alternators can produce about 120RMS @ ~ 3 to 6Amps, maybe more. I base this on a time when auto supply's sold a switch kit that disconnected the alternator from the car's electrical system and fed the output to a 120V outlet. People were running things like 120VAC electric power tools that employed universal motors. This type of motor can withstand large changes in voltage and can operate on DC too. BTW, this contraption was never a good idea!
     
    Last edited: Jan 3, 2011
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I meant for you to use lower voltages with the alternator field windings. They're optimized to interact with the rotor, not as for your intended use. I honestly don't know what kind of a field you'd wind up with, it was just a rough idea.
     
  11. Munk

    Thread Starter Member

    Jan 2, 2011
    36
    0
    @Sgt.Wookie
    Hopefully a rotating magnetic field!

    @CDRIVE
    When you said "this contraption was never a good idea", what concerns are coming to mind?
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I wasn't referring to your project. I was referring to alternator 120V power supply kit. People fried there auto electrics, voided warranties and plugged devices into them that were not compatible.
     
  13. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    You could use a BLDC (BrushLessDC) motor driver for model cars/aircraft to start with. You could even use the internal coils of a BLDC motor if you can find a cheap one to scavenge from. This would be far cheaper and safer than working with line level voltages.

    Total cost would be around $100 or less if you hunt a bit for used. The comment on the video stated a BLDC Motor driver is what the YouTube demonstration used.
     
  14. Munk

    Thread Starter Member

    Jan 2, 2011
    36
    0
    Sorry for the delay, recently moved to a new place. Ok, I like the idea of a using BLDC motor, but I am concerned that, without the right one, there will not be enough power to spin the object. The object is solid stainless steel, about 30 grams, but is not a magnet. What sort of BLDC motor should I be looking for? A wheelchair motor perhaps? How about a ceiling fan motor?
     
  15. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Just the driver, and the coils underneath the surface. The steel is magnetized by induction and reacts. You need variable speed 3 phase, as it cannot start instantly.

    The BLDC driver creates 3 phase current (between 7 and 12V for hobby size) to run through coils (which can be scavenged from motors, as mentioned). Each coil turns on in turn, "pulling" the egg around. If the frequency is too high, the "egg" will just heat up instead of moving.

    That's why the videos start out slowly, and sometimes with a 'nudge' from the operator.
     
  16. Munk

    Thread Starter Member

    Jan 2, 2011
    36
    0
    Do the drivers normally allow for frequency adjustments, or is that accomplished with another device?
     
  17. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    The drivers are speed controlled, which changes the frequency.

    The speed control is the "standard" R/C servo PWM/PPM with 1.5mS center time at 50Hz.
     
  18. Munk

    Thread Starter Member

    Jan 2, 2011
    36
    0
    From what I understand, 35-40 hz is the ideal frequency for this sort of thing...so I think 50 hz would probably work alright. With that being said, its not essential that the object self-starts when spinning (it wouldn't be a problem if I had to do the initial spin, so long as object could stay in the spin afterward). Would a driver still be necessary, or would the stator / coils be enough to do this with a 1 phase current?
     
  19. Munk

    Thread Starter Member

    Jan 2, 2011
    36
    0
  20. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
Loading...