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IamSmooth

Mon Jan 11 2010, 01:42AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I saw a video showing a small piece of metal being levitated while being melted. All I could see that was different was there was two turns for the work coil, and several spiral turns in the opposite direction below.

Is this all that is necessary. Has anyone here attempted induction levitation?

radiotech

Mon Jan 11 2010, 02:05AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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this thread" Strange idea:Can a transformer power a coilgun/railgun effectively?" in electromagnetic projectile forum on 30 Dec. there is
details on levitating copper ring at 60 Hz.

IamSmooth

Sat Jan 16 2010, 11:55PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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I just wanted to update people that I have successfully levitated and heated various metals in my induction heater. I had the stuff levitating in a very stable magnetic field. I blew a few mosfets, diodes and resistors in the learning process. I have it down now, and need to move to a larger service line in order to get to the really hot temperatures. I will then take some video and pictures.

this is the general tutorial

Dinges

Sun Jan 17 2010, 06:22AM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
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Nice!

I've seen videos of levitation using an induction heater too. But I wonder.... is it done for practical reasons (if so, which?) or because of the fun factor of levitating things in mid-air?

How come levitation melting caused FETs etc. of the induction heater to fail?

I've experimented a bit with levitation too, though using much more low-tech means.... 4 NdFeB magnets and a thin slice from a carbon brush

http://sci-toys.com/scitoys/scitoys/magnets/pyrolytic_graphite.html
(no need for pyrolytic graphite, I used an old carbon brush)

Peter.

Steve Conner

Sun Jan 17 2010, 12:08PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I got stuff to levitate briefly with my old induction heater setup. The secret is to have a few turns at the top of the work coil wound in the opposite direction to the main coil, as the Ameritherm video shows. This "bucking coil" partly cancels the field from the main work coil, creating a stable region where the workpiece can float.

I built a coil as close as I could to the one shown in the video, out of 3/16" copper tubing. I had to disable the current limiter and crank up the work coil current well above what I considered to be a safe limit, and even then it would only work with a small, hollow aluminium workpiece (a chainring bolt from a bike) After about 10 seconds, the bolt melted, fell out of the coil, and my inverter exploded.

Out of interest Iamsmooth, why did you choose the series feed configuration for your heater? Most people seem to use the LCLR, following Richie Burnett's example, but I've always preferred the series feed. Ameritherm's smaller units use it as well.

IamSmooth

Sun Jan 17 2010, 06:08PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

Steve McConner wrote ...

I built a coil as close as I could to the one shown in the video, out of 3/16" copper tubing. I had to disable the current limiter and crank up the work coil current well above what I considered to be a safe limit, and even then it would only work with a small, hollow aluminium workpiece (a chainring bolt from a bike) After about 10 seconds, the bolt melted, fell out of the coil, and my inverter exploded.

Out of interest Iamsmooth, why did you choose the series feed configuration for your heater? Most people seem to use the LCLR, following Richie Burnett's example, but I've always preferred the series feed. Ameritherm's smaller units use it as well.

So far, with 1.5kw I have gotten solid aluminum to melt. It, too, dripped out the center, but my mosfets remained fine. I have gotten a 3/8" section of copper to float and heat. I have almost gotten a solid chunk of copper to levitate, but I need to take this to higher power, which I don't have in my basement.

I just want to retract something. I realize in my "melting aluminum" video this is zinc. When I went to get another piece at the hardware store I read the package. Those white vapors were zinc oxide. Nice. When aluminum melted it just went "silver" and did not glow.

My mosfets exploded during some earlier runs because the resonance lock was not achieved. I am guessing the bucking coil affects the inductance in such a way to make the lock range very arrow. As the piece dances in the air the resonance point moves. I cranked up the power in my garage without knowing I was off because I wasn't watching with my scope. Bang. When I used my regular heating coil the lock is very stable. I have to look into this.

Why did I choose series? It seemed simpler. Only on C and L component; the tank is completely isolated from the inverter. I don't know if this is better, but it certainly was easy to setup.

Mates

Sun Jan 17 2010, 06:52PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

This is not a true levitation and probably it is also a bit out of topic but I didnâ€™t want to start a new thread. Anyway, an interesting video:

Dinges

Sun Jan 17 2010, 10:14PM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
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Imsmooth wrote ...
I am guessing the bucking coil affects the inductance in such a way to make the lock range very narrow.

Not sure I agree there. All coils have induction - as long as the induction of two physical different coils is the same, they should both resonate at the same frequency? (Q-factor could be different, but don't see how that affects resonant frequency significantly)

As I understand it, lock range (2Fl) of a 4046 PLL is determined by the value of the resistors R1 and R2 (as connected to pin 11 and 12). Or would it be capture range (2Fc) that matters in this case?

Or maybe the loop filter is too slow to stabilize to the resonant frequency? Just guessing aloud here - am pretty new to phase locked loops.

I wonder if a series diode (along with your already present freewheel diode) would have saved the inverter. From what I understand, the addition of freewheel diodes without a Schottky series diode is a pretty useless thing due to 'current crowding' of the MOSFET body diode.

Peter.

IamSmooth

Sun Jan 17 2010, 11:59PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

My mosfets blew because I was out of resonance. The piece of steel shorted some of the coils, lowering the inductance. This put me in a zone where capacitative reactance was greater than inductive, and this is bad for the mosfets.

So far I have gotten it very stable that nothing is blowing, even after the piece falls out. I am still working on getting copper to melt. THe problem is as I turn up the power the piece is being pushed higher out of the coil, reducing the coupling with the workpiece.

Dinges

Mon Jan 18 2010, 01:56AM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
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Posts: 36

wrote ...
My mosfets blew because I was out of resonance.

They shouldn't have to blow - you can protect against this. See below.

wrote ...
This put me in a zone where capacitative reactance was greater than inductive, and this is bad for the mosfets.

I believe this is exactly what the series-Schottky and parallel freewheel diode are supposed to protect against:

http://www.richieburnett.co.uk/sstate3.html

Have been reading old threads of this forum. One thing I found in one of them was that, without the series Schottky diode, the freewheel diode was basically useless, as the FET's bodydiode crowded all the current. Think it was Steve Connor who even did an experiment to illustrate that, but could've been someone else too.

Peter.

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