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Size of induction heater work coil

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IamSmooth

Tue Nov 17 2009, 02:54PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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what size (turns and diameter) does everyone use for their coils?

Adrenaline

Tue Nov 17 2009, 04:05PM

Registered Member #235 Joined: Wed Feb 22 2006, 04:59PM
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I am currently using 9 turns of 1/4" diameter tubing with an ID of 2". I am probably going to move up to 11 turns to bring the frequency down a bit, operating at about 180kHz right now.

IamSmooth

Tue Nov 17 2009, 05:34PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Changing the coil parameters changes its inductance and, hence, the resonant frequency. Other than these effects, does the number of turns affect how it transfers energy to the workpiece? So, if I have a coil 2" ID with 5 turns operating at a Fres of 40kHz and a 2"/10 turn coil operating at a Fres of 40kHz what is the difference on how the object heats? Or, is there no significant difference?

Adrenaline

Tue Nov 17 2009, 06:32PM

Registered Member #235 Joined: Wed Feb 22 2006, 04:59PM
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You know your workpiece is heated by 2 effects, eddy current heating and hysteresis loss, for ferromagnetic pieces.

The magnitude of the eddy currents is derived from transformer action between your N turn work coil and the 1 pseudo-turn of the workpiece. So, the more turns in your coil the more current flowing in the workpiece.

I do not have an immediate clear picture on the hysteresis loss. However, more turns in your coil will generate a larger magnitude of magnetic field, therefore it should generate more hysteresis loss.

The coupling coefficient does play a role, however I believe the coupling is quite low and coil dimensions wouldn't have a huge effect.

Edit: Qualified first sentence with ferromagnetism.

klugesmith

Tue Nov 17 2009, 07:35PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Adrenaline wrote ...
The magnitude of the eddy currents is derived from transformer action between your N turn work coil and the 1 pseudo-turn of the workpiece. So, the more turns in your coil the more current flowing in the workpiece.

You are assuming constant current in the work coil. If its -voltage- is invariant then more turns -> fewer volts per turn -> fewer volts per turn in workpiece -> less current in workpiece.

I do not have an immediate clear picture on the hysteresis loss. However, more turns in your coil will generate a larger magnitude of magnetic field, therefore it should generate more hysteresis loss.

Same here, you are assuming constant current. If work coil voltage is invariant then more turns -> less magnetic flux -> less hysteresis loss.
Also, for a given flux density, lower frequency -> less hysteresis loss power.

IamSmooth

Tue Nov 17 2009, 07:53PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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A lot of variables. How low of a frequency can one go for induction heating to work? Right now I have 141khz. I can design the tank for 40khz. I can design it for 10khz. I figure with less switching losses and greater skin depth I can get more current into the tank at a lower Fres. Is this assumption correct?

IamSmooth

Thu Nov 19 2009, 05:40PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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I would think that lowering the turns would increase the voltage induced in the "one-turn" workpiece, as long as the magnetizing current did not get too high. Is three turns doable? Will having a smaller diameter coil transfer more energy to the workpiece?

teravolt

Fri Nov 20 2009, 05:42AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

less inductance meens higher frequency and more energy but harder on components. if it were my setup
I would shoot for 80k to 100khz. some idustrial setups can have 1"* 1/2" turn to 3 turns with a 1"diamiter. try 2-2.5" dia. with 6 turns of 1/4 to 5/16 tubing. if it works well you will need to cool the tubing. what type if caps do you intend to use?

IamSmooth

Fri Nov 20 2009, 03:58PM

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

I'm using Illinois Capacitor FPA snubber power film capacitors. They are rated for 3000vdc/750ac, 14A, 85degC.

Right now I can get a nut almost white hot with 141kHz using 5 turns 3/8" copper tubing at 2" diameter with a 1uf capacitor bank. My bank is made from 20 capacitors. I haven't implemented the water cooling, but it is set up for it with the copper plumbing. Right now the mass of copper allows me to run it for a few minutes without it getting crazy hot.

I am building an inverter in order to try out an entrainment frequency locking solution. I want to see how well this works. I want my capacitor bank to be around 4uf and the coil will be somewhere between 0.8-1uH (4 turns) for a frequency near 80kHz. I am hoping this will decrease switching loss and increase the voltage imparted on the workpiece for better heating.

Gabriel35

Sat Nov 21 2009, 01:53AM

Registered Member #2310 Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169

Hi Iam!
Do you have a video of your work?
I would like to see it in action...
Here you can see mine,

What do you think?

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