Induction heater coil modification

Veketti

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Dear All,

Recently I bought from ebay 20A ZVS induction heater and my purpose was to annel brass with it. Problem that I faced is that it is very inefficient to brass. Works well on steel though. So I decided to make new coil for it to concentrate the power to smaller area. Ok, so I bought some brake line copper (seems to be Cu-Ni instead of pure copper). I thought that as long as the coil has same inductance it would work. I was wrong. Both are now ~1.3uH but induction heater seems like shorted and doesn't start to oscillate with this self made coil.

My question to someone who understands how these work is what is the important factor that needs to be met with smaller diameter heating coil to make it work? Could it be the resistance of the coil? The original is 2.43 mohm and fabricated is 27.5 mohm.

Here's the picture of the device and the coils. Self made looks like birds nest but it wasn't too easy to wound three layers without shorting the coils. :lol:


Thank you in advance. Really appreciate your help.

mtwieg

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It could be that the coil's Q is not high enough. But your coil looks like its inductance should be much lower than the original. And any shorting between turns will drastically reduce the effective inductance.

BradtheRad

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seems to be Cu-Ni instead of pure copper

Nickel is magnetic. It belongs to the family of ferromagnetic metals. That may alter operation of your inductive heater. An internet search turns up several discussions.

- - - Updated - - -

(I meant references about nickel being magnetic. Didn't look at discussions about inductive heaters.)

c_mitra

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So I decided to make new coil for it to concentrate the power to smaller area.

Something is not clear. The coil is the primary of a RF transformer. When you put a iron nail in the hole, the nail acts like the secondary. It produces eddy current, hysteresis etc. That causes the heating.

The whole energy is concentrated onto the sample. You need not do anything else to focus the energy.

If it is working well for magnetic materials, the major cause of heating is hysteresis. If it is working for other materials well, then the eddy current is the cause of heating.

Veketti

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It could be that the coil's Q is not high enough. But your coil looks like its inductance should be much lower than the original. And any shorting between turns will drastically reduce the effective inductance.

Indeed, I measured both coils and original has Q = 21 and fabricated Q = 3.

- - - Updated - - -

Something is not clear. The coil is the primary of a RF transformer. When you put a iron nail in the hole, the nail acts like the secondary. It produces eddy current, hysteresis etc. That causes the heating.

The whole energy is concentrated onto the sample. You need not do anything else to focus the energy.

If it is working well for magnetic materials, the major cause of heating is hysteresis. If it is working for other materials well, then the eddy current is the cause of heating.

Ok, did I understood right that it's useless what I'm doing? Smaller coil doesn't make it any more effective, meaning it wont concentrate the eddy currents to smaller area thus making it heat small dia object faster?

c_mitra

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Ok, did I understood right that it's useless what I'm doing? Smaller coil doesn't make it any more effective, meaning it wont concentrate the eddy currents to smaller area thus making it heat small dia object faster?

Right, at least to the first order.

Consider the microwave oven. With nothing in the cavity (that is the cooking chamber), it does supply only marginal power that consumed very little energy. Once you put a cup of water, it absorbs the microwave and "sucks" energy into the water mass. The magnetron knows that and produces more energy and sends it to the cavity. As if the energy is directed to the cup of water!

Veketti

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So it left me wonder is there different kind of induction heaters which are more effective on non ferrous metals? Or is it just so that those seen on youtube videos are just much more powerful..

BradtheRad

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I don't know the explanation but it might be deduced (induced? ha ha) by comparing the concepts below. Some depend on eddy currents, some make use of magnetic metals.

* stovetop induction cooker (pot must be ferromagnetic)

* Certain metals heated in microwave oven produce extreme heat, and may even spark

* Neodymium magnet drops slowly through the middle of a copper pipe (requires tight-clearance), but not a steel pipe.

* Neodymium magnet is lifted vertically when held against spinning aluminum turntable, but not a steel turntable.

TL;DR Summary

FUN induction heater circuit.

If you have not built an induction heater yet it is a fun project. Build the small circuit with 6 yellow capacitors first.

Induction heater is a fun project. I built a small induction heater then I wanted a more powerful one. A small unit is so simple you can build it in about 2 hours work. A larger more powerful unit will take longer. If you take your induction camping it works great to run it on your 12v car battery.

I built my largest induction heater from a microwave oven transformer. Remove the v secondary coil, replace it with enough #12 solid copper wire to get 12v to 15vdc. I have about 15 MOTs they all have 100 turns on the primary coil. Math is easy for the secondary coil running on 120vac on the primary = about .833 vac per turn on the secondary.

I have not experimented with a flat induction coil the value must be 2. uh minimum value. Buy a, resistor, capacitor, inductor, meter about $35. on ebay to test your coils. The choke coil needs to be large enough not to saturate or it will stop working at a certain power range then current suddenly goes to maximum then mosfets explode. Very low internal mosfet resistance works best it prevents mosfets from over heating easy.

Here is my induction circuit, leave off the transformers so you can run it on a car battery if that is what you want. My small induction heater will heat a 1/4" steel rod red hot in about 3 minutes. My w induction heater heats a 1/4" steel rod red hot in 7 seconds. You don't need w to heat food it will burn before food has time to cook.

My induction heaters all have the same circuit drawing for the exception of larger power supplies and larger L2 choke coil for higher power units. If you change #10 circuit copper wire to a smaller wire skin effect goes up and the circuit efficiency goes down. My induction heat is several times smaller than the w power supply as you can see in the photos.

Be sure to connect the caps in a box shape like a Rail Gun cap bank charge and discharge is much faster. Build the small unit with the 6 yellow color caps first. I use insulated Romex solid copper house wire. Be sure to use insulated wire if you accidently touch the osc coil with metal osc stops then 1 of the mosfets will explode.

You can build the small circuit for $15. The larger circuit only cost be extra for a 100a bridge rectifier and the meter. I salvage parts from old TV and old computer power supplies. 10 mosfets are $6 free shipping on ebay if you order from a seller in China.