Induction heater coil modification

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.

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.

Veketti said:

seems to be Cu-Ni instead of pure copper

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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.

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

Veketti said:

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

Click to expand...

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.

mtwieg said:

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.

Click to expand...

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

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c_mitra said:

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.

Click to expand...

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?

Veketti said:

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?

Click to expand...

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!

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..

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.

This is not a good way to anneal brass, brass has VERY little magnetic properties, so it will not work well in an induction heater unless the frequency is very high (>1MHz), better simply to heat it on an element.