Indukcion heter problem

[balazs]

Hi
I built this indukcion heter:https://teslascience.wordpress.com/how-to-construct-simple-powerful-induction-heater/
I replaced the mosfets with IRFP460.
The drive and inverter work perfectly, the GDT signal is nice and there is no ringing in it.
The problem is that when I set the resonant frequency to a resonant frequency with an empty coil, the inverter consumes 0.40 A at 15 V, but when I put something in the coil, the power consumption decreases and consumes only 0.14 A 15 V .
What’s even weirder is that the resonant frequency doesn’t tune even when I’m doing something on the work coil. I adjust the resonance frequency by looking at the signal on an oscilloscope and when the amplitude of the sinusoidal signal is greatest. The resonant frequency is 72 khz.
Thanks in advance for the answers

[MRMILSTAR]

This posting really belongs in the "Electronic Circuits" section. You will get better results there.

[davekni]

The problem is that when I set the resonant frequency to a resonant frequency with an empty coil, the inverter consumes 0.40 A at 15 V, but when I put something in the coil, the power consumption decreases and consumes only 0.14 A 15 V .

Drive from the half-bridge is in series with the resonant circuit in this design.  It is normal for current to drop (impedance increase) as Q drops due to load when series-driven.

What’s even weirder is that the resonant frequency doesn’t tune even when I’m doing something on the work coil. I adjust the resonance frequency by looking at the signal on an oscilloscope and when the amplitude of the sinusoidal signal is greatest.

With ferromagnetic (steel) loads, it is possible for the frequency decrease due to magnetic properties to roughly match the frequency increase due to eddy currents, resulting in little net frequency change.

[balazs]

Tanks for the reply
Why does the impedance decrease when I put something in the work coil when it does not change the resonant frequency. Is there anything that can be done to reduce empty consumption?

[davekni]

Why does the impedance decrease when I put something in the work coil

The impedance seen by the drive circuit is increasing, not decreasing.  Higher impedance draws less current at fixed drive voltage as you measured.

when it does not change the resonant frequency.

Already tried to answer this.  Not sure what more to say.

Is there anything that can be done to reduce empty consumption?

That is a difficulty with series drive as in this design.  Requires more complex control circuitry or manual adjustment with a variac.  For loads of different shapes or materials that do happen to alter frequency, you could solve the empty power issue by tuning to the loaded frequency.

Parallel drive is much more common for DIY induction heaters, as in this long thread:
https://highvoltageforum.net/index.php?topic=530.0

If you do want to try electronic current control, a UD2.9 or similar skip-pulse DRSSTC driver could be made to work.  Would require changing to full H-bridge power stage rather than your existing half-bridge.

[balazs]

Why does the impedance change if the resonance frequency does not change?
If I put a lot of metal into the work coil, the consumption drops to 0.003A and the resonant frequency remains at 71 khz, while if the work coil is empty, it consumes 0.3A. Because of this, it hardly heats large amounts of metal.
And it certainly doesn't have to work that way.
I’m sorry to ask a lot back, I just don’t fully understand what is causing this.

[petespaco]

Maybe I am missing something here, but are you watching your power supply voltage as put workpieces into the work coil?  Could it be that your power supply is becoming overloaded and going into current limit.

[davekni]

If I put a lot of metal into the work coil, the consumption drops to 0.003A and the resonant frequency remains at 71 khz, while if the work coil is empty, it consumes 0.3A. Because of this, it hardly heats large amounts of metal.

I wonder if you are confusing resonant frequency with operating frequency.  Operating frequency of this circuit is set by the 20k potentiometer.  If you adjust the pot for 71kHz, it will stay at 71kHz.  When you add metal, the resonant frequency is likely changing.  Operating frequency will not change to the new resonant frequency until you adjust the potentiometer.  Adjust it to find the new resonant frequency.  Leave it at the loaded resonant frequency, and current will likely drop when metal is removed.

BTW, the ZVS parallel drive circuits in the thread I linked previously automatically run at resonant frequency.  They have no frequency adjustment potentiometer.

[Da_Stier]

One small addition, that I think will further explain, waht davekni just wrote:

In the most common / simplest ZVS incution heater like this one:



The resonant frequency and the operating frequency are basically the same, since the work coil is the component, that determins the oscillation or atleast a part of your resonant circuit.

In your circuit, you drive the work coil with a fixed frequency from the 555 timer circuit.
The resonant frequency of your work coil will of course change, however the only effect will be a impedance missmatch between your driver and your coil.
This is the reason why your input power goes down, since the energy can't be transferred into the coil anymore.

To get the effect, you are expecting, you will need a kind of a feedback loop, that automatically tunes your master oscillator to the resonant frequency of your work coil.





Greetings,
Michael

[balazs]

The power supply I use is 10A while the device consumes a total of 450mah, so unfortunately the power supply is not overloaded.
I do not confuse the resonant frequency with the operating frequency. I know that the drive produces the operating frequency and it must be set to reach the resonant frequency of the resonant circuit. Later, you want to replace the driver, which automatically finds the resonant frequency, there is only one arduino in the circuit that performs this operation, and the designer does not share the program running on the arduino. What you don’t want to start writing until it works with this simple switch.
I would clarify that most of the metals in the work coil slightly reduce the resonant frequency, 72 khz for an empty work coil, 69 kHz for a work coil full of metal, but the consumption of an empty work coil is still 300mah if a lot of iron is put in the work coil, and I set the operating frequency to 69 khz, it only consumes 40 mAh, which is very little. I measure the resonance frequencies with an oscilloscope.
For that i didn't build a ZVS because I want to upgrade to 11 kW later.

[Uspring]

The work coil together with the metal in it can be viewed as a transformer, with the work coil being the primary winding and the metal a secondary winding with a resistor connected to it.
When you drive a sinusoidal current through the primary, it will induce a voltage in the secondary. The secondary voltage will cause a current in the metal, which again will induce a voltage in the primary. That voltage would not be there if there weren't any metal in the coil. This voltage adds up to the voltage of the coil without the metal and will look mostly like it is being caused by a resistor in series with the coil.
And when you add a series resistor into the series tank, the current will drop.

[balazs]

And what can be done to avoid this? Because the person on the site does not have such problems.

[johnf]

Balazs
unless you fully understand how a circuit works it is not a good idea to change components, in this case the IRFP250 for the IRF 460.
These are very different in their specifications.
Teslascience obviously has got good knowledge and had made his one work
I suggest you copy his one exactly and get it working, then you can start exchanging components.

[davekni]

I doubt the IRF460 change is the issue. Likely just need much higher bus voltage.  If that isn't enough. then a lower transformer turns ratio.

Issue of higher current when unloaded has no simple fix with this circuit.  Need to turn off before removing load.