High resistance in Resonant LC circuit

Thread Starter

electricalnoob

Joined Nov 16, 2023
2
Hello,

I have an RF generator that outputs power at 6.78Mhz with an output impedance of 50 ohms. I want to use this for induction heating and have seen it work great for my application.

I have wound my coil around the piece I want to heat and measured :
inductance (1.9940micro Henries)
impedance (85.145 ohms)
and resistance (5 ohms) at the 6.78Mhz using a Keyence E4990A impedance analyzer.

I then select and measure my capacitors
280 pF,
84.6 ohms impedance and
2.34 ohm resistance.

My issue is, I then put the capacitors and inductor in the analyzer connected in parallel and I see a high resistance of 431.68 ohms? where is this coming from?
My impedance is 431.68 ohms, the resistance is 431.68 ohms and my phase angle is 0.045 degree implying I have successfully found resonance at 6.78Mhz but for some reason my purely resistive circuit is unexpectedly high and i need it to be at the 50 impedance to match my power supply . This is even before I put it into my circuit that connects to the power supply.

I have two circuits from the vendor that are 50ohms impedance and 0 phase angle and have variations in coil size & turns and they heat the part very fast. When I go to make my own this high resistance still allows the part to heat but it is noticeably slower despite being very similar to the two physically different circuits i received.
I am a mechanical engineer learning all this on the fly, any help would be appreciated. Attached are the data sheets of the caps i used which should be rated for this.
 

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nsaspook

Joined Aug 27, 2009
12,782
What you're measuring at parallel resonance is the loss factor expressed as resistance. As power is transferred to the work piece by induction this resistance will fall to some lower value but it might not be matched for max power transfer without some tuning.
https://www.uihm.com/en/Induction-H...ails-of-High-Frequency-Induction-Heating.html
In the parallel resonant tank circuit the work coil can be thought of as an inductive load with a "power factor correction" capacitor connected across it. The PFC capacitor provides reactive current flow equal and opposite to the large inductive current drawn by the work coil. The key thing to remember is that this huge current is localised to the work coil and its capacitor, and merely represents reactive power sloshing back-and-forth between the two. Therefore the only real current flow from the inverter is the relatively small amount required to overcome losses in the "PFC" capacitor and the work coil. There is always some loss in this tank circuit due to dielectric loss in the capacitor and skin effect causing resistive losses in the capacitor and work coil. Therefore a small current is always drawn from the inverter even with no workpiece present. When a lossy workpiece is inserted into the work coil, this damps the parallel resonant circuit by introducing a further loss into the system. Therefore the current drawn by the parallel resonant tank circuit increases when a workpiece is entered into the coil.
 

Thread Starter

electricalnoob

Joined Nov 16, 2023
2
What you're measuring at parallel resonance is the loss factor expressed as resistance. As power is transferred to the work piece by induction this resistance will fall to some lower value but it might not be matched for max power transfer without some tuning.
https://www.uihm.com/en/Induction-H...ails-of-High-Frequency-Induction-Heating.html
Thanks for the response! first time posting so I appreciate it. I am taking sometime to read through that article and learn more as this is mostly new to me so hopefully i will get somewhere on my own. What I find strange is the workpiece, coil, and capacitance I received and measured is already "tuned" to 50 ohms with the work piece in it and without applying power. It makes me think I need to achieve the 50 ohms prior to applying power rather than having a different impedance measurement and hoping it will drop to the correct matching impedance when I turn it on. Any thoughts? I will try to take a closer look at the pre-tuned circuitry.
 

nsaspook

Joined Aug 27, 2009
12,782
I've no idea what type of vendor coils you have because you supplied no information about them or images of your coil build or test configuration setup with or without a work load but your Keyence E4990A impedance analyzer provides the power to make the measurements. The actual level of power doesn't make much difference in measuring impedance until we have physical changes due to losses.
 

Janis59

Joined Aug 21, 2017
1,821
The capacitors You may NOT use the ordinary ones. It will explode in the seconds because in the resonant tank of inductance heating are flowing megaVARs of reactive energy or even hundreds of MVAR. Thus, 1 step - measure the readymade coil inductance, 2 step - calculate the wished capacitance at wished frequency, 3 step - calculate the volts * amperes and choose capacitor from high reactive power series, where one capacitor may cost as well 7000 $ or even 70 000 $ (sorry, such is a life!).
Only way around is that used frequently at DIY constructions - divide the capacitance between 20-50 strong built capacitors what adittionally are immersed in destillate. Or other more rarely used cheap solution, use Rogers 5880 biclad teflon-based PCB plates for 380 $ per A3 size, where uniform capacitance is between 1.8 and 2,2 pF/cm2. It allows aboy 20 A per inch squared under strong wind or 5A without of external wind and have Q-factor over the 2000. To be different with ordinary pcb (FR-4) what Q-factor is hard under 100, so much it is lossy.
 
Last edited:

Janis59

Joined Aug 21, 2017
1,821
Coils may be made of ordinary santech (Chineese) pipes 6 mm, 8 mm, 10 mm, 12 mm. Water must circulate as at 3 kW as at 60 kW machines according to our experience. We are making our coils uninhibitelly. Only the trick is to make beforehand the plywood form for bending with roll.

By the way - do You ABSOLUTE sure Your machine not content the capacitors somewhere inside?? Yet we have one of our 100 kW machines with two-box diagram, where second box are solely capacitors thus may be lost. But all other our machines contain capacitors in the unit under the cover.
 

nsaspook

Joined Aug 27, 2009
12,782
The capacitors You may NOT use the ordinary ones. It will explode in the seconds because in the resonant tank of inductance heating are flowing megaVARs of reactive energy or even hundreds of MVAR. Thus, 1 step - measure the readymade coil inductance, 2 step - calculate the wished capacitance at wished frequency, 3 step - calculate the volts * amperes and choose capacitor from high reactive power series, where one capacitor may cost as well 7000 $ or even 70 000 $ (sorry, such is a life!).
Only way around is that used frequently at DIY constructions - divide the capacitance between 20-50 strong built capacitors what adittionally are immersed in destillate. Or other more rarely used cheap solution, use Rogers 5880 biclad teflon-based PCB plates for 380 $ per A3 size, where uniform capacitance is between 1.8 and 2,2 pF/cm2. It allows aboy 20 A per inch squared under strong wind or 5A without of external wind and have Q-factor over the 2000. To be different with ordinary pcb (FR-4) what Q-factor is hard under 100, so much it is lossy.
https://www.vishay.com/docs/22131/twx-twxf.pdf
 
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