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I implemented a parallel LC induction heater using theory. I made a simple square wave Inverter providing a frequency of 20kHz to the tuned LC circuit. The problem I'm facing is that my tank is not drawing any current although the tank is at resonance with the supply. The picture of the output wave across the coil wrt ground is attached. Should I attach a resistance in parallel to the tank? I don't have any resistance in parallel to that. The picture of the circuit is also attached. Circuit of the parallel tank

Voltage waveform on the tank circuit. It is at 20kHz, the oscilloscope was fluctuating because of switching harmonics.

EDIT:

@Marla - I have a variable dc power supply with a built-in ammeter and voltmeter. It has a max rating of 30V at 15A. So, that's where I'm measuring the current. The scope is stating the wrong frequency due to switching harmonics (stays constant at 20.2kHz then fluctuates then goes back to 20.2kHz). The switching frequency of Mosfets is 20.2kHz. Component values => L=1.9uH ; C=33uF; fsw=20.2kHz.

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  • \$\begingroup\$ Component values? Also your scope says 10 kHz. How do you measure current? You said tank isn't drawing current. Edit this into you question. \$\endgroup\$
    – Marla
    Commented Jan 15, 2019 at 18:34
  • \$\begingroup\$ Also, what load do you have in coil? Photographs will help. \$\endgroup\$
    – Marla
    Commented Jan 15, 2019 at 18:36
  • \$\begingroup\$ @Marla - the load was the tip of cast iron pliers. The inner dia of the coil is around 2.5inches and its an 8SWG copper wire wound into a 7 turn coil to give 1.9uH inductance. \$\endgroup\$
    – Sajeer
    Commented Jan 15, 2019 at 18:59
  • \$\begingroup\$ Have you confirmed that both the left side of coil and right side of coil are being switched? \$\endgroup\$
    – Marla
    Commented Jan 15, 2019 at 19:51
  • \$\begingroup\$ Yes. Otherwise it wouldn't go in negative polarity. \$\endgroup\$
    – Sajeer
    Commented Jan 15, 2019 at 19:57

1 Answer 1

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For parallel resonant heating circuits, you need to include a matching inductor in the inverter output in series with the parallel LC tank, something like as shown in the following image:

Parallel resonant induction heater with matching inductors

Image source: https://www.uihm.com/en/Induction-Heating-Technology/Base-details-of-High-Frequency-Induction-Heating.html

Having an inductor on only one side is in fact sufficient. The matching inductor helps pushing energy into the parallel resonant circuit against the rejecting voltages present across the LC tank making it a current source inverter.

That being said, too much inductance would also prevent current flow to the LC tank and cause power to be reduced. Try with few turns without any magnetic core at safe low voltage and increase further to arrive at an optimal value as per need.

Hope this helps.

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  • \$\begingroup\$ Please add a citation or link for the image you copied into your answer. Our site policy is that all material copied into an answer must have proper attribution. \$\endgroup\$ Commented Jan 15, 2022 at 19:28

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