Induction heater o/p power issue (inverter working well)

HELLO ALL RESPECTED MEMBERS







In these days i am making induction heater having o/p power about 500W.i just googled all thing knows richhie's web page,neon john's website all.



I made inverter having frequency about 15Khz-100Khz.I am using IGBT module model:150GB128 in H bridge topology.O/P of inverter is nice.But i cant heat my work piece to red color(much hot). i can convert water into steam but no more then it.





Follwoing are information of my inputs:


300V DC
25Khz resonace frequency
primary current: 10A



now my questions are that my inverter takes 10A of primary current though why cant it heat my work pice to high temp?

and what is purpose of two capacitors shown in red rectangle in image........why cant connect load simillarly as we connect motor to h bridge?


i am using matching transformer core taken out from TV flyback primary turns 20 secondary turns 2. having 10 no of 470nf capacitors and i uses parallel resonace






plz help




-Regards

The purpose of the two caps is to provide a RF centre tap for the DC supply. Suppose they were replaced with an actual centre tap, then when the upper transistor comes on, the current will flow through the load,from +300V to centre tap. When the lower transistor comes on the current flow through the load is the other way around, from the centre tap to earth. The centre tap has been replaced by the two capacitors which are in series with the load so must be of low impedance and high current rating.
One clue might be how much the supply current changes when the iron bar is stuck in the coil. Without the iron, the current should be low, introducing the iron should then increase the current
You could put a known quantity of water in a cup and time it until it has boiled away. Then you can work out the power to do this and find out how much power you have got.
Frank

- - - Updated - - -

More thoughts:- Change the oscillator frequency to bring the output circuit on tune. I do not think that 500W will heat much iron to red heat, try a loop of thin garden fence wire as a starter.
Frank

First of all thanks for such good explanation.


When i put irom work pice in coil current flactuats to 5 A from 4A at supply voltage 270v dc


If i increase frequeny about 25khz from 20khz current is surpisingly reaches to 10A at 270v.......but heating not increases....it remains as it is but primary of coupling transformer burns

- - - Updated - - -

Electronics_chaitanya said:

First of all thanks for such good explanation.


When i put irom work pice in coil current flactuats to 5 A from 4A at supply voltage 270v dc


If i increase frequeny about 25khz from 20khz current is surpisingly reaches to 10A at 270v.......but heating not increases....it remains as it is but primary of coupling transformer burns

Click to expand...

It continioulsly takes current about 6A from supply at 30% duty cycle if i make duty cycle to 50% current goes to 15A

I was unsure of your series tuned output circuit. If your components have low losses, at resonance, they would present a short circuit!, could this be the reason for your primary overheating as you approach the resonant frequency?
At a guess you are after a 1500 :1 impedance transformation ( 300V@ 2A to .1 ohms), or 40:1 turns ratio. I think that the unloaded PA current, which should be low, to the loaded PA current is a good indicator of the efficiency of the circuit.
I would try the circuit as a parallel resonant circuit to see if things improve.
Frank